Photonic Materials: Recent Advances and Emerging Applications 9815049763, 9789815049763

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Photonic Materials: Recent Advances and Emerging Applications Edited by Aavishkar Katti

School of Physics Dr. Vishwanath Karad MIT World Peace University Pune India

& Yogesh Sharma

Faculty of Science, SGT University Gurgram-122505, India Department of Physics India

Photonic Materials: Recent Advances and Emerging Applications Editors: Aavishkar Katti and Yogesh Sharma ISBN (Online): 978-981-5049-75-6 ISBN (Print): 978-981-5049-76-3 ISBN (Paperback): 978-981-5049-77-0 © 2023, Bentham Books imprint. Published by Bentham Science Publishers Pte. Ltd. Singapore. All Rights Reserved. First published in 2023.

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CONTENTS FOREWORD ........................................................................................................................................... i PREFACE ................................................................................................................................................ ii LIST OF CONTRIBUTORS .................................................................................................................. v CHAPTER 1 PHOTONIC CRYSTAL INSTRUMENTS ................................................................. Muhammad A. Butt INTRODUCTION .......................................................................................................................... PC SENSING INSTRUMENTS .................................................................................................... PC OPTICAL LOGIC GATES ..................................................................................................... PC OPTICAL POWER SPLITTER AND POL. SPLITTER .................................................... PC POLARIZATION MAINTAINING INSTRUMENTS ......................................................... PC BASED LASERS ...................................................................................................................... CONCLUDING REMARKS ......................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 2 ALL-OPTICAL LOGIC GATE USING PHOTONIC CRYSTALS FOR ULTRAFAST TELECOMMUNICATION APPLICATIONS ......................................................................... Margarat Michael, B. Elizabeth Caroline, J. Vidhya, M. Saravanan and P Nithyavalli INTRODUCTION .......................................................................................................................... RELATED WORKS ....................................................................................................................... LIGHT PROPAGATION IN PERIODIC MEDIA ..................................................................... TYPES OF PHOTONIC CRYSTALS .......................................................................................... 1D PhC .................................................................................................................................... 2D PhC .................................................................................................................................... 3D PhC .................................................................................................................................... PRINCIPLE OF OPERATION ..................................................................................................... PWE Method ........................................................................................................................... Solutions of Maxwell’s Equations in Frequency Domain ............................................. FDTD Method ......................................................................................................................... DESIGN OF PROPOSED ALL-OPTICAL XOR LOGIC GATE ............................................ PROPOSED ALL-OPTICAL XOR LOGIC GATE USING NANORESONATORS ............. CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 3 PRESSURE DEPENDENT REFLECTANCE AND TRANSMITT- ANCE PROPERTIES IN 1D- PHOTONIC CRYSTAL CONTAINING GERMANIUM (GE) .................. Sanjeev K Srivastava, Yogesh Sharma and Mirza Tanweer Ahmad Beig INTRODUCTION .......................................................................................................................... THEORETICAL MODEL ............................................................................................................ RESULTS AND DISCUSSION ..................................................................................................... Effect of Hydrostatic Pressure on Reflectance Properties of Normal PC Structure ............... Effect of Hydrostatic Pressure on the Transmission mode of Conjugate PC Structure .......... CONCLUSION ...............................................................................................................................

1 1 5 7 9 11 14 16 17 17 17 17 21 22 23 24 25 25 26 27 28 29 29 30 32 37 40 40 40 40 40

43 43 45 47 48 49 52

CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ...............................................................................................................................

53 53 53 53

CHAPTER 4 RECENT ADVANCES IN GRAPHENE BASED PLASMONICS ........................... Tista Basak and Tushima Basak INTRODUCTION .......................................................................................................................... THEORETICAL FRAMEWORK OF PLASMONS IN GRAPHENE ..................................... Electronic Structure of Graphene ............................................................................................ Optical Response and Dispersion Relation of Graphene Surface Plasmons .......................... Semi-classical Model .................................................................................................... Random Phase Approximation(RPA) ............................................................................ TYPES OF GRAPHENE SURFACE PLASMONS .................................................................... COUPLING OF SURFACE PLASMONS WITH PHOTONS, PHONONS AND ELECTRONS .................................................................................................................................. BEHAVIOUR OF SURFACE PLASMONS IN GRAPHENE WITH DIFFERENT DIMENSIONALITIES ................................................................................................................... Characteristics of Surface Plasmons in 2D Bilayer Graphene ................................................ Characteristics of Surface Plasmons in 1D Graphene Nanoribbons (GNRs) and 0D Graphene Quantum Dots (GQDs) ........................................................................................... CURRENT APPLICATIONS OF SURFACE PLASMONS IN GRAPHENE ......................... CONCLUSION AND FUTURE PERSPECTIVES ..................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ...............................................................................................................................

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CHAPTER 5 THIRD GENERATION SOLAR CELLS - PROMISING DEVICES TO MEET THE FUTURE ENERGY NEEDS ......................................................................................................... Ram Chhavi Sharma INTRODUCTION .......................................................................................................................... Basic Parameters ..................................................................................................................... Third Generation Solar Cells .................................................................................................. Organic Solar Cells ................................................................................................................. Main Features of Conjugated Polymers .................................................................................. Principle of Operation ............................................................................................................. Perovskite Solar Cells ............................................................................................................. Dye Sensitized Solar Cells (DSSCs) ....................................................................................... Quantum Dot Solar Cells ........................................................................................................ CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENTS ........................................................................................................... REFERENCES ............................................................................................................................... CHAPTER 6 RECENT ADVANCES OF GRAPHENE IN SOLAR CELL APPLICATIONS .... Chandra Kamal Borah and Sanjeev Kumar INTRODUCTION .......................................................................................................................... APPLICATION OF GR IN VARIOUS TYPES OF SOLAR CELLS ....................................... Gr in Heterojunction Silicon Solar Cell .................................................................................. Graphene in Dye-Sensitized Solar Cells .................................................................................

57 61 61 63 64 66 68 68 72 72 73 74 76 76 76 76 77 85 85 87 90 90 91 92 94 96 96 98 98 98 98 98 101 101 103 103 106

Graphene in Perovskite Solar Cells ........................................................................................ Graphene in Other Types of Solar cells .................................................................................. CONCLUSION ............................................................................................................................... ABBREVIATIONS ......................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 7 A REVIEW ON THE MATERIALS AND APPLICATIONS OF NANOPHOTONICS ............................................................................................................................... Athira Jayaprakash, Joshua Nigel and Ishu Sharma INTRODUCTION .......................................................................................................................... Materials ................................................................................................................................. Two-dimensional Materials .......................................................................................... Single Layered Graphene .............................................................................................. Transition Metal Dichalcogenides ................................................................................ Photonic Crystals .......................................................................................................... Dielectric Nanostructures ............................................................................................. Fabrication Techniques ........................................................................................................... Top-down Methods ........................................................................................................ Bottom-up Methods ....................................................................................................... APPLICATIONS ............................................................................................................................ Absorbers ................................................................................................................................ Graphene Photodetectors ........................................................................................................ 2D TMDCs Based LEDs ........................................................................................................ Biosensing ............................................................................................................................... CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 8 REVOLUTIONARY FUTURE USING THE ULTIMATE POTENTIAL OF NANOPHOTONICS ............................................................................................................................... Sumaya Khan and Ishu Sharma INTRODUCTION .......................................................................................................................... Photons and Electrons: Similarities and Differences .............................................................. Photons and Electrons: The Constriction in Various Facets ........................................ Propagation of sub-atomic Particles through a Classically Forbidden Zone .............. Localization Under a Periodic Potential ...................................................................... Nanoscale Optical Interactions ............................................................................................... Axial Nanoscopic Localization ..................................................................................... Lateral Nanoscopic Localization .................................................................................. New Cooperative Transitions ................................................................................................. Applications ............................................................................................................................ Synchronous Oscillations of Delocalized Electrons on Nanoparticles and Surfaces ... Fluorescence-Based Systems ........................................................................................ Semiconductor Nanocrystals: Single-photon Sources .................................................. Semiconductor Nanocrystals: New Fluorescent Labels for Biology ............................ Nano-Based Semiconductor Crystals: a New Active Component for Excimer Lasers Organic Light Emitting Diode ......................................................................................

106 107 108 108 109 109 109 109 116 117 119 119 120 122 124 125 127 128 129 130 130 130 131 132 133 133 133 133 133 141 142 142 143 144 145 146 146 148 150 151 152 153 153 154 155 155

DISCUSSION .................................................................................................................................. CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 9 A SIMULATIVE STUDY ON ELECTRO-OPTIC CHARACTER- ISTICS OF INALGAAS/INP FOR FIBER OPTIC-BASED COMMUNICATIONS UNDER NANOSCALE WELL THICKNESS LAYERS .............................................................................................................. Pyare Lal and P. A. Alvi INTRODUCTION .......................................................................................................................... SIMULATED HETEROINTERFACE NANOSTRUCTURE, AND THEORETICAL DETAILS ......................................................................................................................................... COMPUTATIONAL RESULTS AND DISCUSSION ................................................................ CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGMENTS .............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 10 TWO-DIMENSIONAL MATERIALS FOR ADVANCEMENT OF FIBER LASER TECHNOLOGIES .................................................................................................................... Kavintheran Thambiratnam, Norazriena Yusoff, Siti Aisyah Reduan, Muhamad Zharif Samion, Shok Ing Ooi and Harith Ahmad INTRODUCTION .......................................................................................................................... 2D Material-Based Saturable Absorbers for Fiber Lasers ...................................................... 2D Chalcogenides ................................................................................................................... Metal Monochalcogenides (MMs) ................................................................................ Transition Metal Dichalcogenides (TMDs) .................................................................. MXenes ................................................................................................................................... OPERATING PRINCIPLES FOR PULSE GENERATION IN FIBER LASER TECHNOLOGY ............................................................................................................................. Q-switching Technique ........................................................................................................... Mode-locking Technique ........................................................................................................ CONFIGURATION OF 2D MATERIALS AS SATURABLE ABSORBERS ......................... GENERATION OF Q-SWITCHED PULSES IN FIBER .......................................................... The 1.0 μm Wavelength Region ............................................................................................. The 1.5 μm Wavelength Region ............................................................................................. The 2.0 μm Wavelength Region ............................................................................................. GENERATION OF MODE-LOCKED PULSES IN FIBER LASERS ..................................... The 1.0 μm Wavelength Region ............................................................................................. The 1.5 μm Wavelength Region ............................................................................................. The 2.0 μm Wavelength Region ............................................................................................. CONCLUSION, CHALLENGES, AND FUTURE PERSPECTIVES ...................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ...............................................................................................................................

156 158 158 158 158 158

160 161 162 167 174 174 174 174 174 177 177 179 180 181 184 186 187 187 189 190 192 192 194 195 197 197 199 200 202 202 203 203 203

CHAPTER 11 OPTICAL PROPERTIES OF HOLLOW-CORE BRAGG FIBER WAVEGUIDES ....................................................................................................................................... 214

Ritesh Kumar Chourasia, Nitesh K. Chourasia and Narendra Bihari INTRODUCTION AND MOTIVATION ..................................................................................... THEORETICAL MODELLING OF THE PROPOSED STRUCTURE .................................. HANKEL FORMALISM OF THE PROPOSED STRUCTURE .............................................. NUMERICAL RESULTS AND DISCUSSION ........................................................................... CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 12 PHOTONIC NANOSTRUCTURED BRAGG FUEL ADULTERATION SENSOR ................................................................................................................................................... Ritesh Kumar Chourasia, Nitesh K. Chourasia, Ankita Srivastava and Narendra Bihari INTRODUCTION .......................................................................................................................... MODELLING OF THE BFW PHOTONIC NANOSTRUCTURE ........................................... Hankel Function Formalism (HFF) and Transfer Matrix Methodology (TMM) in Cylindrical Coordinates .......................................................................................................... Various Predictive Models ...................................................................................................... Fuel Energy Adulteration Sensor Performance Parameter ..................................................... NUMERICAL RESULTS AND DISCUSSION ........................................................................... CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ............................................................................................................................... CHAPTER 13 MODELLING FABRICATION VARIABILITY IN SILICON PHOTONIC DEVICES. ................................................................................................................................................ Mursal Ayub Hamdani and Gausia Qazi INTRODUCTION .......................................................................................................................... PHOTONIC DEVICE LEVEL OPTIMIZATION ...................................................................... Iterative Optimisation Algorithms .......................................................................................... Empirical Optimisation Algorithms .............................................................................. QR-code Structure Algorithms ...................................................................................... Irregular Structure Algorithms ..................................................................................... Deep Neural Networks Assisted Silicon Photonics Design .................................................... Multilayer Perceptron ................................................................................................... Convolution Neural Network ........................................................................................ PHOTONIC CIRCUIT LEVEL OPTIMIZATION .................................................................... Stochastic Collocation Method ............................................................................................... Polynomial Chaos Expansion ................................................................................................. Layout Aware Variational Analysis ........................................................................................ CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENT ............................................................................................................. REFERENCES ...............................................................................................................................

215 216 220 222 232 232 232 232 232 237 238 242 242 246 248 248 258 259 259 259 259 265 265 266 268 268 268 269 271 271 272 274 275 276 277 281 281 281 281 281

CHAPTER 14 INTRODUCTION OF SMART MATERIALS: THE ART TO OUTRIVAL TECHNOLOGY ...................................................................................................................................... Claire Mary Savio and Ishu Sharma INTRODUCTION .......................................................................................................................... PREPARATION METHODS ........................................................................................................ Combustion Synthesis Method ............................................................................................... Preparation of Piezoelectric Materials .................................................................................... Vacuum Induction Melting Method ........................................................................................ Using Molecular Complexes ................................................................................................... Mixed Oxide Technology ....................................................................................................... SMART MATERIALS IN ELECTRICAL ENGINEERING .................................................... Conductive Inks ...................................................................................................................... Muscle Wire ............................................................................................................................ Electro-textiles ........................................................................................................................ Light Diffusing Acrylics ......................................................................................................... Smart Grids ............................................................................................................................. Applications in Other Fields ................................................................................................... Structural Engineering ............................................................................................................ Self-Repair .............................................................................................................................. Defense and Space .................................................................................................................. Nuclear Industries ................................................................................................................... Biomedical Applications ......................................................................................................... Reducing Electronic Waste ..................................................................................................... Reducing Food Waste ............................................................................................................. Health ...................................................................................................................................... The Ageing Population ........................................................................................................... Civil Engineering .................................................................................................................... Soft Robotics ........................................................................................................................... Future Prospects ...................................................................................................................... Cardiac Tissue Engineering .................................................................................................... Civil engineering ..................................................................................................................... Swarm Robotics ...................................................................................................................... Soft Robotics ........................................................................................................................... Hydrogels ................................................................................................................................ CONCLUSION ............................................................................................................................... CONSENT FOR PUBLICATION ................................................................................................ CONFLICT OF INTEREST ......................................................................................................... ACKNOWLEDGEMENTS ........................................................................................................... REFERENCES ...............................................................................................................................

284 284 289 289 290 291 292 292 294 294 295 295 295 295 296 296 296 297 297 297 298 298 298 298 299 299 299 300 300 300 301 301 301 302 302 302 303

SUBJECT INDEX .................................................................................................................................... 

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FOREWORD I feel immense pleasure to write the foreword to the book, titled “Photonic materials: recent advances and emerging applications” edited by Dr. Aavishkar Katti and Dr. Yogesh Sharma. One of the editors, Dr. Katti is already known in the science community as he has authored a research monograph “Optical Spatial Solitons in Photorefractive Materials” on the photorefractive solitons and their various applications, which is published by Springer, Singapore. He is an expert in photorefractive materials and non-linear dynamics. The other editor, Dr. Sharma has been deeply involved in research on band gap engineering in magnetic photonic crystals. Both editors are well known to me as they have obtained their doctoral degrees from Banaras Hindu University. This book describes current and cutting-edge research in the diverse area of photonics. There are fourteen chapters in the book covering theoretical, computational, and experimental research in photonic crystals, nonlinear optical materials, solar cells, semiconductor heterostructures, nano photonics, graphene-based photonics, and silicon photonics among other topics. Near the beginning, the chapters discuss optical logic gates, power splitter, polarizer, all-optical XOR gate, and optical properties of one-dimensional layered structure containing germanium. This optical XOR gate would replace the XOR gate based on semiconductors in the near future. The effect of the photovoltaic field on phase shift grating formed by nonlinear photorefractive materials is well described in one of the chapters. When you will further delve deeper into the book, you will find chapters based on graphene plasmonics, third-generation solar cells and the use of graphene in solar cells. Solar cells are always looked at as an alternative to conventional energy sources since they are used for energy tapping through the Sun. The use of graphene for increasing the efficiency of solar cells has been investigated. Nowadays, nanophotonics has aroused the interest of the scientific research community. A few chapters focus on the properties and applications of optical materials used for nanophotonics. Recent research on fiber Bragg gratings has been beautifully captured in subsequent chapters while novel materials have been investigated in the next chapters. The applications of mono chalcogenides, transition metal dichalcogenides, and MXenes from fibre laser have been discussed. Some smart materials in photonics have also been reviewed. Lastly, the book includes Monte Carlo, stochastic collocation, and polynomial chaos expansion techniques for modelling of photonic integrated circuits. This book is useful for beginners and advanced researchers in differentfields of theoretical or experimental optics and photonics, and material science. Graduates in physical sciences who are interested to pursue research in photonics will be highly benefitted from this book. I wish the book all the success and hope that it is useful for its target audience.

Dr. Surendra Prasad Professor Department of Physics Institute of Science Banaras Hindu University Varanasi-221005 India

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PREFACE Is photonics the new electronics? If we compare the basic elements in electronics viz. the electron with the basic unit in photonics such as photon, soliton and plasmon, we find an uncanny similarity with device applications. This is reinforced if we go on further and compare other elements like electrical cables and optical fibres or plasmonic waveguides, electrical generators and lasers or masers, electric circuits and optical circuits and finally conventional transistors and optical transistors. It can be clearly inferred that photonics has clear analogues for all tools of electronics. It is due to these similarities that the photonic community believes that photonic devices will be able to replace electronic devices entirely. In fact, even now, photonic devices are ubiquitous in fields like, biomedicine, where lasers are used to treat many diseases; aerospace technology, dealing with laser altimeters, laser radars, etc.; in engineering, where photonics is central to manufacturing MEMS and lasers are used for photonic devices, etc.; in information technology for data storage, optical switching, and data transmission using optical fibers among many other applications of practical importance. Such photonic devices encompass a diverse variety of materials like photonic crystals, nonlinear optical crystals like photorefractive crystals and liquid crystals, optical metamaterials, semiconductor laser materials, electro-optic and magneto-optic materials, photonic polymers, and photonic crystal fibers among many others. In the present book, we present the latest trends and research in the broad field of photonics and photonic materials applications. The chapters are categorized as follows: We shall first consider Photonic Crystals. Chapter 1 summarizes recent developments in the field of photonic crystals by presenting the utmost frequent and necessary optical devices established based on PCs such as optical logic gates, optical power splitters, polarization splitters, sensing devices, and lasers. In comparison to conventional photonic devices, these devices have greater efficiency and a small footprint. In Chapter 2, a novel design for an alloptical XOR gate using 2D photonic crystals has been proposed and investigated. Initially, the XOR gate is designed and simulated by using the FDTD method. The proposed XOR logic is achieved without nano-resonators and then with nanoresonators to get enhanced performance metrics in the form of high contrast ratio. Chapter 3 investigates and studies the effect of hydrostatic pressure on the reflectance and transmittance properties of the one-dimensional PC containing germanium (Ge). They use the transfer matrix method to calculate the transmittance and reflectance spectra. Plasmonics is an emerging and fast-growing branch of science and technology that focuses on the coupling of light to the free electron density in metals, resulting in strong electromagnetic field enhancement due to the confinement of light into sub-wavelength dimensions beyond the diffraction limit. Chapter 4 provides a comprehensive description of the theoretical approaches adopted to investigate the dispersion relation of graphene surface plasmons, types of graphene surface plasmons and their interactions with photons, phonons and electrons, experimental techniques to detect surface plasmons, the behaviour of surface plasmons in graphene nanostructures and the recent applications of graphene-based plasmonics. Renewable energy is the future in a power-hungry world. Solar Cells and Materials are hence forth going to play a vital role in the energy sector. In Chapter 5, the third generation

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solar cells, in regard to materials, production, fabrication process, energy payback time, efficiency and applications have been critically analyzed. Chapter 6 gives a brief overview of the recent research work on graphene in solar cell applications. It is notable that graphene has been used in heterojunction solar cells, GaAs solar cells, dye-sensitized solar cells, Perovskite solar cells, polymer solar cells, and organic solar cells and hence such a review will be useful for further research on graphene-based solar cells to achieve higher efficiency. Nanophotonics is a component of the broad field of nanotechnology which studies the characteristics of light on nanometer scales. It can also be said to be a study of interactions of objects of nanometer dimensions with light. Chapter 7 and Chapter 8 focus on the recent developments in nanophotonics. The various materials used for nanophotonics, their properties and different applications have been elucidated quite comprehensively. Chapter 9 investigates the electro-optic characteristics of a heterogeneous nanostructure for graded fibre optic cables based on shortwave infrared light communication systems under several number of nanoscale well-thickness layers. Some novel photonic materials are considered next. 2D materials are believed to be the future solution to various photonics and opto-electronic technologies including fiber laser. In Chapter 10, the application of monochalcogenides, transition metal dichalcogenides and MXenes is reviewed from the viewpoint of fiber laser technology. It covers the fundamental knowledge about these materials, the operating principle of Q-switching and mode-locking, and the configuration of 2D materials as saturable absorbers. The utilization of these materials as saturable absorbers in a wide range of fiber laser systems including Ytterbium-, Erbiumand Thulium-doped fiber laser is also discussed. Smart materials are those materials whose properties are changed upon application of an external stimulus. Devices using smart materials might replace more conventional technologies in a variety of fields. Smart Materials are attractive due to their light weight, sensing capability, lower component size, and complexity combined with design flexibility, functionality and reliability. Bragg Fibers have tremendous practical applications hence spanning a large body of research. In Chapter 11, the propagation and dispersion properties of hollow-core Bragg fibre waveguides for both high and low refractive index contrasts of cladding materials are explored and compared. In Chapter 12, attractive research is presented to review the biological motivation behind the development of multilayer photonic nanostructure and various types of fuel adulteration detection optical sensors using various sensors-based techniques and compare with the Bragg Metal-Polymer nanocomposite optical sensor. Silicon photonics is an area that relates to the investigation of photonic systems using silicon as an optical medium. Silicon photonics allows for high yield and complex integration with large processing, packaging, and testing availability. Chapter 13 analyzes different approaches to modeling fabrication variations in photonic integrated circuits, such as Monte Carlo, Stochastic Collocation, and Polynomial Chaos Expansion. Finally, Chapter 14 gives a comprehensive review of different types of smart materials, their preparation, characteristics and applications.

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In summary, we would like to state that the book tries to give a snapshot of current exciting research going on in the field of photonics incorporating different types of photonic materials. Photonics and photonic materials are a veritable ocean of which this is a humble attempt to sample a drop. We hope that this piques the interest of new researchers across the world and that they are encouraged to pursue research work in this fascinating field of photonics. In addition, we are hopeful that the book proves useful for scientists, university professors and industry professionals with a keen interest in photonics.

Aavishkar Katti School of Physics Dr. Vishwanath Karad MIT World Peace University Pune India Yogesh Sharma Faculty of Science, SGT University Gurgram-122505, India Department of Physics India

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List of Contributors Ankita Srivastava

Department of Physics Instititute of Science , Banaras Hindu University, Varanasi-221005, India

Athira Jayaprakash

Department of Engineering and Technology, Amity University, Dubai, U.A.E

B. Elizabeth Caroline

Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamilnadu, India

Chandra Kamal Borah

Centre for Advanced Research, Department of Physics, Rajiv Gandhi University, Arunachal Pradesh-791112, India

Claire Mary Savio

Department of Engineering and Technology, Amity University, Dubai, U.A.E

Gausia Qazi

Department of Electronics and Communication, National Institute of Technology, Srinagar, India

Harith Ahmad

Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia

J. Vidhya

Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamilnadu, India

Khan Sumaya

Department of Engineering and Technology, Amity University, Dubai, U.A.E

Kavintheran Thambiratnam

Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia

Pyare Lal

Department of Physics, School of Physical Sciences, Banasthali Vidyapith304022, Rajasthan, India

Margarat Michael

Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamilnadu, India

M. Saravanan

Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamilnadu, India

Mirza Tanweer Ahmad Beig

Department of Physics, Faculty of Science, SGT University, Gurgram122505, India

Muhammad A. Butt

Samara National Research University, Russia Institute of Microelectronics and Optoelectronics,Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland

Muhamad Zharif Samion Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia Mursal Ayub Hamdani

Department of Electronics and Communication, National Institute of Technology, Srinagar, India

Narendra Bihari

University Department of Physics, Lalit Narayan Mithila University, Darbhanga-846004, India

Joshua Nigel

Department of Engineering and Technology, Amity University, Dubai, U.A.E

vi Nitesh K. Chourasia

School of Physical Sciences, Jawaharlal Nehru University, New Delhi110067, India

Norazriena Yusoff

Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia

P. Nithyavalli

Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamilnadu, India

P. A. Alvi

Department of Physics, School of Physical Sciences, Banasthali Vidyapith304022, Rajasthan, India

Ram Chhavi Sharma

Department of Physics, Faculty of Science, SGT University, Gurugram122505, Haryana, India

Ritesh Kumar Chourasia

University Department of Physics, Lalit Narayan Mithila University, Darbhanga-846004, India

Sanjeev K Srivastava

Department of Physics, Amity Institute of Applied Sciences, Amity University Uttar Pradesh, Noida-201310, India

Sanjeev Kumar

Centre for Advanced Research, Department of Physics, Rajiv Gandhi University, Arunachal Pradesh-791112, India

Siti Aisyah Reduan

Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia

Ishu Sharma

Department of Engineering and Technology, Amity University, Dubai, U.A.E

Shok Ing Ooi

Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia

Tista Basak

Mukesh Patel School of Technology Management Engineering, NMIMS University, Mumbai 400056, India

Tushima Basak

Mithibai College of Arts, Chauhan Institute of Science and Amrutben Jivanlal College of Commerce Economics, Vile Parle, Mumbai 400056, India

Yogesh Sharma

Department of Physics, Faculty of Science, SGT University, Gurgram122505, India

Photonic Materials: Recent Advances and Emerging Applications, 2023, 1-20

1

CHAPTER 1

Photonic Crystal Instruments Muhammad A. Butt1,2,* Samara National Research University, Samara, Russia Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland 1 2

Abstract: Photonic crystals (hereafter represented as PCs), a synthetic dielectric formation that employs periodic and random changes in the refractive index to control the transmission of light, were presented by Yablonovitch and John in 1987. The capability to change the transmission of the electromagnetic wave in these formations on a miniature scale is used by photonic devices built on PCs. Electromagnetic waves scatter within the PC, and destructive intrusion happens at particular wavelengths, resulting in a photonic bandgap like the energy bandgap of electron waves in a semiconductor (hereafter denoted as SC). Because of the possibility of constructing a photonic bandgap, it may be feasible to influence light transmission. Instruments with tiny footprints are also feasible. In recent years, several fascinating PC-based devices, such as sharp bent waveguides (henceforth denoted as W/G), μ-resonator cavities, and Y-branches, have been demonstrated. These remarkable properties have the potential to result in the growth of a dense integrated circuit. Though PC technology is still in its infancy, and more study is needed in this field, this chapter summarizes recent developments in this sector by presenting the utmost frequent and necessary optical devices established on PCs such as optical logic gates, optical power splitters, polarization splitters, sensing devices, and lasers. In comparison to conventional photonic devices, these devices have greater efficiency and a small footprint.

Keywords: Photonic crystal, Sensor, Optical logic gate, Laser, Polarization splitter, Polarization-maintaining devices. INTRODUCTION The discovery of PCs in 1987, as described by Yablonovitch [1] and John [2], has flickered a great deal of curiosity. Electromagnetic waves scatter inside the PC, and for specific wavelength ranges, destructive interference occurs, ensuring the formation of a photonic bandgap, which is analogous to the energy bandgap of electron waves in an SC. It may be feasible to regulate light transmission because Corresponding author Muhammad A. Butt: Samara National Research University, Russia and Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland; E-mail: [email protected]

*

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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of the probability of creating a photonic bandgap. Light steering, negative refraction, and self-collimation are just a few of the distinguished utilizations where PCs are used. PCs show how to get great performance in sensing applications with a compelling resolution. Several photonic formation proposals established on various platforms have been extensively investigated and used in detection utilities. PCs demonstrate strong optical confinement to a tiny volume, allowing the identification of biochemical species classified on the nm scale. PC W/Gs have recently been investigated for use in microfluidic [3] and biochemical sensing [4]. Fabrication methods include molecular beam epitaxy, chemical vapour deposition, metal-organic chemical vapour deposition, and holographic ultraviolet beam exposure to photosensitive materials. In two coordinate axes of 2Dphotonic bandgaps, the periodicity may be detected, while homogeneity can be found on the third axes. These kinds of structures can be made using dry reactive ion etching (RIE) or wet electrochemical etching. The first technique has a shallow etching depth and allows for nanometer-level precision in the hole size. Wet electrochemical etching has the potential to produce deep trenches, making the technique suitable for manufacturing assemblies with a high aspect ratio, however, the dimension of the etched cavities is unpredictable. 1D-PC formation is composed of a regular variation of the refractive index (RI) in the path of light transmission, but it offers a regular medium in the other two routes [5]. The RI of 2D-PCs varies in two directions but does not alter in the third. This may be shown by making trenches in a medium with a high RI, such as silicon [6]. 3D-PC formations may be created by changing the RI in all three spatial directions, such as a stack of spheres made of a dielectric medium positioned in the air [7]. Light transmission in a periodic formation, like electron transmission, may be investigated using a regular arrangement of atoms. The PCs are also frequently mentioned in principles like the Bloch theorem and Brillouin zones. Fig. (1) shows a graphic of the 1D, 2D, and 3D PC formations. Because of their narrow lattice constant, 1D-PCs, also identified as multilayers, deficient of a broad photonic bandgap, and 3D-PC manufacturing is exceedingly challenging. However, 2D-PCs feature a complete photonic bandgap and are easier to manufacture than 3D-PCs. Consequently, scientists find them more attractive. 2D-PCs are composed of air-holes in a dielectric substratum or cylindrical dielectric rods engrossed in air. The PC’s photonic bandgap may be changed by adjusting the lattice constant, the radius of the rods, and the RI of the dielectric medium. One of the best alternatives for producing a tunable filter for dense wavelength division multiplexing (DWDM) systems is resonant cavities. Cavity structures with an extraordinary Q-factor filter the chosen band of light

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Photonic Materials: Recent Advances and Emerging Applications 3

with an appropriate bandwidth in DWDM systems. A tunable filtering element can also be created by modifying the formation of these cavities.

Fig. (1). Graphical representation of, a) 1D-PC, b) 2D-PC, c) 3D-PC. Dielectric 1 and dielectric 2 represents the high RI and low RI medium, respectively.

When the light is incident on PC, it is reflected from each interface. Under the right circumstances, these reflected waves will interact constructively, according to the Bragg condition. The Bragg formula with modest modifications for PCs is given by [8]: 𝑚𝜆 = 2𝑛𝑒𝑓𝑓 𝑑

(1)

Where m is the diffraction order, λ is the wavelength of the reflected light, neff is the effective refractive index of the regular formation, and d is the crystal’s lattice period in the path of light transmission. When this stipulation is satisfied, an extraordinary reflection for the specified spectrum is seen. To show the Bragg reflection in the 400 nm to 700 nm band, the PCs require a sub-micrometre period (ʌ). When light passes through the PC, a specific spectrum is reflected, which is reliant on ʌ and neff. The photonic stopband is the spectrum range with the highest reflection (means no transmission). The photonic bandgap, instead, shows the spectrum range that is not acceptable to flow into the assembly, ensuing an extraordinary reflection. The chapter is systematized in the following way. In the first section, the operational mechanism of the novel sensing instruments based on PCs is discussed and recent developments in the sensing area are presented.

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PC optical logic gates (OLGs) are presently among the most popular optical media for researchers to use in the development of optical processing units. These instruments commonly have a power utilization of about μW and a reaction rate of just under a few ps, which contributes to a high switching rate. In the second section, the novel designs of OLGs based on PCs are discussed. In integrated photonics and communication schemes, optical power beam splitters (OPBS) and polarization (hereafter used as pol.) beam splitters (PBS) are essential elements. These elements can be implemented on PCs presenting low transmission loss and eminent pol. extinction ratio (PER). The novel and compact designs of OPBS and PBS are discussed in the third section. In the fourth section, the compact polarization maintaining devices based on PC are discussed. The pol. dependency of PC structures is one of their unique characteristics, and it has been utilized to produce a variety of pol.-maintaining instruments, such as polarizers which are extremely helpful in optical systems because they filter out undesired pol. of light for a particular purpose. In the fifth section, the recent advances in lasers based on PC are presented. Over the past 40 years, scientists have consistently investigated lasers ranging from gas lasers to semiconductor (SC) lasers in terms of working mediums, laser cavities, and pumping mode, propelling the swift expansion of lasers. The SC PC laser was created by combining PCs with classic SC laser mediums to achieve collective control of photonic statuses and trapped electrons. Painter et al. created the original PC laser in 1999. Fig. (2) presents the applications based on PC structures discussed in this chapter.

Fig. (2). Applications of PC structures are discussed in this chapter.

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PC SENSING INSTRUMENTS Photonic sensing instruments have witnessed substantial development in recent years because of the rising need for sensing utilizations in the military, medical management, adequacy of food monitoring, and aerospace, to mention a few [9, 10]. The PC surface is a regular-modulated dielectric nanostructure medium that may be designed to produce a photonic bandgap, which prevents light from transmitting at a precise spectrum. Consequently, the PC surface’s local optical modes may be utilized in life science research as a very sensitive, label-free stage for bioimaging. Label-free tomography of surface-absorbed live cells (containing cell attachment, chemotaxis, and apoptosis) and nanoparticles was executed using third-generation PC-enhanced microscopy. The PC may also be stimulated by a laser light, which combines with the resonant PC mode to generate an electric field augmentation phenomenon, allowing for fluorescence-labelled imaging. PCs provide an intriguing alternative for implementing high-efficiency sensing instruments. PCs have strong optical confinement in a tiny footprint, allowing chemical analytes to be identified. Furthermore, exceptional efficiency in tiny sensor chips may be achieved by employing cutting-edge chemical surface functionalization techniques and integration into microfluidic systems. The identification of dissolved Avidin concentrations of 15 nM or 1 μm/ml was experimentally established [11] using slotted PC cavities with embedded microfluidics. In a study [12], experimental and analytical evidence of extremely high efficiency has been provided. For example, anti-biotin has a detection limit of < 20 pM, which translates to < 4.5 fg of the bound medium on the sensing device surface and < 80 molecules in the integrated μ-cavity modal length. Several criteria, including sensitivity (S), stability, selectivity, and detection limit are examined to assess the efficiency of optical biosensing instruments. The degree of contact between the light and the adjacent matter in 2D-PC is computed as the amount of the slightest modification in λres to the difference in medium RI, generally expressed in the nm/refractive index unit. Sensitivity is articulated as: ∆𝜆

𝑆 = ,

∆𝑛

(2)

where Δλ and Δn are the modifications in λres and RI of the surrounding medium, respectively. Selectivity is the capacity of the surface of the sensing instrument to recognize target analytes in samples including other assortments, which is the major issue of biosensing instruments. The term “stability” refers to the sensing instrument’s sensitivity in the occurrence of the involvement of the adjacent

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medium, which influences the sensing instrument’s accuracy. The detection limit is a generic figures of merit that may be used to evaluate the efficiency of a given biosensing instrument to the efficiency of the other optical biosensing instruments. Typically, the detection limit is defined as the slightest essential RI alteration to cause a significant conversion in the optical signal. Researchers all around the world are doing considerable research on PC-based RI sensing instruments, and many sophisticated sensing instrument topologies, such as integrated μ-cavities and interferometers, have been proposed for RI sensing utilizations. These sensing instruments have several advantages, including minimum sample assembly that does not need fluorescent tagging and excellent sensitivity. The detection method is established by measuring the RI changes of a bulk solution caused by the occurrence of biochemical analytes. To differentiate the concentrations of biological samples, utilizations in gaseous and aqueous environments have been investigated. Using these sensing instruments, it is feasible to determine the molecules and proteins on their surfaces as well as their volumetric density. PCs have recently been widely utilized in a variety of sensing instrument systems. Gas sensing instruments established on PC are proposed for the mid-infrared spectrum since numerous hazardous gases such as CO2, CO and CH4 display absorption lines in this region. A high-accuracy gas index sensing instrument established on a PC air-slot cavity with S=510 nm/refractive index unit was suggested [13]. In a study [14], a surface plasmon resonance (SPR) nanocavity antenna network with a high S=3200 nm/ refractive index unit is offered for gas sensing utilizations. In another study [15], a guided-mode resonance gas sensing instrument with S=748 nm/ refractive index unit is described. Nanoscale PC sensor arrays on monolithic substratum are described [16] as shown in Fig. (3). The sensor design may be utilized as an optofluidic architecture to monitor biological interactions in aquatic atmospheres in a highly parallel, labelfree manner. Arrays of lattice-shifted resonant cavities are side-coupled to a single PC W/G in this design. Each cavity has a marginally varied cavity spacing and is demonstrated to move its λdip independently in response to variations in RI. The extinction ratio of a single well-defined drop is greater than 20 dB. This instrument has a RI sensitivity of 115.60 nm/ refractive index unit and a RI detection limit of about 8.65x10-5. By altering the number of functionalized holes, the sensitivity may be changed from 84.39 nm/ refractive index unit to 161.25 nm/ refractive index unit [16].

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Fig. (3). PC sensor arrays. It is composed of five marginally dissimilar cavities side coupled to a PC W/G [16]. (Reprinted/Adapted with permission from D. Yang, H. Tian and Y. Ji, “Nanoscale photonic crystal sensor arrays on monolithic substrates using side-coupled resonant cavity arrays,” Optics Express, vol. 19, no. 21, pp. 20023-20034, 2011 © The Optical Society).

PC OPTICAL LOGIC GATES PC optical logic gates (OLGs) are presently among the most popular optical media for researchers to use in the development of optical processing units [17]. Using PC architectures, the size of OLGs may be lessened to the order of the wavelength. These instruments frequently have a power utilization of about μW and a reaction speed of just under a few ps, which contributes to a high switching rate. PC OLGs increase the likelihood of fabricating integrated optical circuits. At the speed of light, digital data may be transferred from fibre optics to an electronic processor. The highest switching rate for electrical logic gates, with a standard one-switching power of 0.5 mW, is comparable to 50 ps [18]. The switching rate of OLGs implemented on SCs is limited by the p-n junction and connectivity capacitances, but the switching rate of PC-established OLGs is limited solely by the light speed passing through them. OLGs can perform a variety of logic tasks and have a wide range of utilizations in optical communication. For instance, an AND OLG can be used in address identification, data integrity inspection, and it even supports a sampling gate in optical oscilloscopes. The XOR gate may evaluate data forms for address verification, packet switching, data encryption/decryption, and parity check. The NOT OLG may be utilized as an inverter or switch, while the XNOR OLG can be employed for threshold detector operation.

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Various strategies of OLGs in PCs established on linear and nonlinear constituents are presented. For example, SC optical amplifier (SOA) MZI construction [19], SOA cross-pol. modulation [20], ultrafast nonlinear interferometer [21], electro-absorption modulator [22], and PC W/Gs [23]. In a study [24], a NOR gate established on two Kerr nonlinear PC ring resonators (RRs) with a bit rate of 138.9 Gbit/s is presented. OLGs based on PC were originally suggested in 2006 [25], where the researchers proposed and numerically built an AND gate established on a PC interference W/G between a bent W/G with three entrenched Kerr-type nonlinear rods and a T-branch. Similarly, a design process for OLGs and operations established on threshold logic and nonlinear PC RRs are anticipated [17, 26]. The technological implications of silicon nanocrystals have piqued the interest of researchers. Furthermore, it has high nonlinearity and is compatible with the CMOS manufacturing technique. Using a 2D-PC, the new construction of all-optical NOT, XOR, and XNOR OLGs is given in [27]. This section details the optical performance of the XNOR structure. This OLG is crucial for building logic comparators, full adders, and other 2D-PC based logic circuits. The main concept behind optical XNOR is to utilize a resonant cavity with PCs that have a resonant wavelength of 1550 nm. As illustrated in the graphical description of the construction in Fig. (4a), the XNOR OLG has three inputs (Bias (B), Input-1 (I1), and Input-2 (I-2)), as well as an output port. When “B” =1, “I-1” = “I-2” =0, at a wavelength of 1550 nm, the optical signal at the “B” couples to the RR and flows straight to the output port with an efficiency of 56% as seen in Fig. (4b).

Fig. (4). a) Schematic representation of all-optical XNOR OLG [27]. Optical field mapping for different states for the XNOR OLG, b) B=1, I-1=0, I-2=0, c) B=1, I-1=0, I-2=1, d) B=1, I-1=1, I-2=0, e) B=1, I-1=1, I-2=1 [27]. (Reuse of material which is licensed under CC BY 4.0).

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When “B” =1, “I-1” =0 and “I-2” =1, the signal inputs interfere destructively if the two inputs “B” and “I-2” are both equal to 1, leading in an output signal=0. As illustrated in Fig. (4c), ~0.003% of the signal input power arrives at the output port. When “B” = “I-1” =1, and “I-2” =0, negative interference exists between the signal inputs. As indicated in Fig. (3d), the output power at the output is zero with practically zero efficiencies. When “B” =1, “I-1” = “I-2” =1, the two inputs “I-1” and “I-2” are linked together and stream straight to the output by coupling with the RR with 71% efficacy, preventing negative interference with the signal input of Bias as illustrated in Fig. (4e). PC OPTICAL POWER SPLITTER AND POL. SPLITTER In integrated photonics and communication schemes, OPBS and PBS are essential elements. OPBS splits the power of the incoming light into multiple parts, whereas PBS offers distinct light routing in two polarization states: transverse electric and transverse magnetic [6, 28, 29]. Typically, photonic instruments are designed to function in a single pol. state. If unpolarized or partly polarized light is present, the PBS can split the entering ray of light into two orthogonal pol., ensuring that the pol. is appropriate for the instrument. The PER is a critical factor for determining PBS proficiency. Because of their tiny footprint, PBSs developed on PCs are quite appealing. Light passing through a PC may confront a negative or positive RI. The RI difference, angle of light incidence, and slab thickness all influence the two refraction conditions. By correctly using these features, it is probable to implement straightforward and capable optical modules for light transmission. For example, an optical passive instrument that functions as a PBS is demonstrated, with transverse magnetic and transverse electric-pol. bending in positive and negative directions, respectively. In a study [30], transverse electric and transverse magnetic-pol. of rays of lights are separated via in-plane negative refraction in a 2D-PC. The instrument is said to be capable of working in the 350 nm band with a λcentral of 1500 nm. A compact PBS system with a footprint of 15 μm x 10 μm that is generated by combining a hybrid PC with traditional W/G construction is demonstrated [31]. Planar PCs with a tiny footprint have been proven to have a good conception of pol.-sensitive light broadcast [32]. Though, depending on the W/G employed, the substantial light diffraction that this form of PBS experiences may result in low broadcast for both pol(s).

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In recent years, numerous innovative designs of 2D-PC OPBSs have been presented [33, 34]. A unidirectional OPBS based on 2D-PC is displayed [35]. The frontward broadcast is greatly enhanced in a wide frequency domain by adding elliptical air holes on the heterointerface. The unidirectional OPBS is constructed, and the equivalent light intensities of each ray of light may be fine-tuned by adjusting the area ratio of elliptical air-holes. In PC-1, the incident light travels along the x-axis and strikes the heterointerface in the middle for frontward transmission. After then, some of the light energy is reflected to PC-1 and the rest is carried on to PC-2. Finally, light is propagated from PC-2 and split into two output rays of light, P1 and P2, respectively. However, in PC-2 along the x-axis course, the same input light cannot be propagated, and only a tiny amount of light can be sent within the PBS formation for backward transmission. An OPBS structure based on 2D-PCs that can be appropriate to photonic integrated circuits (PICs) is presented [8]. The power splitting mechanism is similar to that of traditional three-W/G directional couplers (DCs), exploiting the coupling between guided modes maintained by line-defect W/Gs. Simple mode analysis determines the position in transmission direction where an input field is divided into the twofolded image by analysing the dispersion curve and field dispersion of modes. The magnetic field mapping in the splitter structure is presented in Fig. 5(a) [8]. Computational analysis on the design of an exclusive 2D-heterostructure PC capable of splitting two orthogonally pol. of light waves is presented in a study [6]. The instrument is made up of two distinct PC formation patterns as shown in Fig. (5b). The first PC structure is designed to allow the light of both pol(s) to pass through it. The second PC structure, instead, only has a photonic bandgap for transverse electric-pol. of light. Due to the photonic bandgap existing in the second PC construction, these two structures are merged at a 45o angle, resulting in a reflection of self-collimated transverse electric-pol. of light at an angle of 90o. While authorizing the self-collimated transverse magnetic-pol. of the light wave to continue its journey unhindered. The normalized Electric field mapping of light in the heterostructure at the operational wavelength of 1550 nm is shown in Fig. (5c). The suggested instrument has a compact 10.9 μm2 footprint, low transmission loss, and a high PER, making it a perfect contender for use as an onchip pol. division multiplexing (PDM) system. (Fig. 5a) [Reprinted/Adapted] with permission from I. Park, H.-S. Lee, H.-J. Kim, K.-M. Moon, S.-G. Lee, B.-H. O, S.-G. Park and E.-H. Lee, “Photonic crystal power-splitter based on directional coupling,” Optics Express, vol. 12, no. 15, pp. 3599-3604, 2004 © The Optical Society) (Fig. 5a and c) taken from [6], material licensed under CC BY 4.0)

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Fig. (5). a) Magnetic field distribution in the OPBS [8], b) Schematic representation of 2D-heterostructure PBS [6], c) Electric field mapping of transverse electric+ transverse magnetic pol. of light [6].

PC POLARIZATION MAINTAINING INSTRUMENTS The polarization dependency of PC structures is one of their unique characteristics, and it has been used to create a variety of pol.-maintaining instruments, such as polarizers which are extremely helpful in optical systems because they filter out undesired polarization of light for a particular purpose. The polarizer’s operating mechanism is built on the polarization-dependent transmission of a PC W/G, which differs from typical polarizers. It is based on the unique characteristics of 2D- photonic bandgap mediums. Unpolarized light can be decomposed into dual parts: one with the Electric field parallel to the periodic plane (referred to as transverse electric) and the other with the magnetic field parallel to the periodic plane (referred to as transverse magnetic). In 2D- photonic bandgap mediums, the transmission of transverse electric and transverse magnetic pol. of light is independent of one another. It’s worth mentioning that the band structures and photonic bandgaps of transverse electric and transverse magnetic polarization(s) are different. Photonic bandgaps can be either passive or active. The Maxwell equations may adequately explain the contact of passive or active photonic bandgaps with the incident light. Multiple scattering and diffraction, like Bragg reflection and diffraction gratings, interact with and inside a photonic bandgap. Defects (imperfections) in a photonic bandgap can have a big influence on the electromagnetic wave around it. Consequently, unique optical resonances

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have been discovered in these photonic bandgaps, and their characteristics may be customized by selecting the appropriate bandgap and generated imperfections. The structural characteristics of a typical square-lattice PC W/G are shown in Fig. (6a). The numerical study only considers transverse magnetic polarization. The spatial profiles of the forward and backward transmitted powers are probable to converge due to the reciprocal presence of the W/G structure [36]. The dispersion map generated by the plane wave expansion approach, as illustrated in Fig. (6(b)), may verify this expectation. The W/G mode extends to the k-axis and spans a broad frequency range.

Fig. (6). a) Schematic diagram of a typical PC W/G, b) the dispersion curve of typical PC W/G.

Fig. (7) depicts a standard 2D-PC photonic band design expected along the direction of incoming light. The transmission of an electromagnetic wave is predicted by its frequency and the band assembly of the PC photonic bandgap. Transverse electric and transverse magnetic polarized light waves can also travel in the same region as the transverse electric and transverse magnetic bands. In the overlapping zone of transverse electric and transverse magnetic photonic bandgaps, however, both transverse electric and transverse magnetic waves are prohibited from travelling. Only transverse electric (transverse magnetic) waves may propagate in the imbricated area of the transverse electric (transverse magnetic) bands and transverse magnetic (transverse electric)-photonic bandgap because transverse magnetic (transverse electric) polarization of light cannot transmit in the section of its photonic bandgap [37]. As a result, the transmission wave is polarized smoothly and has only one polarization. This is how the photonic bandgap polarizer operates.

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Fig. 7. A 2D- photonic bandgap PC with distinctive photonic band architectures is probable in the path of incoming light. In 2D- photonic bandgap crystals, the transverse electric and transverse magnetic poarization. of light is separated. Photonic bands are shown by the hatching regions. Between the bands, photonic bandgaps may occur.

The degree of polarization and transmittance are commonly used to describe a polarizer’s effectiveness. The degree of polarization can be written as: 𝐷𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑝𝑜𝑙. =

𝐼𝑇𝐸 −𝐼𝑇𝑀 𝐼𝑇𝐸 +𝐼𝑇𝑀

,

(3)

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Where ITE and ITM are the intensity of the propagating transverse electric and transverse magnetic pol. of light. The degree of polarization for unpolarized and completely polarized light is 0 and 1, respectively. The ratio of the intensity of the

transverse electric-polarized part at the output to the intensity of the transverse electric-polarized part at the input is denoted as the polarization transmittance “T”. 𝑇=

𝐼𝑇𝐸 (𝑜𝑢𝑡𝑝𝑢𝑡) 𝐼𝑇𝐸 (𝑖𝑛𝑝𝑢𝑡)

, For a flawless polarizer, T=1 is possible.

Numerous photonic instruments use transverse magnetic-pol. of light function well [38]. In addition, transverse magnetic-polarization is better for detection purposes than transverse electric-polarization of light because its evanescent field infiltrates deeper through the top and bottom cover layers. Transverse magneticpol. of light is also used in polarization multiplexing systems to allow most of the channel volume. As a result, it is recommended that instruments that allow the transverse magnetic pol. of light are essential for the development of operational PICs. To make use of the big difference between the refractive index of Si to SiO2, strong birefringence, considerable optical nonlinearities, and mature CMOS manufacturing methods, most PICs are currently built on the SOI platform [39]. Transverse electric and transverse magnetic polarization of bandpass instruments established on a hybrid plasmonic W/G with a PER of 20 dB has been suggested [40, 41]. Another type of transverse electric-pass polarizer has been discovered; this time established on a hybrid plasmonic Bragg grating W/G. Over a bandwidth of 160 nm, the PER value of greater than 17 dB is computed [42]. The transverse magnetic-pass polarizer is made by customizing imperfection modes in the photonic bandgap structure is presented [43]. A transverse magnetic -pol-maintaining instrument established on a 1D-PC W/G which offers a PER of 28.5 dB is presented in [5]. Fig. (8a) shows the graphical illustration for the instrument and the Electric field mapping of the transverse electric and transverse magneticpol. of light at the operational wavelength of 1550 nm is shown in Figs. 8 (b and c), respectively. PC BASED LASERS The origination and growth of lasers have altered human productivity, life, research, and development. Over the last four decades, scientists have consistently investigated lasers ranging from gas lasers to solid and liquid lasers to SC lasers

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in terms of working mediums, laser cavities, and pumping mode, propelling the swift expansion of lasers. A diverse range of lasers serves critical roles in several sectors. Owing to their small footprint and high performance, SC lasers are extensively employed in optical broadcastings, computing, storage, display, and pumping sources.

Fig. (8). a) transverse magnetic -polarization-maintaining W/G established on SOI platform, b) Electric field mapping of the transverse electric-polarization of light at the operational wavelength of 1550 nm, c) Electric field mapping of the transverse magnetic –polarization of light at the operational wavelength of λ=1550 nm.

The SC PC laser was created by combining PCs with classic SC laser mediums to achieve a collective hold of photonic positions and trapped electrons. Painter et al. created the original PC laser in 1999 [44]. At ambient temperature, a 1550 nm PC μ-cavity was realized using the dipole mode of a triangular lattice imperfection cavity. PC edge-emitting (EE) lasers can be created by combining the μ-cavity and line imperfection W/Gs. Sugitatsu et al. demonstrated an EE laser using the slow-light effect of the PC W/G band edge in 2004 [45]. Watanabe and Baba developed an EE laser in 2008 via the connection of a PC μ-cavity with a high Q and imperfection W/G [46]. Recently, Zheng’s group has also presented a variety of PC lasers to enhance the efficiency of the SC laser, such as electrical injection

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PC vertical cavity and lateral cavity surface-emitting lasers, PC high beam value lasers and so on. Purcell first described the modulating impact of the μ-cavity on the coupling between the EM-field and mediums in 1946 [47]. The Purcell effect describes how the magnitude of the wavelength increases the spontaneous emission in the cavity. The Purcell factor is comparative to the μ-cavity’s quality factor (Q-factor) and inversely proportional to the mode volume (V). The number of modes supported by the μ-cavity decreases as V decreases, leading to a rise in the impulsive emission coupling coefficient, even equal to 1 [48]. It is advantageous to obtain a laser with an extremely small threshold. The vertical-cavity-surfae-emitting laser (VCSEL) is intended to be the primary laser that reduces the dimension of the optical mode to the scale of the optical wavelength while maintaining a low loss. In addition, the μ-disk laser contains an optical μ-cavity with a high Q-factor and an insignificant V of a cubic wavelength. An extraordinary Q-factor and a smaller V can be produced by leveraging the PC’s bandgap characteristic. 3D-PCs, for example, entirely block light in all three dimensions of space. In theory, this type of defect structure can be used to create threshold-less lasers. Although the 3D-PC is challenging to construct, a variety of active and passive instruments based on a more basic 2D-PC already show promise in the field of photonic amalgamation, which is now a hotspot of study. The μ-cavity laser established on a 2D-PC slab has an elevated Q-factor and insignificant V, and it can also fine-tune the laser wavelength by marginally modifying the laser geometry; hence, it has a lot of utilizations in on-chip embedded light sources [49]. Scientists distorted the complete lattice of the H1 cavity to achieve the singlemode and enhanced Q-factor, ensuing in the equilibrium modification of the triangular lattice and the parting of the deteriorated dipole mode into x- and y-pole modes [50]. The 2D-PC laser with an H3 cavity was also investigated by Gang et al, [51]. The H3 defect cavity was designed by eliminating 19 air holes in the centre of the flawless PC structure. The laser with a peak wavelength of 1500 nm was created with a mean pumping power of 90 μW and the side mode suppression ratio (SMSR) was 13.8 dB. CONCLUDING REMARKS This chapter deals with the recent development in the field of photonic crystals (PCs) and some novel instruments based on these structures are discussed which include sensors, optical logic gates, polarizers, power splitters and lasers. PCs are synthetic dielectric structures that employ periodic and random changes in the

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Photonic Materials: Recent Advances and Emerging Applications 17

refractive index to control the propagation of the electromagnetic wave. These structures were first presented by Yablonovitch and John in 1987. The ability to change the transmission of the electromagnetic wave is used by photonic devices based on PCs. The propagating wave scatters within the PC, and destructive interference takes place at a specific range of wavelength, resulting in a photonic bandgap. The photonic devices based on PCs are compact and highly efficient which makes them suitable for modern-day applications. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

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21

CHAPTER 2

All-optical Logic Gate Using Photonic Crystals for Ultra-Fast Telecommunication Applications Margarat Michael1,*, B. Elizabeth Caroline1, J. Vidhya1, M. Saravanan1 and P. Nithyavalli1 Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamilnadu, India 1

Abstract: Most major high-speed applications, such as communications, environmental monitoring, transportation, smart homes, industries and gadgets are enabled by recent photonic technology. Basic all-optical logic gates are used in the development of image sensors, ultra-fast optical devices, and positioning equipment in high-speed applications. Among different technologies proposed for all-optical implementation, Semiconductor Optical Amplifiers (SOA) have been widely adopted. They have attractive features such as wide gain bandwidth, low power consumption, compactness and strong non-linearity. SOA still has a limitation that its spontaneous emission noise restricts the performance. The semiconductor optical amplifiers with quantum dots exhibit higher saturation output power, lower current density threshold, wider gain bandwidth, and low noise figure than conventional SOA. Quantum Dot Semiconductor Optical Amplifiers (QDSOAs) also have limitations like large size, high power consumption and spontaneous emission of noise. Photonic Crystal (PhC) is an artificial material that is suitable to overcome all drawbacks of SOA and QDSOA due to its simple structure and compactness, high speed, low power consumption, and low loss. PhC-based structures allow propagation of light in a controlled manner with its periodic crystal arrangements having dissimilar diffraction index. PhCs are considered to be a suitable structure for designing all-optical devices with compactness. In this chapter, an all-optical XOR is designed. Initially, the XOR gate is designed and simulated by using the FDTD method. The proposed XOR logic is achieved without nano-resonators and then with nanoresonators to get enhanced performance metrics in the form of high contrast ratio. The contrast ratio is 260 dB for the XOR gate with a delay time of 0.19 ps. The proposed XOR logic gate has potential practical applications for high speed applications of telecommunication systems.

Keywords: Finite Difference Time Domain (FDTD) Method, Nanoresonator, Photonic Crystal (PhC), Plane wave expansion (PWE) Method, XOR Gate. Corresponding author Margarat Michael: Department of Electronics and Communication Engineering, IFET College of Engineering, Villupuram, Tamil Nadu, India; E-mail: [email protected]

*

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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INTRODUCTION In a communication network, the current electronic technologies have serious limitations when large amounts of information need to be transmitted. The current electronic technology results in a limited amount of communication speed limit and computation time. Thus, to overcome this problem, all-optical logic gates are used. Ultra-compact all-optical logic gates have become attractive devices for real-time optical processing and communications. All-optical logic gates are designed to avoid complications and speed limitations due to the need for optic-electric-optic conversion [1]. Thus, numerous technologies have been described to design and develop all-optical devices. PhC devices are nanostructured optical media that can guide, limit and control the light propagation in the waveguide. Such nanostructures are constructed and manufactured to be used in future generation photonic circuits. Conventional photonic devices such as SOA cannot control the intended optical modes in smallscale circuits, because of the limiting factors such as total internal reflection and high loss in refraction. There has been explicit attention paid to PhC structures due to the fabricating feasibility of such material with the silicon and also because of the superior performance metrics such as lesser loss and higher light confining capability of input signals [2, 3]. A class of PhCs in two dimensions have nurtured a rapidly increasing interest over numerous applications with novel phenomena such as strong light confinement, slow light, spatial dispersion, and filtering [2 5]. The photonic bandgap (PBG) is formed in the PhC structure because of the periodic interaction within the structure. The frequency signals in the PBG span cannot propagate within the structure. By introducing a defect within the structure, the mode of light can be localized and limited in the lattice. This can let the PhC structures a strong capability to control the modes, limit and guide the input light and has improved the use of such structures in producing optical elements. The possibility of limiting optical modes is increased by extending the PhCs into two- or three-dimensional structures. Currently, PhC based logic gates have become an attractive waveguiding medium to create all-optical devices [2, 4]. By utilizing the PhC structures, the optical logic gates dimension can be reduced to the order of the wavelength of light. Also, these devices lead to increased switching speed with the microwatts power consumption and its response time over the output in the order of lesser than a few picoseconds. Digital data can be transmitted at the speed of light to an electronic processor by an optical fiber. However, for electrical logic gates, the maximum

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Photonic Materials: Recent Advances and Emerging Applications 23

switching speed is equal to 2 × 105 Hz (50 ps) for 0.5mW average switching power. The switching speed of logic gates made up of semiconductor material is limited by interlinking capacitances and p-n junction, whereas switching speed in an optical logic gate is limited only by light speed propagating through it. Periodic dielectrics [6] or magnetic structures demonstrate a photonic bandgap in a PhC arrangement. An array of light wavelengths that cannot pass through the PhC is called Photonic Band Gap [7]. The propagation of electromagnetic waves within certain frequency bands is forbidden, the principle is that the electromagnetic waves cannot propagate through the periodic structures resulting in various optical effects. Light waves with frequencies lying within the bandgap will get reflected by the PhC. PhCs are nanostructures fabricated by means of the interrupted arrangement of different refractive index materials. PhC can have a period of single (1D), dual (2D) or full three (3D) dimensions. 1D PhCs are an alternating sequence of layers with different dielectric constants. 2D PhCs consist of periodic rods in a dielectric medium. 3D PhCs having a periodicity in the refractive index in all three dimensions are very difficult to fabricate, but can have huge potential in areas of optical computation. RELATED WORKS PhCs extend an adaptable method for the propagation of light and controlling emission [8] by changing the lattice constant value of the crystal structure. A photonic structure is a regularly repeating structure consisting of two materials or more of different dielectric constants. There have been proposed systems designing an all-optical logic gate using the 2D PhCs. Fariborz Parandina [9] designed structures of NOT, XOR, and NOR with a very low power transfer delay of 0.1 ps and a contrast ratio of about 30dB. Golnaz Tavakolia [10] realized a structure of XOR and XNOR cascading two resonant rings. The delay time for the XNOR and XOR logics is 2.5 and 1.5 ps, respectively, the working bit rates for the XNOR and XOR logics are 400 and 666 Gbit/s. Ahmad MohebzadehBahabady [11] designed a structure for NOT and XOR gate comprising three waveguides and a nanoresonator. The response time and contrast ratio (CR) for the XOR logic gate were found to be 0.466ps and 19.95 dB, respectively. Sandip Swarnakar designed an XOR gate using Photonic Crystal Ring Resonator [12]. The square lattice PhC is made up of Silicon in Silica and the contrast ratio is calculated to be 8.37 dB. In this chapter, the all-optical logic XOR gate in a 2D PhC is proposed. The interference method is used to obtain the logic effect. The simulation of the proposed device is carried out using the Finite Difference Time Domain (FDTD) method of Rsoft Photonics CAD.

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LIGHT PROPAGATION IN PERIODIC MEDIA Recently, the material control of optical features has become an interesting field of study for researchers. One good way to manipulate and control light transmission is utilizing semiconductors with the periodic arrangements called as PhCs. The motivation of scheming and fabricating PhCs is to design structures over which control of photons can be achieved in a limited space for an extended time, but with inner limitations in light confinement over a few wavelengths. Such behaviour of PhC is formulated with Maxwell equations by assuming isotropic, linear and transparent materials in a diverse dielectric medium as follows: 𝛻 . 𝐻(𝑟, 𝑡) = 0

𝛻 × 𝐸 (𝑟, 𝑡) + 𝜇0

(1) 𝜕𝐻(𝑟,𝑡) 𝜕𝑡

=0

(3)

𝛻 . [𝜀(𝑟)𝐸(𝑟, 𝑡)] = 0

𝛻 × 𝐻(𝑟, 𝑡) − 𝜀0 𝜀(𝑟)

(2)

𝜕𝐸(𝑟,𝑡) 𝜕𝑡

=0

(4)

where, H(r,t) and E(r,t) are the magnetic and electric fields, μ0 = 4 π ×10−7 and ε0 = 8.854 × 10−12 are vacuum permeability and permittivity, respectively. The dielectric constant is related to position vector in a PhC, thus ε(r) = ε(r + a), where a is the lattice structure constant. The PhC wave equation is represented by combining the above expressions and assuming the light speed at vacuum as c = 1/ √ε0μ0, 𝛻×(

1 𝜀(𝑟)

2

𝛻 × 𝐻(𝑟)) = (𝜔𝑐) 𝐻(𝑟)

(5)

The electric field of the propagating wave can be calculated with

𝐸 (𝑟) =

𝑖 𝜔𝜀0 𝜀(𝑟)

𝛻 × 𝐻(𝑟)

(6)

The light beam striking at the periodic structure is partially reflected under a specific condition. The intensity and direction of reflected light depend on the refractive index dispersal and periodicity of the PhC lattice, respectively. Bragg’s law describes the properties of reflected light from the periodic structure,

𝑚𝜆 = 2𝑛𝑒𝑓𝑓 𝑑 𝑠𝑖𝑛 𝜃

(7)

where λ is the reflected light wavelength, m is the diffraction order, neff is the periodic structure’ effective refractive index, d is the plane spacing between the

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Photonic Materials: Recent Advances and Emerging Applications 25

lattice, and θ is the angle of glancing between the crystal diffraction planes and incident light and. If the PhC structure is near perfect and the Bragg’s condition is also satisfied, then the light scattering will be possibly very small. The photon interaction with periodic atom arrays in the PhC creates bands of light energy. The light incident over the specific wavelength range in correspondence to the stop band is very weak within the periodic structure. The stopping band’s existence indicates the availability of a gap in the permitted frequencies where propagation of electromagnetic modes is prohibited. The photon interaction with a dielectric periodic lattice is analogous to electron interaction behaviour with a periodic atomic lattice. The concepts like energy-frequency and emitted momentum being constant can be said to be analogous here. Despite a few similarities between PhC and semiconductors, it is notable to point out that the propagating light is being absorbed in the semiconductor material at a specific wavelength or energy over a particular bandgap but in a PhC, the photonic bandgap refers to a frequency region where light propagation is prohibited. TYPES OF PHOTONIC CRYSTALS According to the structure periodicity, PhCs are classified into three different categories namely one-dimension PhC (1D), two-dimension PhC (2D) and Threedimension PhC (3D). The geometrical representation of 1DPhC, 2DPhC and 3DPhC are given in Fig. (1). where different colours signify material with variable dielectric constants.

Fig. (1). (a) 1DPhC (b) 2DPhC (c) 3DPhC.

1D PhC Simple 1DPhCs in periodic dielectric stack arrangements have been utilized for a significantly longer period. Their selective wavelength reflection properties enable them to be used in an extensive application range including Fabry-Perot cavities, high-efficiency mirrors, distributed feedback lasers and optical filters. As shown in Fig. (5a), the basic PhC model is an alternating arrangement of two

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varieties of dielectric materials. When light is incident on such an arrangement, it gets reflected at each interface. If each layer’s thickness is chosen suitably, then the reflected fields are in-phase with each other and result in strong reflectance interfering constructively(Bragg reflection). In contrast to 2D and 3D PhCs, 1D PhC Bragg reflection happens regardless of the contrast index, though more periods are required to attain high reflectance with small contrast. Since the optical dielectric materials’ absorption is very less, mirrors produced from dielectric arrangements are highly efficient and possible to design with 100% reflectivity of the light incident within a small frequency range. The foremost limitation of 1D PhC dielectric mirrors is that the device operation is limited over a specific angle range near normal incidence. The Fibre Bragg grating (FBG) is another recent 1DPhC application, where the fibre core refractive index is periodically varied along its axis, typically with a sinusoidal profile. The properties are fundamentally the same as the previous case but it is more complex due to the continuous variation of the refractive index instead of a discrete variation. The key difference is that the FBG refractive index contrast is so small comparatively (∆n ≤ 0.5%), very narrow operational bandwidth and a requirement of thousands of periods to get the required reflectance properties. They are an essential part of optical fibre systems, used in filters, dispersion compensation and an extensive range of applications. 2D PhC Both 2D PhC and 3D PhCs can be assumed as generalizations of 1D PhCs in which a full 2D or 3D material bandgap appears if the condition for Bragg reflection is satisfied in 1D and simultaneously in all the directions of signal propagation in which a periodic structure is present. It occurs in 2D periodic lattices providing sufficiently large index contrast, but for 3DPhC structures only specific types of lattice geometries display this necessary property, and only then it is for sufficiently larger index contrasts. Instead of uniform dielectric layer stacks, 2D PhCs typically contain dielectric cylinder arrays in a background of homogeneous dielectric material, as illustrated in Fig. (5b), even though with many possible geometries, 2D bandgaps occur with sufficiently larger refractive index contrast between the background and cylinders of propagation in the plane of periodicity which is perpendicular to the direction of the rod. Light at the bandgap frequencies undergoes Bragg reflection in each direction because of the cylinders in periodic array arrangements. However, similar to 1DPhC case where the possibility of light propagates in 2D, in a 2DPhC, propagation occurs in parallel to the cylinders through a non-periodic direction. Hence, there is a need for an alternative confinement method in the third dimension to reduce excessive

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Photonic Materials: Recent Advances and Emerging Applications 27

losses due to scattering and diffraction. Similar to semiconductor devices, considerable interest in PhC arises not only with bandgap but for the ability to introduce structural defects for creating local defect states in the bandgap material to produce regular lattice differently. For example, the elimination of a cylinder from a 2D PhC makes a resonant cavity or point defect, and the deletion of an array of cylinders can produce a propagating waveguide to support mode propagation. Based on this concept, numerous potential applications have been demonstrated and proposed. The next class of 2D PhC application utilizes the unique feature of the mode propagation that exists outside the bandgap of PhC without defects. The discrete transversal symmetry of PhCs enforces severe phase conditions over the supporting field distributions. Consequently, only a distinct number of modes are maintained for any specified frequency and propagation of light in such Bloch modes has different features for the light in a homogeneous medium. 3D PhC 3D PhCs are known to be more challenging to fabricate. Completely new techniques of fabrication are required for 3D PhC as compared to 2DPhC. A wide variety of 3DPhC geometries revealing complete bandgaps have been demonstrated theoretically and experimentally. For example, a “woodpile” 3DPhC is illustrated in (Fig. 5.3). Due to the challenges involved in the highquality fabrication of structures with features in optical wavelength scale, initial PhC experiments were accomplished at mid-infrared and microwave frequencies. With the improvement of processing materials and fabrication methods, small structural designs have become possible, and the first 3DPhC at telecommunication frequency bandgap was reported in 1999. Since then, many lattice geometries at similar operating frequencies have been reported. The introduction of intentional defects and waveguiding in 3DPhCs has not developed as rapidly as in 2DPhCs, mainly due to the difficulties in fabrication and the additional complex geometry requirements to realize 3D bandgaps. Theoretical research has demonstrated the potential for new photonic design circuits, but only a limited experimental outcome has been reported so far. Although most of the current interest in PhCs was towards telecommunications-based applications, the unique concept of directing spontaneous emission has not been overlooked. Recent experimentations demonstrate both enhancement and inhibition of spontaneous emission originated from quantum dots implanted both in 2DPhC and 3DPhCs. At radiation over black-body range frequencies, the existence of a photonic bandgap has been revealed to change the thermal emission features for heated tungsten in 3DPhCs. 3DPhCs formed in materials with low-index contrast such as polymer or silica are also useful for applications where the requirement of prohibiting light propagation in all polarizations and directions is not essential.

28 Photonic Materials: Recent Advances and Emerging Applications

Michael et al.

Super prism experimentation has been designed in polymer 3DPhCs then both nonlinear and liquid crystal tuning have been used to demonstrate the tunable bandgap effects. Passive PhCs have been exposed to display a multitude of fascinating phenomena, including slow light (SL) propagation in waveguides with line defects. It was recommended that by integrating an active material inside the waveguide, SL could be used to improve the material effective gain, which has several interesting applications, including the possibility for ultra-compact all-optical amplifiers for photonic chips integration. PRINCIPLE OF OPERATION There are two methods used for the analysis of 2D PhCs based structures. The two methods are: 1. Plane wave expansion (PWE) method. 2. Finite difference time domain method. PWE method solves Maxwell’s equations in the frequency domain and the second method FDTD solves Maxwell’s equation in the time domain. Firstly, Maxwell’s equations need to be introduced and then their solution in frequency and time domain will be explained. Light propagation characteristics of PhCs are explained by solving following Maxwell’s equation in a linear isotropic dielectric medium.

𝛻 × 𝐸 = −𝜕𝐵/𝜕𝑡

(8)

𝛻 × 𝐻 = 𝐽 + 𝜕𝐷/𝜕𝑡 𝛻 · 𝐷 = 𝜌

(9) (10)

𝛻 · 𝐵 = 0

(11)

and the constitutive relations 𝐽 = 𝜎𝐸

(12)

𝐷 = Ɛ0 𝐸

(13)

𝐵 = 𝜇0 𝐻

(14)

where Ɛ0, μ0 are the permittivity and permeability of the vacuum.

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Photonic Materials: Recent Advances and Emerging Applications 29

PWE Method The plane wave expansion method is widely used for getting dispersion relation and mode profiles inside PhCs structures. PWE gives a graph between frequency f v/s wave vector k. This is a frequency domain method in which the Bloch theorem is used for solving the Eigenvalue problems and solutions to these problems are obtained as a superposition of plane waves. Solutions of Maxwell’s Equations in Frequency Domain For isotropic, linear, dispersive and transparent materials, the magnetic field density and electric field density are related to magnetic and electric fields through the following equations, respectively: (15)

𝐵 = 𝜇 𝑟 𝜇0 𝐻 𝐷 = 𝜀𝑟 𝜀0 𝐸

(16)

Now, if it is assumed that there is no free charge or current in the structure, then ρ = 0 and J = 0. With all the above assumptions and relations, Maxwell’s equations (8)-(11) can be rewritten as, 𝛻 × 𝐻 = 𝜀𝑟 𝜀 0

𝜕𝐸 𝜕𝑡

𝛻 × 𝐸 = −µ𝑟 µ0

𝜕𝐻 𝜕𝑡

(17) (18)

𝛻∙𝐻=0

(19)

𝛻∙𝐸=0

(20)

The electric and magnetic field are a function of space and time. Maxwell’s equations are linear so the dependence of time can be separated from the dependence of spatial by expanding E and H fields into a set of harmonic modes. Harmonic modes can be written as a mode profile times a complex exponential as below:

𝐻(𝑟, 𝑡) = 𝐻(𝑟)𝑒𝑥𝑝 (−𝑖𝜔𝑡) 𝐸 (𝑟, 𝑡) = 𝐸 (𝑟)𝑒𝑥𝑝 (−𝑖𝜔𝑡)

(21) (22)

Operating equations (17) and (18) with curl on both sides, we get, ∇ × (∇ × 𝐻(𝑟)) + 𝑖𝜔𝜀𝑟 𝜀0 𝐸(𝑟) = 0

(23)

30 Photonic Materials: Recent Advances and Emerging Applications

Michael et al.

∇ × (∇ × 𝐸(𝑟)) − 𝑖𝜔µ0 𝐻(𝑟) = 0

(24)

Now we divide Equation (23) by Ɛr and then taking the curl again, we have, 𝛻×(

1 𝜀𝑟

𝛻 × 𝐻(𝑟)) + 𝑖𝜔𝜀0 𝛻 × 𝐸(𝑟) = 0

(25)

Using equation (24) in the above equation, we have 1

𝛻 × ( 𝛻 × 𝐻(𝑟)) = 𝜔2 𝜀0 𝜇0 𝐻(𝑟) 𝜀 𝑟

Putting, 𝑐 =

1 √𝜀0 𝜇0

(26)

, we get, 1

𝜔 2

𝛻 × ( 𝛻 × 𝐻(𝑟)) = ( ) 𝐻(𝑟) 𝜀 𝑐 𝑟

(27)

Similarly, we can write the equation for the electric field 1

𝜔

2

𝛻 × (𝛻 × 𝐸(𝑟)) = − ( ) 𝐸 (𝑟) 𝜀 𝑐 𝑟

(28)

Equations (27) and (28) are examples of eigenvalue problems having (ω/c)2 as eigenvalues. One of these equations is solved to obtain the eigenfrequencies for given dielectric constant Ɛr. Eigenvectors corresponding to the eigenfrequency give us the required information about the mode profile. FDTD Method To solve electromagnetic problems, the FDTD method was proposed by Kane Yee in the year 1966 [13]. Various improvements to the novel Yee’s algorithm have been presented since then for increasing the strength and accuracy. Yee’s formalism is best suited for the problems of electromagnetic theory wherein Maxwell’s curl equations are solved in the time domain to provide straightforward solutions. A popular and robust approach that has been widely adopted in calculating both the transmission spectra and the electromagnetic field distribution in PhC is based on a numerical solution of Maxwell equations using the FDTD method. When Maxwell’s differential equations are examined, it is seen that the change in the Efield in time (the time derivative) is dependent on the change in the H-field across space (the curl) and vice versa. The FDTD method solves Maxwell’s equations by

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Photonic Materials: Recent Advances and Emerging Applications 31

first discretizing the equations via central differences in time and space and then numerically solving these equations. The equations are solved based on Yee’s mesh and compute the E-and H-field components at points on a grid with grid points spaced Δx and Δy apart in the case of 2D. The E-field and H-field components are then interlaced in all two special dimensions. The time evolution of the electromagnetic field is determined at every point within the computational domain in an iterative process. Maxwell’s equations in a lossless medium are expressed as, 1

𝜕𝐻

=− 𝛻×𝐸

𝜕𝑡 𝜕𝐸 𝜕𝑡

(29)

𝜇

1

= 𝛻×𝐻

(30)

𝜀

The scalar equations are obtained by expanding the curl operators in Cartesian coordinates, 𝜕𝐸𝑋

𝜕𝑡 𝜕𝐸𝑦 𝜕𝑡 𝜕𝐸𝑧

𝜕𝑡 𝜕𝐻𝑋

𝜕𝑡 𝜕𝐻𝑦

𝜕𝑡 𝜕𝐻𝑧

𝜕𝑡

1

=

1

=

𝜀 1

=

𝜀

=

𝜕𝐻𝑋

𝜕𝑍 𝜕𝐻𝑦

(

µ

1 µ

𝜕𝑋 𝜕𝐸𝑧

(

µ 1

𝜕𝑦

(

1

= =

𝜕𝐻𝑧

(

𝜀

𝜕𝑦

𝜕𝐸𝑋

(

𝜕𝑍

𝜕𝐸𝑦

(

𝜕𝑋

𝜕𝐻𝑦

−

𝜕𝐻𝑧

−

− − −

−

)

(31)

)

(32)

)

(33)

)

(34)

)

(35)

𝜕𝑍

𝜕𝑋 𝜕𝐻𝑋

𝜕𝑦 𝜕𝐸𝑦 𝜕𝑍

𝜕𝐻𝐸𝑧 𝜕𝑋

𝜕𝐸𝑋 𝜕𝑦

(36)

)

These equations are discretized over time and space by using the following central difference scheme, 𝜕

𝜕𝑋

∆𝑥

𝑓 (𝑥 ) =

∆𝑥

𝑓(𝑥+ 2 )−𝑓(𝑥− 2 )

(37)

∆𝑥

Hence the previous set of equations are transformed into,

|𝐸𝑥 |𝑛+1/2 | |𝑛−1/2 𝑖,𝑗+1/2 = 𝐸𝑥 𝑖,𝑗+1/2 +

∆𝑡 𝜀𝑖,𝑗+1/2

𝑛 |𝐻𝑧 |𝑛 𝑖,𝑗+1 −|𝐻𝑧 |𝑖,𝑗

[

∆𝑦

]

(38)

32 Photonic Materials: Recent Advances and Emerging Applications 𝑛+1/2

𝑛−1/2

|𝐸𝑦 |𝑖+1/2,𝑗 = |𝐸𝑦 |𝑖+1/2,𝑗 + |𝐻𝑧 |𝑛+1 𝑖,𝑗

=

|𝐸𝑧 |𝑛+1 𝑖+1/2,𝑗+1/2

|𝐻𝑧 |𝑛𝑖,𝑗 =

µ𝑖,𝑗

=

𝜀𝑖+1/2,𝑗

[

[

∆𝑦

+

∆𝑡 Ɛ𝑖+1/2,𝑗+1/2

+

|𝐻𝑦 |𝑖,𝑗+1/2 = |𝐻𝑦 |𝑖,𝑗+1/2 +

∆𝑥

𝑛+1/2

|𝐸 𝑥 |𝑖,𝑗+1/2 −|𝐸𝑥 |𝑖,𝑗−1/2

|𝐻𝑥 |𝑛𝑖+1/2,𝑗 𝑛

𝑛 |𝐻𝑧 |𝑛 𝑖,𝑗 −|𝐻𝑧 |𝑖+1,𝑗

∆𝑡

𝑛+1/2

|𝐸𝑧 |𝑛−1/2 𝑖+1/2,𝑗+1/2

|𝐻𝑥 |𝑛+1 𝑖,𝑗+1/2 𝑛+1

+

∆𝑡

[

Michael et al.

−

𝑛+1/2 𝑛+1/2 −|𝐸𝑦 | 𝑖+1/2,𝑗 𝑖−1/2,𝑗

|𝐸 𝑦 |

∆𝑥

𝑛 𝑛 −|𝐻𝑦 | 𝑖+1,𝑗+1/2 𝑖,𝑗+1/2

|𝐻𝑦 |

∆𝑡 µ𝑖+1/2,𝑗 ∆𝑡 µ𝑖,𝑗+1/2

∆𝑥

𝑛+1/2

[

(39)

]

−

𝑛 |𝐻𝑥 |𝑛 𝑖+1/2,𝑗+1 −|𝐻𝑥 |𝑖+1/2,𝑗

∆𝑦

(40) ] (41)

𝑛+1/2

|𝐸𝑧 |𝑖+1/2,𝑗−1/2 −|𝐸𝑧 |𝑖+1/2,𝑗+1/2 ∆𝑦 𝑛+1/2

[

]

]

(42)

𝑛+1/2

|𝐸𝑧 |𝑖+1/2,𝑗+1/2 −|𝐸𝑧 |𝑖−1/2,𝑗+1/2 ∆𝑦

]

(43)

The field distribution of TE wave and TE wave field distribution is calculated by using the above equations. The superscripts indicate the time steps and the subscripts indicate the coordinates of the field quantities. Δt and Δx and Δy are the time step unit and space step unit, respectively. The linear dimensions of the space grid are fractions of the wavelength. Typically, the grid spacing can solve the numerical dispersion in time, and therefore usually be satisfied as Δ ≤ λ/10. In these equations, the temporal change in the E field is dependent upon the spatial variation of the H field, and vice versa. Hence, the recurrent computation of the electric field is carried out first and then the magnetic field starts from one side of the computation mesh and moves to another side. The values of the electric and magnetic field components are taken from the internodes (i+1/2, j+1/2) and (i-1/2, j-1/2) to allow central difference approximation. The computation is carried out repeatedly for different time moments until the required computation time is achieved. DESIGN OF PROPOSED ALL-OPTICAL XOR LOGIC GATE The intended logic of the XOR gate is obtained through the expression given below, Y = A𝐵 + 𝐴B = A ⊕ B The designed logic gate has a high contrast ratio that has the high distinction between logic ‘0’ and logic ‘1’. The proposed structure for designing an alloptical XOR logic consists of an array of 14*19 silicon rods in an air background with a square lattice as illustrated in Fig. (2). The gate comprises dual input waveguides, input waveguide A and input waveguide B and an output waveguide Z. The radius of the silicon rod is 0.169a, where a is the period of the structure.

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Photonic Materials: Recent Advances and Emerging Applications 33

The period is defined as the distance between the radii of the two rods. The value of the lattice constant is 560 nm.

Fig. (2). 2D Photonic structure of all-optical XOR gate.

The basic principle of the XOR logic gate is that, once the inputs are put in with the phase difference of 0̊ & 180̊ and different input levels, constructive interference occurs resulting in high output. When the inputs are the same, they cancel out due to the phase difference, resulting in no output. The data signals are assigned with the phase difference of about 0̊ &180̊ to obtain the logic at the output. This will result in logic ‘0’ or logic ‘1’at the output of negligible or maximum intensity respectively. Simulation results are carried out using the FDTD method. The wavelength at which maximum output is obtained is 1700 nm. The EM wave propagation is in

34 Photonic Materials: Recent Advances and Emerging Applications

Michael et al.

the direction of X and Z planes where magnetic fields are analogous to the Si rods axis [14]. According to courant condition, it was assumed as [15] 𝑐∆𝑡
5 GPa. The value of the quality factor of the proposed PC corresponding to different hydrostatic pressure is also tabulated in Table 3. Further, we observe that the shift in the position of transmission mode is very sensitive to the variation in applied hydrostatic pressure. Variation of the peak wavelength of transmission modes at different hydrostatic pressure is shown in Fig. (6). Such a type of PC structure is very useful in designing a pressure sensor with very high sensitivity. Also, it can be used in designing a tunable narrow transmission mode with high-quality factor.

Fig. (6). The shift in the position of transmission peak with hydrostatic pressure (P).

CONCLUSION In conclusion, a theoretical investigation and study of hydrostatic pressure effect on the reflectance and transmittance properties of the one-dimensional PC (1DPC) containing semiconductor layer have been discussed and presented in this chapter. In this study, two types of PC structures have been considered. As a first case, a 1D PC structure composed of alternate layers of germanium (Ge) and air having a finite number of layers has been taken. In the second case, we have taken the structure, by breaking its periodicity such that each part is a mirror image of each other. In order to calculate the transmittance and reflectance spectra of the

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Photonic Materials: Recent Advances and Emerging Applications 53

proposed PC, the transfer matrix method (TMM) has been used. From the analysis of reflectance curves, it is found that the width of reflection bands decreases with an increase in the hydrostatic pressure. Further, it has been observed that when the applied pressure increased, the position of reflection bands shifted towards the lower side of the wavelength. Analysis of transmittance spectra of conjugate PC structure shows that the transmission peak is blue shifted with an increase in the hydrostatic pressure. Moreover, the transmission mode is very sensitive to the variation in the applied pressure. The proposed structure can be useful in designing tunable optical reflectors and pressure sensors with very high sensitivity and tunable narrow transmission modes with high-quality factors. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT One of the author (Sanjeev K Srivastava) is thankful to the Amity Institute of Applied Sciences, Amity University Uttar Pradesh, Noida, India, for providing the necessary facilities for this work. REFERENCES [1]

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Reflectance and Transmittance

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[22]

Y-H. Chang, Y-Y. Jhu, and C-J. Wu, "Temperature dependence of defect mode in a defective photonic crystal", Opt. Commun., vol. 285, no. 6, pp. 1501-1504, 2012. [http://dx.doi.org/10.1016/j.optcom.2011.10.053]

[23]

B. Suthar, and A. Bhargava, "Bhargava., “Temperature dependent tunable photonic channel filter”", IEEE Photonics Technol. Lett., vol. 24, no. 5, pp. 338-340, 2012. [http://dx.doi.org/10.1109/LPT.2011.2178401]

[24]

V. Skoromets, H. Němec, C. Kadlec, D. Fattakhova-Rohlfing, and P. Kužel, "Electric-field-tunable defect mode in one-dimensional photonic crystal operating in the terahertz range", Appl. Phys. Lett., vol. 102, no. 24, pp. 241106-1, 4, 2013. [http://dx.doi.org/10.1063/1.4809821]

[25]

S. Srivastava, "Electrically controlled reflection band and tunable defect modes in one-dimensional photonic crystal by using potassium titanyl phosphate (KTP) crystal", Journal of Nanoelectronics and Optoelectronics, vol. 11, no. 3, pp. 284-289, 2016. [http://dx.doi.org/10.1166/jno.2016.1895]

[26]

F. Segovia-Chaves, and H. Vinck-Posada, "Dependence of the defect mode on the temperature and the angle of incidence in a one-dimensional photonic crystal", Optik (Stuttg.), vol. 163, pp. 16-21, 2018. [http://dx.doi.org/10.1016/j.ijleo.2018.02.035]

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S. Tao, D. Chen, J. Wang, J. Qiao, and Y. Duan, "A high sensitivity pressure sensor based on twodimensional photonic crystal", Photonic Sens., vol. 6, no. 2, pp. 137-142, 2016. [http://dx.doi.org/10.1007/s13320-016-0316-x]

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A.Y. Herrera, J.M. Calero, and N. Porras-Montenegro, "Pressure, temperature, and thickness dependence of transmittance in a 1D superconductor-semiconductor photonic crystal", J. Appl. Phys., vol. 123, no. 3, pp. 033101-1, 5, 2018. [http://dx.doi.org/10.1063/1.5009708]

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CHAPTER 4

Recent Advances in Graphene Based Plasmonics Tista Basak1,* and Tushima Basak2,* Mukesh Patel School of Technology Management & Engineering, NMIMS University, Mumbai400056, India. 2 Mithibai College of Arts, Chauhan Institute of Science and Amrutben Jivanlal College of Commerce & Economics, Vile Parle, Mumbai400056, India. 1

Abstract: Plasmonics is an emerging and fast-growing branch of science and technology that focuses on the coupling of light to the free electron density in metals, resulting in strong electromagnetic field enhancement due to confinement of light into sub-wavelength dimensions beyond the diffraction limit. The development of novel photonic and optoelectronic devices based on metal-based plasmonics is however plagued by the high loss at optical frequencies, originating partly from inter-band electronic transitions and lack of electrical tunability, practically limiting their potential applications in the terahertz (THz) and mid-IR spectrum range. The recent successful exfoliation of graphene from graphite has rendered a breakthrough in the realm of plasmonics due to its phenomenal properties such as exceptionally tight light confinement, extremely long plasmon lifetime, high carrier mobility leading to a relatively low level of losses, strong optical nonlinearity and electrostatically as well as chemically tunable response. These versatile features of graphene can effectively address the challenges faced by metals, and hence the physics and potential applications of graphene-based plasmonics have triggered increasing attention of industry, academic and research fraternity in recent years. This chapter provides a comprehensive description of the theoretical approaches adopted to investigate the dispersion relation of graphene surface plasmons, types of graphene surface plasmons and their interactions with photons, phonons and electrons, experimental techniques to detect surface plasmons, the behaviour of surface plasmons in graphene nanostructures and the recent applications of graphene-based plasmonics.

Keywords: Graphene, Graphene Nanostructures, Mid-infrared Photonics, Plasmonics, Quasi-Particles, Tunability, Terahertz Photonics.

Correspondence: Corresponding Author(s): Tista Basak, Mukesh Patel School of Technology Management & Engineering, NMIMS University, Mumbai 400056, India; , Tushima Basak, Mithibai College of Arts, Chauhan Institute of Science and Amrutben Jivanlal College of Commerce & Economics, Vile Parle, Mumbai 400056, India; Emails: [email protected], [email protected] Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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Photonic Materials: Recent Advances and Emerging Applications 57

INTRODUCTION A serious setback to technological progress in recent years is critically associated with the almost static growth of microprocessor performance. A cutting-edge solution to circumvent this persistent issue is to switch from the conventional electronic to the photonic mode which can have a pioneering impact on next generation computational and communication systems. Among all the photonics related research domains, the field of plasmonics offers the most promising option to attain this goal. Plasmonics involves the study of light-matter interaction at the nano-scale limit. It exploits the phenomenon of coherent and collective oscillation of charge carriers excited by an electromagnetic field [1, 2] at the boundary between two media having a positive and negative magnitude of permittivity (e.g., interface between metal and dielectric), termed as surface plasmon resonance (SPR), for a varied range of nano-photonic applications [3, 4]. Such free electron oscillations propagating on the surface of continuous thin films are designated as surface plasmon polariton (SPP), while those confined to nano-structures are termed as localized surface plasmon resonance (LSPR). Plasmons can also be induced by electron beams in the bulk of large materials, denominated as bulk plasmons, which evanesce severely due to heavy energy loss. The excitation of SPP and LSPR at characteristic frequencies results in a strong local electromagnetic field enhancement due to confinement of light into sub-wavelength dimensions, allowing the modulation of light beyond the diffraction limit. This appealing feature of SPR has attracted immense scientific interest in plasmonic based research over the past few years. The dawning age of surface plasmons can be tracked down to the year 1902 when Wood [5] observed an irregular distribution of light intensity reflected by a metallic grating, termed as Wood anomaly. In 1904, Maxwell-Garnett [6] proposed a theory based on an effective dielectric constant to justify the colours emitted by glasses having small metallic particles. The Maxwell-Garnett theory was followed by Mie [7] theory in 1908, which attempted to explain the colour of metallic colloidal particles based on light scattering and absorption properties of an arbitrary sized spherical particle. A plausible explanation of Wood's anomalies, theoretically proposed by Fano [8] in 1941, suggested that quasi-stationary (Sommerfeld’s type) electromagnetic waves with substantial tangential momentum on the surface of metals was responsible for the anomalous distribution of light reflected by a metallic grating which could not be elucidated by Rayleigh's approximation [9]. A comprehensive understanding of the theory of plasma oscillations in metals was realized in 1952 when Bohm and Pines [10 - 13] propounded a quantum-mechanical theory incorporating long-range electron

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correlations to explain the correlation between the discrete and collective behaviour of the free electrons. They showed that the free electrons oscillate collectively for distances d > λD, where λD is the Debye length; while for d < λD, it could be approximated as an ensemble of almost free discrete particles. This theory could conclusively explain the experimental observation of energy loss (in keV) of electrons in metallic films by Ruthemann and Lang [14, 15]. In 1957, Ritchie [16] computed the dispersion relations of SPPs in metallic films to predict that when an electron propagates in thin films, anomalous energy (depending on the thickness of films) is lost both at and below the resonant frequency of plasmon. The predictions of Ritchie was experimentally corroborated by Powell and Swan [17] from the measurements of electron energy loss spectra of aluminium foils. The next significant development in this field was achieved in the era of 70s by the seminal works of Teng and Stern [18], experimental demonstrations employing the attenuated total reflection (ATR) method by Otto [19], and also Kretschmann and Raether [20], followed by the discovery of the surface-enhanced Raman scattering by Fleischmann et al. [21]. From this time forth, several outstanding researchers have gradually achieved significant developments in the domain of surface plasmons. According to Maxwell's electromagnetic theory [1, 2, 22], the electric field of SPPs traversing along the boundary of a dielectric and a semi-infinite metallic medium is 𝑗

𝑗

(1)

𝐸𝑗 = (𝐸𝑥 , 0, 𝐸𝑧 )𝑒𝑥𝑝{𝑖(𝑘𝑆𝑃𝑃 𝑥 − 𝜔𝑡)}𝑒𝑥𝑝(−𝛼𝑗 |𝑧|)

where, kSPP and ω designate the wave-vector and frequency of SPPs, the superscript j = d and j = m represents the dielectric and metal, respectively, while αj is termed as the decay constant. The y-component of the magnetic field satisfies the same condition, implying that surface plasmon polaritons can be excited only by polarized transverse magnetic (TM) field. The amplitude of the electromagnetic field associated with SPPs decreases exponentially along with the normal interface between metallic medium and dielectric surface. The decay constants αd and αm for the dielectric surface and metal obey the relationship as shown below: 𝛼𝑑 =

𝜔

[

𝜀𝑑 2

𝑐 𝜀𝑑 +𝜀𝑚

]

1 2

and

𝛼𝑚 =

𝜔

[

𝜀𝑚 2

𝑐 𝜀𝑑 +𝜀𝑚

]

1 2

(2)

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Photonic Materials: Recent Advances and Emerging Applications 59

where, εm and εd, represent the dielectric function of metallic medium and dielectric surface, respectively. The distance normal to the interface between metallic medium and dielectric surface, at which the SPP field amplitude decreases by a factor of 1/e is defined as skin-depth (z) and can be determined from the decay constants according to the relations zd = -1/Re(αd) and zm = 1/Re(αm) inside the dielectric and metal, respectively. The theoretical dispersion relationship between the frequency (ω) and wavevector (kSPP) of SPPs, derived from surface mode solutions of Maxwell’s equations by applying necessary boundary conditions is specified by: 1

𝑘𝑆𝑃𝑃 =

𝜔

𝜀 𝜀 2 [ 𝑚 𝑑] 𝑐 𝜀𝑑 +𝜀𝑚

(3)

Since the dielectric function is a complex quantity, kSPP is also complex, with its positive real component denoting wave-propagation, and its imaginary term, ′′

designated as 𝑘𝑆𝑃𝑃 , representing internal wave-absorption by the metal. Due to this absorption, an SPP propagating parallel to the metal-dielectric boundary ′′ suffers from energy loss and consequently, its intensity decreases as 𝑒 −2𝑘𝑆𝑃𝑃𝑥. The distance at which the SPP intensity is diminished by a factor of 1/e is designated ′′ )−1 as propagation length (L), which is equal to (2𝑘𝑆𝑃𝑃 . The schematic plot of SPP dispersion curve is depicted in Fig. (1).

Fig. (1). The dispersion curve of SPP.

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For small values of kx, the SPP dispersion curve approaches the air light line (which denotes the relation between frequency and wave-vector in vacuum) given by 𝜔

𝑘𝑥 = ( ) √𝜀𝑚 𝑐

(4)

For large values of kx, the SPP dispersion curve deviates and approaches the horizontal line ω=ωSPP asymptotically, where the plasmon frequency ωSPP satisfies the condition εωSPP=-εm. The domains of the kx-ω space on the left- and right side of the air light line are called the radiative and non-radiative regions, respectively. The SPP dispersion curve lies in the non-radiative domain and hence, the SPPs cannot be directly excited by light. The essential requirement of matching the wave-vectors of SPPs and light for exciting the surface plasmons can be experimentally achieved by the prism, topological defects and periodic corrugations. The theoretical SPP dispersion relation (Eqn. 3) is generally fulfilled by conventional metals like Ag, Au, Al, Cu, Cr, Mg etc., as the real part of the dielectric function, εm, of these materials, has negative magnitude due to the abundant free electrons and small magnitude of its imaginary component. For these metals, the dielectric function, εm, obtained from the Drude model of free electrons is: 𝜀𝑚 = 1 −

𝜔02 𝑖𝜔

(𝜔2 + 𝜏 )

(5)

where τ denotes the relaxation time of electrons. However, metal-based plasmonic devices are plagued by significant ohmic and radiative energy losses notably at visible [23, 24] and ultra-violet (UV) frequencies, originating partly from interband electronic transitions, and exhibit poor tunability [25, 26]. These drawbacks severely limit their performance and potential applications. This has compelled the scientific community to explore alternative plasmonic materials. In this context, graphene [27], a two-dimensional (2D) allotrope of carbon with atoms arranged in a regular honeycomb lattice (Fig. 2a), has emerged as an excellent alternative for plasmonic material [28 - 42] due to its exotic mechanical, electrical, magnetic and thermal properties. The charge carriers (both electrons and holes) of graphene with zero effective mass, called Dirac fermions, exhibit a

Graphene Based Plasmonics

Photonic Materials: Recent Advances and Emerging Applications 61

small plasmon damping rate due to their ultra-high mobility, resulting in longer propagation length and plasmon lifetime, even at room temperature [43]. The carrier densities in graphene can be dramatically modulated by electrical gating and chemical doping [44 - 50], thereby substantially increasing the tunability of its plasmon spectrum. In particular, the exceptional potential of graphene is underlined by its significant high absorbance (≈ 2.3%) [33] of incident white light. Additionally, the plasmons in graphene exhibit stronger spatial light confinement (within volumes-106 times smaller than the diffraction limit) [33] compared to conventional metal plasmons, thereby enhancing its applicability in the fields of metamaterials [34, 45, 51 - 54], plasmonic wave-guides [55 - 57], light modulation [58 - 60], optical signal processing [61], sensing [62], quantum optics [63, 64] and nonlinear photonics [65, 66]. THEORETICAL FRAMEWORK OF PLASMONS IN GRAPHENE Electronic Structure of Graphene The electronic configuration of carbon atoms in graphene is 1s2 2s2 2p2, leading to sp2 hybridization involving one s and two p (px, py) orbitals of its outermost shell. This results in the formation of a trigonal planar structure comprising a σ bond between adjacent carbon atoms (atomic spacing 'a' ≈ 1.42 Å) [43] in the x-y plane, which renders high mechanical stability to the lattice structure. The energy bands corresponding to the σ bonds are filled in agreement with Pauli’s exclusion principle and hence constitute the lower energy levels of the valence band. The unaffected electron in the pz orbital, which is normal to the trigonal planar structure, forms a delocalized π bond [67] by binding covalently with adjoining carbon atoms and imparts distinctive optoelectronic and photoelectric properties to graphene. The first Brillouin zone of graphene in reciprocal space (Fig. 2b) contains four characteristic points - Γ point at the cell centre, two Dirac points (K and K′ at the inverse corners), and M at the midpoint of the edge defined by K and K′. The six vertices (Dirac points) in the Brillouin zone of graphene constitute the Fermi plane, while the Fermi surface is determined by K and K′. (Figs. 2 a, b and c) reprinted from [43]: A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, "The electronic properties of graphene," Reviews of modern physics, vol. 81, no. 1, pp. 109-162, 2009. Copyright 2009 by the American Physical Society).

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Fig. 2. (a) Honeycomb lattice structure of graphene. (b) First Brillouin zone of graphene in reciprocal space. (c) Energy band structure of graphene. (d) 3D picture of electronic dispersion of graphene with zoomed view of energy bands near the Dirac point.

(Fig. 2c) Reproduced from [68]: I. E. Abbott’s, "Graphene: exploring carbon flatland," Phys. Today, vol. 60, no. 8, p. 35, 2007 with permission from AIP Publishing.) The energy band structure of graphene (Fig. 2c) can be computed by employing a semi-empirical tight-binding Hamiltonian. The tight-binding approximation [43] considers only the π-bands and neglects the σ-bands since they lie far away from the Fermi energy level. The pz electrons forming the π-bonds are considered to be independent as the overlap of their orbitals with the other valence (s, px, py) orbitals is forbidden by the symmetry. The energy dispersion relationship of π-electrons in a monolayer pristine graphene sheet derived from the semi-empirical tight-binding approximation for ћ=1 is expressed as:

Graphene Based Plasmonics

Photonic Materials: Recent Advances and Emerging Applications 63

𝐸 ± (𝒌) = ±𝑡[3 + 𝑓(𝒌)]1/2 − 𝑡 ′ 𝑓(𝒌) √3𝑘 𝑎

(6)

3𝑘 𝑎

𝑦 𝑥 where, 𝑓(𝒌) = 2𝑐𝑜𝑠(√3𝑘𝑦 𝑎) + 4𝑐𝑜𝑠 ( 2 ) 𝑐𝑜𝑠 ( 2 ), represents the wavevector, kx and ky denote the components of the wave vector in the x and y directions respectively. t and t′ denote the nearest and next-nearest neighbour hopping energy, respectively.

The two energy bands [E+(k) and E-(k)] are symmetric at the Fermi energy (zero energy reference) level when t′ is zero, while the particle-hole symmetry is broken for t′ not equal to zero. The lower filled E- valence band (labelled as π band) and the empty higher energy E+ conduction band (denoted as π* band) are in contact with one another at the Dirac point [69], while the dispersion structure is conelike near the Dirac point Fig. (2d). This unique intersection of the valence band and conduction band at the Dirac point results in a zero bandgap, conferring semimetallic nature to graphene. Also, this unique intersection results in linear energy dispersion close to the Fermi level which is expressed by Dirac's relativistic equation, wherein the electrons are considered to be 2D massless fermions moving with velocity -106 ms-1. The dynamics of the massless Dirac fermions determines the phenomenal electronic and optoelectronic properties of graphene. The energy band-structure of the σ electrons cannot be accounted for by the tightbinding model and are dealt with the first-principles approach. The position of the Fermi level in graphene is critically governed by electronic perturbations induced by defects, disorder, chemical doping, electronic corrugations, interfacial hybridization, and anisotropic charge-induced dipoles, which can significantly impact its electronic structure. Optical Response and Dispersion Relation of Graphene Surface Plasmons The optical properties of graphene can be derived from its in-plane conductivity σ(k,ω), which depends on the dispersion relation. In recent years, several theoretical studies based on a Semi-classical model, Random-phase approximation (RPA), tight-binding approach, first-principles computation, Dirac equation continuum model and experimental Electron Energy Loss Spectroscopy (EELS) measurements have substantially investigated the dispersion relation of surface plasmons in graphene. We now provide a comprehensive description of the most extensively employed Semi-classical model and Random-phase approximation for theoretical computations.

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Semi-classical Model According to the semi-classical model, the in-plane conductivity, σ, of graphene derived from the Kubo formalism [70] in the absence of magnetic field, is given by 𝜎(𝜔, 𝜇, Γ, 𝑇) = −

𝑖(𝜔+2𝑖Γ)𝑒 2 𝜋ℏ2

1

∞

𝜕𝑓(𝐸)

× [(𝜔+2𝑖Γ)2 ∫0 𝐸 (

𝜕𝐸

−

𝜕𝑓(−𝐸) 𝜕𝐸

∞

) 𝑑𝐸 − ∫0

𝑓(−𝐸)−𝑓(𝐸) 𝐸 2 ℏ

(𝜔+2𝑖Γ)2 −4( )

(7)

𝑑𝐸 ]

where ω is the incident light frequency, T is the temperature, μ is the chemical potential, is the rate of phenomenological scattering, with vF and μm corresponding to the Fermi velocity and carrier mobility of electrons in graphene, respectively, and the Fermi distribution function denoted by: 𝑓(𝐸) =

1

(8)

(𝐸−𝜇) [1+𝑒 𝑘𝐵 𝑇 ]

where kB denotes the Boltzmann constant. The second term in Eqn. (7) indicates the interband electron transition while the first term (σintra) [67] represents the process of intraband electron-phonon scattering, given by: 𝜎 𝑖𝑛𝑡𝑟𝑎 = 𝑖

𝑒 2 𝑘𝐵 𝑇

[

𝜇

𝑖 𝜋ℏ2 (𝜔+𝜏) 𝑘𝐵 𝑇

+ 2𝑙𝑛 (

1 𝜇

+ 1)]

(9)

𝑒 𝑘𝐵 𝑇

in which the real part implies the energy dissipation or absorption on account of intraband electrons. Eqn. (9) is valid for   EF with 𝜇 ≈ 𝐸𝐹 [1 − (𝜋𝑘𝐵 𝑇/𝜇)2 /12] for the very small magnitude of kBT/μ. The chemical potential, μ, of graphene can also be computed from the carrier density, n, according to the expression, 𝑛=

∞ 2 ∫ 𝐸{𝑓(𝐸) ℏ2 𝜋𝑣𝐹2 0

(10)

− 𝑓(𝐸 + 2𝜇)}𝑑𝐸

In the case of highly doped graphene, |𝜇| ≫ 𝑘𝐵 𝑇, 𝜇 ≈ 𝐸𝐹 ≈ √ℏ2 𝜋𝑣𝐹2 𝑛, 𝑛 ≈

𝜇2 ℏ2 𝜋𝑣𝐹2

.

The corresponding intraband term exhibiting Drude-like nature and interband contribution is approximately expressed as: 𝜎 𝑖𝑛𝑡𝑟𝑎 =

𝜇𝑒 2

𝑖

ℏ2 𝜋 (𝜔+ 𝑖 ) 𝜏

and 𝜎 𝑖𝑛𝑡𝑒𝑟 =

𝑒2 4ℏ

𝑖

ℏ𝜔−2|𝜇|

𝜋

ℏ𝜔+2|𝜇|

[𝜙(ℏ𝜔 − 2|𝜇|) + 𝑙𝑛 |

|]

(11)

Graphene Based Plasmonics

Photonic Materials: Recent Advances and Emerging Applications 65

where 𝜙(ℏ𝜔 − 2|𝜇|) denotes a step function. From these expressions, it is evident that the contribution of intraband transition to the conductivity of graphene is dominant in the terahertz to the far-infrared frequency domain on account of the difficulty in exciting interband electronic transition by incident photons in this frequency range. However, the role of interband electronic transition in determining the conductivity enhances gradually from the near-infrared to the optical range. Hence, both the intraband and interband terms have to be evaluated for computing the conductivity in this range. These conclusions have been validated by experimental results [69]. The surface plasmons in graphene can propagate in two possible modes transverse magnetic, TM (s-polarized) and transverse electric, TE (p-polarized). The type of mode supported in graphene depends on the magnitude of the imaginary component of its conductivity, designated as σ′. When graphene behaves as a semiconductor, valid either when it is pristine or has shallow chemical potential, the interband electronic transition contributes dominantly to its conductivity and σ′ < 0, resulting in the existence of only TE surface plasmon waves. However, when the characteristics of graphene are similar to metals, effective for high chemical potential, the intraband transition is the primary contributor to its conductivity and σ′ > 0, leading to the presence of only TM modes in graphene. Also, the TE modes in graphene occur in the far- and nearinfrared domain while the TM modes lie in the terahertz and far-infrared band for carrier concentrations [67] of 10111014 cm-2. The dispersion relation of the TE and TM surface modes is expressed as: 𝑘 𝑇𝐸 =

𝑘 𝑇𝑀 =

𝜔

2

(12a)

√1 − (𝜎𝑍0 ) 2

𝑐

𝜔 𝑐

2

√1 − (

𝜎𝑍0

2

(12b)

)

Where, Z0 denotes the intrinsic wave impedance. The wavelength of the graphene surface plasmons, λSP, obtained from the above dispersion relations (Eqn. 12) is given by: 𝜆𝑆𝑃 = 𝜆0

𝜇𝑒 2 𝜋ℏ2 𝜀

1

𝑖 0 𝑐(𝜀𝑟1 +𝜀𝑟2 ) (𝜔+ ) 𝜏

(13)

where, εr1 and εr2 represent the relative permittivity of the upper and lower medium surrounding graphene.

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This implies that the dispersion relation and characteristics of surface plasmons in graphene can be controlled by regulating the relative permittivity or the chemical potential. Random Phase Approximation(RPA) The RPA methodology, initiated by Bohm and Pines [10 - 12], eliminates the complex nature of many body electron interactions by assuming that each independent electron moves in a self-consistent field comprising the external field and the effective field of all electrons. A detailed kinetic approach for computing the quantum dielectric function of free electron gas was developed by Lindhard [71] by adopting the RPA approach. In the case of graphene, the polarizability, dielectric function and nonlocal conductivity can be computed by employing the RPA method. These quantities determine the dynamic and static screening of Coulomb interaction in graphene, which is significantly different from the screening behaviour exhibited by classical 2D systems. Physical properties of graphene such as plasmon dispersion and elementary excitation spectra can be computed from dynamic screening, while its transport characteristics can be obtained from static screening. The local RPA model calculates the dispersion relation by taking into account only the long-wavelength plasmons (q→0) while the more precise nonlocal RPA model also includes short-wavelength plasmons corresponding to non-zero values of q [72]. The dynamic longitudinal screening function (dielectric function) [73], within the RPA, is given by 𝜀(𝑞, 𝜔) = 1 + (

2𝜋𝑒 2 𝜅𝑞

) Π(𝑞, 𝜔)

, (14)

where κ is the background lattice dielectric constant of graphene and Π(q, ω) is the 2D graphene polarizability, which can be obtained from the non-interacting bare bubble diagram. The 2D graphene polarizability is entirely different from the analogous 2D Lindhard function (applicable to free electron gas) since the band dispersion in graphene is linear near the Dirac points which is in stark contrast to the parabolic energy dispersion of free electrons in conventional 2D systems. The plasmon dispersion mode can be computed from the zeroes of the dynamical longitudinal dielectric function, ε(q,ω). According to the local RPA model (q→ 0), the limiting forms of 2D graphene polarizability corresponding to the highand low-frequency domains are: Π(𝑞, 𝜔) ≈

𝐷𝐸𝐹 𝑣𝐹2 𝑞2 2𝜔2

[1 − (

𝜔2 4𝐸𝐹2

)],

2𝐸𝐹 > 𝜔 > 𝑣𝐹 𝑞

(15)

Graphene Based Plasmonics

Photonic Materials: Recent Advances and Emerging Applications 67 𝜔

Π(𝑞, 𝜔) ≈ 𝐷𝐸𝐹 [1 + 𝑖 (

𝑣𝐹 𝑞

)],

𝑣𝐹 𝑞 > 𝜔

(16)

where, 𝐷𝐸𝐹 denotes the density of states at the Fermi energy (EF) and vF is the 2D Fermi velocity. The dispersion of surface plasmon modes 𝜔sp0(q) for q→0 in monolayer graphene is given by 𝜔𝑠𝑝0 (𝑞) = √𝑞𝜔𝑝

(17)

1

(18)

where, 𝜔𝑝 = (𝑔𝑣 𝑔𝑠 𝑒 2

𝐸𝐹 2 ) 2𝜅

Here, gv=gs=2 represents the valley and spin degeneracy of graphene, respectively. The local plasmons of graphene obey the same dispersion, q, as the classical 2D plasmons. However, the frequency of plasmons, ωp, in graphene is proportional to n1/4, where n denotes the carrier density, which is in contrast to the n1/2 dependence exhibited by the frequency of classical 2D plasmons. This variance is due to the quantum relativistic behaviour of graphene. The incorporation of nonlocal effects in RPA (i.e., nonlocal RPA model) in 2D classical systems increases their plasmon frequency at finite q values, ωsp (q), with respect to ωsp0 (q), in accordance with the relation, 𝜔𝑠𝑝 (𝑞) 𝜔𝑠𝑝0 (𝑞)

3

=1+ (

𝑞

4 𝑞𝑇𝐹

)

(19)

where, qTF = (gvgsme2)/κ is the normal 2D Thomas-Fermi wave-vector. However, the inclusion of these higher order terms of q (nonlocal effects) in graphene decreases the frequency of its plasmons at non-zero q values, compared to its corresponding frequency for long-wavelength plasmons, as per the expression, 𝜔𝑠𝑝 (𝑞) 𝜔𝑠𝑝0 (𝑞)

=1−

𝑞0 𝑞 2 8𝑘𝐹

(20)

where kF is the Fermi momentum and q0 is the graphene Thomas-Fermi wavevector given by 𝑞0 =

𝑔𝑣 𝑔𝑠 𝑒 2 𝑘𝐹 𝑣𝐹 𝜅

(21)

The dispersion relation of graphene surface plasmons computed by employing the

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RPA model is quite consistent with the results of the EELS experiment [74]. However, the RPA methodology cannot precisely model the dispersion relation of surface plasmons in graphene due to its two basic presumptions of many-body interaction effect and infinite electronic relaxation time. TYPES OF GRAPHENE SURFACE PLASMONS Plasmon waves in graphene can be supported by both its outer (σ and π) electrons. The intraband transitions account for the low energy (< 3 eV) plasmons (termed as 2D plasmons), while two other types of plasmons, termed as π and π + σ plasmon, exist at higher energies. In pristine graphene, intraband transitions are forbidden while they are allowed in doped graphene due to their partially filled π* band. Consequently, 2D plasmons in THz and IR range occur in doped graphene, while only π and π + σ plasmons at energies > 4.5 eV appear in pristine graphene. In particular, 40% of incident electromagnetic radiation in the THz range can be absorbed by moderately doped graphene, making it a potential contender in designing THz metamaterials and electromagnetic interference (EMI) shielding materials. The plasmon frequency in graphene can be enhanced from THz to midIR range by reducing the size of graphene sheet from 10-6 m to 10-7 m. The plasmons in graphene can be probed by diverse direct and indirect experimental techniques such as EELS measurements on epitaxial graphene samples and exfoliated graphene sheets, optical measurements on graphene microstructures such as graphene microribbons and microdisks, angle-resolved photoemission spectroscopy (ARPES), inelastic light scattering and scanning tunnelling spectroscopy [48]. The plasmons in graphene can also be detected by increasing the concentration of dopants in graphene, assembling multilayer of graphene, and employing external photonic structures (e.g., Fabry−Perot cavity having an opaque reflector and a lossless dielectric spacer). COUPLING OF SURFACE PLASMONS WITH PHOTONS, PHONONS AND ELECTRONS The interaction of surface plasmon with photon results in the formation of surface plasmon polariton (SPP). In pristine graphene, surface plasmon polariton cannot be excited by direct incident light due to the higher momentum of the surface polariton in comparison to that of incident light with the same frequency. Some of the most frequent experimental techniques for exciting SPPs include attenuated total reflection (ATR) [75], scattering from topological defects [76, 77] and Bragg scattering from diffraction grating [78] or a periodic corrugation on the surface of graphene [79, 80]. Other methodologies to excite surface plasmons in graphene include using a dipole emitter [33] at short distances from doped graphene, modelling the graphene geometries into ribbons, nanodisks, and antidots [45, 46,

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81] and electrostatic inhomogeneous periodical doping of pristine graphene [82, 83]. The properties of SPPs can be efficiently regulated by tuning the frequency of incoming radiation, the relative permittivity of the substrate and electrostatic gate voltages. In addition to light-matter coupling, graphene also supports interactions of surface plasmons with photons and electrons, which are discussed subsequently. The interaction of plasmons with phonons (quantized vibrations of crystal lattice) in graphene exhibits an unusual nature that has fundamental implications in governing its characteristics such as nonadiabatic Kohn anomaly [84] and breakdown of the Born-Oppenheimer approximation [85]. The abrupt changes in the screening of atomic vibrations by gapless electrons at specific points of the Brillouin zone, governed by the shape of the Fermi surface, lead to cusps in the phonon dispersion of a material, termed as Kohn anomalies. The electronic bandgap in graphene is zero only at the two equivalent Dirac (K and K′) points, implying that Kohn anomalies can manifest as linear kinks in the dispersion of the longitudinal optical (LO) and transverse optical (TO) phonon branches at Γand K points, respectively (Figs. 3 and 4a). Both these kinks occur for phonons with wave-vector q = 2kF, where kF is the wave-vector of the Fermi surface. The occurrence of these cusps in the in-plane optical phonon branches (LO and TO) is confirmed by experimental measurements such as Raman Spectroscopy [85], ARPES [86], EELS [31] and inelastic X-ray scattering [IXS] [87] as well as theoretical studies. The coupling of plasmons with phonons also accounts for the acoustic-like quasi-linear dispersion of plasmons, detected by EELS measurements. (Reprinted from [87]: A. Grüneis et al., “Phonon surface mapping of graphite: Disentangling quasi-degenerate phonon dispersions,” Physical Review B, vol. 80, no. 8, p. 085423, 2009. Copyright 2009 by the American Physical Society).

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Fig. (3). The experimentally measured (represented by points) and theoretically computed (denoted by lines) phonon dispersion curve of graphene. This Fig is reproduced with permission from [87].

The occurrence of the Kohn anomalies in graphene is strong evidence of the failure of the Born-Oppenheimer approximation, which is possible when the phonons and plasmons have comparable energy ranges. Theoretical computations employing self-consistent linear response methodology have predicted that the hybridization of plasmon and optical phonon modes in graphene is in striking contrast to that observed in conventional systems. In graphene, longitudinal plasmons (LP) couple exclusively to TO phonons, while transverse plasmons (TP) couple only to LO phonons, with the strength of the LP-TO coupling being more significant than TP-LO coupling [88]. This peculiar coupling of plasmon and phonon modes is a more striking manifestation of the failure of the BornOppenheimer approximation as compared to the experimentally measured Kohn anomaly. In free-standing graphene, the first order coupling between plasmons and out-of-plane phonons is severely inhibited due to mirror symmetry about the horizontal plane. However, when graphene is grown on a substrate, this first order coupling between plasmons and out-of-plane optical phonons (ZO mode) is allowed leading to Kohn anomaly for the ZO mode (Fig. 4b) as demonstrated by high-resolution electron energy loss spectroscopy (HREELS) measurements [89] on single layer graphene grown on Pt(111). Even though extensive studies have been done to investigate the interaction between plasmons and phonons in

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graphene, a detailed understanding of the phonons involved in the coupling and the strength of the plasmon-phonon interaction is still lacking.

Fig. (4). Dispersion of the (a) longitudinal optical (LO) phonon and (b) out-of-plane optical (ZO) phonon, near the Γ point showing kink at q = 0.13 Å-1. The experimental data and theoretical fitting are represented by red dots and black line, respectively.

(Reprinted from [89]: A. Politano, F. de Juan, G. Chiarello, and Herbert A. Fertig, “Emergence of an out-of-plane optical phonon (ZO) Kohn anomaly in quasifreestanding epitaxial graphene,” Physical Review Letters, vol. 115, no. 7, p. 075504, 2015. Copyright 2015 by the American Physical Society) In the earlier sections, we have described the non-interacting single-particle model of pristine graphene in which the dispersion relation of the filled valence band and empty conduction band is linear and form two cones that intersect at k = 0 in momentum space, with the corresponding energy termed as the Dirac crossing energy (ED) (Fig. 5a). However, this model neglects the interaction between charge carriers and plasmons which collectively couple to create a quasiparticle, termed as plasmaron. These interactions drastically alter the topology of the dispersion bands in the vicinity of the Dirac crossing, as confirmed by ARPES measurements performed on quasifreestanding graphene, grown epitaxially on Hterminated silicon carbide and doped chemically with potassium atoms [90]. The ARPES experiment detected a splitting of the single point Dirac crossing at energy ED into three different well-defined crossings: the crossing of the hole band at E0, the ring like an intersection of the hole and plasmaron bands at E1 and the plasmaron band crossing at E2, resulting in a diamond-like shape, with its width determined by two momenta k± (Fig. 5b). The plasmaron, at a definite momentum k, comprises strongly coupled holes and plasmons having momentum k+q and -q, respectively. It has higher binding energy than a bare plasmon or hole, due to the excess energy required for the generation of a plasmon along with a hole. The creation of a plasmaron requires the fulfilment of two conditions: (1) pseudospin and momentum conservation, which is possible only when k and q are

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parallel; (2) same group velocity for plasmons and holes. These conditions ensure that the plasmaronic quasiparticle is discrete in phase space.

Fig. (5). (a) The (non-interacting) single-particle picture of linear band dispersion of graphene. (b) Schematic representation of the Dirac energy spectrum by considering the interactions between charge carriers and plasmons.

BEHAVIOUR OF SURFACE PLASMONS IN GRAPHENE WITH DIFFERENT DIMENSIONALITIES In graphene, the attributes of surface plasmons can be efficiently modulated by varying the dimensionality and geometric configuration of graphene. Characteristics of Surface Plasmons in 2D Bilayer Graphene The structural difference between bilayer and monolayer graphene leads to a multitude of distinct electronic and optical properties. The first sign of the different nature of plasmonic behaviour in bilayer graphene having Bernal ABtype stacking (compared to monolayer graphene) is revealed by the occurrence of two prominent peaks in optical conductivity in the IR range. The first peak corresponds to an optically active phonon mode at energy ≈ 0.2 eV. The Fanotype line profile and intensity of this peak is strongly dependent on the applied gate voltage [91, 92]. The second peak at energy ≈ 0.4 eV [93, 94] is attributed to interband transition between the two nested bands, created by interlayer coupling

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in bilayer graphene. Theoretical computations based on RPA methodology propose that these two peaks can efficiently tune the plasmonic characteristics of bilayer graphene, leading to the interference of IR active phonon with quasicontinuum electronic transitions (Fano-type resonances), massive plasmonic amplification of phonon absorption in IR range, a narrow optically transparent window, and the emergence of a new higher energy plasmonic mode in comparison to that of the classical plasmon [95]. In addition, experimental observations using IR nanoimaging technique have confirmed that plasmons in this type of bilayer graphene exhibit stronger light confinement in comparison to monolayer and weakly-coupled randomly-stacked double-layered graphene on account of higher interlayer electron tunnelling. Also, plasmons in the midinfrared range can be effectively switched off through electrostatic gating over a wider voltage range in contrast to the narrow voltage span observed for single layer graphene [96]. Characteristics of Surface Plasmons in 1D Graphene Nanoribbons (GNRs) and 0D Graphene Quantum Dots (GQDs) It has been described earlier that the direct excitation of 2D plasmons by incident electromagnetic radiation in single layer graphene is a highly challenging problem in photonics. This technical challenge can be efficiently resolved by reducing the graphene dimensionality to 1D graphene nanoribbons which have a higher local density of optical states leading to stronger light-matter interactions. The infrared nanoimaging technique has observed that the confined configuration of GNRs generates well-defined plasmonic mode patterns and strong electromagnetic field enhancement which change in a well-defined manner with the GNR width and the infrared excitation wavelength (Figs. 6 and 7a). Further, GNRs exhibit 1D plasmonic modes propagating along their edges termed edge plasmons (Fig. 7b). The wavelength of the 1D edge plasmons is 0.906 times the wavelength of 2D surface plasmons [97, 98] due to the effective reduction of Drude weight in GNRs. Experimental investigations [99] also revealed that the width of the armchair edge plasmon is narrower than the width of the zigzag edge plasmon. Theoretically, the extra plasmon broadening in zigzag terminated GNRs has been attributed to the presence of localized states in these systems [100, 101]. In addition, reduced plasmon damping and larger plasmon propagation length can be achieved in 1D GNRs as compared to 2D graphene [102, 103]. (Reprinted (adapted) with permission from [104]: Z. Fei et al., “Edge and surface plasmons in graphene nanoribbons,” Nano letters, vol. 15, no. 12, pp. 8271-8276, 2015 Copyright 2015 American Chemical Society.”) The surface plasmons in graphene exhibit distinct behaviour when their

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dimensionality is further reduced from 1D to 0D nanostructures such as micro/nano-disks, -antidots, -rings, etc. The finite-size effect in graphene microdisks leads to broadening of the spectral band in the lower energy resonance domain and splitting of the photoabsorption strength line. In addition, the nature of the shift (red/blue-shift) of plasmon excitation spectra is also critically dependent on the geometries of the 0D GQDs. The coupling of surface plasmons between graphene nanostructures on the same plane is weaker in comparison to that in stacked configurations [81, 105]. Further, the plasmonic interactions can be regulated by electrostatic/chemical doping or hybridizing graphene with other 2D or conventional plasmonic materials.

Fig. (6). (a) Schematic illustration of the infrared nanoimaging arrangement for observing plasmons in GNRs. (b) The AFM phase images of GNRs with varying widths. (c-f) Images of near-field experimental data of GNRs with varying widths under ambient conditions. (g-j) Line profiles measured along the red dashed lines in (c−f), perpendicular to GNRs. Black dashed lines and red/green arrows mark the boundaries of the ribbons and the locations of the principal/inner fringes, respectively. The blue arrows in (j) denote the FWHM of the single fringe. (Reprinted (adapted) with permission from [104]: Z. Fei et al., “Edge and surface plasmons in graphene nanoribbons,” Nano letters, vol. 15, no. 12, pp. 8271-8276, 2015 Copyright 2015 American Chemical Society”).

CURRENT APPLICATIONS OF SURFACE PLASMONS IN GRAPHENE The extraordinary properties of surface plasmons in graphene can be efficiently modulated to design devices such as metamaterials [34], biological and gas sensors [35 - 37], high-speed transistors [38], phototransistors [39], energy storage [40], displays [41] and flexible devices [42]. A few of these applications are described below. Recently, heavily boron doped silicon quantum dots were used to fabricate hybrid graphene-based phototransistors (Fig. 8a). The photodetection range of this

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resulting phototransistor was extended from the normal UV-vis domain into the near-IR range due to sub-bandgap optical absorption caused by electron transitions in boron doped silicon quantum dots. Further, the photodetection range of this phototransistor was extended to the mid-IR range by the LSPR of boron doped Si quantum dots.

Fig. (7). (a) The black curves represent the line profiles of infrared near-field amplitude s(ω) measured with continuous wave (CW) lasers at specific frequencies ω along the red dashed line in Fig. (6c). (b) The black and red lines indicate the line profiles of 2D surface plasmon modes and 1D edge modes, respectively. The boundaries of the GNRs and the principal peaks in the line profiles are denoted by black dashed lines and arrows, respectively. (Reprinted (adapted) with permission from [104]: Z. Fei et al., "Edge and surface plasmons in graphene nanoribbons," Nano letters, vol. 15, no. 12, pp. 8271-8276, 2015 Copyright 2015 American Chemical Society.")

Fig. (8). Schematic representation of (a) graphene-based hybrid phototransistors with B-doped Si QDs, (b) graphene based plasmon device for gas identification [37, 39].

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(Fig. 8a Reprinted (adapted) with permission from [39]: Z. Ni et al., “Plasmonic silicon quantum dots enabled high-sensitivity ultrabroadband photodetection of graphene-based hybrid phototransistors,” ACS nano, vol. 11, no. 10, pp. 98549862, 2017. Copyright 2017 American Chemical Society.”) (Fig. 8b Reprinted from [37]: H. Hu et al., “Gas identification with graphene plasmons,” Nature communications, vol. 10, no. 1, pp. 1-7, 2019. (reused material is under a CC BY 4.0 license) Furthermore, the strong electromagnetic field confinement exhibited by graphene plasmons and large physisorption of gas molecules on the GNRs has been effectively used to fabricate GNR sensors (Fig. 8b) for determining gas molecules such as SO2, N2O, NO and NO2 by observing their rotational-vibrational modes. CONCLUSION AND FUTURE PERSPECTIVES This chapter provides a comprehensive overview of the new perspectives in the alluring field of plasmonics with the advent of graphene. It has been emphasized that the unique properties of graphene such as exceptionally tight light confinement, extremely long plasmon lifetime, high carrier mobility leading to relatively low level of losses, strong optical nonlinearity and electrostatically as well as chemically tunable response lend it a remarkable edge over traditional metal plasmonic materials. The tremendous progress made in the theoretical methodologies, understanding of interactions of graphene surface plasmons with photons, phonons and electrons, experimental techniques and applications of graphene-based plasmonics have been reviewed in this chapter. In addition, the plausibility of tuning the properties of graphene surface plasmons by varying the dimensionality of graphene has been discussed. It is expected that this chapter will be of great significance for a deeper understanding of the physics of graphenebased plasmonics, thereby, opening up new frontiers in the development of novel devices using graphene plasmonics. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none.

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CHAPTER 5

Third Generation Solar Cells - Promising Devices to Meet the Future Energy Needs Ram Chhavi Sharma1,* 1

Department of Physics, Faculty of Science, SGT University, Gurugram-122505, Haryana, India Abstract: Energy is the basic input for the improvement of the social status of human beings and the development of a nation. At present, we are observing a shift in the use of energy from non-renewable to the renewable energy due to exhausting natural resources of non-renewable energy and other environmental and climatic concerns. Solar energy resource is an inexhaustible source of energy. The development of first generation solar cells using silicon material in the middle of the nineteenth century introduced a new era in the renewable energy transformation process when the first solar cells were flown on the fourth satellite, the Vanguard-I in 1958. But despite abundant material resources, high stability and good performance, this technology could not fulfill the energy need except a fraction due to very long payback time. The second generation solar cells are also not very encouraging due to the scarcity of materials and their toxic nature. The third generation solar cells, due to extremely low energy payback time and unlimited availability of material are promising devices to contribute significantly in solar energy conversion, despite limitations of poor stability and low efficiency. The present chapter critically analyses the third generation solar cells, in regard to materials, production, fabrication process, energy payback time, efficiency and applications.

Keywords: Efficiency, Energy Payback Time and Applications, Renewable Energy, Solar Cells. INTRODUCTION Energy is the basic input to the national economy, both agricultural and industrial, apart from being an instrument for improving the quality of life. The development of any nation and society as a whole depends on the energy resources available and affordable technologies for usable energy conversion. The energy demand is increasing. The question is, how we are going to meet the growing energy demands of the future and what is the right choice for utilizing the resource base. The need of the time is to develop promising new technologies and even new Corresponding author R.C.Sharma: Department of Physics, Faculty of Science, SGT University, Gurugram-122505, Haryana, India. Email: [email protected] *

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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physical and chemical processes for the establishment and operation of efficient systems to generate, accumulate, transform and transport energy into its various forms [1]. Decreasing trends in non-renewable energy resources and their negative social, health, and environmental impacts on account of unsustainable patterns of energy extraction and use are apparent [2, 3]. The significant global warming due to the emission of greenhouse gases and climate change put a threat to the sustainability and existence of entire species on the globe. The currently available data on non-renewable energy resources indicate that to meet the future energy demand, large-scale alternative methods of producing the vast quantities of energy will be needed to sustain and enhance our standard of living [4, 5]. We are fortunate enough that nature has provided us with alternative sources of energy, such as wind, geothermal, biomass, and solar energy. Among these energy resources, solar is the most promising because every hour the energy absorbed by Earth’s atmosphere from the sun is sufficient to satisfy global energy needs for a year. Solar power is a renewable resource that is available everywhere in the world but in varying degrees depending on the geographical status of a country and for India, solar power is the best choice. Among the advantages are; inexhaustible source of power (as long as the sun shines), a sustainable and environmentally friendly method of producing energy, small and highly modular, no fuel costs and relatively low operation and maintenance costs. The first photovoltaic effect was discovered by Becquerel in 1839 and since then, solar energy has been of research interest in the scientific community. In 1877, the photovoltaic effect was observed in solidified Selenium [6]. In 1883, Charles Fritts developed the first Selenium solar cell based on a thin layer of gold, which has a power conversion efficiency of less than 1%. Since then, research has exploded to find the most efficient and cost-effective solar cells. Due to the design and synthesis of the novel compounds, understanding and controlling the film morphology and elucidating the device mechanism, the PV cell technologies are improving and are usually classified into three generations. The first generation solar cells are mainly based on silicon wafers and typically demonstrate the performance of about 15-25%. Good performance and high stability are the main advantages, while very large payback time, rigidity and large energy requirement in their production are the main disadvantages [7]. The second-generation solar cells are based on inorganic semiconducting materials with higher absorption coefficients like amorphous silicon, polycrystalline semiconductors, Copper Indium Gallium Selenide (CIGS) and Cadmium Telluride (CdTe). They demonstrate the typical performance of about

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10-15%. They are thin film solar cells since they use direct bandgap materials and can be made much thinner than first generation solar cells. However, the production of these solar cells requires a large amount of energy to go through the vacuum processes and high-temperature treatments. Also, the materials used in these cells are scarce elements and this is a limiting factor in both the price and their commercialization [7, 8]. The third-generation solar cells are a mix of many types of solar cell technologies mostly processed from solution. These include; organic solar cells, perovskite solar cells, dye-sensitized solar cells, multi-junction solar cells, and quantum dot solar cells. The experimental multi-junction solar cells hold the world record in solar cell performance, plus novel devices in general. A new class of thin film solar cells currently under investigation are perovskite solar cells which show huge potential with record efficiencies beyond 20% on a very small area. The present book chapter critically analyses the third generation solar cells, in regard to materials, production, fabrication process, energy payback time, efficiency and applications. The main focus will be kept on low cost PV technology, which is getting a lot of attention from academic researchers and there is adequate industrial interest too. This article will also provide you with enough motivation that there is a need for low cost PV technology and the development of solar cells to meet the future energy demands. Basic Parameters To understand the power production capability of solar cells, it becomes essential to learn about their I-V characteristics. I-V Characteristics determine the parameters like open circuit voltage, short circuit current, fill factor, power conversion efficiency and external quantum efficiency. The rating of the solar panel is determined by these parameters which are illustrated in Fig. (1) and briefly described below. The current equation of a solar cell, when illuminated by light, is given by; 𝑞𝑉

𝐼 = 𝐼𝑜 [𝑒𝑥𝑝 (

𝑛𝑘𝑇

) − 1] − 𝐼𝐿

(1)

where IL is the light generated current. It is the photocurrent when the external voltage is zero. It has the effect of shifting the I-V curve down. The Power Conversion Efficiency (PCE) is related to the short-circuit current density (Jsc) and the open circuit voltage (Voc) by:

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Fig. (1). A typical I-V Characteristics and power characteristics of a solar cell indicating open circuit voltage, short circuit current and Maximum power point [9] (used with permission; licensed material under CC BY NC SA).

PCE (η) = P max./P in = (FF X J sc X Voc.)/Pin

(2)

where Pin is the incident power from solar irradiation and Fill Factor (FF) is a measure of the squareness of the J vs. V characteristics. 𝐹𝐹 =

𝐽𝑚𝑎𝑥 ×𝑉𝑚𝑎𝑥 𝐽𝑠𝑐 ×𝑉𝑜𝑐

(3)

The fill factor in (3) represents the maximum power that can be extracted from a solar cell. The short circuit current density (Jsc) and open circuit voltage both depend on the value of the bandgap of material (Eg). Since the photon energy (E) is inversely proportional to the wavelength, so Jsc generally increases with increasing wavelength across the visible and infra-red regions of the solar spectrum provided that E is greater than Eg. Also the value of Voc decreases with the decrease of band gap Eg. This indicates that there is an optimum value of Eg to achieve optimum PCE value. The maximum efficiency for a solar cell with an Eg of 1.1 eV was calculated by Shockley and Queisser to be 30%. The intensity of light absorbed in a solar cell decreases exponentially with material thickness so the thickness of the photoactive layer compared to the absorption length (1/ α, where α is the absorption coefficient in cm-1) is another key parameter governing the PCE for a solar cell. The solar cells having relatively low α values need a greater thickness of their photoactive layers (order of

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hundreds of micrometres to millimetres). This increases the material and production costs of solar cell significantly. External Quantum Efficiency (EQE) which is the ratio of the rate of electrons collected as photocurrent and the Rate of Photons incident on a solar cell is an important parameter that measures the solar cells' behaviour in a specific range of wavelength to maximize the utilization of solar irradiance. The short circuit current density in terms of EQE is given by 𝜆

𝐽𝑠𝑐 = 𝑞 ∫𝜆 2 𝐸𝑄𝐸(𝜆)∅𝐴𝑀1.5 (𝜆)𝑑𝜆 1

(4)

The variation of quantum efficiency with wavelength for an ideal solar cell is shown in Fig. (2).

Fig. (2). Quantum Efficiency of an Ideal Solar cell [10].

The energy payback time (EPBT) is another important consideration that determines the commercialisation of any technology for the generation of energy. For a PV module, it is the amount of time a module must produce power to recover the energy it took to produce the module initially. In the production of solar energy, the solar cell should have minimum EPBT. The energy payback time depends on the materials used in the solar panel, power conversion efficiency and geographical location and related solar irradiation. The EPBT for various solar cell technologies is given in Fig. (3). It can be seen that the EPBT is minimum for organic solar cells.

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Fig. (3). Estimates of Energy payback time and Record Efficiency for the manufacture of various solar technologies. (calculations based on data from [11, 12]).

Third Generation Solar Cells The main aim of advancement in solar cells technology over the existing technologies is to improve power conversion efficiency over a wider band of solar energy, making solar cells less expensive, decreasing energy payback time and making use of materials without any toxicity so that it can full fill the energy requirement of more and more people, and may be used in diversified fields. With this aim, the third generation solar cells technology come into the picture. It includes organic solar cells, perovskite solar cells, dye sensitized solar cells, multi-junction solar cells, and quantum dot solar cells. Organic Solar Cells Polymer solar cells are a photovoltaic technology that uses hydrocarbon based organic molecules or macromolecules to harvest energy from light. The key advantages of polymer solar cells include higher flexibility, potentially less expensive to manufacture, low material cost, lower weight and independence of scarce resources. The material used to convert the solar light into electric energy in polymer solar cells is a conjugated polymer that exhibits semiconducting behaviour. This was discovered by Alan J. Heeger, Alan MacDiarmid, and Hideki Shirakawa and received the Nobel Prize in Chemistry in the year 2000. Polymer solar cells are fabricated from the solutions, which are made of organic solvents which can be printed or coated.

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For many years, polymer solar cells could not keep up the first and second generation solar cells technologies on both power conversion efficiency and stability Today, performances of roughly 12% have been demonstrated for polymer solar cells with a lifetime of 3-5 years. However, polymer solar cells have yet to need intensive research for large scale commercialization. Main Features of Conjugated Polymers In a conjugated polymer system, alternating single and double bonds are found between the carbon atoms. In the backbone of the conjugated polymers, where each carbon atom binds to three adjacent atoms, leaving an electron free per carbon atom in the pz orbital. The overlapping of these pz orbitals leads to the formation of pi bonds along the conjugated polymer backbone. This delocalizes the pi electrons along the length of the polymer. These pi electrons fill up the band and therefore these polymers are intrinsic semiconductors. The filled pi band is called the Highest Occupied Molecular Orbital (HOMO) and the empty pi band is termed the Lowest Unoccupied Molecular Orbital (LUMO). The pi electrons can be excited without changing the chemical structure of the polymer. This means that it is possible to promote an electron from the HOMO to LUMO level upon excitation. The bandgap in the conjugated polymers depends on the size of the polymer and hence any change in the polymer chain will affect the position of local HOMO and LUMO levels. The schematic of the mechanism of an organic solar cell is shown in Fig. (4).

Fig. (4). Mechanism of an Organic Solar Cell [13]. (reused content licensed under CC BY 4.0).

The organic semiconductors have low dielectric constants and so electrons and holes are tightly bound by Columbic force of attraction. These tightly bound

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electrons and hole pair are called excitons. The binding energy of these excitons is in the range of 0.3 to 1.0 eV. Principle of Operation When solar radiations are incident on organic solar cell, excitons are generated inside the donor material. The binding energy of the excitons prevents their dissociation into free electrons and holes, which causes the flow of current in the external circuit. On dissociation of excitons, the electrons are transferred to the highly electronegative acceptor material. The force for separation of electron and hole in the exciton is determined by the energy difference between the LUMO of the donor material and the LUMO of the acceptor material. The isolated electrons and holes are then freely delocalized and are collected at the anode and cathode, respectively. For efficient operation of an Organic solar cell, the acceptor material must have strong electronegativity and high electron mobility in the carbon shell. The invention of C60 fullerene and its derivatives as acceptor materials [14] which possess these properties has been one of the major milestones in OPV technology and becomes a standard acceptor in the OPVs. Also, the separation between an exciton generation site and a donor/acceptor interface is of the order of 5-10 nm. But for most of the photons to be absorbed, the thickness of the active layer should be of the order of 100 nm. So for maximum absorption of photons and to enhance the efficiency of organic solar cell, the donor-acceptor structure in the active layer becomes all important. Intensive research is going on in this direction, by forming single layer junctions, bilayer junctions and bulk heterojunctions with relative merits and demerits. In a bulk heterojunction based polymer solar cell, the absorption layer consists of a nanoscale blend of donor and acceptor materials. Ideally, the domain sizes of this blend are on the order of nanometers, allowing for excitons with short lifetimes to reach an interface and dissociate due to the large donor-acceptor interfacial area. The nanostructure in the bulk heterojunction is, however, critical, since large enough domain sizes are needed to form a percolating network that allows the donor materials and the acceptor materials to reach their respective electrodes. Without this network, charges might be trapped in a donor or acceptor-rich domain and undergo recombination. The main advantage of bulk heterojunctions is that the thickness of the active material layer can be increased for effective photon absorption without the limitation caused by the short exciton lifetime. The problem is that the nanostructural morphology of the bulk heterojunctions is difficult to create in a reliable way and the formed nanostructure may not be thermodynamically stable.

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However, the nano-structure is critical to photovoltaic performance. Fig. (5) shows the standard and inverted device architecture for a heterojunction organic solar cell.

Fig. (5). Heterojunction organic solar cell (a) Standard device architecture (b) inverted device architecture [15]. (Reuse of material which is licensed under CC BY 4.0).

As we have seen, polymer solar cells are a technology with some very specific drawbacks, mainly stability and efficiency, but also some clear advantages, mostly production speed, material abundance, and low processing energy. The challenges of efficiency and stability have seen dramatic improvements over the years, and today record efficiencies are on the order of 12%, while multiyear lifetimes are reached. The improvement in power conversion efficiency of organic solar cells with time is presented in Fig. (6).

Fig. (6). Development in Laboratory Organic Solar Cell PCE (calculations based on data reported in [16 18]).

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One key advantage of polymer solar cells is that the cells can be assembled in modules during the manufacturing itself and the manufacturing process are solution-based (spin coating, inkjet printing, doctor blading, screen printing, etc.). In this way, infinitely long solar cell modules can be produced. This allows extremely fast installation times for polymer solar cells. Organic solar cells can be printed atop flexible and transparent PET substrate. These type of solar panels can be folded and carried along for charging electronics devices [19]. It can be integrated with clothes, a backpack and on tents. Thus, the main target of this technology is niche applications where a life span of 3-5 years is good enough. The development of a Tandem solar cell, where two sub-cell can complement each other spectral response, is one of the most effective ways of improving the PCE of Organic Solar Cells (OSCs) [19, 20]. The operation mechanism of tandem solar cells is presented in Fig. (7). Perovskite Solar Cells In the third generation, Perovskite solar cells are another new technology that has seen an incredibly fast development, improving PCE from 3.8% to 22.1% in time span from 2009 to 2016. These solar cell has the potential of achieving even higher efficiencies as the bandgap is tunable and can be optimized for the solar spectrum. Also, the technology is solution-processable and so it shares the fabrication advantages of organic solar cells and thereby promises low production costs. The material used as an absorber in perovskite solar cells is methylammonium lead tri-halide (CH3NH3PbX3, where X is chlorine, bromine or iodine). Depending on halide content the bandgap can be varied between 1.5 and 2.3 eV.

Fig. (7). Operation mechanism of Tandem Solar cell [20]. (Reprinted (adapted) with permission from K. Zhang, L.Ying, H. L. Yip, F. Huang, and Y. Cao, ACS Appl. Mater. Interfaces, 12, 36, 39937-39947, 2020. Copyright 2020 American Chemical Society).

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A common concern about the perovskite solar cells is the inclusion of lead as a component of the perovskite materials. Lead is a highly poisonous heavy metal that has serious environmental concerns and this is one of the biggest problems with perovskite solar cells. In an attempt, Lead has been replaced by Tin as a component of perovskite material, such as CH3NH3SnI3 but this lowers the powerconversion efficiencies. Another concern with perovskite solar cells is the stability which is mainly influenced by environmental changes (moisture and oxygen), thermal changes, heating under applied voltage, and photo-degradation. In a study [21], the stabilizing effect of fluorolymeric coating has been investigated to verify the stability of the coated Perovskite Solar Cells (PSCs) under real outdoor atmospheric conditions, where temperature variations, precipitation phenomenon and pollution are typically encountered. The PSCs were subjected to highly variable climatic conditions, as outdoor temperatures ranged from ‒3 to +27°C and 27 days over 92 were characterized by heavy rain and storms. The front/back-coated PSCs exhibited long-term stability retaining 95% of their initial efficiency as they protect the perovskite from UV-radiation, and convert it into exploitable visible photons, acting as a moisture barrier thereby preventing the hydrolytic phenomenon of the material and keeping the front electrode clean by means of the self-cleaning characteristics of this fluorinated polymer. The photocurable fluoropolymers coating maintains stability and also improves the power conversion efficiency of PSCs. The improvement in PCE (%) with fluoropolymeric layer coating laden with different amount of V570 is presented in Fig. (8).

Fig. (8). Variation in Power Conversion efficiency of perovskite solar cells with photocurable fluoropolymers (based on data reported in [21]).

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Dye Sensitized Solar Cells (DSSCs) Another technology that is gaining momentum is Dye sensitized solar cell, whose modern version was invented in 1988 by Brian O’Regan and Michael Gratzel at UC Berkeley. Its operation is based on the formation of a semiconductor between a photo-sensitized anode and an electrolyte. The attractive features of dye sensitized solar cell are; production is simple using conventional roll-printing techniques, semi-transparent and semi-flexible which offers a variety of uses not applicable to traditional solar cell systems, and most of the materials are low-cost. Some of the materials used in the current DSSCs need replacement on account of their high cost, less abundance, and long-term stability. By optimizing the material and structural properties using Ru(II) dyes, the PCE of existing DSSCs reaches up to 12% [22]. The construction and working principle of Dye synthesized solar cell is shown in Fig. (9).

Fig. (9). Construction and working principle of Dye synthesized solar cell [22] (Reuse of material which is licensed under CC BY 4.0).

Quantum Dot Solar Cells In quantum dot (QD) solar cells, nanocrystals of semiconducting metal chalcogenides including cadmium sulfide (CdS), cadmium selenide (CdSe), lead sulfide (PbS), lead selenide (PbSe), and other materials function as the lightabsorbing material in the device. The methods of production of these cells are low-cost solutions based which are amenable to high-speed printing techniques. The bandgap of the active material depends on the size of quantum dots and can be tuned by changing the size of the quantum dots. In a recent study [23], high efficiency and flexible CsPbI3 QD solar cells have been demonstrated using hybrid interfacial architecture. The key factors which

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determine the device efficiency are; efficient charge transfer at hetero-interfaces and suppression of interfacial charge recombination. To achieve this, the fullerene derivative [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) has been introduced into the CsPbI3 QD layer leads to the formation of a hybrid heterojunction interfacial connecting layer. The band energy levels diagram of control and target devices of QD solar cells are presented in Figs. (10 and 11) respectively. The organic molecule PCBM not only suppresses the interfacial charge recombination but also enhances the QD film quality and improves the contact between the active layer and the electron transport layer (ETL). As a result, the CsPbI3 QD solar cells demonstrated an impressive efficiency of 15.1% (stabilized power output of 14.61%). Also, flexible perovskite QD solar cells with a PCE of 12.3% with improved mechanical flexibility relative to similar thin-film perovskite compositions have been successfully demonstrated. This opens a new route to improving the performance of QD photovoltaic devices [23].

Fig. (10). Band Energy Level Diagram of the control devices [23] (Reuse of material which is licensed under CC BY 4.0).

Fig. (11). Band Energy Level Diagram of the target devices [23] (Reuse of material which is licensed under CC BY 4.0).

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CONCLUSION In summary, among the renewable energy resources, solar is the best option. Encouraging improvements have been made in the past few years in organic solar cells toward high power conversion efficiency, long-lasting operation, and largearea processing, showing a bright future for large-scale applications. In the last decade, the PCE of organic solar cells reached up to 17.3% from about 5% and stability has considerably improved. Also, perovskite solar cells gain tremendous momentum towards increasing PCE. The increase in PCE from 14.5% to about 17% with fluoropolymeric layer coating laden with different amount of V570 is highly encouraging. The tuneable energy bandgap of organic solar cells and Quantum Dot solar cells is of significance to produce multi electron-hole pairs per incident photon and enables tandem applications which are not possible in the case of technologies using fixed bandgap materials. It is expected that the third generation solar cells bring about substantial changes to the global energy landscape in the near future. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENTS The author is grateful to SGT University Gurugram for providing an excellent research environment and encouragement during the course of this work. REFERENCES [1]

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L. Hu, Q. Zhao, S. Huang, J. Zheng, X. Guan, R. Patterson, J. Kim, L. Shi, C.H. Lin, Q. Lei, D. Chu, W. Tao, S. Cheong, R.D. Tilley, A.W.Y. Ho-Baillie, J.M. Luther, J. Yuan, and T. Wu, "Flexible and efficient perovskite quantum dot solar cells via hybrid interfacial architecture", Nat. Commun., vol. 12, no. 1, p. 466, 2021. [http://dx.doi.org/10.1038/s41467-020-20749-1] [PMID: 33473106]

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101

CHAPTER 6

Recent Advances of Graphene in Solar Cell Applications Chandra Kamal Borah1,* and Sanjeev Kumar1,* Centre for Advanced Research, Department of Physics, Rajiv Gandhi University, Arunachal Pradesh-791112, India 1

Abstract: There has been incredible progress so far in graphene (Gr)-based solar cells and this is going to continue well into the future. Therefore, it is important to get an idea of the recent progress of graphene-based solar cells in the last decades. In this chapter, a brief overview of the recent research on Gr in solar cell applications has been outlined. It is prominent that Gr has been used in heterojunction solar cells, GaAs solar cells, Dye-sensitized Solar cells (DSSC), Perovskite solar cells, Polymer solar cells, and organic solar cells. In these solar cells, Gr has been utilized either as an absorber layer, hole transport layer, or electron transport layer. However, Gr has been used in the form of thin film, flakes, or quantum dot form. About 25% output efficiency has been observed in Gr-based solar cells so far. This chapter gives an overview of the Grbased solar cell with efficiencies to further continue the research on Gr-based solar cells to achieve higher efficiency.

Keywords: Absorber Layer, DSSC, Electron transport Layer, Heterojunction, Hole transport Layer, Perovskite, TCE. INTRODUCTION With the rise of the world population by 0.7% per year and the development of industries, the consumption of energy increases gradually. As a result of this, the conventional and non-renewable energy sources (i.e. fossil fuels) diminish day by day. Alternatively, the burning of fossil fuels adversely affects the environment by releasing greenhouse gases. Considering the energy crisis and the crucial impact on our environment, human begins to think of alternative, renewable, low cost, and environment-friendly energy sources that are capable of fulfilling the higher energy demand in the future. Solar energy is one of the renewable and freely available energy sources [1 - 3]. Corresponding authors Chandra Kamal Borah and Sanjeev Kumar: Centre for Advanced Research, Department of Physics, Rajiv Gandhi University, Arunachal Pradesh-791112, India and Centre for Advanced Research, Department of Physics, Rajiv Gandhi University, Arunachal Pradesh-791112, India; E-mails: [email protected] and [email protected]

*

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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Since the discovery of energy generation from sunlight by Edmund Becquerel in 1839 [4], the Photovoltaic (PV) solar cell technology opened the era for energy generation. Solar cells are the devices that converted sunlight directly into electricity. At present time, silicon-based solar cells have captured 80% market share as silicon is abundant and easily available [5]. Besides, the performance of this silicon-based solar cell is reliable. Moreover, III-V material-based solar cells have also shown their efficiency about 32.8% [6]. Beyond these, the comparatively new and promising Perovskite and Dye-based solar cells have compatibility in terms of their efficiency, low cost, and easy fabrication process [7 - 8]. Although these conventional solar cells are highly efficient, yet some drawbacks limit their application extensively. For example, the process cost of silicon [9] and III-V materials is very high [10]. The Perovskite materials are unstable [11] and the materials used in conventional dye-based solar cells are not cost-effective [8]. Therefore, to overcome these challenges we have to realize such a material that can be easily integrated with the mature and well-established solar cell technology to overcome these difficulties. Over the last few years, twodimensional graphene (Gr) has emerged as a potential material for solar cell application. Graphene is an allotrope of carbon consisting of carbon (C) atoms arranged in a honeycomb structure [12]. Fig. (1) illustrates the structure of 2D-Gr.

Fig. (1). Molecular structure of 2D-Graphene.The Carbon atoms are arranged in a honeycomb structure.

In 2004, Andre Geim and Kostya Novoselov [13], for the first time demonstrated the 2D-Gr exfoliated from highly oriented pyrolytic graphite. After the discovery of Gr, from near and far in 2010, research had been started on the potential application of Gr on solar cells due to its exciting and interesting optoelectronic properties. Gr exhibits semi-metallic behavior and many interesting properties such as more than 90% optical transparency, sheet resistance as low as 10 Ω/sq, and charge carrier mobility of 105 cm2/Vs and tunable bandgap as well as work function. These properties ensure the applicability of Gr in solar cells [14].

Cell Applications

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APPLICATION OF GR IN VARIOUS TYPES OF SOLAR CELLS Gr in Heterojunction Silicon Solar Cell The first generation heterojunction solar cells are composed of n-type silicon and p-type silicon. Although these solar cells are highly efficient yet the processing cost is very high. To reduce its cost, the research on the application of Gr in Si solar cells has been started. Gr is a semi-metallic material with a zero bandgap. Therefore, most of the research from 2010 to 2020 has been carried out on the Gr/Si schlocky junction solar cells. Here, Gr has been used as an emitter layer. However, Gr has also been used as a Transparent conducting electrode (TCE). So far maximum output efficiency (η) of 15% has been reported as per our knowledge. To achieve this efficiency, the study on the effect of doping on Gr and anti-reflecting coating on Gr/Si structure has also been carried out during the past years. In Table 1, research progress on Gr/Si-based solar cells from 2010 to 2020 has been listed. Table 1. Research Progress of Gr/Si solar cell. Year

Structure

Gr layer (layer=L)

η Ref. (%)

2010

Al/Gr/p-cSi/Al

1-4 L, CVD grown

0.01 [15]

2010

Au/Gr/n-Si/ (Ti/Pd/Ag)

> 3L,CVD grown

2011

Gr/n-SiNW/( Ti/Pd/Ag)

Multilayer, CVD grown, Treated with SOCl2

2.86 [17]

1.5

2011

Ag /Gr/nSiNW/Ag

1L, CVD grown

2.15 [18]

2012

G/n-Si/ (Ti/Pd/Ag)

3-5 L, CVD grown, Boron doped

2012

(Ag/Ti)/Gr/n-Si(Pillar-array) /(Ti/Pd/Au)

1L, CVD grown, Treated with HNO3

4.35 [20]

3.4

[16]

[19]

2012

Ag/Gr/P(VDF-TrFE)/GO/n-Si/(In-Ga)

1L, LPCVD grown

4.14 [21]

2012

Ag/Ti)/Gr/n-Si(Pillar-array) /(Ti/Pd/Au)

2~3 L, CVD grown, Treated with HNO3

7.72 [22]

2012

(Au/Cr)/Gr/n-Si/---

1L,CVD grown, TFSA amide doped

2013

---/Gr/n-Si/(Ti/Au)

1-5 L, CVD grown, Treated with HNO3

9.63 [24]

2013

----/Ag/Gr/Si/--- (TiO2 coating)

1L, CVD grown, Treated with HNO3

14.5 [25]

2013

Ag wire/Gr/n-Si/(cu foil+ silver paste)

3L, CVD grown, Treated with SOCl2

5.95 [26]

2013

--/Gr/n-Si/(Ti/Au)

4-7L, CVD grown, Treated with SOCl2

9.27 [27]

8.6

[23]

104 Photonic Materials: Recent Advances and Emerging Applications

Borah and Kumar

2013

Ag/Gr/n-cSi/---

1L, CVD grown

4.2

2013

(Ti/Au)/Gr/n-Si/(In-Ga)

4L, CVD grown, AuCl3 doped

10.4 [29]

2014

Ag/Gr-AgNW/Al2O3/ n-Si/ InGa

1L, CVD grown, AgNWs network is soldered using graphene oxide (GO) flakes

8.68 [30]

2014

Ag/Gr/n-Si/Ga–In

Electrochemical exfoliation

4.41 [31]

2014

Ag/Gr/nSi/pSi/

1L Gr, APCVD grown, Gr as Front contact

8.94 [32]

2014

TiO2 layer/Ag/Gr/Go/n-Si/Ga–In

6-7L, LPCVD grown, Treated with HNO3

5.2

2014

Ag/Gr/GO/Si/InGa

1L, LPCVD grown

6.18 [34]

2014

Ag/Gr/n-cSi/

4L, CVD grown

1.48 [35]

2015

AgNW/GNW/ Si/ (Ga-In)

Few Layer, RF sputtering

2015

Ag/Gr/P3HI/Flexible Si

Few layers, CVD grown, Treated with HNO3

2015

Ag/GNW/Si/ (E-GAIn)

Few layer, PECVD grown, Treated with HNO3

2015

Ag/PMMA-Gr/nSi fim/ GaIn on PET

single layer, Flexible device

5.09 [39]

2015

TiO2/CeG/nSi/---

CNT embroidered Gr (CeG), CVD grown, Treated with HNO3

15.2 [40]

2015

Ag/Gr/MoS2/nSi/GaIn

7L, LPCVD grown

6.56 [41]

2016

(Ti/Au)/Gr/GQD/nSi/InGa

Few layers, CVD grown,3-6 nm 11.43 [42] size layer of Graphene quantum dot, Treated with HNO3

2016

(Ti/Au)/Gr/nSi/(Ti/Au)

1L, CVD grown, NiO nano particle 4.91 [43] coated Gr

6.6

[28]

[33]

[36]

8.26 [37] 5.1

[38]

2016

(Ti/Au)/Go/G/nSi/Al

1L, CVD grown

10.6 [44]

2016

Ag/Gr/FG/Si/In-Ga (Flurographene)

1L,LPCVD grown, Treated with NHO3

13.38 [45]

2016

(Ti/Au)/PMMA/G/nSi/(Ti/Au)

1nm L, LPCVD grown, Treated with NHO3

13.34 [46]

2016

Ag/PMMA/Gr/spiro-OMeTAD/ nSi/GaIn

1L, LPCVD grown, Treated with NHO3

10.97 [47]

2017

(Ag/AuCl3/Gr/nSi/Ag) PMMA coating

1L, LPCVD grown

12.5 [48]

2017

Au/Gr/PSi/nSi/GaIn (Porous Silicon)

1L, CVD grown, Au Nano 10.69 [49] Particles+ bis(trifluoromethanesulfonyl)-amide doping,

2017

(Ag/PtNP/Gr/nSi/InGa)-TiO2 coating

1L, commercial Gr

2017

Metal/PMMA/Gr/Al2O3/nSi film/a-S:H/(Cr-Ni)

10 mm, Doped with AuCl3

7.04 [50] 7.4

[51]

Cell Applications

Photonic Materials: Recent Advances and Emerging Applications 105

2017

(Ag/Gr/P3HT/Si/InGa) Coating with (PDMS/TiO2)

CVD grown, 12.95 [52] bis(trifluoromethanesulfonyl)amide + poly(3-hexylthiophene-25-diyl) doped

2017

Ag/Gr/MoO3/a-Si/InGa (PMMA coating)

1L, CVD grown, Treated with HNO3

12.2 [53]

2018

(Au/Ti)/AlOx/n-Gr/ p-Si/Al

1L, PECVD grown

12.5 [54]

2018

(Cr/Au)/Gr/Al2O3/ n-Si/Al

Multilayer, CVD grown

2018

(Au/Cr)/Gr/n-Si/Al (V2O5 coating)

1L, CVD grown

2019

Graphite/Gr/nSi/Al

Multilayer, CVD grown

2019

(Cr/Au)/Gr/nSi/Al

4 nm Gr, Doped with HNO3

2020

Au/Gr/CQDs/Si/InGa (TiO2 coating)

2020 Al/Gr/p-a:Si:H/i-a-Si:H/n-cSi/i-a-Si:H/n-SiOx/AZO/Al (ZnS+MgF2) coating

8.4

[55]

3.03 [56] 7.9

[57]

9.18 [58]

3L, PECVD

9.97 [59]

Few layer graphene and used as TCE

15.21 [60]

Graphene in GaAs Solar Cell In comparison with commercially available Si, Gallium Arsenide (GaAs) has higher saturated electron mobility and direct bandgap. Due to the radiation resistance nature, GaAs solar cells have advantages over Si solar cells for space applications [61]. Unfortunately, Gr/GaAs-based solar cells have not been studied in comparison to other solar cells. However, this type of solar cell has reached the maximum output efficiency of 18.5% [62]. The Gr/GaAs structure is almost similar to the Si-based solar cells where Gr has been used as an emitter layer and GaAs as an absorber layer. It has been observed that doped with bis(trifluoromethanesulfonyl)-amide (TFSA) [62], Au Nano-particles (AuNP) decorated with [63] Gr layer and TiO2 coating as an anti-reflecting layer [64] increase the efficiency than the simple Gr/GaAs based solar cells. Table 2 demonstrates the research process of Gr/GaAs-based solar cells. Table 2. Research Progress of Gr/GaAs solar cell. Year

Structure

Gr layer

η (%) Ref.

2013

Ag/graphene/GaAs/Au

1-2L Gr, CVD grown

1.95 [61]

2015

TiO2/Ag/Gr/P3HT/GaAs/(Ge/Au)

1L Gr, LPCVD grown Gr is doped with TFSA

13.7 [64]

2015

Ag/Gr/GaAs/Au

1L Gr, CVD grown Gr is doped with TFSA

18.5 [62]

2015

Ag/SiG/GaAs/Au

1L Gr, CVD grown, Gr is doped with silicon

4.5

[65]

106 Photonic Materials: Recent Advances and Emerging Applications 2015

Ag/Gr/h-BN/GaAs/Au

Borah and Kumar

1L, CVD grown

7.10 [66]

2016 Ag/AuNPs/Gr/n-GaAs/n-AlGaAs/n-GaAs/Au

1L, CVD grown

16.2 [63]

2018

1L, CVD grown

10.8 [67]

(Ti/Pt/Au)/Gr/GaAsNW/ (Ti/Pt/Au)

Graphene in Dye-Sensitized Solar Cells Dye-Sensitized solar cells (DSSC) are another kind of solar cell that is different from conventional solar cells. In DSSCs, the dye-loaded and TiO2 coated transparent conducting glass is used as a photo-anode, whereas Pt-coated transparent conducting glass is used as a counter electrode (CE). An iodine-based electrolyte is sandwiched between these two electrodes. The Pt used in traditional DSSCs is rare and highly expensive. Therefore, research has been carried out to find the alternative of Pt. So far, a few research works have been carried out on the application of Gr in DSSC. Mostly, Gr has been used as a CE material, however, Gr has also been used in photo-anode. However, the output efficiency of Gr-based DSSC is very less than the other types of solar cells. Table 3 demonstrates the recent research and output efficiency of Gr-based DSSCs. Table 3. Research Progress of Gr based DSSC. Year

Structure

Gr Layer

η (%) Ref.

2011

FTO/TiO2/N719dye/Electrolyte/(Gr/Pt)/FTO

2014

ITO/TiO2/N719dye/Electrolyte/(Gr/Pt grid)/ITO

CVD grown

0.4

2014

PET/TiO2/N719dye/Electrolyte/(Gr/ Ni grid)/PET

CVD grown

0.25 [69]

2016

FTO/TiO2/N719-dye /Electrolyte/Gr/FTO

Gr ink

1L Gr, modified Hummers method 6.09 [68]

3.5

[69] [70]

2019 ITO/TiO2/N719-dye /Electrolyte/(CoNiSe/Gr)/ITO Composite, hydrothermal method

9.42 [71]

2020

in-situ polymerization

7.45 [72]

-------

3.37 [73]

FTO/TiO2/N3 dye/Electrolyte/ (PANi/Gr)/FTO

2020 FTO/(TiO2/SnO2/Gr)/N719-dye /electrolyte/Pt/FTO

Graphene in Perovskite Solar Cells Due to the rapid progress in stability and performance, the organohalide metal Perovskite solar cells have attracted remarkable interest from many researchers. Easy fabrication, low cost of processing, high carrier mobility, and strength and board absorption of light make it an ideal candidate in solar cell applications. To achieve higher efficiency, the selection of a proper electron transport layer (ETL) and hole transport layer (HTL) is important [74]. Recently, research has been carried out on the application of Gr in Perovskite-based solar cells. The recent progress of graphene-based Perovskite solar cells is shown in Table 4. From the

Cell Applications

Photonic Materials: Recent Advances and Emerging Applications 107

table, it has been observed that in most structures, the thin film or graphene quantum dots are used and graphene is used as a composite material in most Perovskite solar cells. However, more than 20% output efficiency has been observed in graphene-based Perovskite solar cells so far. Table 4. Research Progress of Gr based Perovskite solar cells. Year

Structure

Gr layer

η (%)

Ref.

2013

FTO glass/(TiO2+Graphene)/(Al2O3+CH3NH3PbI3-xClx)/CH3NH3PbI 3-xClx /Spiro-OMeTAD/Au

Exfoliation method

15.6

[75]

2014

FTO glass/Compact TiO2/(CH3NH3PbI3/Graphene QD/ TiO2)/Spiro-oMeTAD/Au

Single/Few 8.81%-10.15% [76] layer QD, electrochemical method

2015 (PDMS/PMMA/Graphene)/PEDOT:PSS/Spiro-OMeTAD/CH 1-4L graphene, 8.74%-12.37% [77] CVD grown 3NH3PbI3-xClx/TiO2/FTO/Glass 2016

FTO glass/(mTiO2/Graphene)/MAPbI3/ spiro-OMeTAD/Au

Graphene ink

16.30

[78]

2016

FTO-glass /(mTiO2/Graphene)/MAPbI3/Gr Oxide /spiroOMeTAD/Au

Graphene ink

18.19

[78]

2017

Glass/FTO/c-TiO2/ (Graphene+mTiO2)/MAPbI3/Spiro-OMeTAD/Au

------

12.5

[79]

2017 Glass/FTO/c-TiO2/(Graphene+mTiO2)/GO Li/MAPbI3/SpiroOMeTAD/Au

------

12.6

[79]

2017

FTO glass/(Graphene+TiO2)/CS0.1MA0.3P-bI3/SpiroMeOTAD/Au

------

7.34 - 15.42

[80]

2017

Glass/(AuCl3-Gr)/PEDOT:PSS/MAPbI3/PCBM/Al

1L Gr, CVD grown

11.5-17.9

[81]

2018

Au/HTL/Perovskite/Gr-Quantum dot/mp-TiO2/c-TiO2/FTO

------

20.45

[82]

2019

FTO-galss/(graphene/ZnO)/MAPbI3/Spiro-OMeTAD/Au

1L Gr

21.03

[74]

2019

Quartz/FTO/TiO2/CH3NH3PbI3/Gr/ Au

7L Gr, CVD grown

7.1

[83]

Exfoliation method, few layer Gr

25.9

[84]

2020 (Glass/FTO)/c-TiO2/m-TiO2/(Cs0.06FA0.78MA0.16)Pb(Br0.17I0.83)3 /Spiro-OMeTAD/ITO/(Front contact + metal grid)/HJT Si cell

Graphene in Other Types of Solar cells Graphene has also been used in other types of solar cells such as organic solar cells or polymer solar cells. Various forms of graphene, like Nanoribbon forms or quantum dots have been used as electron transfer layer (ETL) in such types of

108 Photonic Materials: Recent Advances and Emerging Applications

Borah and Kumar

solar cells. Table 5 gives examples of various types of solar cells. Table 5. Gr based other type solar cells. Type of Solar Cell

Structure

Gr Layer

η (%) Ref.

Polymer

Glass/Au-doped Gr/PEDOT:PSS/ WO3/SMPV1:PC71BM/Ca/Al

Gr-Nano Ribbon

8.48

[85]

Polymer

Graphene/PEDOT:PSS/P3HT:PCBM/LiF/Al

Gr solution

0.13

[86]

Organic

Glass/Graphene/PEDOT:PSS/P3HT:PCBM/Al:Ca

Multilayer Gr, CVD grown

1.17

[86]

Organic

Au/(PMMA/Gr)/PEDOT:PSS (Au)/P3HT:BCBM/ZnO/Pt

Multilayer Gr, CVD grown

3.2

[87]

CONCLUSION In the last few decades, graphene-based solar cells have achieved tremendous progress and this progress continues to the near future. The various parameters of Graphene can be optimized to achieve higher efficiency at a low cost. Moreover, the improvement of the performance of existing solar cells by graphene could play an important role in rapidly advancing PV technology. ABBREVIATIONS CNT

Carbon Nano Tube

E-GAIn

Eutectic Gallium Indium

GWFS

Graphene Woven Fabrics

GNW

Graphene Nano Wire

GO

Graphene Oxide

HJT

Hetero Junction Tandem

NP

Nano Particle

NW

Nano Wire

P (VDF-TrFE) Vinyl Fluoride-Tetrafluorethryl SMPV1

2,6-Bis[2,5-bis(3-octylrhodanine)-(3,3-dioctyl-2,2':5,2”-terthiophene)]-4,8- bis((5ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene

TFSA

Bis(trifluoromethane sulfonyl) amide

TCE

Transparent Conducting Electrode

QD

Quantum Dot

η

Output efficiency

Cell Applications

Photonic Materials: Recent Advances and Emerging Applications 109

CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

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CHAPTER 7

A Review on the Materials and Applications of Nanophotonics Athira Jayaprakash1, Joshua Nigel1 and Ishu Sharma1,* 1

Department of Engineering and Technology, Amity University, Dubai, U.A.E Abstract: Recent developments in nanotechnology have resulted in significant technical improvements in devices based on light's interaction with nanomaterials. As a result, nanophotonics has seen a significant increase in attention among researchers. The significance of low energy consuming information processing at high rates of speed has pushed the use of light for information transmission and processing forward. Nanophotonics hence introduces ways of integrating a wide range of systems that can produce, regulate, amplify and process light waves that are at superfast accelerations, as energy demands and interaction time decrease with a decrease in the particle dimensions of the nanomaterials. Nanophotonics, also known as nano-optics, is a branch of nanotechnology that studies characteristics of light at nanoscale dimensions and the interrelationships of nano-scale materials with light. Nanophotonics is a subfield of nanotechnology and a discipline of optoelectronics. On a dimension considerably smaller than the wavelength of light, it presents new opportunities for exploring concepts of interaction between the propagating light and matter. Fundamental properties of nanomaterial-light interactions, such as nanometer photon confinement and change in optical, chemical and physical properties of the material in nanorange, continue to provide numerous possibilities for real-life applications. The optical characteristics of materials can hence be enhanced by these materials having dimensions smaller than the wavelength of light. Electromagnetic waves are diffracted and dispersed if the material has dimensions in the range of the light wavelength or a portion of the wavelength, and the numerous waves produced interfere with each other. Controlling the spatial distribution of light, as well as its phase, polarization, and spectral distribution may be accomplished by understanding such materials. Moreover, materials with lower dimensions can be used to make extremely condensed sophisticated systems in a variety of industries, including information technology, optical interactions, photovoltaic energy, image processing, medical and surveillance. This chapter reviews the various materials used for nanophotonics and their properties as well as their nanophotonics application.

Keywords: Nanophotonics, Nanotechnology, Photonic Devices. Corresponding author: Ishu Sharma: Department of Engineering, Amity University, Dubai, U.A.E; E-mail: [email protected]

*

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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INTRODUCTION There has been a huge influx and range of photonics technologies throughout the last five to ten years. This is responsible for the recent improvements in computerized development tools including subsequent affordability, as well as the advent of novel nanotechnology approaches and the implementation of innovative morphological and electrical analytical techniques. The emergence of submicrometre and nanometer technologies having proportions with the range of wavelength of light is at the leading edge of these advancements [1]. Nanoscience and nanotechnology refer to the science and engineering associated with the matter at the nanometric scale, such as atomic, molecular or macromolecular structures. Photonics is the science and engineering associated with the generation, control and detection of photons [2]. Photonics and electronics are analogous fields of science as within photonic applications, photons fulfill the same function electrons do within electric circuits. Likewise, as transistors were one of the turning points in electronics, lasers were the same with respect to photonics. Nanophotonics is therefore the field concerned with optical phenomena and materials in which the length-scale is smaller than the wavelength of optical radiation [1, 2]. Light-matter interactions occur at the nanoscale, at which the length scale is equal to or smaller than the wavelength of radiation. It poses challenges to fundamental science while also opening the door to technological innovations. It encompasses the investigation of novel optical interactions, materials, manufacturing techniques, and models, as well as the exploration of organic and inorganic, or chemically manufactured structures such as holey fibers, photonic crystals, subwavelength structures, quantum dots, and plasmonics. Researchers could perhaps start modifying and influencing photon interactions at nano-scale as required for specific applications by studying how they function, which may lead to ideas such as a novel approach to diagnose and cure cancer or optical quantum computers. During the next 10 years, nanophotonic approaches guarantee dramatic decreases in energy required for systems, more intensively comprehensive technologies with reduced power consumption, better accuracy for image processing and morphogenesis, and control technologies with higher accuracy and precision [3]. Developments in morphological characterization techniques including atomic force microscopy, nano-secondary ion mass spectrometry, scanning electron microscopes and transmission electron microscopes have been aided by the growth in nanophotonics. The capacity to link the dimensions, elemental composition and morphology of nanostructures to observable optical characteristics has been greatly aided by these devices.

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Optoelectronic devices, bioinformatics, photonics, and nanotechnology are all significant surge topics in nanophotonics. Increasing proficiency in combining nanomaterials and optoelectronics has lately proven to be crucial, resulting in the emergence of frontiers that challenge basic research. The fundamental aspects of nanophotonics are the three types of confinement which have also been summarized in Fig. (1).

Fig. (1). Key confinements in nanophotonics [4].

●

●

●

Nanoscale confinement of matter – Physical structures have their dimensions limited to the nanometric scale via specialized synthesis techniques, such as photolithography and chemical vapour deposition. These physical constraints alter the optical properties of materials by varying the bandgap, optical resonance and excitation phenomena, influencing light-matter interactions to this scale. Nanoscale confinement of radiation – Confining light to nanometer-sized dimensions far less than the wavelength of light, for example via the use of the near-field optical transmission model. Nanoscale confinement of optical processes – Control of the spatial confinement of photochemical and photophysical processes to allow for nanofabrication techniques [4].

As photons traverse space at the speed of light, photonics presents us with the opportunity to significantly increase the efficiency and speed of current technologies.

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Photonic technology has advanced significantly over the recent decades to the point where it can supplement or even supplant its electronic counterparts, further proliferating into commercial and residential usage. The coming century will depend on photonics just as much as the previous century was dependent on electronics. This article reviews the various materials used for nanophotonics, common fabrication techniques and their properties as well as their nanophotonics application. Materials Two-dimensional Materials The discovery of single and multilayered graphene was a remarkable moment in the field of two- dimensional materials. Even now, there is a rising trend in the number of two-dimensional materials and the development of various devices are predicated to continue [5]. The fact that these two-dimensional materials have both, strong in-plane connections and comparatively weak out-of-plane van der Waals interactions is one of the reasons why academia is so interested in exploring them. It is these properties that allow multilayered two-dimensional materials to be exfoliated down to a single atom in thickness. While most past research focused on the electronic properties of these materials, the optoelectronic properties are nowadays particularly prominent [6]. Most photonic characteristics have even been used as representations for the two- dimensional materials that they belong to. Graphene is one of the initial two-dimensional materials that have been thoroughly investigated. The honeycomb single layer of carbon molecules has generated countless fascinating possibilities in nanophotonics and nanotechnology due to its unusual band gap at the boundary of two-dimensional quantum confinement. Another two- dimensional material having a honeycomb arrangement, hexagonal boron nitride, which can be seen in Fig. (2), is similar to graphene. Possessing a high 6eV bandgap energy, it is an exceptional piezoelectric material for other nanostructured materials, improving the optical and electrical efficiency of various systems [7]. A further type of two-dimensional material that has lately gained popularity is layered transition metal dichalcogenides (TMDCs), – for example MoS2. Aiding to its discrete and direct bandgap having a single layer structure, this group of single layered chalcogenides has the energy band gap of around 2eV and perhaps, even more, providing an avenue to design and build systems that might achieve unique properties for the photonic application. Furthermore, the valley coherence and valley-selective circular dichroism found in different single layer transition metal dichalcogenides provide interesting possibilities for the study of new optical and electronic properties [8, 9].

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Fig. (2). Monolayer lattice structures of significant two-dimensional materials (a) graphene (b) boron nitride (c) Molybdenum disulfide.

Single Layered Graphene Graphene is among the biggest innovative two-dimensional nanomaterials in history, with zero-bandgap energy. Most significant nanophotonic device technologies are enabled by the distinctive optical characteristics of layered materials. Properties such as degree of interaction with incident light across a large energy region and excellent charge transport make graphene a viable option for increased applications in a large range of operating conditions. The enhanced optical process in graphene was originally credited towards the conventional photovoltaic effect, which occurs in bulk semiconductors such as GaAs and Si and involves the separation of electron-hole pairs by an incorporated electromagnetic field, resulting in output current [10, 11]. Once the Fermi level of graphene is lined with Dirac point energy, as illustrated in Fig. (3), a monolayered graphene sheet captures roughly two per cent of downward incident light across a wide range of wavelengths owing to inter-band interactions. Therefore these characteristics make a potential photodetector material, particularly in the near-IR to the visible region of the spectrum [12]. The potential to employ graphene as a hole pumping semiconductor and an electron pumping cathode is well known. These characteristics are due to graphene's unique structural and electrical properties. Graphene's momentum space Minkowski diagrams features two conical shapes linked at a single position, opposite from insulators, which have energy levels divided by bandgap energy, and conductors, which have a half full band. As a result, there is no space between the bands or a band that is only half filled. It is possible to manipulate the deviation between the Fermi energy (EF) and the energies of a stationary electron in zero pressure closely outside a solid

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surface. Work function (Fig. 3) is the change between Fermi energy and energy of a stationary electron, which is also described as the lowest energy required to extract an electron from an exterior of the material. The work function is a surface feature, not a basic attribute of a bulk component. Since graphene's work function is around 4-5 eV (also in the region of graphite), its use as an electrode must be limited, but its usage as an anode necessitates a high work function (to enhance hole pumping effectiveness) [13]. As a result, employing graphene in two distinct electronic components requires various electronic activity changes. Furthermore, there are certain methods for creating a direct and adjustable energy bandgap in formerly no bandgap graphene, which might contribute to laser light photonic technologies. Based on Pauli's exclusion principle, once the Fermi-level is shifted further far from Dirac point energies by Fermi energy, the graphene should behave near to invisible to photons having energies lower than twice that of Fermi energy. Optical modulation schemes and switches might be made with this variable absorption characteristic. The appropriate bandgap in graphene may be introduced with the right amount of dopants, allowing graphene to be used in nanoelectronic integrated devices. Researchers have found that when a hydrogen atom is attached to the surface armchair graphene nanoribbon, it possesses a direct energy bandgap at the surface [14]. Furthermore, the energy bandgap may be generated and controlled by converting single layer graphene to its limited nanoribbon structure. These graphene nanoribbons are two-dimensional strands of graphene nanostructure with a morphology comparable to that of an unwrapped carbon nanotube. Armchair graphene nanoribbon and zigzag graphene nanoribbon are the two primary forms of graphene nanoribbons. Armchair graphene nanoribbon has semiconductor characteristics, while zigzag graphene nanoribbon has unique physical and chemical properties. This characteristic is solely determined by the way the graphene sheet splits across the axis. The bandgap grows exponentially when the nanoribbon thickness is reduced. It was previously discovered that adding a transversal magnetic flux to the ribbon cross-section causes the energy band gap to be controllable, i.e. the bandgap can be tuned by altering the amount of the external magnetic field [15]. The development of effective and controllable nanophotonic devices and systems that function with the highest precision is an essential element of an adjustable bandgap of a semiconductor.

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Fig. (3). Graphic illustration of the energy band of (a) increasing the WF of graphene (b) pristine graphene, (c) decreasing the WF of graphene and resistance vs gate voltage dependence for graphene (inspired by [13, 16]).

Transition Metal Dichalcogenides Transition metal dichalcogenides (TMDCs) are composites having the composition MX2, where M denotes a transition metal such as Molybdenum, tungsten, Neodymium, or Rhenium, and X denotes a chalcogen element (sulfur, selenium and tellurium). In most TMDCs, a single layer is made up of an X-M-X composite arrangement. The weak intermolecular forces interact between layers in TMDCs, but the in-plane interaction is a strong covalent bonding [17]. As a result, bulk TMDCs may be separated back to several sheets comparable to graphene, greatly expanding the range of two-dimensional materials. In multilayered structures, several two-dimensional TMDCs, including such Mo and W based dichalcogenides, possess indirect energy bandgap, but in single layer structures, they become direct bandgap materials [18]. Their large and adjustable

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energy bandgap not only creates intense luminescence properties, but moreover opens up a new range of photovoltaic applications, including photodiodes, energy generation, and photocatalytic, with a spectral region that differs from graphenebased systems. Novel photonic characteristics including valley coherence and valley-selective circular dichroism have also been seen in several twodimensional TMDCs, rendering such semiconductors highly intriguing for the development of newer physical processes [19]. Single layered TMDCs are two-dimensional semiconductors with an ultraviolet and visible energy bandgap. This characteristic sets them apart from other twodimensional materials with larger bandgaps, such as graphene and hexagonal boron nitride. Such semiconductors' energy band gap is direct in the single layer range which allows for improved dipole transition interactions with photons. High resonance frequencies with significant interaction affinity in the visible and infrared region are caused by low dielectric effect and high Electrostatic interactions amongst electrons and holes. The optical band gap of TMDCs has been further demonstrated to be adjustable from semiconducting to almost conductive - using an electrostatic force and physical stresses, which can be beneficial for designing the photonic performance for certain applications [20, 21]. Single-layered TMDCs are becoming an important category of two-dimensional materials for photonic devices, such as optical detection and light gathering, photoconductive and modulator, LEDs, and lasers, owing to their single layer structure, strong resonator intensity, and tunable possibilities. Two dimensional TMDCs may be manufactured with variably structured components to generate nanostructures including 2D–2D superconductors, 2D–0D superconductors, 2D–1D superconductors, and 2D–3D superconductors, depending on needs, resources and cost. Fig. (4) shows the Van der Waals heterostructures formed by integrating TMDCs with 0D nanoparticles, 1D nanowires, 3D bulk materials and 2D nanosheets. TMDC heterostructures have been widely used to generate unique photoluminescence characteristics, such as strong quantum photonic qualities in mono photon transmitters. The heterostructures are also used in optoelectronic devices that are extremely photosensitive [23].

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Fig. (4). Van der Waals heterostructures formed by integrating TMDCs with 0D nanoparticles, 1D nanowires, 3D bulk materials and 2D nanosheets (inspired by [22]).

Photonic Crystals Photonic crystals (PC) are synthetic nanostructures of one, two or three dimensions that are utilized to control the direction of propagation of photons. The mechanism for photon control via a PC mimics the use of a semiconductor to control electron mobility within an electron circuit via potential barriers. A PC is specifically engineered to periodic variation in their dielectric structure – repeating regions of higher and lower dielectric constants. Such a periodic structure influences the propagation of photons through the material due to brag scattering at the dielectric interfaces resulting in destructive interference, and the development of ‘forbidden frequencies’ which are also referred to as photonic bandgaps. Photons with energies lying within the photonic bandgap cannot propagate through the material; therefore, the PC provides the user with the opportunity to modulate the flow of light within the material based on the engineered periodicity of the dielectric function [24 - 26]. Defects within the perfect periodicity of a photonic crystal could be used to impart further functionality to a PC: a point defect functions as a microcavity trapping photons, a line defect functions as a waveguide and a planar defect functions as a perfect mirror [27].

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The photonic crystal can be homogenous, by structuring a single compound, or a composite with varying dielectric properties. The scale of this periodic variation determines the wavelength of radiation that the PC can work with: the lattice constant of the photonic crystal must be approximately half the wavelength of the electromagnetic wave that needs to be diffracted. A PC designed to modulate Xrays should have a periodic modulation of the dielectric function on the order of one micrometer, likewise, a PC for infrared radiation should have the modulation on the order of 0.5μm. This is the reason why the fabrication of a PC is extremely complex and difficult, and why sophisticated techniques such as electron-beam or X-ray lithography must be utilized [24, 25, 27]. There can be one-, two- or three-dimensional photonic crystals. In a onedimensional PC, the periodic modulation of the dielectric function occurs in one axis, an example is a Bragg mirror. Likewise, in a two-dimensional PC, the periodic modulation of the dielectric function occurs in two axes and a threedimensional PC, it occurs in all three axes [27]. Dielectric Nanostructures Dielectric materials possess relatively high refractive indices ranging from 3.5 and above for the upper end, 2 to 3.5 for the middle region and 2 or lower for the lower end. The refractive index influences the resonance phenomena of the nanostructure and the localization effect on the electric and magnetic field of radiation. Therefore, this variation of the dielectric properties determines the optical behavior and subsequent nanophotonic application. Well-designed dielectric nanostructures allow for the manipulation of Mie, Kerk or Fano resonances. For instance, some nanostructures can support toroidal and anapole Fano modes, and the coupling of these modes results in the directional control of dark-field scattering and wavelength-conditional routing. Likewise, increasing the refractive index has shown to narrow the Mie-type resonant peaks and result in stronger localization of the light field, while weakening intermode coupling. In addition to this, the intense magnetic resonances in these structures enhance the nonlinear signals of Second Harmonic Generation(SHG) and Third Harmonic Generation(THG) [28 - 32]. There has been a growing interest in silicon nanostructures, particularly silicon nanoparticles, as their resonant modes can be perfectly modeled by the Mie theory. It is this Mie-type resonance that allows for novel light manipulation and localization modes. The refractive index of silicon is higher than 3.5 within the visible range and the extinction coefficient declines to zero at wavelengths lower than 500nm [33]. The high refractive index and intense Mie-type resonance of silicon nanostructures result in strong dark-field scattering in the visible to

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infrared range as well as the fragmentary overlap of the magnetic and electric dipoles. The level of overlap also opens up the opportunity for the directional scattering of light [34 - 36]. Furthermore, other interesting optical phenomena have been observed such as the size-dependence of the magnetic dipole resonance modes, which allows for the whole visible spectrum to be covered depending on the size of the silicon nanostructure [37, 38]. Other high aspect nanostructures with narrow gaps such as nanowires, arrays [39], dimer structures [40] and nanostrips also share the aforementioned characteristics [41]. Tellurium nanoparticles have also been investigated as their refractive index is greater than silicon for wavelengths greater than 600nm, but less when the wavelengths are less than 600nm. Moreover, the extinction coefficient is higher than silicon and the real component of its permittivity is below zero at wavelengths less than 500nm, resulting in metallic properties in this spectral region [42]. In contrast to high refractive index materials, a low refractive index brings about broadening of the electric and magnetic dipole resonant modes in a spectrum. Titanium dioxide shows a refractive index of 2.5-2.8 in the visible range [43]. While this refractive index, within a relative sense, comes in the middle region, the extinction coefficient of titanium dioxide is approximately zero which renders functionality as an ideal building block for metasurfaces with a high-quality factor. These metasurfaces can be used to change the wavefront as well as the polarization of incoming light as it imparts an orbital angular momentum and switches the polarization states [44]. Apart from metasurfaces, akin to the previously mentioned semiconductor nanostructures, TiO2 nanoparticles also exhibit size-dependent Mie resonance which can be utilized for light trapping and amplifying light absorption [44]. Similar to TiO2, boron nanoparticles also have a refractive index in the range of 2.5-3.0. There sonant modes of boron nanoparticles have been shown to encompass the entirety of the visible spectrum. Boron nanoparticles can behave as one-directional nanoantennae due to the high overlap between the electric and magnetic dipole modes resulting in a local broadband electromagnetic interaction based on the Kerker effect [45]. Silicon nitride (Si3N4) and silicon carbide (SiC) are other mid-range dielectric materials having refractive indices of 2.4 and 2.6, respectively [46]. The broadening of the electric and magnetic dipoles plays a role in these materials functioning as effective SHG amplifiers. In addition to this, SiC has a unique property which is the creation of surface phonon polariton modes that are effective in confining infrared light via optical phonons.

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Barium titanate nanoparticles are another appealing mid-range dielectric with a refractive index of 2.4-2.7. The tetragonal structure of if BaTiO3 provides it with effective second-order nonlinear coefficients of d15 13.7 pm V-1, d32 14.4 pm V-1, and d33 5.5 pm V-1. Furthermore, Mieresonances can further enhance the SHG signals [47]. The magnetic dipole and quadrupole modes have been shown to play a major role in the SHG process and BaTiO3 also demonstrate spectral tunability for the SHG enhancement. More focus has been placed on the incorporation of plasmonic materials with barium titanate to further strengthen the SHG enhancement [48]. Dielectric materials with refractive indices lower than 2 are unable to produce obvious Mie resonances; however, the optical interactions of these materials are still significant when it comes to other optical phenomena [49]. Germanium nanostructures also have a place in nanophotonics with a refractive index of 1.0 and an extinction coefficient which is higher than silicon by 0.5-2.0. Directional scattering due to coupling of electric and magnetic modes has been observed in germanium nanoparticles [50]. In addition to this, the larger k value of germanium (compared to silicon), in conjunction with Mie resonance, results in enhanced absorption and allows for creating perfect absorption layers [51]. Germanium also has very large nonlinear coefficients which enhance third harmonic generation via magnetic resonance. Lastly, germanium nanostructures with a specific aspect ratio can produce Fano resonance in the anapole mode, further promoting third harmonic signals. In the case of silicon dioxide, which has a refractive index of 1.46, it is the near zero optical absorption that makes it a promising material for optical communication [52]. Polymer nanostructures are also materials with low refractive indices, often lower than 1.8, and there is great interest in this material for nanophotonics due to the ability to engineer the molecular orientation of the polymer during synthesis [53]. Fabrication Techniques In recent years, there have been significant advancements in the field of nanotechnology which have resulted in the development of a wide variety of fabrication techniques. However, due to the specifically tailored physical properties of the nanophotonic devices in order to function with maximum efficiency within a specific spectral range, only a limited amount of fabrication methods are currently available. Moreover, each technique comes with its own set of advantages and disadvantages and therefore the desired application must be thoroughly considered when choosing the fabrication technique. These fabrication techniques are covered in this section.

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Top-down Methods With the top-down approach, the bulk material is removed in a highly controlled manner in order to achieve the nanostructure. Lithography is a technique in which a substrate is coated with a material, resist, whose solubility changes depending on exposure to a certain stimulus. The resist is then virtually patterned via exposure to the stimulus, and the region of the resist whose properties have been altered as the result of exposure is then removed by solvation or etching in order to realize the pattern within the resist layer. This is then followed by the deposition of the structural material and subsequent removal of the support material. In contrast to conventional photolithography where UV radiation is passed through a photomask onto the resist, electron beam lithography (EBL) is commonly used in nanophotonic synthesis. By scanning the resist layer with a focused beam of electrons, it is possible to draw nano-patterns with a resolution of less than 10nm [54]. Numerous structures have been developed via EBL such as nanowires [55], nanopillars [56], waveguides [57] and photonic crystals [58]. The limitation of the lithographic approach is the high cost, complexity and fixed substrate. Focused ion beam (FIB) milling is another top-down technique in which a focused beam of helium or gallium ions impinges onto the structural material, and the target region can be effectively milled/thinned down into the desired morphology. The amount of material sputtered off during the milling process is directly dependent on the beam current. Sections with the thickness of 10 to 100nm as well as nanochannels smaller than 5nm can be synthesized via this technique [59, 60]. Various nanostructures such as nanoholes, nano grooves, nano cups and nanochannels have been fabricated via FIB [61]. Lastly, there are laser-based techniques, these techniques provide excellent precision, resolution and selectivity. Laser-Induced Transfer (LIT) is a technique in which a laser is an incident on the junction between the printed material and transparent donor substrate in order to transfer material onto another receiver substrate that is in close contact with the donor sample [62]. This technique allows for the laser printing of nanoparticles from metals and semiconductors. Laser ablation is a technique in which ultrashort, microsecond, nanosecond or femtosecond laser pulses are directed onto the substrate material in order to fragment and vaporize it and then collect the ablated material [63]. Laser ablation conducted within a liquid environment is known as Laser Ablation in Liquid (LAL). LAL has numerous advantages such as being chemically simple and clean, relatively low-temperature synthesis, and the morphology and phase of the

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nanostructure can be controlled by varying the laser parameters [63, 64]. A wide variety of nanostructures have been synthesized via these laser techniques such as β-MnO2 nanowires [65], cobalt monoxide nanocrystals [66], TiO2/αFe2O3 coreshell nanoparticles [67] and monocrystalline microspheres of ZnO, CdSe, ZnSe and CeO2 [68] Lastly, there is the process of dewetting in which a thin film is heated resulting in the aggregation of nanoparticles due to the decrease in the total energy of the thin film surface, inclusive of the film-substrate interface. There is a direct relationship between the thickness of the source thin film and the dewetting process. The main controlling parameters are temperature and the properties of the thin film, such as the defects, thickness, thermal coefficient and initial patterns [69]. Bottom-up Methods One bottom up approach is the chemical approach in which the nanostructure is built up, atom-by-atom subsequent to chemical interactions between the source materials in order to produce the desired nanostructure. The most common chemical methods for the synthesis of nanostructures in the field of nanophotonics are solvothermal and hydrothermal synthesis, sol-gel processing and chemical vapour deposition [70]. In the process of sol-gel, a colloidal suspension of the precursors is first produced and left to react in order to produce a gel which can then be processed to produce nanoparticles or produce three dimensional porous structures. SnO2 nanoparticles were synthesized via the sol-gel method, via organometallic precursors, in order to produce nanoparticles with a size range between 15nm to 25nm [71]. In the hydrothermal method, two or more precursors are reacted in the presence of water in order to produce the nanostructure. The reaction vessel is sealed and heated which results in an autogenous high-pressure reaction environment. In the solvothermal process, any solvent is used except for water. Various nanoparticles with diverse sizes and morphology are frequently created using the solvothermal method, for example, TiO2 [72] and ZnO. Chemical vapour deposition is a technique in which the by-product of heated chemical gas-phase reactions is deposited onto a substrate to build up the nanostructure. Using CVD, structures such as silicon nanoparticles [73], silicon carbide nanowires [74] and boron nanoribbons [75] have been produced. Lastly, there is the process of self-assembly in which nanoscale building blocks spontaneously rearrange into the desired formation of the nanostructure. The first approach relies on exposing the building blocks to electric or magnetic fields which results in dipole-dipole interactions between them. Provided these interactions overcome the natural Brownian motion, self-assembly will be initiated. The second approach is to functionalize the surfaces of the nanoscale building blocks with chemical moieties. The resultant chemical interactions

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between the functionalized building blocks (which may be spontaneous or require stimuli such as light) result in the build-up of the nanostructure [76, 77]. APPLICATIONS Absorbers Dielectric materials, with appropriate doping, can be tuned to support plasmonic resonance. The significance of using these materials for absorption and photothermal applications is the fact that they possess features of both, plasmonic materials and semiconductors [78]. Self-doped TiO 1.67 was synthesized via laser ablation to produce a dark-blue color titanium oxide. The material had a strong plasmon resonance due to interband transitions, which allowed for more than 90% absorption in a wavelength range of 300nm to 2000nm for size distribution of 40nm to 340 nm [79]. As mentioned, these materials display properties of plasmonic materials as well as semiconductors; therefore, plasmonic resonance is not the only mechanism for absorption, but strong absorbers have also been realized on the basis of Mie resonances. A dual band metamaterial absorber constituted of highly symmetrical artificial dielectric molecules has also been fabricated– four atoms of two sizes matching to two absorption bands with an absorptivity of near unity. This material can be used as an absorber for a wide range of incidence angles as it is polarization insensitive. The strong absorption and resonances also allow for the development of heat which can be used in photothermal applications, as displayed by S. Ishii et al. in which they used the Mie resonances of silicon nanoparticles to heat water [80]. Photonic crystals have been used in conjunction with the absorber layer of solar cells in order to allow for selective solar absorption on the basis of the dimensions of the periodic structure. In addition to this, photonic crystals can also be used to boost absorption by matching the quality factor [81]. Graphene Photodetectors High speed, broad bandwidth photodetectors are essential for communication, sensing, and digital imaging. Most traditional commercial photodetectors are based on silicon or III-V semiconductors. When photons are absorbed into the photodiode’s depletion region, they excite electron-hole pairs and their separation leads to photocurrent generation. This mechanism (usually called the photovoltaic effect) was also claimed to be the operational principle of graphene-based photodetectors in early research. Photodetectors with a wide range and great performance are critical for telecommunication, sensors, and digital photography. Si or semiconductors such

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as gallium nitride (GaN) or gallium arsenide (GaAs) are used in the majority of industrial photodetectors. Once photons are captured into the depletion area of a photodiode, the photovoltaic effect was suggested to be the underlying mechanism of graphene integrated photodetectors in previous studies [82]. Several electrons and protons can be produced with only a single photon due to Auger-type events in graphene, possibly improving detector performance. The change in resistivity of graphene when subjected to heating could contribute to a photoresponse as well as optoelectronics effects. To summarize, the production of photocurrent in zero bandgap graphene is tougher than that in typical bulk materials with a large energy band gap because of its decreased size and conductive nature. Graphene can capture energy from the mid-infrared(IR) to Ultraviolet(UV) wavelengths due to its zero energy bandgap characteristics and particle interbond interactions [83]. However, the number of generated photocurrents per incident photon in graphene integrated photodetectors has been restricted due to the low optical absorption of single layer graphene due to its inherent low thickness. For starters, integrating graphene alongside photonic nanomaterials can refocus light via the plasmonic resonant frequency, leading to a considerably increased localized electromagnetic field [84]. Multiple color shade identification may also be accomplished by frequency photo response enhancement of plasmonic nanomaterials, in addition to a significant increase in quantum effectiveness. Furthermore, combining nanocrystals with graphene is a great way to improve the responsiveness of graphene photodetectors. Researchers have created a composite photodetector with extremely elevated photodetection efficiency and photoresponsivity by coating graphene with finely dispersed quantum dots(QDs). Photogenerated carriers are used in reaching graphene layers by the QD, which confine all oppositely charged particles in the QD layers, producing a field-effect doping event. Given the considerable time it takes to create a gain, graphene-QD photo-detectors have poor function speed. The QD, instead of graphene, determines the operating range in these systems [85]. 2D TMDCs Based LEDs The use of light-emitting diodes for projection, illumination, and detection is widespread. As single layer TMDCs like tungsten selenide are direct bandgap materials, electron-hole pairs may readily interact to create photons in emission mechanisms. Electroluminescence was seen in monolayer molybdenum(IV) sulfide field effect transistors that were confined to the interface region and developed on a highly p-doped Si wafer [86]. Nevertheless, the photoelectric efficiency of molybdenum (IV) sulfide-based LEDs is poor, and it decreases considerably as charge injection increases. Likewise, the difficulties in obtaining hole conductivity, inefficient connections, and low optic performance of single layer molybdenum disulfide light emitting diodes have restricted their prospective

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uses. Numerous distinct gate voltages are lately been used to develop Tungsten selenide single layer lateral diodes. The p-n and n-p transistors may be characterized via electromagnetic dopants, resulting in effective strong electroluminescence. To achieve circular polarization luminescence, Zhang et al. [87] used the valley degrees of freedom of layered tungsten selenide. The light was generated from electrostatic interactions produced throughout p-I-n junctions. A single layer tungsten diselenide based integrated light emitting diodes was also achieved by Ross et al. [88]. Researchers used tungsten diselenide to make electrostatically generated p-n junctions with a boron nitride sheet beneath as a dielectric substrate. This configuration allows electrons and holes to be efficiently pumped into pathways, resulting in the intense generation of photons. With a 30 nA electric load, the overall photoelectron output may approach ~15 million s-1. They also discovered that within this structure, both electro and photoluminescence emanated from the valley excitions. However, the study into LEDs based on TMDCs is now only in its early stages. To enhance their performance and benefit, more work is required. Additionally, getting significant optical gain in TMDCs would still be far off, and greater study is needed to investigate the feasibility of light emission in these semiconductors. TMDCs have been integrated with several semiconductors with differing electronic band structures to create hybrid materials that may be used as light emitting diodes [89]. MIS (metal–insulator–semiconductor) junctions, p–n (p-n type diode) junctions, and MS (metal–semiconductor) junctions are among the device designs that are available. The most popular construction components for TMDC-based light emitting diodes are p–n diodes [23, 89]. Biosensing Based on the Mie and Kerker theory for resonance, changes in the magnetic or electric resonant modes due to the refractive index of an environment provides the mechanism needed for biosensing. In 2017, R. Quidant et al. developed a nanoresonator using only dielectric materials. They observed varying degrees of shifts in the resonance when the concentration of a specific antigen was varied [90]. The advantages of their sensor, as stated by them, were: having a longer decay period which is useful for multilayer assays, the quality factor of their sensor may be higher than the sensors made using plasmonic materials and higher stability in solution [90]. In 2016, J. Yan et al. developed a biosensor based on silicon nanosphere dimers. They observed that a broader electric dipole mode of resonance was more red-shifted than individual silicon nanospheres under different environmental refractive indices. This red-shift of the electric mode hampered the interaction between

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electric and magnetic dipole modes and therefore significantly enhanced the backward scattering intensity. The figure of merit of the dimer variety of the sensor can reach 116.8 and the sensitivity can reach 1,168,000; thus, allowing for the detection of trace amounts of biomolecules as well as monitoring changes in concentration [91]. CONCLUSION We have discussed the various materials used for nanophotonics and their properties as well as their nanophotonics application. Starting from transition metal dichalcogenides, we have discussed photonic crystals, single layered graphene and dielectric nanostructures. The various fabrication techniques have been studied which are classified into the top down or bottom up approach. Finally, the applications of these materials have been discussed which span a wide variety of fields from absorbers, detectors to LEDs and biosensing. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

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CHAPTER 8

Revolutionary Future Using the Ultimate Potential of Nanophotonics Sumaya Khan1,* and Ishu Sharma1,* 1

Department of Engineering and Technology, Amity University, Dubai, U.A.E. Abstract: As the world is modernizing, it is noteworthy to mention photonics and its categorization based on size. Despite the components of light being invisible to the human eye, nature never ceases to amaze us with its idiosyncratic phenomenon. Furthermore, the manipulation of the matter is confined to the nanoscale as a part of the progression. Adding nanotechnology to photonics emerges out as nanophotonics which is the cutting-edge tech of the twenty-first century. Human beings have acclimated to the concept of photonics, furthermore, nanophotonics is the science of miniaturization study, potentially helping the technology to modify itself into the sophistication of the equipment and thereby be of assistance in various disciplines of science and technology. One can illustrate nanophotonics by considering the fabrication processes of nanomaterials. In variegated applications, these nanoscale processes will refine and produce structures with high precision and accuracy. Meanwhile, groundbreaking inventions and discoveries have been going around, from communications to data processing, from detecting diseases to treating diseases at the outset. As one stresses on the idea of nanophotonics, it never reaches a dead-end, however, this explains how vast the universe and each of the components co-existing are infinitesimally beyond humans' reach. Nevertheless, nanophotonics and its applications bring about remarkable multidisciplinary challenges which require proficient and well-cultivated researchers. Despite the fact it has several advantages, it carries its downside, which requires a detailed analysis of any matter. Using state-of-the-art technology, one can constrict light into a nanometer scale using different principle methodologies such as surface plasmons, metal optics, near field optics, and metamaterials. The distinctive optical properties of nanophotonics call out specific applications in the electronics field such as interaction chips, tiny devices, transistor filaments, etc. When compared to conventional electronic integrated circuits, the pace at which data using nanophotonic devices is sent is exceptionally fast, accurate, and has a better signal processing capability. As a result of the integration of nanotechnology with photonic circuit technology, high-speed data processing with an average processing speed on the order of terabits per second is possible. Furthermore, nano-integrated photonics technology is capable of comprehensive data storage and processing, which inevitably lays the groundwork for the fabrication, quantification, control, and functional requirements of novel optical science and technology. The majCorresponding author Ishu Sharma: Department of Engineering and Technology, Amity University, Dubai, U.A.E. E-mails: [email protected], [email protected]

*

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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ority of applications include nanolithography, near-field scanning optical microscopy, nanotube nanomotors, and others. This explains about the working principle, different materials utilized, and several other applications for a better understanding.

Keywords: Metal Optics, Near Field Optics, Metamaterials, NEMS, Surface Plasmons. INTRODUCTION Nanotechnology merged with photonics, called nanophotonics includes a broad range of recursive physical phenomena, including light-matter dynamics that are well below diffraction boundaries, and has paved the way for novel applications in light absorption, sensing applications, luminescence, optical switching, and media transmission technological advancements [1 - 6]. Growing competence in merging nanotechnology and photonics has recently emerged as a fundamental, emergent frontier, challenging basic experiments and prospects for innovations in our daily lives, and playing a vital role in several optical components [1 - 6]. It presents analytical research on photonic interaction with matter at infinitesimally small sizes, known as nanostructures, in order to build nanoscale devices and equipment to process, develop, lose momentum, influence, and/or control photons by understanding their behaviour when interacting with or otherwise passing through matter [1 - 6]. This multifaceted discipline has also influenced the industry, encouraging researchers to develop new frontiers in designing, applied science, chemistry, physical science, basic materials science, and biomedical technology [1 - 6]. Before understanding how photonics function and their application, it’s important to brief about the similarities and differences between Photons and Electrons. Photons and Electrons: Similarities and Differences Photons and electrons are fundamental components in the language of physics, which display the same behaviour as particles and waves. In terms of Classical physics, photons are described as electromagnetic waves, which carry energy, and electrons as the basic charged particle (lowest mass) of matter. On the contrary, a quantum description indicates that photons and electrons may be processed directly analogously and have many comparable properties [1]. In most contexts, the electrons are marked by substantially greater dynamic values than photons of equal energies (derived from orders of magnitude more rest mass than the relative mass of a photon given by

[1]. Therefore, there is a

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better solution for electron microscopy (in which the electron power and momentum are regulated using the accelerated high volume value) over optical (photon) microscopy because the final microscope resolution is restricted to the wavelength by the phenomenon of diffraction [1]. The wavelength of the photons can be given the equation as: λ=

ℎ 𝑝

=

2𝜋𝑐

(1)

𝜔

The conduction electrons travelling in a solid have momentum values that are relatively high compared to photons, and hence the feature-specific lengths are shorter than the wavelengths of light. A significant consequence of this characteristic is that photons have 'size' or 'containment' effects on greater sizes than electrons [1]. The eigenvalue equation of the photons can be given as: [𝛻 × (

1 𝜀(𝑟)

⍵ 2

(2)

𝛻 × 𝑬)] = ( ) 𝑬 𝑐

And, the free space propagation of photons is given by: 1

𝐸 = 𝐸0 (𝑒 −𝑖(𝑘 . 2

𝑟 − 𝜔𝑡)

+ 𝑒 𝑖(𝑘 .

𝑟− 𝜔𝑡)

)

(3)

where k is the wave vector, a real quantity. Photons and Electrons: The Constriction in Various Facets The spread of photons and electrons can be dimensionally restricted by employing regions with diverse interaction possibilities to reflect or redirect these particles, therefore restricting their spread to a certain route or a certain group of them [1]. In the case of photons, trapping light in a high refractive area or high surface reflectivity can introduce configuration. This secluding area might be a cavity resonator or a waveguide. Considering a thin film, a high refractive index layer carries out the propagation of light, provided the light-guided layer has a high refractive index (n1). The graphic displays the traditional optical image via a ray path, which describes light directing (trapping). For a flat waveguide, the containment is in the vertical position only (x-direction). The light propagates in the z-direction, whereas in the case of a fiber or a channel guide, the path is followed in the x and y directions.

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An example of a light-containing optical medium in every dimension is the microsphere [1]. The light is limited by the contrasting index of the directing medium and the surrounding media. Thus n1 (refractive index of the medium) /n2 (refractive index of the medium surroundings) serves as a scattering potential and prevents the spread of light [1]. The z-axis serves as the direction of propagation wherein the nature of light allows it to behave like a plane wave with a propagation constant k, cognate to the propagation vector k for free space [1]. The electrical field transmission has a distinct spatial profile in the confining orientation. Therefore, for a waveguide, the enclosure of the electric field E (spatially variable depending only), is amended as the guidance in a fiber or platform guide (Rectangular or Square Medium guidance) that contains two-dimensional containment. The function a(z) represents the electrical amplitude which is constant in the zdirection (where there is no loss). In the confining plane, the f (x, y) function depicts the electric field distribution. For a planar waveguide that generates containment in the x-direction only, x is confined to a space distribution by the confining potentials. If a Gaussian beam is launched in the waveguide, it will display the distinctive propagation in the y-direction. Propagation of sub-atomic Particles through a Classically Forbidden Zone Photons and electrons are contained entirely in the containment areas in a conventional image. Classical physics also forecasts that the electron is entirely contained inside the walls after it is caught within the barrier height where the energy ξ of an electron is lower than the potential energy V owing to this barrier. But this is not predicted by the wave depiction [1]. Light can therefore leak into the region outside the waveguide, a prohibited classical area. This light leak produces a magnetic field termed an evanescent wave. The field propagation in the vicinity outside the waveguide is not like a plane wave with the wavevector k as the actual quantity. The amplitude of the electric field extending into the classically prohibited area decays from the frontier of the guiding area exponentially by distance x into the medium with the lower refractive index [1]. −𝑥

𝐸𝑋 = 𝐸𝑂 exp ( ) 𝑑𝑝

(4)

EO is the electric field of the waveguide at the boundary. The parameter dp, also known as the penetration depth, is defined as the separation at which the electric field magnitude reduces to 1/e of EO.

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Localization Under a Periodic Potential When exposed to a periodic potential, photons and electrons exhibit similar behaviour. A semiconductor crystal, which is made up of a periodic arrangement of atoms, provides an example of an electron exposed to a periodic potential. The electrons remain able to travel freely across a lattice of atoms, but while they do so, they are subjected to a strong Coulomb contact by the nucleus of the atom at each lattice site [1]. The refractive index contrast (n1/n2), where n2 is the refractive index of the packing spheres and n1 is the refractive index of the interstitial medium (which can be air, a fluidic medium, or, more ideally, a substance with extreme refractive index), serves as a periodic arrangement. In both situations, the periodicity has a distinct length scale. The atomic arrangement (lattice spacings) of an electronic (semiconductor) crystal is on the sub-nanometer scale. This array of configurations correlates directly to X-Rays with respect to the electromagnetic waves, and X rays can be diffracted on crystal lattices to generate Bragg scattering of X-ray waves [1]. The equation of Bragg, which determines the direction in space in which the diffraction occurs, is given to the wavelength, m is the diffraction order, n is the index of the refraction, and θ is the incidence angle [1]. 𝑛𝜆 = 2𝑑 𝑠𝑖𝑛 𝜃

(5)

The very same Bragg dispersion creates optical wave propagation diffraction for a photonic crystal. For instance, using Eq. (2), a separation of the lattice structure should be 200 nm (distance between centres of packed spheres) to generate a Bragg light dispersion of the wavelength of 500 nm [1]. The Schrödinger energy solution now applied to periodic V power results in the electronic belt splitting for a free electron [1]. The lower band which has the highest occupied energy is called the valence band and the higher band where the electrons are in action is called the conduction band. As for highly conjugated complexes with alternative single and dubbing links, this may also be defined in the languages of chemists. The Hückels theory forecasts several molecular orbits that are like the valence band tightly spaced and are occupied and equivalent to that of the conductive band, and a set of molecular orbitals that are vacant and closely separated. Both bands are separated by a forbidden gap of energy wherein this separation of width is called the bandgap [1]. The energy from the bandgap is frequently denoted as Eg and plays a significant role in the electro-optical characteristics of the semiconductor [1].

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Nanoscale Optical Interactions Axial Nanoscopic Localization Evanescent Wave The evanescent wave is a spatially focused energy oscillating electric and/or magnetic field around the source. From the waveguide surface, this field penetrates the lower refractive index surrounding the media, where it declines in the axial direction exponentially (away from the waveguide). This evanescent field stretches between 50 and 100 nm to produce optical interactions at the nanoscale [1]. Furthermore, it is employed with excellent surface selectivity for the detection of fluorescences. Evanescent wave coupling guides have been suggested for sensor use, where sensing results in a shift in the photon tunneling from one channel to another [1]. A further example is the complete internal reflection, by a prism of n1 reflection index, to the environment with a lower refractive index, which is a geometry creating an evanescent wave [1]. In a suitably narrow incidence angle, the light at the interface refracted and passed partly into the second medium. However if the angle of incidence is greater than a value of θ = θc, the light reflects in the interface. This approach is known as total internal reflection (TIR) [1]. The following equation indicates the critical angle θc 𝜃𝑐 = 𝑠𝑖𝑛−1 (

𝑛2 𝑛1

(6a)

)

The light is fully reflected internally in the prism from the prism/environment interface. A standard n2 glass prism is around 1.52, whereas an aqueous buffer may be 1.33, which yields a critical angle of 61°, for example, the n2 of the surrounding environment [1]. Even under TIR, a part of the incoming energy penetrates the surface of the prism as an evanescent wave and comes into touch with the prism surface in the surroundings. As explained above, its electric field amplitude Ez decays into the surrounding medium with the lower refractive index n2 as exp(–z/dp), i.e., exponentially, with distance Z [1]. The term dp, also called the penetration depth, can be given as: 𝑛

𝑑𝑝 = 𝜆/ [ 4𝜋𝑛1 {𝑠𝑖𝑛2 𝜃 − ( 2 )2 }1/2 ] 𝑛1

(6b)

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Total internal reflection may be used for microscopy, and an image of the fluorescent-tagged biological close to the surface is obtained. There are several advantages associated with this technique, namely low fluorescence background, no out of focus fluorescence and minimum cell exposure in any other plane of the sample, except at the Surface Plasmon Resonance contact (SPR) [1]. As a matter of principle, the SPR approach extends the above-mentioned evanescent wave interaction, except in the absence of an electrical-metal contact to the waveguide or prism. Surface plasmons are electromagnetic radiations that spread along the boundary of a metallic film with a dialectal substance, such as organic films [1] As surface plasmons spread in a metal foil over frequencies and wave vectors where no light spread in any of the two media is permitted, no direct surface plasmon arousal is feasible. Total reflection is attenuated, the most frequent way to build a Surface Plasmon Wave (ATR) [1]. In order to induce surface plasmons, Kretschmann–Raether configuration ATR setup is commonly utilized. A slide is microscopically covered by a thin metal coating (usually a 40- to 50nm-thick gold or silver film by vacuum deposition). The microscopic slide now has an indexable liquid or a polymer layer in connection with a prism. The prism is affected by a p-polarized laser beam [1]. The laser beam is checked for reflection. At some point, the electromagnetic wave pairs as a surface plasmon to the interface. At the same time, an evanescent field spreads from the contact and extends to approximately 100 nm below the metal surface. The reflected light intensity (the ATR signal) decreases with this angle [1]. The angle is thus given by the relationship: (7)

𝑘𝑠𝑝 = 𝑘𝑛𝑝 𝑠𝑖𝑛𝜃𝑠𝑝

where ksp is the wavevector of the surface plasmon, k is the wavevector of the bulk electromagnetic wave, and np is the refractive index of the prism [1]. The surface plasmon wavevector ksp is given by 1

𝑘𝑠𝑝 = (𝜔/𝑐)[ (𝜀𝑚 𝜀𝑑 )/(𝜀𝑚 + 𝜀𝑑 )]2

(8)

where ω is the optical frequency, c is the speed of light, εm and εd are the relative dielectric constants of the metal and the dielectric, respectively, which are of opposite signs. In the context of a bare metal layer, εd (or the squared refractive index for a dielectric) is the dielectric constant of air, and the property of reflectivity drop occurs within a certain angle. This angle shifts in the case of metal covered with another layer of dielectric material [1].

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The coupled angle of deposition of Langmuir-Blodgett polydiaketylene, poly-4BCMU monoclave. It also shows the altered SPR curve at the right [1]. The angle of reflection minimum, the minimal reflectivity value, and the breadth of the sound curves are measured in this experiment [1]. These observables are utilized to construct a resonance curve computer fit using a minimum quadrature fit with the Fresnel reflecting formulation, with three parameters: the actual, the imagined components of the refractive index, and the dielectric layer thickness [1]. The above-mentioned equations illustrate that the change in surface plasmon resonance angle is produced, respectively, by the change in εm and εd of dielectric constants of the metal and films cover, which corresponds to minimal reflectivity (for simplification, the subscript sp is deleted). And therefore the resonance angle is given by, 2 𝑐𝑜𝑡 𝜃 𝛿𝜃 = (2𝜀𝑚 𝜀𝑑 (𝜀𝑚 + 𝜀𝑑 ))−1 (𝜀𝑚 𝛿 𝜀𝑑 + 𝜀𝑑2 𝛿𝜀𝑚 )

(9)

Since |𝜀𝑚 | ≫ |𝜀𝑚 | , changes in θ are significantly more sensitive to changes in εd (i.e., the dielectric layer) than changes in εm [1]. This approach thus seems best suited for obtaining εd (or a change in the refractive index), depending on interactions or structural interference in the dielectric layer. Another means of visualizing the Surface Plasmon Resonance's great sensitivity to fluctuations in dielectric optical characteristics above the metal is to take into account the force of the evanescent field in the dielectric, which is stronger by magnitude than in an optical waveguide in typical evanescent sources. This increased surface plasma evanescent wave can provide a considerably greater intensity non-linear visual process [1]. Lateral Nanoscopic Localization A lateral nanometric light containment may be accomplished utilizing a geometric near the field in which the specimen is located in a fragment of the light's wavelength from the source or aperture under optical illumination [1]. The near field geometry creates spatially confined optical interactions through electrical field distribution around a nanoscope structure. The electric field distribution, also spatially located, has a considerable evanescent nature — which is, because of the imaginative wave vector character, fading exponentially [1]. A near-field geometry is also known as using a near-field scanning optical microscope, abbreviated as NSOM or sometimes as SNOM. A submicron size of 50 to 100 nm, such as a tapered optical fiber opening tip, is utilized to confine

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light to more typical NSOM methods [1]. A nano metal tip, such as those used in tunnelling microscopes, STM scans, or nanoparticles (e.g., metal nanoparticles) will be employed near the material in the case of an apertureless NSOM configuration in order to boost the field locally [1]. Quantum Confinement A nano-crystal semiconductor (SNC) or a quantum dot (QD) is a semiconductor nano-based crystals or nanoparticles, the excitons of which are restricted to all three dimensions. As a consequence, they exhibit characteristics between semiconductor materials and discrete molecules [7]. Size effects in semiconductor crystals with a measurement of 10–100 nm are found, but in quantum size effects, nanocrystallites have features smaller than 10 nm. There are two particular explanations for the physical and chemical characteristics of nanocrystalline particles [7]. Structures limited to nanoscale sizes in one, two, or all three dimensions are referred to as quantum-confined materials [8]. Quantum confinement must occur at a length shorter than the de Broglie wavelength of electrons for thermal energy in the media. One or perhaps more dimensions must be smaller than 10 nm for effective quantum confinement [8]. Quantum-confined structures have a major influence on their optical characteristics. Energy quantization into discrete levels has application areas in the construction of new solid-state lasers. Multiple Quantum Wells (MQM) structures are formed when two or more quantum wells are placed alongside. The only motion in the Z- direction is possible. Particle in a Box describes the motion of electrons and holes travelling in the Z-direction in a narrow bandgap material [8]. First, the high volume-to-surface ratio leads to a large number of atoms on the crystalline grid surface. Second, because of the three-dimensional containment of the charger carriers, the electronic strips are divided into separate levels of energy [7]. This leads to the quantum load containment leading to enhanced bandage with reduced particle size. Around 25 years ago, the quantum containment effect was reported in low-dimensional semiconductor devices. In a nanocrystal, the bulk crystalline building is retained. Nanocrystals nonetheless have molecular and discrete electrical conditions with strong and size-based characteristics due to the amount of containment [7]. During the past decade, extensive research has been carried out in order to study the size-dependent characteristics of semiconductors, including absorption and luminescence [7].

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Nanoscopic Interaction Dynamics Nanoscopic Interaction Dynamics describes and enhances local interactions with a certain radiative transition (emission with a specific wavelength). An example of this is the utilization of a low-frequency phonons hosting environment (gate vibrations) to considerably minimize multiphoton relaxing of exciting energy in rare-earth ions to increase the emission efficiency. Based upon the sensitivity to nanoscale interaction in rare-earth transitions, the type of electronic interaction is solely regulated by the nanocrystalline environment. This gives a possibility for various device applications to adopt a glass or plastic media storing those nanocrystals. Electronic nanoscale interactions also generate novel optical transitions and increased optical communication between different electronic centers. New Cooperative Transitions Two surrounding species can interact in a collection of ions, atoms, and molecules, producing new optical absorption strips or allowing novel processes of multiphoton absorption. Here are some instances. An example is the production of biexcitons, such as Copper chloride, in a semiconductor or a quantum framework. This generates additional optical absorption and biexcitonic emissions, the energy of which is less than that of the two individual excitons. This energy difference aligns with the two excitons' binding energy. A bond of multiple excitons has been used to extend the term Biexciton to multi-exciton or exciton string [1]. For a molecular system the development of multiple kinds, such as a J-group of teeth, is an analog. The dipoles of different colourants are aligned head-to-head. Another form of electronic nanoscale interaction that gives birth to a new optical shift appears in the nearest nearby vicinity of the electron-donating group or molecule within a Nanoscopic Distance (electron acceptor). Examples include organometallic complexes that link an inorganic (metallic) ion to multiple organic groups (ligands) [1]. The new optical transitions between these sorts of organometal complexes include the transference of metal to ligand (MLCT) charge, or in certain cases a reverse charge transfer, caused by the absorption of light. Another example of this is an intermolecular organic donor (D)–acceptor (A) complex producing charging species D+A in the excited state [1]. These charging transfer complexes have a strong visible colour generated from a new transition to the visible charge, even though the D and A components are colorless, and hence do not absorb the visible spectrum range separately [1].

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Another kind of co-operative transition is the production of dimers in the ground electronic state, involving an exciting species A (typically designated A*) and an additional species B. This production of excited dimers, caused by optical absorption, may be described as: .

𝐴 → 𝐴∗ , 𝐴∗ + 𝐵 → 𝐴𝐵

(10)

An excimer is formed when A and B are the same excited state dimer. If A and B are distinct, an exciplex is termed the resultant heterodimer. It should be noted that there is no electron (charge) movement between A and B in an exciplex [1]. They are still neutral, however, bound by favourable nanoscope interactions in the exciplex state. The optical emission from this excimeric or exciplex state is significantly red-shifts in comparison to the emissions from monomeric A* form (for longer wavelength or lower energy). In addition, the excimer or exciplex emissions are quite large and uncharacteristic [1]. The excimer and exciplex emissions are a very sensible sample for the nanoscale structure and orientation around a molecule and have been widely utilized to test local and dynamic biological activities. Another example is illustrated by rare earth-ion pairs, in which an ion absorbs energy and transmits it to another ion, which further captures another photon in an additional higher level of the electron. In comparison with excitation, the emission may then be turned into energy [1]. APPLICATIONS In medicine, photonic nanotechnologies are a new and potentially effective method to protect, detect and treat diseases [9]. Lighting may be used to link diagnostic and treatment with a fusion of therapeutics with diagnostic treatment, thanks to its high maximum speed handling and the distant character of optical techniques (including patient prescreening and therapy monitoring). The premise that diagnostic, therapy and therapy guidance are three discreet and individual stages is strongly linked to limitations in medical practice [9]. Theranostics combine the three aforementioned phases with a single procedure to alleviate some of the specificity and sensitivity of therapeutic goods that are now available. There is a growing need to improve the capacity of theranostic processes in which nanophotonic sensing devices can simultaneously detect several gene-connected and nanodevice situations using light and light-activated therapies with real-time monitoring of drug activity [9].

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Synchronous Oscillations of Delocalized Electrons on Nanoparticles and Surfaces Surface plasmons are charged vibrations that eventuate at the junction between conductors and dielectric materials. They can take many different forms, from freely travelling electron density waves along the metallic surface to confined electron oscillations on metal nanoparticles (NPs). When light travels through a nanoparticle made of metal, it generates dipole moments that fluctuate at the frequency of the incident wave, causing secondary rays to be dispersed in all directions. Localized surface plasmon resonance (LSPR) refers to the collective oscillations of free electrons in the conduction band [9]. Light on a nanoparticle causes electrons of conduction to inclusively vibrate at a resonant frequency determined by various factors of the nanoparticle such as size, shape, subject matter, interparticle spacing, and surroundings (dielectric properties) [9]. Because of these Surface Plasmons Resonance modes, nanoparticles absorb and scattering of light takes place so powerfully that single nanoparticles may be seen with the naked eye using dark-field (optical scattering) microscopy. Plasmonic Nanoparticles, which do not blink or fade like fluorophores, provide a practically infinite photon supply for monitoring molecule binding for prolonged periods. Localized Surface Plasmons Resonance can be designed to detect Deoxyribonucleic Acid or proteins by changes in the surrounding refractive index following adsorption of the target molecule to the surface of the metal in nanoparticle-based colourimetric tests for identification. Gold nanoparticle colloids have been frequently utilized in molecular diagnostics due to their strong SPR in the visible, resulting in very brilliant hues [9]. In addition, gold nanoparticles (AuNPs) functionalized with single-stranded DNA have the potential of amalgamating to a complementary target for the identification of particular sequences of nucleic acid in organic specimens [9]. Plasmonic Nanoparticles have indeed been utilized as very bright labels in immunoassays and biochemical probes. In situ hybridization and immunohistology, tests have also been reported to employ colloidal silver plasmon resonant particles (PRPs) encased with standardized agents as highly targeted labels [9]. Most prominently, a nanoparticle-based Biobar code for identification of proteins has been devised, which relies on magnetic microparticle probes with antibodies that bind specifically to a target of interest, as well as nanoparticle sensor encoded with DNA that is distinctive to the protein target molecule and antibodies that can sandwich the target acquired by the microparticle biosensors [9].

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Fluorescence-Based Systems Quantum dots (QDs) are nanoparticles made of semiconductors with narrow, adjustable, symmetrical emission spectra and high quantum yields, which, along with their compatibility with DNA and proteins, make QDs excellent fluorescent label replacements. QDs have long been proposed for nucleic acid characterization, for example, QDs have long been proposed for nucleic acid characterization, cadmium selenide/zinc sulphide QDs for Single nucleotide polymorphism detection on the human TP53 gene, multi-allele detection of hepatitis B and C viruses, and in situ detection of chromosomal abnormalities and mutation. QDs have been further utilized as chemical sensing particles, with a typical fluorescence resonance energy transfer device adopting a dark quencher at a protein binding site connected to a surface of Quantum Dot [9]. When the analyte is present, the quantum dots emission is quenched and when the analyte is removed, the emission is restored. A simpler method was utilized to detect adenine using fluorescent Zinc Sulphide nanoparticles at pH 7, utilizing adenine's ability to quench emission of the quantum-dot-like nanoparticles [9]. Several studies have been published on the modulation of fluorophores in the presence of nanoparticles (e.g., gold, silver, and quantum dots), an interaction that has found its use in application in a variety of systems to detect biologically relevant targets, with a particular emphasis on AuNPs due to its versatility of functionalization with biological molecules [9]. Numerous methodologies for DNA detection based on fluorescence quenching have been studied, including fluorophore-labelled single-stranded DNA electrostatic force of attraction to get adsorbed onto gold nanoparticles, carbon nanotubes, and carbon nano clots, where the presence of a complementary target causes desorption of the newly formed dsDNA from the nanostructures due to electrostatic discrepancy between the nanostructured materials [9]. In addition, fluorescence quenching of fluorophores near metallic nanoparticles synthesized with thiol-modified oligos in various crystal structures has been examined [9]. Tang and colleagues presented a technique for probing hydroxyl radicals utilizing an AuNP-oligonucleotide-FAM combination, in which the hydroxyl radical stimulates strand breaking and, as a result, the release of FAM, restoring previously quenched fluorescence [9]. Semiconductor Nanocrystals: Single-photon Sources Contemporary forms of energy, such as laser diodes or lasers, produce radiation that is well characterized by Maxwell's equations [3]. However, several practical applications in the relatively young field of quantum information need optical

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sources with low intensity but a regulated quantity of photons. The emitting pulses with only one photon are the most important among these sources [3]. This is especially true in quantum cryptography, where the fundamental goal is to secure communications by employing the idea of quantum measurement intelligently [3]. The presence of isolated single-photon emitting materials that display “photon antibunching,” or a dark period between two subsequent photon emissions, is a requirement for producing single-photon sources. Single-photon sources might be made with semiconductor nanocrystals. Compared to other materials, they have numerous benefits. The stability of dye molecules is lower than that of radiation. Furthermore, unlike epitaxy-derived quantum dots, they may be utilized at ambient temperature [3]. Semiconductor Nanocrystals: New Fluorescent Labels for Biology Within the intriguing field of microscopy, there has long been a link between biology and optics. Due to the sheer growing interest in modern biotechnology, this synergy has grown. Biophotonics is a particularly dynamic branch in this regard, with better and updated detection techniques. The absorption and luminescence spectra of dye molecules are very narrow, with the maxima slightly shifted in energy. At low energy, the luminescence spectra are kind of asymmetrical in nature and display shoulders. These qualities make them appealing as biological identifiers, but they also contribute to several drawbacks, particularly when it comes to the multi-detection of molecules with discriminating based on the scratched molecule's colour [3]. Indeed, the experiment entails attaching various molecules to various biological entities and tracking their interactions using dye molecule luminescence detection. The first challenge in this circumstance is that to stimulate all of the dye molecules, an adjustable source or several sources emitting all wavelengths that dye molecules may absorb are required. The second limitation includes the possibility that detection would be less selective than predicted attributed to the prevalence of long luminescence delays that may provide erroneous information. However, the primary challenge is the loss of dye molecules' luminescence during the process of irradiance, a process known as photodarkening [3].

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Nano-Based Semiconductor Crystals: a New Active Component for Excimer Lasers As the active material which emits radiation in laser sources, quantum dots, also known as auto-assembled or semiconductor nanocrystals, offer two major benefits [3]. First, because the wavelength of emission acts as a function of quantum dot size, adjusting the size of emitting quantum dots allows for adjustable lasers over a wide range of wavelengths. Second, the energy difference between two electronic states is likewise a size aspect, it is conceivable to make laser emitters with a temperature-independent critical concentration of laser emission [3]. Furthermore, under the profound incarceration domain, the difference in energy tween thermal and electronic states is so large that electronic levels adjacent to the bandgap are almost always inhabited. Quantum dots made by molecular beam epitaxy also show the laser effect. Conversely, semiconductor nanocrystals offer numerous benefits over constructed dots in this application. For instance: a very limited particle size is feasible for about 5 per cent. Second, its chemical manufacturing opens up a whole new world of interfacing prospects [3]. Furthermore, the diameters of quantum dots produced by a chemical technique of molecular beam epitaxy are an order of magnitude smaller, and the key risks, which are determined by the framework in which dots are submerged, are larger in semiconductor nanocrystals. Because of these two last aspects of nanocrystals, the profound incarceration domain may be easily produced, making it a more viable starting point for developing autonomous temperature-based laser systems [3]. Despite this, only a few studies have looked at stimulated emission in nanocrystals. In the strong confinement mechanism, nano-based crystals of semiconductors exhibit increased Auger recombination (a non-radiative mechanism), which is a significant drawback [3]. Organic Light Emitting Diode One such application includes LEDS integrating organic elements and is known as an Organic Light Emitting diode. Organic materials are broadly divided into two types: Small Molecule Organic Light Emitting Diode and a polymeric lightemitting diode. Multiple elements are put between a cathode and an anode in both types; subsequently, when electricity flows through, light is generated [10]. These devices have already entered the commercial market as basic displays on consumer items such as Philips electric razor, as well as cameras (Kodak) and flat screens (Sony). The applications for nanophotonics are expanding as the capacity to successfully develop and produce devices at the nanoscale size improves. This science and its ongoing progress serve a wide range of sectors, including computing, telecommunications, biology, and sensing [10].

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Consider a photonic crystal to see the interplay of light and matter in a nanophotonic substance. A photonic crystal is a nanostructured substance that influences the movement of electromagnetic radiation. Photonic crystals have a wide range of uses, including telecommunication, security dyes, and paints. Colour-shifting paints are one of the remarkable examples [10]. A relatively small proportion of photonic crystals is mixed into a base paint, which inevitably results in a varnish that seems to change colours based on two factors: firstly, the type of light shining on the paint, and secondly, the angle of view. As light passes through the crystal, it resonates with the substance's matrix [10]. By altering the environment in which the crystal dwells, the way light interacts with the material may perhaps be altered. An electric field, for example, can be used to vary the speed at which light passes through a medium. Adjustments in frequency or wavelength and intensity can arise from the alteration of photonic materials [10]. DISCUSSION The use of photonics on a nanometer level is a challenge of nanophotonics. As light is reduced to nanoscale size volumes, there are field enhancement effects, which lead to novel optical phenomena that may be used to challenge present technical limitations and offer improved photonic devices [11]. Nanophotonics includes a wide range of subjects including metamaterials, plasmonics, highresolution imagery, nanophotonic quantum, and photonic functional nanomaterials [11]. Nanophotonics has been previously regarded as an extensively academic field and is becoming a major player in developing new products and technologies, ranging from high-performance solar cells to personalized healthcare monitoring systems, which can detect the chemical structure of ultralow concentration levels of molecules [11]. Nanophotonics and the confinement of materials, particularly semiconductors and dielectronic, may be comprehended [12]. Generally, these phenomena are called quantum confinement effects as wavy characteristics of electrons are typically described as quantum phenomena. Secondly, the photonic solids fine idea, in which the spread of the light wave is regulated in a similar way to electron waves in solids, is present in the structured dielectric phenomena. Nanophotonics is mostly nano-structured quantum optics which concerns the changed interaction between light matter and restricted light waves in the nanoscale [12]. Spontaneous emission and light scattering are altered and controlled, as spontaneous emissions of photons and spontaneous dispersion by engineered photon densities of states commonly referred to as electromagnetic mode density can be encouraged or prevented [12]. Nanophotonics is ultimately optical and

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optical design centred on nanostructures of metal dielectricity. In optical research and engineering, metals are often not considered a significant issue. However, there are some wonderful properties of metal-dielectric nanostructures arising from the formation of electron excitations on metal-dielectric interfaces known as surface plasmons [12]. Future advancements and outlook on nanophotonics offer a variety of tools on which to establish: Photonic crystals have optimum dispersion and low loss storage capacities while the optical interaction with a single molecule is the preferred system to manipulate light on ultra-fast time scales and ultrasmall length scales [13]. The finishing touches on all light characteristics are metamaterials and metasurfaces. The nanophotonics industry is in an ideal position with the large light control at the nanoscale already attained, to take the field farther, through couplings of light with other freedom degrees at the nanometric scale [13]. The concomitant controls of closely confined lights, electrons, spins, and/or excitons interacting with light centres around hybrid nanophotonics [13]. For instance, when optical and acoustic vibrations are located at a nanoscale, light may be utilized to monitor mechanical movement and vice versa with optomechanical coupling strength which other geometries cannot achieve. For example, for laser cooling a nanomechanical resonator to its quantum earth status, these interactions were utilized [13]. Furthermore, a dynamic backaction of the plasmon on the vibration of a molecule may be represented in plasmon-enhanced surface-enhanced Raman scattering which paves the way for novel molecular quantum optomechanics [13]. Potential hybrid nanophotonic platforms support pairing of light electron spins that enable built-in quantum nanophotonic networking — for instance by the use of singlephoton emissions from diamond or defect centre colour centres in materials, such as Silicon Carbide [13]. Nanophotonics also plays a vital role in the development of new methods for energy conversion. A further study being undertaken on how to best generate electric power is the recently found plasmon-electric effect in metal nanoparticles and holes, in which optical light directly creates a powerful potential via the offresonant excitement of plasmonic structures [13]. The over energy from hot electrons generated to the optically excited plasmonic nano-structures will be another fascinating issue. Applications may be found in energy collection as well as on surface catalysis supported by plasmon for the production of solar fuel [13]. More broadly, photochemistry and catalysis with plasmon support is a study subject that offers numerous prospects for exploration. The immediate influence

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of nanophotonic research on optoelectronic integration was the primary driving element in combining electrical and photonic dimensions. The discovery of new nanostructures which allow a non-reciprocal flow of light leads the door to onchip all-optical isolation, which can only propagate light in an entirely integrated lighting communication network in a specific direction [13]. In current history, the manipulation of light at the nano-level by scientists has led to a continual stream of basic findings of the interaction between light and matter at a deep subwavelength scale. This intensive research effort will undoubtedly offer great potential for nanophotonic technologies in the following years. CONCLUSION Optics and light science, is a vibrant area of study with basic advances and innovative applications that continue to amaze ten years on end. Incandescent bulbs are being swapped by efficient lights in solid-state and solar power technologies are on their approach to price parity with electricity from fossil fuel. The discovery of laser and optical fiber has modernized communications technology. Many of these improvements were caused by greater control over light flow at lengths below the wavelength. Squeezing light into nanoscale dimensions also offers the potential of densely integrated optical circuitry, which can address major bandwidth and power dissipation issues in the current integrated electronic circuit technologies. More generally, the discipline of nanophotonics aspires to break the diffraction limit of Abbe and build technology that allows light to be manipulated at a profound wavelength. With photons shrinking to the nanoscale scale that eventually approaches the electrons' wave function, basic new science is predicted and major technological progress appears. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

P.N. Prasad, Nanophotonics, John Wiley and Sons, pp. 1-8, 2004. [http://dx.doi.org/10.1002/0471670251]

[2]

N. Dai, "Nanophotonics", Nanotechnol. Rev., vol. 4, no. 3, pp. 207-207, 2015.

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[http://dx.doi.org/10.1515/ntrev-2015-0030] [3]

H. Rigneault J.M. Lourtioz C. Delalande, A. Levenson, Nanophotonics, ISTE, pp. 13-17, 2006.

[4]

W. Joseph, “Fundamentals and Applications of Nanophotonics”, Woodhead Publishing Series in Electronic and Optical Materials, 85, Elsevier, 2016.

[5]

K. Eric Drexler, “Productive nanosystems: the physics of molecular fabrication”, Physics Education, Vol 40, No .4, pp. 339-346, 2005. [http://dx.doi.org/10.1039/C9TC04187G]

[6]

M.A. Iqbal, N. Ashraf, W. Shahid, M. Awais, A.K. Durrani, K. Shahzad, and M. Ikram, "Nanophotonics: Fundamentals, Challenges, Future Prospects and Applied Applications", Nonlinear Optics., IntechOpen: London, United Kingdom, 2021.https://www.intechopen.com/online-first/77354 [Online] [http://dx.doi.org/10.5772/intechopen.98601]

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G. Ramalingam, P. Kathirgamanathan, G. Ravi, T. Elangovan, B.A. Kumar, N. Manivannan, and K. Kasinathan, "Quantum Confinement Effect of 2D Nanomaterials", Quantum Dots - Fundamental and Applications., IntechOpen: London, United Kingdom, 2020. https://www.intechopen.com/ chapters/70534 [Online] [http://dx.doi.org/10.5772/intechopen.90140]

[8]

L. Liu, Introduction to Nanophotonics.https://nanohub.org/resources/7670/download/Introduction_ to_Nanophotonics.pdf

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J. Conde, and J. Rosa, “Nanophotonics for Molecular Diagnostics and Therapy Applications”, International Journal of Photoenergy, Vol no. 2012, pp. 1-11, 2012.

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G.D. Phelan, Nanophotonics Principles and Applications, 2008. https://www.techbriefs.com/ component/content/article/tb/supplements/ptb/features/applications/ 11254

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G. Badenes, S. Bekk, and M. Goodwin, Nanophotonics: A forward Look, The Nanophotonics Europe Association, 2012.

[12]

S.V. Gaponenko, “Introduction to Nanophotonics", Physics Today, Vol 64, No. 3, pp. 56-57, 2011.

[13]

A.F. Koenderink, A. Alù, and A. Polman, “Nanophotonics: Shrinking light-based technology", Science, Vol 348, No. 6234, pp. 516-521, 2015.

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CHAPTER 9

A Simulative Study on Electro-Optic Characteristics of InAlGaAs/InP for Fiber Optic-based Communications under Nanoscale Well Thickness Layers Pyare Lal1,* and P. A. Alvi1 Department of Physics, School of Physical Sciences, Banasthali Vidyapith-304022, Rajasthan, India 1

Abstract: The paramount goal of this fundamental explanatory book chapter has been to investigate a simulative study on EO (Electro-Optic) characteristics of InAlGAs/InP heterogeneous nanostructure for GFOCs (Graded Fiber Optic Cables) based SIL (Shortwave Infrared Light) communication systems under several numbers of NWTLs (Nanoscale Well Thickness Layers) in the photonic material based emerging nanotechnological sciences. The energy values in eV of C-V (Conduction-Valence) band offsets with SN (Step Normalized) width and the maximum value of quasi-Fermi energies in eV with various NWTLs have been illustrated graphically under the exploratory simulation in this chapter. Under this simulative investigation, the computational performances of SIL gain amplification with photon’s wavelength and values of carrier concentration per unit volume for several NWTLs have been properly calculated. Next, other various critical parameters such as modal confinement SIL gain amplification and A-G (Anti-Guiding) parameter with values of current per unit area of the cross-section for various values of NWTLs have been calculated cumulatively. Moreover, the performances of differential SIL gain amplification with carrier densities per cubic cm for various NWTLs have been illustrated. It has been distinguished by SIL gain spectra that the peaks of SIL gain spectra are enhanced with a decrease in the value of NWTLs and have been shifted towards the low value of the wavelength of lasing due to enhancement in energy separation values between quasi-Fermi energy levels. In the exploratory investigation through the results, the crest values of SIL gain amplification are ~ 6100/cm and ~ 5100/cm at the photon wavelengths ~ 1332 nm and 1553 nm respectively for 4 nm and 6 nm values of NWTLs. The SIL of maximum intensity emitted by the proposed heterogeneous junction based nanostructure of wavelengths ~ 1332 nm and 1553 nm has been largely utilized in the GFOCs-based SIL communication systems through the process of TIRs (Total Internal Reflections) with no attenuation loss of SIL signals in dB/km because of diminished net dispersions, scattering and net absorptions in the photonic material. Corresponding Author: Pyare Lal: Department of Physics, School of Physical Sciences, Banasthali Vidyapith-304022 (Rajasthan) India; E-mails: [email protected], [email protected] *

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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Keywords: A-G Parameter, C-V Energy Band Offsets, Differential SIL Gain Amplification, GFOCs, InAlGaAs, InP, Modal Confinement SIL Gain Amplification, Net SIL Gain Amplification, NWTLs, SIL Signal Loss, TIRs. INTRODUCTION It has been noticed that, since the last few years, the GFOCs (Graded Fiber Optic Cables) based SIL (Shortwave Infrared Light) communication systems and heterogeneous junction inspired nanostructures have made a critical contribution to nano-optoelectronics and photonics nanosystems due to their unidirectional nature of propagating light. The nanophotonic materials based on a heterointerface nanostructure (such as AlGaInAs, AlGaInN, AlAsInP and GaAsAl, GaAs, etc.) inspired emerging technologies and have tremendous applications in various fields such as in medical research area, industrial field, area of remote sensing, aerospace field, SIL emitters, SIL lasers, SIL detectors, and SIL communications by GFOCs nanosystems etc. In emerging nanotechnological based photonics, the various EO characteristics of III-V photonic materials based on nanoheterointerface structures [1 - 7] have been simulated and investigated experimentally and theoretically by researchers in recent times. The photonic material InAlGaAs/InP based heterointerface nanostructure plays an important role in the SIL emitters due to their high temperature tolerance performances. This is because such type of photonic material has higher energy values of CBOs (Conduction Band Offsets) than VBOs (Valence Band Offsets) so electrons have been confined properly resulting in minimal leakage due to vaporization. This III-V group InAlGaAs/InP photonic material-based heterointerface nanostructure has been utilized in SIL applications because firstly, the emitted SIL is safe to our eyes and secondly, loss effects like dispersion and scattering are negligible in the lasing process. The EO properties [8 - 10] of MQL (Multi Quantum-well Laser), BTL (Bipolar Transistor Laser) and JDL (Junction Diode Laser) based on III-V photonic material AlGaInAs-InP have been investigated recently by several authors. In this chapter, the fundamental EO characteristics of InAlGAs/InP heterogeneous junction-based nanostructure for GFOCs based SIL communication systems under several numbers of NWTLs have been investigated by the computing process. The energy values in eV of C-V band offsets with SN width and the maximum value of quasi-Fermi energies in eV with various NWTLs have been illustrated graphically under the exploratory simulation in this chapter. Under this simulative investigation, the computational performances of SIL gain amplification with photon’s wavelength and values of carrier concentration per unit volume for several NWTLs have been properly calculated. Next, other various critical parameters, such as modal confinement SIL gain amplification and A-G parameter with values of current per unit area of the cross section for various values of NWTLs have been calculated cumulatively.

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Moreover, the performances of differential SIL gain amplification with carrier densities per cubic cm for various NWTLs have been illustrated. It has been distinguished by SIL gain spectra that the peaks of SIL gain spectra are enhanced and diminished as per the value of NWTLs and have been shifted towards the low value of the wavelength of lasing due to enhancement in energy separation values between quasi-Fermi energy levels. It can be seen that the crest values of SIL gain amplification have been found to be ~ 6100/cm and ~ 5100/cm at the photon wavelengths ~ 1332 nm and ~ 1553 nm respectively for 4 nm and 6 nm values of NWTLs. The SIL of maximum intensity emitted by the proposed heterogeneous junction based on the nanostructure of wavelengths ~ 1332 nm and ~ 1553 nm has been largely utilized in GFOCs-based SIL communication systems through the process of TIRs with no attenuation loss of SIL signals in dB/km because of diminished net dispersion, scattering and net absorption in the nanophotonic materials. SIMULATED HETEROINTERFACE THEORETICAL DETAILS

NANOSTRUCTURE,

AND

Taking into account the dimensional performances of nanoscale order, the simulative type heterogeneous interface-based structure has been proposed by computing-based nanotechnologies. The proposed heterogeneous interface-based nanostructure is a simulative structure. This simulative structure has a total of five RNLs (Refractive-index Nanoscale Layers) in which one QNL (Quantum-well Nanoscale Layer) is sandwiched between two RNBLs (Refractive-index Nanoscale Barrier Layers) hence this simulated system is enveloped by two RNCLs (Refractive-index Nanoscale Cladding Layers), such that the whole system is grown simulative on the substrate of InP layer. The entire details of compositional and dimensional parameters have been illustrated in Table 1. Table 1. The details of parameters of the simulative proposed heterogeneous interface-based nanostructure. Five types of specified RNLs

The percentage value of x and y for specified RNLs (In1--yAlyGaxAs)

Values of thickness of specified RNLs in (nm)

Values of Photon's wavelength in (nm)

Energies values of BOs in (eV)

RNCL (C.B.)

0%, 48%

10

0837

0.2354

RNBL (C.B.)

34%, 25%

5

1035

0.1525

QNL

21%, 08%

6

1554

0.0514, - 0.0514

RNBL (V.B.)

34%, 25%

5

1035

- 0.1525

RNCL (V.B.)

0%, 48%

10

0837

-0.2354

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In the above table, the energies values of CBOs and VBOs of five specified RNLs have been illustrated. The higher energy values of CBOs have a very critical role in the performance and stability at higher temperatures. The schematic diagram of the proposed heterointerface-based nanostructure showing CBOs, VBOs and corresponding energy envelope wave functions for conduction electrons and valence holes is exhibited in Fig. (1).

Fig. (1). Schematic diagram of proposed heterointerface based nanostructure showing CBOs, VBOs and corresponding energies envelope wave functions for conduction electrons and valence holes.

For longer confinement of energy envelope wave functions, higher values of CBOs and VBOs are required. The values of QCF (quantum-well confinement factor) are influenced by the confinement of energy envelope wave functions. The QCF-inspired enhancement of SIL gain is termed modal confinement SIL gain in photonics. The OCs (Optical Characteristics) of several types of intensities of gains such as strain-influenced SIL gain, A-G factor-inspired light gain and electrochemical affected optical type gain [11 - 14] have been optimized due to their substantial type unique light performance. Basically, the intensity of SIL gain depends on the photonic materials as well as heating effect, internal and external strain effects and NWTLs. Thermal effects and NWTLs influenced SIL gain expression as a function of photon's energies is explained by [15], which is given by,

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G (h ) 

  h  f q 2h  1  exp 2 2neff hm0  0 c   kbT

  

Lal and Alvi

(1)

(h / 2 )dk x dk y M b fc fv    2 nc ,nv 4 L  ({h nc  h nv  h sg }  h ) 2  (h / 2 ) 2 W 2

Hereis termed as the energy of SIL-photon, is termed as the charge of the electron, is called Planck’s constant, is the mass of the electron, is termed as permittivity in free space, is known as the effective index of refraction, is termed as Boltzmann constant, is known as momentum matrix element, is known as width of QWL, are called Fermi functions of electrons and holes, is termed as energy separation between quasi Fermi levels and is the life time of SIL-photon. By the above mentioned gain relationship (1), it has been predicted that the SIL gain values diminish exponentially with the effect of temperature but get enhanced exponentially with energy separation between the quasi Fermi energies levels of C-V bands. Moreover, according to this relationship, the gain performance has an inverse dependence with values of NWTLs. In terms of SIL gain intensities, the QCF is approximately equal to the fraction ratio of the value of modal confinement SIL gain and the value of material SIL gain. The value of QCF never becomes greater than unity, hence the value of modal confinement type SIL gain always becomes less than the value of material SIL gain. In other words, when the volume of the modal active region is divided by the volume of the entire active region, then QCF is approximately obtained. The exact expression of QCF in terms of the squared electric field is given by equation (2).

 

 ( z) .dz  ( z) .dz

Lw 2

 Lw / 2 





2

2

(2)

Here is the width of the active layer and is termed as the electric field in the zdirection. In general, QCF influenced SIL gain is proportional to concentrations of carriers due to enhancement in separations between energies values of quasi Fermi type energy levels, but some gains have inverse dependence on concentrations of carriers, hence these type gains are termed as differential type SIL gains. The below mentioned equation represents the differential type SIL gain relationship as a function of photon's energies.

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G ' (h ) 



dG(h ) 8 2 mr h 2   Mb dN ch3 Lw E'

(3)

 df (h ) dfv (h )  ' '  c    L(h )dh dN   dN

Here is the width of the active layer and is termed as Lorentz line shape function, is the carrier densities and c is termed as the speed of SIL in a vacuum. The above-mentioned differential SIL gain relationship (3) has also been affected by BTME (Bulk Type Momentum- matrix Element), the value of Line shape functions and NWTLs as well as a change in the value of IORs (Index Of Refractions). In the proposed heterointerface based nanostructure, the value of IORs has been enhanced from QNLs to RNCLs, i.e. the value of IORs has been diminished from RNCLs to QNLs. In other words, the QNL has a minimum value of IOR and RNCL has a higher value of IORs. The change in IORs with respect to concentrations of carriers is given by the differential equation (4), which is called DRE (Differential Refractive-index Equation) and it is also affected by BTME, nature of photonic materials and temperature effects. n ' (h )  

  Mb E'

2

dn(h ) 4 2 mr h  dN ch 4 Lw  df (h ) dfv (h )  ' ' '  c    (h  h )  L(h )dh dN   dN

(4)

Here L w is the width of the active layer and L(h ' ) is termed as Lorentz line shape function, N is the carrier densities, c is termed as the speed of SIL in vacuum, λ is called wavelength of SIL-photon, and  is the life time of SIL-photon. The differential equations (3) and (4) both have a substantial role in the determination of A-G factor or parameter. Whenever the ratio of differential equations (4) and (3) is multiplied by the double value of the wavevector of the propagating SIL wave, then the A-G factor can be determined by mathematical approach. In terms of DRE, wavevector and differential type SIL gain, an exact equation of A-G factor has been expressed by the below relationship, 

 dG(h , N ) 4  1    n ' (h , N )    ' dN  G (h , N )  





(5)

Here n ' is the differential index of refraction of layer and G ( h ' ) is termed as differential SIL gain, N is the carrier densities, and λ is called wavelength of SILphoton.

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The equation (5) of A-G factor provides a very vital role in the investigation of all type SIL gains. This factor is influenced by strain effects and temperature effects as well as concentration effects of carriers. According to different types of lasing situations, the A-G factor becomes negative, positive or zero. A-G parameter tuned frequencies of ROs (Relaxation Oscillations) have significant contributions in the calculation of current values in terms of the threshold condition. The frequency value of ROs in GHz is proportional to current (in mA). The frequencies of ROs in terms of several parameters [16 - 20], such as A-G parameter, differential SIL gain parameter, and peak SIL gain parameter etc. have been simulated and investigated by most of the researchers. The frequency relationship of ROs in terms of differential type SIL is given by below formula, 1/ 2

1 fr  2

 (cP )    {G ' (h , N )}  (n )  p  

(6)

Here L w is the width of the active layer and G ( h ' ) is termed as differential SIL gain, N is the carrier densities, c is termed as the speed of SIL in a vacuum,  p is the lifetime of SIL-photon and P is the optical power. The values of frequency relationship of ROs and power of output light both have been enhanced due to an increase in value of CIs (Current Injections) in mA. The value of CIs in mA at which both, the frequency relationship of ROs and power of output light tend to vanish is termed as the threshold value of CIs in mA, and this situation is called the threshold condition for lasing action. The threshold value of CIs also depends on the NWTLs, losses of internal and mirror types, SIL gain performances and length of cavity. Hence, the relationship of CIs in terms of threshold situation is given by the equation,     nJ L L  1 1 1       int    1 I th   0 w   exp  ln 2 L R1 R2       nG0 ( J )  

(7)

Here L w is the width of the active layer and  is termed as QW optical confinement factor, n is termed as the number of QWs,  int is the internal loss in SIL,  is the quantum efficiency, L is called cavity length and R1 and R2 are termed as facet’s reflectivity. The role of equations (6), (7) and light output power have great importance in the determination of SIL signal attenuation. It has been noticed that the SIL attenuation (in dB/km) is the largest at the wavelength of ~ 1420 nm but the lowest at wavelengths of ~ 1332 mm and 1553 nm. The equation of SIL signal

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Photonic Materials: Recent Advances and Emerging Applications 167

attenuation in terms of light powers and length of GFOCs has been expressed by the following logarithmic relation (8), P  10  A     log10  in L  Pout

  

1

(8)

Here is termed as the length of optical fiber, Pin, and Pout are termed as input and output powers respectively. COMPUTATIONAL RESULTS AND DISCUSSION In photonics, the enhancements of SIL are the net performance of the rate of SEs (Stimulated Emissions) that a typical SIL photon generates as it propagates for a given distance. Generally, in the heterointerface nanostructures, the SIL amplification has been caused by photon-induced transition of electrons from the C to V-bands in the proper active region of QNL. Whenever, the rate of downward transitions exceeds the rate of upward transitions, there will be a net generation of SIL photons and hence, an SIL gain can be achieved [11, 12]. The SIL gain is exponentially enhanced with a decrease in the temperature values. In other words, the gain value tends towards zero as the temperature value increases and tends towards infinity. At absolute zero temperature, the gain value is always positive and maximum. It has been predicted by various investigations and studies that the SIL gain enhancement value is also affected by bulk momentum matrix elements. Sometimes SIL gain value depends on the modal confinement parameter with a diminished value than material SIL gain value. The modal confinement parameter has a critical role in the computing of modal confinement SIL gain per cm in emerging photonic material-based nanosystems. The various EO properties of SIL, NIL, and UVL for GaAlAs/GaAs, AlGaInAs/InP and AlGaN/AlN have been investigated theoretically by us [21 - 23] in recent times. The intensity of SIL gain enhancement has a maximum value whenever the maximum value of energy occurs between quasi Fermi levels of C-V bands. The energy levels at which the probability of finding electrons is maximum after applying an external electric field are called QFLs (Quasi Fermi Levels). The QFLs have a very crucial role in the finding of SIL gain. The graphical representation of maximum energy separations (in eV) between Fermi-energy levels (quasi type) with various NWTLs (in nm) of InAlGaAs/InP type heterointerface based proposed nanostructure has been shown in Fig. (2). It has been observed in (Fig. 2), that the maximum values of QFEs(Quasi Fermi Energies) of C-V bands have been enhanced with an increase in the value of NWTLs.

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Fig. (2). The graphical representation of maximum energy (in eV) separations between Fermi-energy levels (quasi) with various NWTLs (in nm) of InAlGaAs/InP type heterointerface based proposed nanostructure.

In the QNL, the QFEs are also influenced by spacing between energy subbands. The energy difference between subbands has an inverse dependence on the value of NWTLs. The spectral type performances of intensities of SIL gain with photon's wavelength (in micron), and in the inset picture, the graphical performances of A- G parameter with values of injection current densities (in Acm-2) for various NWTLs of III-V photonic material based heterointerface nanostructure have been illustrated in Fig. (3). It has been distinguished by SIL gain spectra shown in Fig. (3), that the peaks of SIL gain spectra are enhanced with a decrease in the value of NWTLs and have been shifted towards a low value of wavelength of lasing due to enhancement in energy separation values between quasi-Fermi energy levels. In the exploratory type investigation through the results, the crest values of SIL gain amplification have been found ~ 6100/cm and ~ 5100/cm at the photon wavelengths ~ 1332 nm and 1553 nm respectively for 4 nm and 6 nm values of NWTLs. The SIL of maximum intensity emitted by the proposed heterogeneous junction based nanostructure of wavelengths ~ 1332 nm and 1553 nm has been largely utilized in the GFOCs-based SIL communication systems through the process of TIRs (Total Internal Reflections) with no attenuation loss of SIL signals in dB/km. This is because of diminished net dispersions, scattering and net absorptions in the photonic material-based emerging nanotechnological research field.

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Fig. (3). The variation in intensities of SIL gain with photon's wavelength (in micron), and in the inset picture the variation of A- G parameter with values of injection current densities (in Acm-2) for various NWTLs of III-V photonic material based heterointerface nanostructure.

Next, Fig. (4) shows several types of proportional and saturated behaviors of values of SIL gain intensities (in cm-1) with the value of carrier densities (in 1018 × cm-3) for several NWTLs of proposed III-V photonic material InAlGaAs/InP based heterointerface nanostructure. The inverse behavior of the saturated values of SIL gain intensities (in cm-1) with various values of NWTLs (in nm) of the proposed III-V photonic material-based heterointerface nanostructure is shown in Fig. (5).

Fig. (4). The behavior of values of SIL gain intensities (in cm-1) with the value of carrier densities (in 1018 × cm-3) for several NWTLs of proposed III-V photonic material based heterointerface nanostructure.

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Fig. (5). The functional dependence of saturated values of SIL gain intensities (in cm-1) on various values of NWTLs (in nm) of the proposed III-V photonic material based heterointerface nanostructure.

When the SIL gain is multiplied by the modal confinement parameter then the modal confinement SIL gain is achieved. The simulatory performances of reciprocal behaviors of modal confinement SIL gain intensities (in cm-1) with various values of NWTLs (in nm) at injection current densities of 300 Acm-2 for proposed III-V photonic material InAlGaAs/InP based heterointerface nanostructure are shown in Fig. (6).

Fig. (6). The variation of modal confinement SIL gain intensities (in cm-1) with various values of NWTLs (in nm) for proposed III-V photonic material InAlGaAs/InP based heterointerface nanostructure.

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In general, the derivative value of modal confinement SIL gain with respect to injection carrier densities is called modal differential SIL gain. The inverse behaviour of peak values of differential SIL gain (in cm2) with various NWTLs of the proposed III-V photonic material based heterointerface nanostructure is exhibited in Fig. (7). In Fig. (7), the value of differential SIL gain has been diminished with enhancement in the value of NWTLs on account of a decrease in energy spacing between the energy subbands.

Fig. (7). The inverse behaviour of peak values of differential SIL gain (in cm2) with various NWTLs of proposed III-V photonic material based and heterointerface nanostructure.

In the determination of threshold injection current densities, the values of ROFs and output light power have a very important role. The values of frequencies relationship of ROs and power of output light both increase with an increase in the value of CIs (Current Injections) (in mA). The value of CIs in mA at which both, the frequency relationship of ROs and power of output light tend to vanish is termed as the threshold value of CIs in mA, and this situation is called the threshold condition for lasing action. The threshold value of CIs also depends on the NWTLs, losses of internal and mirror types, SIL gain performances and length of the cavity.

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The graphical performance at several values of ROFs (in Hz) with injection current densities (in Acm-2) for various NWTLs of III-V photonic material based heterointerface nanostructure has been shown in Fig. (8). It has been predicted by curves of Fig. (8) that the value of ROFs increases with an increase in injection current. Moreover, the bottom of curves has been shifted towards higher values of NWTLs. Further, (Fig. 9) shows the appropriate inverse behaviour of variation of ROFs (in Hz) with various values of NWTLs of III-V photonic material based heterointerface nanostructure. The frequency value of ROs in GHz has proportional behaviour with current values in mA. The frequencies of ROs in terms of several parameters, such as A-G parameter, differential SIL gain parameter, and peak SIL gain parameter, etc. have been simulated and investigated by most of the researchers. Finally, the differential equation of SIL gain and differential equation of index of refraction both have a substantial role in the determination of the A-G factor or parameter.

Fig. (8). The graphical performance of several values of ROFs (in Hz) with injection current densities (in Acm-2) for various NWTLs of III-V photonic material based heterointerface nanostructure.

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Fig. (9). The appropriate inverse behaviors of graphical performance of ROFs (in Hz) with various values of NWTLs of III-V photonic material based heterointerface nanostructure.

Whenever the ratio of differential equations of the index of refraction and differential equation of SIL gain is multiplied by a double value of the wavevector of the propagating SIL wave, then the A-G factor has been determined by mathematical approach. The variation in maximum A-G parameter with various values of NWTLs of III-V photonic material InAlGaAs/InP based heterointerface nanostructure has been shown in Fig. (10). It can be inferred from Fig. (10) that the value of the maximum A-G parameter always becomes positive and it has been enhanced from 0.5 to 12 with an increase in NWTLs.

Fig. (10). The proportionalbehaviors of graphical performance of maximum A-G parameter with various values of NWTLs of III-V photonic material based heterointerface nanostructure.

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CONCLUSION Considering the photonic material-based emerging nanotechnological sciences, GFOCs-based SIL communication systems under several NWTLs, the simulative type studies on EO properties of InAlGAs/InP heterointerface nanostructure have been investigated. The energies values in eV of C-V band offsets with SN width and the maximum value of quasi Fermi energies in eV with various NWTLs have been illustrated graphically under the exploratory simulation in this chapter. Under this simulative investigation, the computational performances of SIL gain amplification with photon’s wavelength and values of carrier concentration per unit volume for several NWTLs have been properly calculated. Next, other various critical parameters such as modal confinement SIL gain amplification and A-G parameter with values of current per unit area of cross section for various values of NWTLs have been calculated cumulatively. Moreover, the performances of differential SIL gain amplification with carrier densities per cubic cm for various NWTLs have been illustrated. It has been distinguished by SIL gain spectra that the peaks of SIL gain spectra are enhanced with a decrease in the value of NWTLs and have been shifted toward the low value of wavelength of lasing due to the enhancement in energy separation values between quasi Fermi energy levels. In the exploratory investigation, the crest values of SIL gain amplification have been found to be ~ 6100/cm and ~ 5100/cm at the photon wavelengths ~ 1332 nm and 1553 nm respectively for 4 nm and 6 nm values of NWTLs. The SIL of maximum intensity emitted by the proposed heterogeneous junction based nanostructure of wavelengths ~ 1332 nm and 1553 nm has been largely utilized in the GFOCs-based SIL communication systems through the process of TIRs with no attenuation loss of SIL signals in dB/km because of diminished net dispersion, scattering and net absorption in the photonic material. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGMENTS The authors are very grateful to Banasthali Vidyapith for providing computer related appropriate facilities in the School of Physical Sciences. REFERENCES [1]

P.A. Alvi, "PyareLal, S. Dalela, M. J. Siddiqui, “An Extensive Study on Simple and GRIN SCH based In0.71Ga0.21Al0.08As/InP Lasing heterostructure", Phys. Scr., vol. 85, 2012.035402

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[http://dx.doi.org/10.1088/0031-8949/85/03/035402] [2]

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J.J. Geuchies, B. Brynjarsson, G. Grimaldi, S. Gudjonsdottir, W. van der Stam, W.H. Evers, and A.J. Houtepen, "Quantitative Electrochemical Control over Optical Gain in Quantum-Dot Solids", ACS Nano, vol. 15, no. 1, pp. 377-386, 2021. [http://dx.doi.org/10.1021/acsnano.0c07365]

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P. Lal, R. Yadav, M. Sharma, F. Rahman, S. Dalela, and P.A. Alvi, "Qualitative analysis of gain spectra of InGaAlAs/InP lasing nano-heterostructure", Int. J. Mod. Phys. B, vol. 28, no. 29, 2014.1450206 [http://dx.doi.org/10.1142/S0217979214502063]

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P. Lal, "An Investigation of Optical Gain of Nanomaterial AlGaAsIn/InP under CTLSs in Optical Communications", Journal of Atomic, Molecular, Condensed Matter and Nano Physics, vol. 7, no. 3, pp. 189-195, 2020. [http://dx.doi.org/10.26713/jamcnp.v7i3.1544]

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G. Bhardwaj, P. Lal, V. Mishra, and P.A. Alvi, "Numerical simulation of optical properties of compressively strained grin- InGaAlAs/InP type-I nano-heterostructure", Mater. Today Proc., vol. 44, pp. 4847-4849, 2021. [http://dx.doi.org/10.1016/j.matpr.2020.11.698]

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P. Lal, "An Investigation of Optical Gain of Nanomaterial AlGaAsIn/InP under CTLSs in Optical Communications", Journal of Atomic, Molecular, Condensed Matter and Nano Physics, vol. 7, no. 3, pp. 189-195, 2020. [http://dx.doi.org/10.26713/jamcnp.v7i3.1544]

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CHAPTER 10

Two-Dimensional Materials for Advancement of Fiber Laser Technologies Kavintheran Thambiratnam1,*, Norazriena Yusoff1, Siti Aisyah Reduan1, Muhamad Zharif Samion1, Shok Ing Ooi1 and Harith Ahmad1 1

Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia Abstract: Two-dimensional (2D) materials such as graphene, chalcogenides, topological insulators, black phosphorus, and MXenes have of late become the focus of intense research efforts due to the excellent and unique optoelectrical properties these materials possess. This is due to the unique properties these materials possess, such as tunable bandgaps, high mobility in the energy bandgap, third-order nonlinearity, and nonlinear absorption that can be tailored to suit the specific needs of different optical applications. These properties have allowed for the development of fiber optic-based pulsed laser systems with better integration and flexibility capabilities as well as improved performance as compared to their bulk counterparts. In this chapter, the development of optical fiber pulsed lasers that incorporate selected 2D materials, particularly 2D chalcogenides that encompass metal monochalcogenides (MMs), and traditional metal dichalcogenides (TMDs) and MXenes is reviewed. This chapter will cover the fundamental aspects of the aforementioned materials, the operating principles of Q-switching and mode-locking, and the configuration of these 2D materials as saturable absorbers (SAs). The main section of this chapter will focus on the current status of the development of Q-switched and mode-locked optical fiber laser systems using 2D material-based SAs. Finally, the chapter will explore the perspectives and challenges on the future of the potential applications of these 2D materials in pulsed optical systems.

Keywords: Fiber Lasers, Nonlinear Optics, Optical Systems, Two-Dimensional Materials. INTRODUCTION In recent decades, optical fibre laser technologies have expanded rapidly into a commercial business worth more than $800M/year and an annual growth rate of approximately 13%, which is the highest among the various laser technologies [1]. This record represents the remarkable achievement of fiber laser technologies Corresponding author Kavintheran Thambiratnam: Photonics Research Centre, Universiti Malaya, Kuala Lumpur – 50603, Malaysia, Email: [email protected] *

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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and was driven mainly by the flexibility, compactness and alignment-free properties afforded by optical fibers and systems developed using optical fibers. Of the many types of fiber laser technologies, pulsed fiber laser technologies, in particular, have proven to be very useful for a variety of applications such as material processing [2, 3], skin treatment [4, 5] and medical applications [6, 7]. The efficient generation of pulses from fiber laser systems however required the use of use of an optical modulating optical modulating element known as saturable absorber (SA) which exhibited power dependent absorption properties. The SA could be obtained as either active or passive devices, but the latter was mostly preferable due to its simplicity and cost-effectiveness. Currently, semiconductor saturable absorber mirrors (SESAMs) based on III–V semiconductors have been the primary choice for passive SA devices. These devices have high performance and stability, but with the limitation of being complex and fragile and having a costly fabrication process. In addition, they also possess a relatively narrow tuning range dictated by the bandgap energy of the semiconductor absorber. These limitations have thus shown the need to develop a more cost-efficient, broadband saturable absorber for pulsed laser generation in fiber laser systems. While there were many efforts to develop a more cost-effective and viable SA, it was the discovery of graphene by Andre Geim and Konstantin Novoselov in 2004 that showed the potential of twodimensional (2D) materials as SA devices [1]. 2D materials are highly suitable for fiber laser technologies due to the fiber compatibility which allow an alignmentfree, all-fibre format. Furthermore, they offer easy integration into the fiber laser system as the SA can be formed by sandwiching the 2D material between two fibre connectors or drop-casting it onto the surfaces of a specially designed fiber. By 2009, graphene had already been heavily utilized as an SA in fiber laser systems due to its easy fabrication procedure and unique optical properties. Nevertheless, single-layer graphene suffered from a low-modulation depth, which was insufficient for ultrafast laser generation. The stacking of few-layers graphene could enhance the modulation depth but at the same time it also increases its nonsaturable loss. As a result of this, various 2D materials other than graphene have been studied and tested for pulsed fiber laser generation. These new emerging 2D materials include chalcogenides [8 - 12], topological insulators (TIs) [13], black phosphorus (BP) [14 - 16] and MXenes [17 - 20]. These materials show distinct optical properties and are thus suitable to be used for the generation of a versatile pulsed fiber laser system. Among these 2D materials, chalcogenides including metal monochalcogenides (MMs) and transition metal dichalcogenides (TMDs) as well as MXenes are

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currently attracting the greatest research interest in the field of fiber lasers. TMDs are particularly interesting for application as an SA in pulsed fiber laser systems due to their layer-dependent optical properties [21, 22] and ultrafast carrier dynamics [23 - 25]. On the other hand, MMs provide alternative structural features than that of TMDs, together with large material options. Alternatively, MXenes are widely explored due to their broadband nonlinear optical response and high optical nonlinearity [18, 26]. In this chapter, the development of fiber lasers based on MMs, TMDs and MXenes is reviewed. A discussion on the operating principle of these SAs for Qswitching and mode-locking in fiber laser system, the integration configuration of the saturable absorber and experimental setup for pulsed laser generation at different wavelength ranges is also presented. Lastly, the challenges and future perspective of the MMs, TMDs and MXenes saturable absorber for the advancement of fiber laser technologies are discussed. 2D Material-Based Saturable Absorbers for Fiber Lasers It is well established that the pulsed fiber laser can be operated in both Qswitching and mode-locking modes. These pulsed lasers can then be classified further as active and passive systems, depending on the technique used to generate the pulses. An active pulsed laser is one that used several optical devices to generate pulses, such as electro-optic or acousto-optic modulators. This technique is very effective in controlling most of the parameters of the generated pulses; however, the resulting bulky setup, as well as high cost, became a major inhibitor for the widespread use of this approach. On the other hand, passive pulse lasers were usually realized by means of SAs, which overcome the issues of bulk and cost but at the same time offer less control on the output parameters of the pulses generated therein. However, in most real-world applications, the pros of this approach far outweigh its cons, and thus research efforts are now focused on the development of passively pulsed fiber lasers. Like graphene, 2D materials exhibit interesting characteristics that make them highly desirable for photonics applications, such as strong light absorption. This is a unique behaviour of 2D materials and it is not exhibited by their bulk material counterparts [27]. Furthermore, 2D materials also demonstrate a great linear energy-momentum dispersion correlation which permits broad wavelength range coverage ranging from ultraviolet to far-infrared [28]. Layered 2D materials possess an excellent potential for incorporation into optoelectronic devices due to their good mechanical flexibility, strong intra-layer bonding and great robustness [29]. Fig. (1) shows some of the examples of 2D materials that have been used as SA materials for pulsed fiber laser generation.

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Fig. (1). Example of 2D materials-based SA from different groups with their corresponding FESEM image.

2D materials can be classified into a few general groups including black phosphorus, the graphene family which includes graphene oxide, reduced graphene oxide and hexagonal boron nitride, TIs, the 2D chalcogenides family including TMDs, semiconducting dichalcogenides, metallic dichalcogenides, and layered semiconductors as well as 2D oxides that include layered copper oxides and hydroxides. Fig. (2) describes the various categories of 2D materials. We will focus on the discussion of the fundamental knowledge of 2D chalcogenides groups that include MMs, and TMDs as well as MXenes. While these are far from being the only 2D materials used as SAs, they are currently the most commonly used and thus can serve as a foundation for all other 2D materials as well. 2D Chalcogenides Chalcogenides are a group of materials that compromise one or more chalcogen elements, namely sulphur (S), selenium (Se) and tellurium (Te). These elements are usually incorporated with other element such as germanium (Ge) [30], gallium (Ga) [31], silicon (Si) [32], antimony (Sb) [33], tin (Sn) [34], molybdenum (Mo) [35], tungsten (W) [36] or aluminium (Al) [37] to form a chalcogenide. Chalcogenides, especially in their 2D form, have very interesting opto-electronic properties. 2D Chalcogenides are classified by their sandwich units of X-M-X or X-M-M-X atomic layers which form a close-packed structure. Here, M refers to

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the metal atom while X represents the chalcogen atom. These sandwich units are aligned with one another by a van der Waals gap along the crystallographic cdirection. The inner surfaces of the layers are constructed by the chalcogenide atoms that are closely packed and chemically saturated. Within the sandwich units, there is a strong covalent bond as well as several electrostatic contributions due to the ionicity of the M–X bonds. The metals are mostly present in a trigonal prismatic coordination to allow more covalent chalcogenides to optimize the covalent overlap, whilst the octahedral coordination is preferred for ionic as in to minimize the electrostatic repulsion [38]. Due to their unique arrangement, an uncommon physical phenomenon occurs in chalcogenides materials that range from atypical fascinating electronic, thermal, and optical properties to novel forms of superconductivity and magnetism [39]. Therefore, rapid advances in understanding quality, and fabrication of 2D layered chalcogenides have been observed recently and as such, they are now used in a wide variety of new device applications. In the development 2D materials for photonics application, 2D chalcogenide materials including MMs and TMDs have been widely utilized as SAs in Q-switched and mode-locked fiber lasers [12, 40, 41].

Fig. (2). Classification of layered 2D materials into a few groups.

Metal Monochalcogenides (MMs) The MMs on the other hand are built up from two elements that are: the metal atom from group IVA or IIIA (M = Si, Ge, Sn, Ga. In, etc.) and the chalcogen

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atom (X = S, Se, or Te), with the general formula of MX. Naturally, 2D MMs are highly anisotropic in view of the fact that they demonstrate in-plane structural anisotropy along with the armchair and zigzag crystalline directions [42]. The monolayer MMs also disclose an orthorhombic crystal structure (Pnma space group) with low crystal symmetry C2υ. The new order spin–orbital coupling parameters and polarization properties can be observed upon breaking this inversion symmetry. It has been anticipated that various intriguing phenomena occurred in monolayers of MMs, for instance, valley physics [43], spontaneous polarization and bulk photovoltaic effect [44], giant piezoelectricity [45], ferroelasticity [46], and ferroelectricity [47]. On top of that, monolayer MMs also exhibit large optical second harmonic generation (SHG) with the susceptibility strength of more than one order of magnitude higher than that in TMDs materials [48], thus showing promising potential in nonlinear optoelectronic applications. The most inspiring characteristics of these MMs materials are good photoconductivity, large carrier mobility, tunable energy gap based on the number of layers and unique 2D structure with large surface area. Due to their unique properties and other features such as the low-cost, earthabundant, and environment friendly nature, the 2D MMs materials have gained worldwide attention in recent decades and they are used in various fields of applications that have been rapidly explored. For instance, the study conducted by Qui et al. has revealed the potential of one of the MMs materials known as tin selenide (SnS) for gas sensor application [49]. This is due to the unique 2D monolayer and superior oxidation resistance of SnS as well as a large active surface area that is highly desired for improving the adsorption of gas molecules. In another study by Brent et al., they reported the electronic properties of monolayer or fewlayer MMs by theoretical and experimental works [50]. Their finding exposed that SnS manifests a direct and indirect bandgap within the range of 1.0 to 2.3 eV which covers a part of infrared and visible range. Moreover, MMs materials have also shown significant nonlinear optical characteristics as verified by the research conducted by Zhou et al. [51]. They have revealed that the germanium sulfide (GeS) possesses a broad-band nonlinear optical absorption ranging from 400 to 800 nm and an extraordinary nonlinear optical response which is dependent on the wavelength and excitation intensity. Based on the result obtained in this study, it has been proven that the few layers GeS can be a promising candidate for the nanophotonic applications in particular for a saturable absorber and optical diode development. Even more, the applicability of 2D MMs materials for implementation as transistors, sensors, and photodetectors also has been addressed in a number of studies. Several methods have been introduced to separate ultrathin layers of MMs from their bulk form. These methods can be separated into two categories namely the

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top-down and bottom-up approaches. In the top-down approach, the mechanical (ME) or so-called Scotch tape delamination, liquid phase exfoliation (LPE), and electrochemical techniques were employed in order to exfoliate the thin 2D MMs from their parent layered bulk crystals. The ME method was used to produce small amounts of large and high-quality pristine nanosheets, though it requires a large size of starting crystals and is not scalable. The LPE method on the other hand has shown great potential for the scaled up manufacture of 2D materials that are beneficial for the future electronics industry based on 2D devices. For the bottom-up approach, techniques such as wet chemical synthesis (WCS), pulsed laser deposition (PLD) and chemical vapour deposition (CVD) are prime examples. These methods involve the chemical reactions of certain precursors at given experimental conditions and make the bottom-up approach favourable for the synthesis of ultrathin and single-layer of 2D MMs. Fig. (3) gives an overview of 2D MMs materials showing the structure, and several properties and applications.

Fig. (3). Overview of the structure, properties, and applications of 2D MMs materials [12, 45, 52 - 54].

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(reuse from [12] © 2020 IEEE. Reprinted, with permission, from Harith Ahmad, Rizal Ramli, Aizuddin Ahmad Kamely, Muhammad Zharif Samion, Norazriena Yusoff, Leonard Bayang, Siti Nabila Aidit, and Kavintheran Thambiratnam. “GeSe Evanescent Field Saturable Absorber for Mode-Locking in a Thulium/Holmium Fiber Laser.” IEEE Journal of Quantum Electronics 56, no. 5 (2020): 1-8.) (Reprinted from [45]: Fei, Ruixiang, Wenbin Li, Ju Li, and Li Yang. “Giant piezoelectricity of monolayer group IV monochalcogenides: SnSe, SnS, GeSe, and GeS.” Applied Physics Letters 107, no. 17 (2015): 173104., with the permission of AIP Publishing.) (reuse from [52, 53, 54] of material which is licensed under CC BY 4.0) Transition Metal Dichalcogenides (TMDs) The most commonly used member of 2D chalcogenide materials as an SA is the TMD and this is due to their substantial optoelectronic properties. The general formula of TMDs is represented as MX2 where M refers to the transition metal atom of Group IVB (Ti, Zr, Hf) or group VIB (Mo, W) while X indicates the chalcogen atom of either S, Se, or Te. The possible elements that can constitute TMDs have been highlighted in Fig. (4a). The TMDs materials exhibit a layered structure of strong X-M-X bonds in which one transition metal atom (M) is sandwiched between two chalcogen atoms (X) that are tightly held together through intense interactions with covalent bonding. Meanwhile, the individual MX2 layers are held together through a relatively weak interlayer Van der Waals forces. This is shown in Fig. (4b). Similar to other 2D materials, the electronic and optical properties of TMD can be tuned by adjusting the thickness of the formed layers, or more accurately to control the number of layers being formed. Unlike their bulk counterparts, layered TMD materials exhibit a direct optical bandgap with strong light-matter interaction, good stability, and high nonlinear optical response. Furthermore, the highly desirable qualities of a non-zero bandgap and third-order optical nonlinearity are observed in TMDs. In fact, 2D TMDs are already finding a wide range of applications such as in photodetectors, transistors, energy storage and nonlinear optical devices. In the last several years, TMDs have been intensively studied to demonstrate ultra-fast electron relaxation and broadband nonlinear optical response which makes them capable to be used as passive Q-switches, optical limiters, and mode-lockers. Owing to their peculiar properties, TMDs have application prospects in many fields as for instance in electronics, optoelectronics, sensing and energy storage applications.

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Fig. (4). (a) Periodic table showing the possible elements to prepare TMDs material. (b) The crystal structures of the layered TMDs material. Reprinted with permission from [55]. (reuse of material which is licensed under CC BY 4.0).

The atomic-thin layers of 2D TMDs material can be synthesized by separating the single or few layers from their bulk crystal which can be done by various techniques such as ME, CVD and LPE. The mechanical exfoliation technique is commonly employed to obtain single-layer of TMDs with good quality. However, this technique suffers from several drawbacks as such the size of the TMDs material is relatively small, thus hindering real device development. The same issues also arise for the LPE technique. Therefore, a solution processing approach is more appropriate in order to obtain a high quantity of single or few-layer of TMDs. Besides that, the CVD technique can also be used to achieve a large-area growth, in which two parts are involved in this technique. The first part is the sulfurization (or selenization) of metal thin films and the the second part is the vapour phase reaction of metal oxides with chalcogen precursor. However, the direct sulfurization (or selenization) of a metal oxide thin film has several limitations, such as the difficulty to control the thickness of the pre-deposited metal oxide or metal thin film, which affects the wafer-scale uniformity.

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Generally, the CVD technique is the most favourable approach for wafer scale fabrication and real device production despite its drawbacks in terms of the sample’s uniformity. MXenes Transition metal nitrides, carbonitrides and carbides or commonly known as MXenes, are among the latest member of 2D family. MXene holds a chemical formula of Mn+1XnTx (n can be ranging from 1 to 3) where M represents the transition metals such as chromium (Cr), molybdenum (Mo), niobium (Nb), tantalum (Ta), vanadium (V), titanium (Ti), etc, X is the carbon and/or nitrogen and Tx is the surface terminations group for example oxygen (-O), fluorine (-F) or hydroxyl (-OH) terminations. As such, the typical structure of MXene is made up of stacking of n+1 layers of M covering the n layers of X under the arrangement of [MX]nM. In the simplest word, MXene exhibits the crystal pattern of hexagonal dense packing structure in which the M atoms are located in the outer layer and the X atoms sandwiched between two layers of M atoms. Generally, MXenes can be synthesized by selectively etching the A layers from their Mn+1AXn phase precursors, where A is aluminium (Al). The wet chemical etching of the A atomic layers from 3D MAX phase has been common practice in obtaining MXenes. This is due to the fact that the layer-to-layer bonding in MAX phase is much weaker than the intralayer, hence much more easily to be removed with the aid of an etchant element. Research has revealed that the etching agents and synthesis method have a direct effect on the structural defects, morphology, and surface groups of MXenes. In the typical procedure, the MAX phase is immersed in acid in favour of breaking the bonds between the M and A element. However, most of the synthesis methods cause the generation of intercalated water, poisonous gases, and abundant hydroxyl groups on the MXene surfaces as it mainly depends on water as the main solvent and fluoride-based compounds as etchants for the selective etching process [56]. Therefore, several novel methods have been introduced to overcome these issues, thus making an effective and greener exfoliation process, and with abundant terminations-containing MXenes. The examples of these novel methods include the use of fluoride-containing acidic solutions and their derivative methods [57], CVD based methods [58], hydrothermal synthesis [56], and alkali etching methods [59]. Despite the drawbacks, acid etching and its derivative methods are still being widely used in current research. Theoretically, MXenes possess infinite lateral dimensions with atomic scale thicknesses and terminations which allow each member of the MXene family to have different properties. In general, MXene has unique optical, electromagnetic,

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mechanical, and gas sensitive properties [60, 61]. Based on the experimental and theoretical results, it is evident that there are several factors affecting the exceptional properties of MXenes, for instance, the elemental abundance, diverse chemical compositions, tunable surface functionalities, high electrical conductivity, large surface dimensional ratio, and exceptional strength and stability [62]. MXenes have become one of the promising candidates for energy storage applications, electromagnetic interference shielding, and transparent conductive electrodes due to the fact that they possess the combined properties of good hydrophilicity and high electrical conductivities. Additionally, as MXene is a hybrid material, it exhibits properties that combine the aspects of both metals and ceramics in particular good conductivity, high damage tolerance, resistance to thermal shock, and readily machinable [63]. Interestingly, the nonlinear optical properties of MXene are adjustable with a large nonlinear absorption coefficient (10−13 esu) that is higher than any other 2D materials, thus making it a potential candidate as an optical modulator for generating the Q-switched and mode-locked laser [64]. Owing to their peculiar and fascinating properties, MXenes have been widely used in numerous applications especially in optoelectronics, energy storage, spintronics and catalysis, and environmental and biological fields [65]. Besides that, their substantial applications have been proven in various emerging fields such as in photoluminescent quantum dots [66], conversion of light-to-heat for energy harvesting [67], photothermal therapy [68], biosensors, chemical catalysts [69], water purification [70], composite reinforcement [71], electromagnetic interference shielding [72] and electrochemical capacitors [73]. OPERATING PRINCIPLES FOR PULSE GENERATION IN FIBER LASER TECHNOLOGY In order to develop pulsed output in fiber lasers, two techniques can be used. These techniques produce different types of pulses, where one technique produces slightly longer pulses with higher pulse energies while the other produces extremely short pulses with higher peak powers. Various methods can be utilized in both of these techniques, including the use of SAs based on 2D materials. Q-switching Technique Q-switching is a method used to produce high energy pulsed laser outputs by actively or passively manipulating the resonator loss. In other words, they modulate the quality factor (Q-factor) of a laser cavity. The Q-factor can be described by the following equation [74]:

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𝑸=

𝟐𝝅𝑬𝒔 𝑬𝒍

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(1)

whereby Es is the energy stored in the laser cavity and El is the energy emitted per cycle. From Equation 1, it can be discerned from the inverse relationship between the Q-factor and El that in a high cavity loss value, the Q-factor is small. As the loss is removed or reduced, the Q-factor quickly switches to a higher value. Effective Q-switching is determined by the pumping rate and the Q-switching mechanism. Firstly, the rate at which the gain media is pumped must be higher than the rate for the upper laser level to spontaneously decay. The high pumping rate ensures a sufficient build-up of population inversion in the cavity. Secondly, the mechanism of the Q-switching i.e., the resonator loss modulation must operate rapidly in comparison to the build-up of laser oscillations. The generation of the Q-switched pulses can be explained by discussing its gain and loss temporal evolution. During the initial stage of the Q-switching operation, a high loss level is maintained in the laser cavity that exceeds the condition for lasing. Light oscillation is obstructed, and no laser is generated from the cavity. With the gain medium continuously being pumped, the light trapped in the cavity significantly builds up the population inversion of the gain medium. At a certain threshold, the high cavity loss is rapidly reduced to a considerably small value. Following this, the gain exceeds the cavity loss that results in an instantaneous build-up of laser oscillation. The ‘stored’ energy is released in a large, single pulse that peaks when the gain drops to the same level as the cavity loss. The pulse is now capable of depopulating the upper lasing level that will drop the gain below the lasing threshold and cause the lasing action to stop. This process repeats to produce multiple pulse outputs that are called Q-switched pulse trains. The parameters that are used to measure the performance of Q-switched pulsed are the repetition rate, pulse width, pulse energy, and peak power. Generally, Qswitched lasers have repetition rates within the kilohertz (kHz) range meanwhile the pulse width is in the microsecond (µs) range. In comparison to mode-locked pulses, Q-switched pulses have a longer pulse width and a slower pulse repetition rate. This clear difference between both techniques is mainly due to the fact that Q-switching is dependent on the lifetime of the electron in the excited state within the gain medium. For example, an erbium-doped fiber gain media that has several millisecond lifetimes is not short enough to generate a high frequency Q-switched laser. However, Q-switching can be of value with its simplistic requirements whereas the generation of mode-locked lasers is sensitive to the balance between the intracavity dispersion and nonlinearity of the medium.

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As previously mentioned, the resonator losses can be modulated by active or passive methods, i.e. active and passive Q-switching. The active Q-switching technique involves the use of active components that require an external energy input, typically electrical energy, to drive the loss modulation. These active components are usually quite expensive, bulky, and must be connected to the laser cavity to create the loss mechanism, for example, electro-optic and acousto-optic modulator, and mechanical rotating chopper [75 - 77]. On the other hand, the passive Q-switching technique ‘self-drives’ the loss mechanism by taking advantage of the optical saturable absorption phenomenon. Here, the various SAs play their role, especially those formed by 2D materials such as TMDs, TIs, black phosphorus and MXenes [78 - 82]. These passive SAs induce Q-switching in the laser cavity quite differently than active Q-switching, whereby the operator ‘switches’ the loss value by modulating the external driving force. The initial state of passive Q-switching is similar to active Q-switching, whereby the loss mechanism is at a high level, thus inhibiting lasing. As light propagates in the cavity, the electrons populating the SA are excited to a higher energy level creating an extremely large population inversion. As the SA is continuously populated, its absorption drops which allow the gain of the cavity to rise and overcome the initial loss originating from the SA. Finally, the SA is bleached by the generated photons and emits a high energy pulse similar to the process mentioned earlier. The passive Q-switching technique can be desirable as it does not require a piece of external equipment to drive the loss modulation. The SAs also allow for a much more compact configuration at a lower cost. Mode-locking Technique Due to the fundamental nature of mode-locked laser generation and its application in various critical fields such as optical telecommunications, ultrafast sensing and spectroscopy, manufacturing, and biomedical research [83, 84], mode-locking has been the focus for laser researchers and engineers. In contrast to Q-switching, the mode-locking technique generates pulsed laser typically with ultrafast outputs that can reach extremely short pulses within the picosecond and femtosecond region. A mode-locked fiber laser can be obtained by ‘locking’ the multiple longitudinal modes oscillating within the cavity into a fixed mode relationship with each other. These ‘locked’ modes interfere with each other and lead to the formation of a stationary wave in the time and space domain that is observed as fast-running pulses of light This mode-locked output can be described in the time domain by considering the summation of all interfering lasing cavity modes: 𝒊(𝝎𝒏 𝒕+ ⌀𝒏 ) 𝑬(𝒕) = ∑𝑵 𝒏 𝑬𝒏 𝒆

(2)

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whereby the equation describes the amplitude En, angular frequency ωn, and phase φn values of the nth mode. Active and passive approaches can also be applied for mode-locking, as is the case with Q-switching. Active mode-locking, as with Q-switching, also requires external equipment such as amplitude modulators and RF signal generators. For ultrafast mode-locked laser generation, the passive method is much more preferred as active components are limited in the speed of their operation. In the passive method, mode-locking also utilizes SAs as the main mechanism, however, the formation of mode-locked pulses works based on a different principle than Qswitched pulses. The mode-locked laser operation starts at the continuous-wave (CW) lasing regime whereby the SA allows for noise spikes within the CW background to grow faster and gain energy while circulating within the cavity. As the noise spike continues to grow and collect enough energy, it starts to take over and saturate the gain of the cavity causing the CW background to decay. Amongst the noise spikes, the most energetic ones will experience the least absorption due to the effect of the SA and saturate the gain, consequently, eliminating all the competing spikes. This process goes on further until a single circulating pulse is generated. With each cavity round trip, the pulse shape or its width shortens because of the SA effect that favours the high energetic peaks of the pulse than the pulse wings. The pulse will continue to compress until the nonlinear dispersion effect of the light circulating medium induces pulse broadening that stops further pulse shortening. In some cases, strong pulse broadening that is present can even prevent the mode-locking process to start. CONFIGURATION OF 2D MATERIALS AS SATURABLE ABSORBERS In order to integrate 2D materials as saturable absorbers in fiber lasers, various configurations could be utilized. Generally, 2D materials are produced in small nanometre thin sheets that are unsuitable for direct application into laser systems. As such, these 2D materials must be utilized in a configuration that improves optical coupling to generate efficient laser output and ensure sufficient light and material interaction to get the desired effect. Optical coupling structures or photonic devices that have been constructed commonly utilize direct light material interaction or indirect interactions through evanescent wave coupling. One of these photonic devices is the use of photonic crystal fibers (PCFs). As shown in Fig. (5a), the hollow channel of the PCF is filled with the SA material to ensure direct optical coupling. However, the use of PCF together with 2D materials imposes some challenges since the core and air hole are smaller than the standard SMFs. This creates a hurdle to completely dry the solvent that is present inside the PCF core. Any trace amount of unwanted solvent will result in higher insertion loss and will deteriorate the performance of the generated laser. The

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dissimilar structure between PCF and SMF also creates an issue with inter-fiber splicing. Due to this, the PCF-based structure is difficult to be confidently reproduced.

Fig. (5). Configuration of 2D materials as SAs. a) In photonic crystal fibers (PCFs). Reproduced with permission from [85](reuse of material which is licensed under CC BY 4.0), b) Placed between two fiber ferrules. c) As a reflective surface d), Tapered and side-polished fibers.

Fig. (5b) shows another configuration that is simple yet useful for SA coupling. The SA (in its sheet form) is physically placed on the surface of the fiber ferrule and sandwiched with another ferrule to create a closed system. Light emitted from the fiber core will directly interact with the material, then be transmitted to the next fiber connection. A small piece of the material is enough to sufficiently cover the fiber core, which is ~9 µm in diameter. Using this approach it is crucial to be aware of the optical damage threshold of the material. Direct light shining on the material may accumulate heat and damage the material. This difficulty limits the possibility of this method, especially for high power laser applications. Another solution for optical coupling is inspired by the structure of SESAMs. As the name follows, SESAM has reflective structures in the form of gold or silver mirrors that are plated with SA materials as shown in Fig. (5c). The problem with working on reflective structures, the emitted light output from the fiber must be perpendicular to the surface of the mirror to ensure effective light coupling. The

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fiber surface may indirectly create scratches on the mirror surface as they inevitably touch. To alleviate the collimation and contact damage problems, the reflective structure and the SA material can be directly coated onto the surface of the optical fiber but at the cost of reduced efficiency. Fig. (5d) offers a better solution through indirect light interaction by exploiting the evanescent wave of light. By coating the 2D materials on the surface of an exposed fiber such as a microfiber or side-polished fiber, there will be a reduction in the risk of material damage with sufficient light and material interaction. This method also allows for a longer operation with less issue of heat accumulation and has been demonstrated for high-power operation in fiber laser technologies. GENERATION OF Q-SWITCHED PULSES IN FIBER Q-switched pulsed fiber lasers have drawn interest for applications in various fields due to their advantages, namely simplicity, cost-effectiveness and compactness [86, 87]. Pulsed fiber lasers are mostly used in diverse fields such as medicine, material processing, imaging and sensing [88 - 91]. As described in the previous section, Q-switching operations can be generated by two techniques; passively or actively. However, active Q-switched operation is not preferable due to the limitations of having a bulky component, high costing and complexity. Thus, the development of passive Q-switching in fiber laser systems has attracted attention among researchers, especially for applications that require a low-cost and compact system. There have been many previous works reported on the use of 2D materials as SAs operating in various wavelength regions. In this chapter, Q-switched pulsed fiber lasers in the 1.0 μm, 1.5 μm and 2.0 µm wavelength regions will be discussed in terms of the pulsed characteristics with a material consisting of graphene, TMDs, TIs and transition metal oxides (TMOs) in these wavelength regions. The 1.0 μm Wavelength Region For lasing in the 1.0 μm wavelength region, most optical fiber laser cavities utilise ytterbium-doped fibers (YDFs) as the primary gain medium. A typical YDF based laser that is configured to operate at the 1.0 μm wavelength region is shown in Fig. (6).

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Fig. (6). Schematic diagram of Q-switched pulsed fiber laser using YDF and Bi2Te3 SA. (Reprinted with permission from [92]: Salim, M., et al., Bi2Te3 based passively Q-switched at 1042.76 and 1047 nm wavelength. Laser Physics, 2017. 27(12): p. 125102. DOI: 10.1088/1555-6611/aa8577, ©Astro Ltd. Reproduced by permission of IOP Publishing Ltd. All rights reserved)

Table 1 shows the various works that have been reported for the generation of passively Q-switched pulsed fiber lasers in the 1.0 μm wavelength region. These pulsed 1.0 μm fiber laser systems were demonstrated by using a different type of materials such as the SAs. This table focuses on generating Q-switched pulses using ytterbium-doped fiber as the gain medium. As shown in Table 1, the materials which act as an SA have been reported by previous researchers on generating Q-switched pulsed fiber laser in 1.0 μm wavelength region including graphene, TMDs, TIs and TMOs. Graphene reported by L. Zhang et al. [93] generates the highest maximum repetition rate among these reported works in Table 1. Three types of TMDs act as SA are included in the table; molybdenum disulphide (MoS2), molybdenum diselenide (MoSe2) and platinum ditelluride (PtTe2). A previous work by Woodward et al. [94] reported the shortest minimum pulsed duration and the highest maximum average pulse energy by using MoSe2 as compared to other TMDs based SAs. Bismuth telluride (Bi2Te3) and bismuth selenide (Bi2Se3) are two types of TIs that were used as SAs by previous research on generating the Qswitched pulsed in fiber laser system. From the table, Bi2Se3 [95] reports a higher maximum output power as compared to material Bi2Te3 [92]. On the other hand, lutetium oxide (Lu2O3) [96] and iron(II, III) oxide (Fe3O4) [97] are from the group materials known as TMO. Both materials are able to generate Q-switched pulses with a central wavelength in the 1.0 μm wavelength region. However, the Lu2O3-based SA reports a higher maximum average pulse energy as compared to the other.

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Table 1. Characteristics of passively Q-switched pulsed fiber laser in the 1.0 μm wavelength region.

Type of Material

Centre Wavelength

Repetition Rate

Pulse Duration

Maximum Output Power

Maximum Average Pulse Energy

Ref

Graphene

~1027.03 nm

28.9 kHz – 110 kHz

~3.2 μs -1.3 μs

15.6 mW

141.8 nJ

[93]

MoS2

~1055 nm

6.53 kHz-89.0 kHz

2.68- 4.4 μs

~10.5 mW

100 nJ

[98]

MoSe2

1060 nm

60.0 kHz-74.9 kHz

2.8 μs-1.1μs

8.72 mW

116 nJ

[94]

PtTe2

1066 nm

23.0-33.5 kHz

13.6-5.2 μs

2.48 mW

74 nJ

[99]

Bi2Te3

1042.76 nm, 1047.00 nm

3.79 kHz15.63 kHz

38.40 μs – 96.12 μs

0.06 mW

3.82 nJ

[92]

Bi2Se3

1050.4 nm

23 kHz – 46 kHz

5.44 μs – 13.4 μs

117 mW

89 nJ

[95]

Lu2O3

1067.8 nm

26.25 kHz – 46.68 kHz

4.59 μs – 9.36 μs

~ 2.2 mW – 6.0 mW

128 nJ

[96]

Fe3O4

1039 nm

25.88 kHz – 47.33 kHz

3.78 μs -9.6 μs

~ 0.38 mW 1.01 mW

21.29 nJ

[97]

TMD

Topological insulator

TMO

The 1.5 μm Wavelength Region On the other hand, fiber lasers operating at the 1.5 μm wavelength region use erbium-doped fibers (EDFs) as the gain medium to generate the desired laser output. Table 2 shows previous works reported on the generation of passively Qswitched pulsed fiber laser in the 1.5 μm wavelength region by using a different type of material as SAs. This table summarizes reported works on Q-switched pulses using EDFs as the gain medium and using various 2D materials as the SAs. Graphene, as reported by Ahmad et al. [96] generate the highest maximum repetition rate among these reported works in Table 2. Three types of TMDs that function as SAs are included in the table; MoS2, MoSe2 and palladium disulfide (PdS2). A similar work by Xun Wu et al. [100] reported the shortest minimum pulsed duration and the highest maximum output power by using MoSe2 as compared to other TMD-based SAs. Another group of materials that have attracted attention as SAs are TIs and TMOs. Bi2Te3 and Bi2Se3 are two types of TIs that have been used as SAs in a previous research for generating the Qswitched pulsed in the fiber laser system. From the table, Bi2Se3 [95] reports a higher maximum output power as compared to material Bi2Te3 [101]. Similarly, copper (II) oxide (CuO) [102] and Fe3O4 [103] are TMOs that have been shown to

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be able to generate Q-switched pulsed at the 1.5 μm wavelength region. However, Lu2O3-based SA achieved a higher maximum average pulse energy as compared to the others. Table 2. Characteristics of passively Q-switched pulsed fiber laser in the 1.5 μm wavelength region.

Type of Material

Centre Wavelength

Repetition Rate

Pulse Duration

Maximum Output Power

Maximum Average Pulse Energy

Ref

Graphene

~1551.66 nm

1.387 kHz – 208.00 kHz

~0.412 μs 94.8 μs

94.0 mW

16.26 nJ

[104]

MoS2

~1560 nm

7.758 kHz41.452 kHz

13.534-9.92 μs

~0.77 mW

184.7 nJ

[105]

MoSe2

1563 nm

45 kHz-89 kHz

4.9μs-1.3μs

1.79 mW

-

[100]

PdS2

1567 nm

17.2-26.0 kHz

12.6-4.5 μs

0.393 mW

15.1 nJ

[106]

Bi2Te3

1566.9 nm

2.154 kHz12.82 kHz

49 μs – 13 μs

19.56 mW

1.525 μJ

[101]

Bi2Se3

1560.33

14.96 kHz – 62.5 kHz

2.1 μs – 7.56 μs

112 mW

6.1 nJ

[95]

CuO

1560.0 nm

69 kHz – 83 kHz

4.8 μs – 2.6 μs

5.5 mW

66 nJ

[102]

Fe3O4

1558.6 nm

21.7 kHz – 128.2 kHz

2.44 μs -613 ns

41.2 mW

321.3 nJ

[103]

TMD

Topological insulator

TMO

The 2.0 μm Wavelength Region The schematic diagram of a typical fiber laser system in the 2.0 μm wavelength region is shown in Fig. (7). In this type of laser, thulium-doped fibers (TDFs) are typically used as the gain medium with SAs to generate a mode-locked output due to their wide emission bandwidth from 1700 to 2100 nm. (Reprinted from Harith Ahmad, et al., Ternary MoWSe2 alloy saturable absorber for passively Q-switched Yb-, Er-and Tm-doped fiber laser. Optics Communications, 2019. 437: p. 355-362. Copyright 2019, with permission from Elsevier) Table 3 shows reported works on the generation of passively Q-switched pulsed fiber laser in the 2.0 μm wavelength region by using a different type of materials as the SAs. The Q-switching operations obtained using these materials were obtained in fiber lasers that utilized TDFs and THDFs as the gain media.

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Fig. (7). Schematic diagram of Q-switched pulsed fiber lasers using TDF and MoWSe2 SA [107]. Table 3. Characteristics of passively Q-switched pulsed fiber laser in 2.0 μm Wavelength Region.

Type of Material

Centre wavelength

Repetition Rate

Pulse Duration

Maximum Output Power

Maximum Average Pulse Energy

Ref

Graphene

2010.4 nm

53 kHz (Max)

1.4 μs (Min)

4.5 mW

85 nJ

[108]

MoWS2

~1985 nm

36.3 kHz (Max)

2.8 μs (Min)

-

86.4 nJ

[112]

MoWSe2

1964 nm

5.2 mW

85.3 nJ

[107]

Bi2Te3

1957.6 nm

13.51 kHz27.7 kHz

4.52 μs – 2.22 μs

-

0.94 μJ

[110]

Bi2Se3

1980 nm

8.4 kHz – 26.8 kHz

4.18 μs (Min)

8.4 mW

313 nJ

[109]

Sm2O3

1935.25 nm

69 kHz – 83 kHz

4.8 μs – 2.6 μs

5.5 mW

66 nJ

[111]

TMD

Topological insulator

TMO

16.6 kHz-61.5 8 μs-2.4 μs kHz

Jiang Liu et al. [108] generate the shortest minimum pulse duration among these reported works in Table 3 using graphene SA. In the case of TMDs, two types of TMDs were used as SA as indicated in the table; MoWS2 and MoWSe2. A previous work by H. Ahmad et al. [107] reported the higher maximum repetition rate and shorter minimum pulse duration by using MoWSe2 as compared to other TMD based SAs. Bi2Se3 [109] was reported to have a higher maximum average pulse energy as compared to material Bi2Te3 [110], while, Sm2O3 [111] was able to generate Q-switched pulsed at with a central wavelength in 2.0 μm wavelength region.

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GENERATION OF MODE-LOCKED PULSES IN FIBER LASERS Mode-locked systems produce ultra-short laser pulses by locking in multiple longitudinal modes in the laser system. These pulses have a duration varying from a few picoseconds to femtoseconds and have become favourable for use in many applications across various fields such as optical fiber communication, biomedical diagnostics, material processing and supercontinuum [113, 114]. This is due to the advantages offered in mode-locked systems that include excellent heat dissipation, an alignment-free system, compact size and the ability to generate a high quality pulse with picosecond durations [115, 116]. Similar to the case of Q-switching, SAs are typically used as nonlinear optical devices to passively induce mode-locking operation in fiber laser systems. They are preferred because of their compactness, simplicity and adaptability as well as their low operating costs as they do not require external active devices to optically modulate the intra-cavity signal. In this regard, there have been various demonstrations of 2D materials as SAs in mode-locked fiber lasers. In this section, mode-locked pulsed fiber lasers operating at three different wavelength regions, namely the 1.0, 1.5 and 2.0 μm, will be discussed in terms of their performance and characteristics using SAs based on graphene, TMDs, TIs and also TMO. The 1.0 μm Wavelength Region As mentioned in the previous section, YDFs are typically used as the gain medium to generate lasers in 1.0 μm due to their ability to provide a broad emission from around 975 to 1200 nm [117]. Fig. (8) shows the schematic diagram of a typical mode-locked fiber laser using YDF as the gain medium and using a 2D material-based SA as the mode-locker. In this particular setup, Cheng et al. [118] demonstrated the use of a PdS2 that was coated onto a D-shaped fiber to obtain a mode-locking operation at 1033 nm. The PdS2, which belongs to the family of TMD, had successfully generated stable mode-locked pulses in the picosecond range with a high signal-to-noise ratio (SNR) of 65 dB. Additionally, Rusdi et al. also obtained a mode-locking operation at 1090 nm using a MoS2 from the family of TMD. The MoS2 was prepared through the mechanical exfoliation method and then integrated into a YDF laser cavity to perform as the SA. These results show the reliability of using TMDs in fiber laser systems and for further use in optoelectronics applications as they have unique optical, mechanical, and electrical capabilities.

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Fig. (8). Schematic diagram of a typical mode-locked pulsed fiber laser using YDF and (a) PdS2 SA [118].(reuse of material which is licensed under CC BY 4.0).

Apart from the use of TMDs in 1.0 μm fiber lasers, other 2D materials such as graphene, TIs and TMOs were also reported and demonstrated as SAs for generating mode-locked pulses. Table 4 shows some of the various parameters of works reported on the passive generation of mode-locked outputs at the 1.0 μm wavelength region using these 2D materials. From the table, it is seen that the shortest pulse width of 1.67 ps was obtained using a tungsten trioxide (WO3) SA from the TMO family. In this demonstration by Al-Hiti et al. [119], the WO3 was fabricated as a thin film composite and easily inserted into the YDF laser cavity to also obtain stable mode-locked pulses with a comparable SNR of 55 dB. The use of TIs such as Bi2Se3 and Bi2Te3 as SAs produced mode-locked pulses with the highest pulse energy of 7.23 nJ and the highest output power of 33.7 mW, respectively. Dou et al. [120] prepared the Bi2Se3 by using a unique method to place the pure TI onto the fiber end facet. This method eliminates the issue of having a low damage threshold in thin film forms, which relieves heat deposition and thus improves the damage threshold of the SA.

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Table 4. Characteristics of passively mode-locked pulsed fiber lasers in the 1.0 μm wavelength region. Operating Repetition Output SNR Pump Pulse Pulse Rate Power Value Power Width Energy (MHz) (mW) (dB) (mW)

Group of material

Centre Wavelength (nm)

Graphene

1069.8

115

0.9

580 ps 0.41 nJ

0.37

-

[121]

PdS2

1033

160

24.4

375 ps 0.64 nJ

15.7

65

[118]

MoS2

1090

689

13.2

32.5 ns

1.48 nJ

20

29

[122]

Bi2Se3

1031.7

153

44.6

46 ps

0.756 nJ

33.7

58

[120]

Bi2Te3

1064.3

160

1.10

1.11 ns

7.23 nJ

1.41

64

[123]

WO3

1065

92

10

1.67 ps

2.16 nJ

21.64

55

[119]

TMD

Topological insulator

TMO

Ref

The 1.5 μm Wavelength Region Demonstrations of mode-locking operation in the 1.5 μm were obtained in EDF laser cavities with the addition of 2D material-based SAs. Table 5 summarizes the works on the generation of passively mode-locked pulsed fiber lasers in 1.5 μm using a different type of material as the SA. Graphene is one of the 2D materials that have long attracted attention in a variety of photonics applications, especially in pulsed fiber laser systems. This is because of its broadband saturable absorption and its simplicity of fabrication. Zhang et al. [124] obtained a mode-locked fiber laser at a center wavelength of 1576.3 nm using an atomic layer graphene with a narrow pulse width of 415 fs and high pulse energy of 7.3 nJ. From the TMD family, Liu et al. [125] achieved a mode-locking operation with the shortest pulse width of 163.5 fs and the highest SNR of 96 dB using a WSe2 that was prepared by CVD and then deposited onto a tapered fiber. A similar method was also carried out by Liu et al. using a MoSe2 [126], which produced mode-locked pulses with a comparable pulse width of 207 fs and SNR of 85 dB. These results further show the suitability of TMDs not only for generating modelocked pulses in the 1.0 μm but in the 1.5 μm as well. In addition to the use of graphene and TMDs for the generation of mode-locked fiber lasers in 1.5 μm, TIs and TMOs have also been demonstrated as SAs in EDF mode-locked laser cavities. For example, Sotor et al. [127] utilized an antimony telluride (Sb2Te3)-deposited tapered fiber to generate mode-locked pulses at 1561 nm with a pulse width of 270 fs. However, the pulse energy of 29 pJ obtained was

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lower compared to the value obtained by the graphene SA, which was likely due to the low operating pump power that causes a low output power. Despite this, the performance of the Sb2Te3 in terms of the pulse width and the pulse energy was still better compared to the use of Bi2Se3 [127]. Furthermore, the use of TMOs such as holmium(III) oxide (Ho2O3) [128] and WO3 [128] has also been successful in generating mode-locked pulses in the 1.5 μm wavelength region, albeit with a longer pulse width of 650 fs and 2.69 ps, respectively. Table 5. Characteristics of passively mode-locked fiber laser in the 1.5 μm wavelength region. Operating Repetition Output SNR Pump Pulse Pulse Rate Power Value Power Width Energy (MHz) (mW) (dB) (mW)

Group of material

Centre Wavelength (nm)

Graphene

1576.3

130

6.84

415 fs

7.3 nJ

-

65

[124]

WSe2

1557.4

630

63.133

163.5 fs

-

28.5

96

[125]

MoSe2

1552

630

64.56

207 fs

-

-

85

[126]

Sb2Te3

1561

30

34.58

270

29 pJ

1

70

[127]

Bi2Se3

1562.4

69.2

22.6

630 fs 15.3 pJ

-

70.1

[129]

Ho2O3

1565.4

62

17.1

650 fs

0.524 nJ

9.01 mW

75

[130]

WO3

1565.08

50

1.85

2.69 ps

1.14 nJ

2.12

53

[128]

TMD Topological insulator

TMO

Ref

The 2.0 μm Wavelength Region In the case of generating mode-locked pulses in the 2.0 μm wavelength region, TDFs are also used as the gain medium in fiber laser systems that have been integrated with SAs. An example of the experimental setup for the 2.0 μm modelocked fiber laser is shown in Fig. (9), where the setup utilized a dual-pump configuration to obtain a higher pump power. Table 6 shows previous reports on the generation of passively mode-locked pulsed fiber laser in 2.0 μm wavelength region by using a different type of 2D materials as the SAs. From the table, it is seen that the shortest pulse width generated for the mode-locking operation was obtained using the graphene-based SA by Sobon et al. [131]. The graphene was fabricated by the CVD on a copper substrate and later immersing it in a poly(methylmethacrylate) solution (PMMA), generating a stable mode-locked output at 1876 nm. A MoSe2-based saturable absorber (SA) in the form of a thin film [132] was also reported to generate mode-locked pulses at 1943.35 nm with a slightly longer pulse width of 980 fs

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compared to graphene. However, the pulse width obtained using the MoSe2 was still shorter compared to another demonstration from the TMD family using WSe2 [133], where the pulse width obtained for the latter was 1.16 ps. The use of other 2D materials such as Bi2Te3 [133], ZnO [134] and WO3 [135] was proven as good SAs, in which they were successful in generating mode-locked outputs at wavelengths above 1900 nm. The pulse width obtained using these SAs however, is slightly longer at around 1 to 1.26 ps.

Fig. (9). Schematic diagram of a mode-locked pulsed fiber laser using TDF and ZnO SA [134]. (Reprinted from Harith Ahmad, et al., Mode-locked thulium doped fiber laser with zinc oxide saturable absorber for 2 μm operation. Infrared Physics & Technology, 2019. 97: p. 142-148., Copyright 2019, with permission from Elsevier). Table 6. Characteristics of passively mode-locked pulsed fiber laser in the 2.0 μm wavelength region. Operating Repetition Pulse Output SNR Pump Pulse Rate Energy Power Value Power Width (MHz) (J) (mW) (dB) (mW)

Group of material

Centre Wavelength (nm)

Graphene

1876

215

41

1943.35

500

23.53

MoSe2 TMD Topological insulator

603 fs

-

980 fs 0.39 nJ

Ref

1.5

70

[131]

8.2

65

[132]

WSe2

1863.96

650

11.36

1.16 ps

Bi2Te3

1909.5

315

21.5

1.26 ps

-

~2

52

[133]

ZnO

1945.45

113

11.36

1.15 ps

56.33 pJ

0.64

50.5

[134]

WO3

1955.3

463

10.98

1.26 ps

0.59 nJ

6.47

54.8

[135]

TMO

-

32.5

53

[136]

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(Reprinted from Harith Ahmad, et al., Mode-locked thulium doped fiber laser with zinc oxide saturable absorber for 2 μm operation. Infrared Physics & Technology, 2019. 97: p. 142-148., Copyright 2019, with permission from Elsevier) CONCLUSION, CHALLENGES, AND FUTURE PERSPECTIVES In this chapter, the application of MMs, TMDs and MXenes was reviewed from the viewpoint of fiber laser technologies. The utilization of these materials as SAs in a wide range of fiber laser systems including Ytterbium-, Erbium- and Thulium-doped fiber based lasers is discussed. Based on the current achievements in the fiber laser research using these materials, it can be easily concluded that MMs, TMDs and MXenes can perform as SAs throughout the wavelength range of 1.0 μm to 2.0 μm and beyond. These serve as the motivation to further explore 2D materials for practical use in commercial products. Nevertheless, more research needs to be done to study the stability performance of these materials at high power laser systems as well as the repeatability of the laser output generated using these materials. In addition, the current research on the application of MMs, TMDs and MXenes as SAs in fiber laser technology is mainly focussing on the experimental demonstration of pulsed laser generation. Further comprehensive studies need to be done in future to study the carrier dynamics of these materials and their effects on the performance of the pulsed laser. By knowing the correlation between the carrier dynamics of these saturable absorbers and the output of the pulsed fiber laser, further optimization can be done to improve the performance of the pulsed laser to meet the requirement for practical use. Besides, various material processing techniques have been explored today to synthesise high quality 2D materials with simple facilities and low production cost. These techniques include various top-down and bottom-up approaches which have different advantages and drawbacks. Despite the wide exploration of the various material processing techniques, further improvements on the current techniques are required for large-scale production of high quality 2D materials for commercialization as the currently available techniques are having limitations for producing high quality, uniform 2D materials at a large scale. Therefore, future research should also focus on identifying a material processing technique that can strike a balance between output quality, repeatability, and the production scale of 2D materials. CONSENT FOR PUBLICATION Not applicable.

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Commun., vol. 437, pp. 355-362, 2019. [http://dx.doi.org/10.1016/j.optcom.2019.01.009] [108] J. Liu, J. Xu, P. Wang, D. Popa, T. Hasan, Z. Sun, W.B. Cho, and A.C. Ferrari, "Graphene-based passively Q-switched 2μm thulium-doped fiber laser", Opt. Commun., vol. 285, no. 24, pp. 5319-5322, 2012. [http://dx.doi.org/10.1016/j.optcom.2012.07.063] [109] Z. Luo, C. Liu, Y. Huang, D. Wu, J. Wu, H. Xu, Z. Cai, Z. Lin, L. Sun, and J. Weng, "Topologicalinsulator passively Q-switched double-clad Fiber laser at 2$\mu $ m wavelength", IEEE J. Sel. Top. Quantum Electron., vol. 20, no. 5, pp. 1-8, 2014. [http://dx.doi.org/10.1109/JSTQE.2014.2305834] [110] H.M.A. Nur, F. Ahmad, N.F.Z. Siti, M.H. Ibrahim, and S.W. Harun, "Bismuth (III) telluride (Bi2Te3) embedded in PVA as a passive saturable absorber in a 2 micron region", Photonics Lett. Pol., vol. 8, no. 4, pp. 101-103, 2016. [http://dx.doi.org/10.4302/plp.2016.4.04] [111] K.N. Rusdi, M.B. Hisyam, M.F.M. Rusdi, N.F. Zulkipli, A.A. Latiff, S.W. Harun, and N. Saidin, "Samarium (III) oxide thin film as a saturable absorber for the passively Q-switched Tm-doped fiber laser", Journal of Physics: Conference Series., IOP Publishing, 2019. [http://dx.doi.org/10.1088/1742-6596/1371/1/012026] [112] H. Ahmad, A.S. Sharbirin, and M.F. Ismail, "Molybdenum tungsten disulphide (MoWS2) as a saturable absorber for a passively Q-switched thulium/holmium-codoped fibre laser", J. Mod. Opt., vol. 66, no. 11, pp. 1163-1171, 2019. [http://dx.doi.org/10.1080/09500340.2019.1609612] [113] Y.X. Guo, X.H. Li, P. Guo, and H. Zheng, "Supercontinuum generation in an Er-doped figure-eight passively mode-locked fiber laser", Opt. Express, vol. 26, no. 8, pp. 9893-9900, 2018. [http://dx.doi.org/10.1364/OE.26.009893] [PMID: 29715934] [114] S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, M. Jablonski, and S.Y. Set, "Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers", Opt. Lett., vol. 29, no. 14, pp. 1581-1583, 2004. [http://dx.doi.org/10.1364/OL.29.001581] [PMID: 15309825] [115] Z. Zhang, C. Mou, Z. Yan, K. Zhou, L. Zhang, and S. Turitsyn, "Sub-100 fs mode-locked erbiumdoped fiber laser using a 45°-tilted fiber grating", Opt. Express, vol. 21, no. 23, pp. 28297-28303, 2013. [http://dx.doi.org/10.1364/OE.21.028297] [PMID: 24514340] [116] S. Yan, J. Zhang, J. Zhou, and W. Zhao, "Characteristics of active mode-locked fiber ring laser based on semiconductor optical amplifier", Passive Components and Fiber-based Devices V., International Society for Optics and Photonics, 2008. [http://dx.doi.org/10.1117/12.802517] [117] R. Paschotta, J. Nilsson, A.C. Tropper, and D.C. Hanna, "Ytterbium-doped fiber amplifiers", IEEE J. Quantum Electron., vol. 33, no. 7, pp. 1049-1056, 1997. [http://dx.doi.org/10.1109/3.594865] [118] P.K. Cheng, S. Liu, S. Ahmed, J. Qu, J. Qiao, Q. Wen, and Y.H. Tsang, "Ultrafast Yb-Doped Fiber Laser Using Few Layers of PdS2 Saturable Absorber", Nanomaterials (Basel), vol. 10, no. 12, p. 2441, 2020. [http://dx.doi.org/10.3390/nano10122441] [PMID: 33291350] [119] A.S. Al-Hiti, A.H.H. Al-Masoodi, M.M. Najm, M. Yasin, and S.W. Harun, "Passively mode-locked laser at 1μm region based on tungsten trioxide (WO3) saturable absorber", Optik (Stuttg.), vol. 231, 2021.166377 [http://dx.doi.org/10.1016/j.ijleo.2021.166377] [120] Z. Dou, Y. Song, J. Tian, J. Liu, Z. Yu, and X. Fang, "Mode-locked ytterbium-doped fiber laser based

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on topological insulator: Bi_2Se_3", Opt. Express, vol. 22, no. 20, pp. 24055-24061, 2014. [http://dx.doi.org/10.1364/OE.22.024055] [PMID: 25321981] [121] L.M. Zhao, D.Y. Tang, H. Zhang, X. Wu, Q. Bao, and K.P. Loh, "Dissipative soliton operation of an ytterbium-doped fiber laser mode locked with atomic multilayer graphene", Opt. Lett., vol. 35, no. 21, pp. 3622-3624, 2010. [http://dx.doi.org/10.1364/OL.35.003622] [PMID: 21042370] [122] M.F.M. Rusdi, A.A. Latiff, E. Hanafi, M.B.H. Mahyuddin, H. Shamsudin, K. Dimyati, and S.W. Harun, "Molybdenum disulphide tape saturable absorber for mode-locked double-clad ytterbiumdoped all-fiber laser generation", Chin. Phys. Lett., vol. 33, no. 11, 2016.114201 [http://dx.doi.org/10.1088/0256-307X/33/11/114201] [123] Y. Peiguang, L. Rongyong, Z. Han, W. Zhiteng, C. Han, and R. Shuangchen, "Multi-pulses dynamic patterns in a topological insulator mode-locked ytterbium-doped fiber laser", Opt. Commun., vol. 335, pp. 65-72, 2015. [http://dx.doi.org/10.1016/j.optcom.2014.09.009] [124] H. Zhang, D.Y. Tang, L.M. Zhao, Q.L. Bao, and K.P. Loh, "Large energy mode locking of an erbiumdoped fiber laser with atomic layer graphene", Opt. Express, vol. 17, no. 20, pp. 17630-17635, 2009. [http://dx.doi.org/10.1364/OE.17.017630] [PMID: 19907547] [125] W. Liu, M. Liu, Y. OuYang, H. Hou, G. Ma, M. Lei, and Z. Wei, "Tungsten diselenide for modelocked erbium-doped fiber lasers with short pulse duration", Nanotechnology, vol. 29, no. 17, 2018.174002 [http://dx.doi.org/10.1088/1361-6528/aaae40] [PMID: 29424706] [126] W. Liu, M. Liu, Y. OuYang, H. Hou, M. Lei, and Z. Wei, "CVD-grown MoSe 2 with high modulation depth for ultrafast mode-locked erbium-doped fiber laser", Nanotechnology, vol. 29, no. 39, 2018.394002 [http://dx.doi.org/10.1088/1361-6528/aad0b3] [PMID: 29968568] [127] J. Sotor, G. Sobon, K. Grodecki, and K.M. Abramski, "Mode-locked erbium-doped fiber laser based on evanescent field interaction with Sb2Te3 topological insulator", Appl. Phys. Lett., vol. 104, no. 25, 2014.251112 [http://dx.doi.org/10.1063/1.4885371] [128] A.S. Al-Hiti, A.H.H. Al-Masoodi, S.W. Harun, M. Yasin, and W.R. Wong, "Tungsten trioxide (WO3) film absorber for generating soliton mode-locked pulses in erbium laser", Opt. Laser Technol., vol. 131, 2020.106429 [http://dx.doi.org/10.1016/j.optlastec.2020.106429] [129] H. Haris, H. Arof, A.R. Muhammad, C.L. Anyi, S.J. Tan, N. Kasim, S.W. Harun, and S. Fu, "Passively Q-switched and mode-locked Erbium-doped fiber laser with topological insulator Bismuth Selenide (Bi2Se3) as saturable absorber at C-band region", Opt. Fiber Technol., vol. 48, pp. 117-122, 2019. [http://dx.doi.org/10.1016/j.yofte.2018.12.002] [130] A. Shakir Al-Hiti, M.F.A. Rahman, S.W. Harun, P. Yupapin, and M. Yasin, "Holmium oxide thin film as a saturable absorber for generating Q-switched and mode-locked erbium-doped fiber lasers", Opt. Fiber Technol., vol. 52, 2019.101996 [http://dx.doi.org/10.1016/j.yofte.2019.101996] [131] G. Sobon, J. Sotor, I. Pasternak, A. Krajewska, W. Strupinski, and K.M. Abramski, "All-polarization maintaining, graphene-based femtosecond Tm-doped all-fiber laser", Opt. Express, vol. 23, no. 7, pp. 9339-9346, 2015. [http://dx.doi.org/10.1364/OE.23.009339] [PMID: 25968764] [132] M. Wu, X. Li, K. Wu, D. Wu, S. Dai, T. Xu, and Q. Nie, "All-fiber 2 μm thulium-doped mode-locked fiber laser based on MoSe2-saturable absorber", Opt. Fiber Technol., vol. 47, pp. 152-157, 2019. [http://dx.doi.org/10.1016/j.yofte.2018.11.032]

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[133] K. Yin, B. Zhang, L. Li, T. Jiang, X. Zhou, and J. Hou, "Soliton mode-locked fiber laser based on topological insulator Bi_2Te_3 nanosheets at 2 μm", Photon. Res., vol. 3, no. 3, pp. 72-76, 2015. [http://dx.doi.org/10.1364/PRJ.3.000072] [134] H. Ahmad, M.Z. Samion, A.A. Kamely, and M.F. Ismail, "Mode-locked thulium doped fiber laser with zinc oxide saturable absorber for 2 μm operation", Infrared Phys. Technol., vol. 97, pp. 142-148, 2019. [http://dx.doi.org/10.1016/j.infrared.2018.12.037] [135] M.H.M. Ahmed, W.A. Khaleel, S.A. Sadeq, M.A.W. Abdul Hadi, N.H. Zainol Abidin, and M.A. Mahdi, "Mode-locked thulium doped fiber laser utilizing tungsten trioxide saturable absorber", Opt. Laser Technol., vol. 136, 2021.106730 [http://dx.doi.org/10.1016/j.optlastec.2020.106730] [136] S. Hou, "Lau Chengjin, Lin Haifeng, Wang Jinzhang, Guo Chunyu, Cheng Jianqun, Zhang Min, and Yan Peiguang, Ultrafast thulium-doped fiber laser mode locked by monolayer WSe 2", IEEE J. Sel. Top. Quantum Electron., vol. 24, no. 3, pp. 1-6, 2017.

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CHAPTER 11

Optical Properties of Hollow-Core Bragg Fiber Waveguides Ritesh Kumar Chourasia1,3,*, Nitesh K. Chourasia2 and Narendra Bihari1,3 University Department of Physics, Lalit Narayan Mithila University, Darbhanga-846004, India School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, India 3 Department of Physics, Samastipur College, Samastipur-848134 (A constituent college of L.N.M.U. Darbhanga-846004, Bihar, India) 1 2

Abstract: The propagation and dispersion properties of hollow-core Bragg fibre waveguides for both high and low refractive index contrasts of cladding materials are explored and compared in this chapter using two design wavelengths: 1550 nm in the near-infrared area and 632.8nm in the visible range. The boundary matching approach was used to build a relationship between the incoming and outgoing light waves employing the transfer matrix method. The observed photonic band gaps are somewhat substantial in high refractive index contrast cladding Bragg fibre waveguides, i.e. HRBFW, and low periodic cladding layers are required to achieve a perfect photonic bandgap. The spectrum range and spectral location of photonic band gaps in both HRBFW and low refractive index contrast cladding Bragg fibre waveguides, i.e. LRBFW, are substantially dependent on the angle of incidence of a light beam, i.e. the optical path of the incident light. The sensitivity of the Bragg fibre waveguide for sensing applications may be determined by measuring the thickness of the photonic bandgap or the spectral shift of the photonic bandgap. HRBFW seems to have a high sensitivity when considering the change in spectral bandwidth of photonic bandgap with core refractive index, which grows with increasing design wavelength. LRBFW has a much higher sensitivity than HRBFW when considering the LBE (Left band edge) and RBE (Right band edge), hence it is suggested for sensing applications. HRBFW directed a greater number of modes than LRBFW, according to the assessment of dispersion characteristics.

Keywords: Photonic nanostructures, Transfer matrix method, Hankel formalism, Sensitivity, Photonic Bandgap. Corresponding author Ritesh Kumar Chourasia: Department of Physics, Samastipur College, Samastipur-848134 (A constituent college of L.N.M.U. Darbhanga-846004, Bihar, India); E-mails: [email protected], [email protected] *

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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INTRODUCTION AND MOTIVATION Due to its advantageous features over traditional fibers, Bragg gratings and the fiber based on other photonic crystals, researchers have been enamoured with Bragg fiber waveguides during the last decade. The theoretical introduction of the Bragg fiber waveguide by Yeh et al. [1 - 3] was followed by the design and viability investigation for optical communications by various investigators, including Doran and Bluow in 1983 [4]. However, towards the turn of the twentyfirst century, an experimental platform for such a waveguide was proven. In a nutshell, a Bragg fiber is a concentric circular cladding in length that surrounds a low index cylindrical dielectric or material free core in the middle. These concentric circular claddings are designed by a periodic arrangement of two different bi-layer materials that are transparent in the visible and near-infrared region. The propagation mechanism of electromagnetic waves (EM waves) in Bragg waveguides is completely different from that of ordinary fiber waveguides based on total internal reflections. The thickness of the layers in the Bragg fiber waveguide is designed to accept the quarter-wave condition. Recurrent Bragg reflections from claddings propagate the EM wave in such waveguides, and total reflections are concentrated in the low index core and further move towards the end of the fiber. Birefringence effect, polarisation effect, dispersion non-linearity, material dispersion, distortion, and various losses due to confinement, modal dispersion and further unusual radiation [4 - 17], which happen often in various classic fibers, are virtually eliminated in such waveguides because of the material free core. In addition, the Bragg fiber waveguide's hollow or low index core makes it a strong contender for a range of other intriguing applications. Biosensors [18 - 25], chemical gas sensors [26], strain sensors [19, 27, 28], and other devices based on Bragg fiber waveguide have recently been created. In addition to these uses, narrowband transmission filters, optical de-multiplexers [28 - 32], temperature sensors [33], and other optoelectronic applications of these waveguides exist. Researchers, on the other hand, have built and investigated the various characteristics of 1-D cylindrical photonic crystals, which are most similar to the design mechanism of Bragg fiber [34 - 36], due to their ease of production (just one step) [35]. Various types of Bragg fiber fabrication are also simple in this context. Researchers have recently been intrigued by the possibility of fabricating such fibers utilising solvent evaporation fiber rolling and drawing techniques [37, 38]. In the drawing approach, they first produced a bilayer preform, then used fiber drawing towers to draw it into length, which is already mentioned in the introduction part of the present chapter. High contrast Bragg fibers can be made with the help of this method in which polymer material is at the low index layer

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and chalcogenide or perovskite material is at the high index layer [39]. Further low contrast Bragg fiber where both low refractive index (R.I). material as well as high R.I. material are of the polymer film, can be fabricated by using the rolling technique [40]. Sensing and optoelectronic are important applications where we can use such fiber waveguides. Recently, due to very specific properties, these fiber waveguides have tremendous applications in high power laser transmission where the non-linear effects are significantly suppressed [41]. They are also promising in various applications such as strain, bio and chemical sensing [26, 39, 40, 42] bending and displacement sensing [40], narrowband transmission filter, optical de-multiplexers, etc. [29, 31, 43, 44]. The optical characteristics of high R.I. contrast and low R.I. contrast Bragg fibers are of concern to the contemporary investigator due to their recent popularity. Since, the photonic bandgap (PBG) mechanism is used to guide EM waves in such a fiber structure, the function of PBG in high and low R.I. contrast Bragg fibers has been investigated, optimised and compared in this chapter. By adjusting the incidence angle of light, the tunable PBG from such fiber structure can be obtained. The PBG is determined for the operational spectral mode, namely the widely utilised 632.8 & 1550 nm for which signal loss window is minimum. The main characteristic parameter, full width at half maximum (FWHM) of the output spectrum is also influenced by the Bragg fiber periodicity. As a result, two types of empty core Bragg fiber structures, HRBFW and LRBFW, have been examined in this chapter. The suggested fiber waveguide's propagation characteristics are studied using the transfer matrix approach. In addition, we have seen the PBG move in response to changes in the core refractive index, which is the biosensing mechanism's primary premise. Furthermore, dispersion analysis is critical for the practical implementation of such fiber waveguide structures. Through proposed HRBFW and LRBFW structures, dispersion analysis of fiber waveguide structure examines the optimization of core radius and cladding thickness, as well as their cutoff condition. In this chapter, we have used the transfer matrix method (TMM) and Hankel formalism (HF) to theoretically analyse the propagation parameters of the Bragg fiber and find the mathematical equation for the various propagation modes in the fiber structure. THEORETICAL MODELLING OF THE PROPOSED STRUCTURE The front view with all details along the length of the Bragg fiber is shown in Fig. (1a). In this front view, nc and rc shows the R.I. and empty core radius while nH , nL and dH , dL depict respectively, the R.I.’s and the respective thicknesses of high and less R.I. cladding layers forming the periodic structure. Further, Λ=dH+dL is periodicity. Assuming that the electro-magnetic components can be represented by spatial-temporal factor of Utz=exp[i(ωt-βz)], β being the propagation constant. With the periodic arrangement, a concentric Bragg fiber must be disigned. In the

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Bragg fiber mechanism, due to the cicular symmetry along the length of the fiber, the EM wave propagating through the fiber structure is assumed to be cylindrical. This cylindrical wave is further supposed to be diverging from the axis of symmetry r = 0, and then it hits on the first cylindrical cladding interface r = rc. Now, applying cylindrically symmetric TMM [45] to calculate the reflectance or transmittance through the designed structure.

Fig. (1a). In this schematic front view of Bragg fiber, the z-axis in the direction of propagation via the BFW is significantly bigger than the other axis. (Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia, Vivek Singh, Estimation of photonic bandgap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier).

Ignoring the same temporal part of all the fields, the source free Maxwell’s equations [46] can be repersented as;   E   jH   H  j E

(1a) (1b)

In cylindrical symmetry (r, θ, z) [46], the finite electromagnetic components for TE or H mode must satisfy the following equations,

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1 E z   jH r r  E z  jH θ r

(2a)

(2b)

 (rH θ ) H r   j rEz   r

(2b)

The differential equation for the designed structure can be expressed by; r

  E z r r  r

  Ez  2 1  E z  r   r  r      

 2 2    r E z  0 

(3)

Further, assuming the electric field component Ez = U(r) Θ(θ) = U(r) eimθ and thus the solution of eq. (3) is expressed as;

U (r )  C1 J m (kr)  C2Ym (kr),

(4)

where C1 and C2 are constants, Jm and Ym are the function of Bessel and Neumann and k is the material wave number. The ‘m’ is the azimuthal mode and taken zero for maintaining cylindrical symmetry throughout the study. In a similar fashion, assuming the magnetic field component Hθ = V(r) eimθ, where V(r) is expressed by,

V (r )   jp(C1 J m' (kr)  C 2Ym' (kr)),

(5)

where p = √(ε/μ) is the material's intrinsic admittance. Equations (4) and (5) are used simultaneously to create a mono layer matrix T1 which must connects the electromagnetic field at its two interfaces. Thus, the matrix of mono layer at the interface (and) is written as [45, 47]: U (r1 ) ˆ U (r0 ) V (r )   T1 V (r )   1   0 

t11 Thus the mono layer matrix Tˆ1   t 21  ' t11 

t12  j

t12  can be expressed as: t 22 

k1r0 [Ym (k1r0 ) J m (k1r1 )  J m' (k1r0 )Ym (k1r1 )],

2

 k1 2 p1

t 21   j



2

(6)

r0 [ J m (k1 r0 )Ym (k1 r1 )  Ym (k1 r0 ) J m (k1 r1 )],

k1r0 p1[Ym' (k1r0 ) J m' (k1r1 )  J m' (k1r0 )Ym' (k1r1 )],

(7a) (7b) (7c)

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t 22 

 2

k1r0 [ J m (k1r0 )Ym' (k1r1 )  Ym (k1r0 ) J m' (k1r1 )],

(7d)

where p1 = √(ε1/μ1). The resemblance of radii dependence is shown in the above equations (7). As a result, the matrix for all existing levels of concentric Bragg fiber structure can be easily constructed. To obtain the complete transfer matrix that connects the material free core to the final interface of the cladding structure, 2N no. of matrices must required: U (rf ) ˆ U (r0 ) V (r )   T V (r )  ,  f   0 

(8)

where T  T Tˆ   11 12   Tˆ2N Tˆ2N 1 ..... TˆN 1 TˆN ..... Tˆ2 Tˆ1 T21 T22 

(9)

The reflection and transmission coefficients can be estimated using this calculated transfer matrix using the following relationships [45]. rd  td 

(2) ' (2) (2) ' (T21'  jp0 C m0 T11 )  jp f C mf (T22'  jp0 C m0 T12 ) (1) ' (2) (1) ' ( jp0 C m0 T11  T21' )  jpf C mf ( jp0 C m0 T12  T22' )

,

4  0 0 , (1) ' (2) (1) ' Kr0 H m(2) (k 0 r0 ) H m(1) (k 0 r0 )[( jp0 Cm0 T11  T21' )  jpf Cmf ( jp0 C m0 T12  T22' )]

(10) (11)

where p0   0  0 and pf   f  f are the intial and final admittance. T11' , T12' , T21' and T22' are the inverse matrix element. K    0 0 is the vaccume wavenumber and (1, 2 ) C ml 

H m(1,2)' (k l rl ) l  0, f , H m(1,2) (k l rl )

(12)

where Hm(1) and Hm(2) are the first and second kind Hankel function. By eqs. (10) and (11), the %R and %T can be calculated as: % R  100  rd

2

, %T  100 

nout td nc

2

(13)

where nc and nout are the core and final R.I. respectively. The effectiveness of suggested Bragg fiber design can be assessed by variety of sensing criteria, with detection accuracy (D.A.), sensitivity (S), and quality parameter (Q.P.) being the most relevant. The ratio of the sensor's measured

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outcome to the change in the physical quantity to be measured is used to determine the sensor's sensitivity. The ratios of the spectral shift in resonance wavelength to the slight change in aqueous medium or analyte refractive index present in the core define the sensor's sensitivity mathematically.

S   res nc

(14)

The real value is described by detection accuracy. The ratio of the shift in resonance wavelength to the full width at half maximum (FWHM) of the photonic bandgap determines the detection accuracy of this sensor. The sensor's total performance in terms of sensitivity and detection accuracy is represented by the quality parameter. HANKEL FORMALISM OF THE PROPOSED STRUCTURE Fig. (1b) shows the waveguide's refractive index profile schematically. Because this waveguide proposed followed both TE and TM modes, however, in our work, we only looked at the H-polarisation (TE Mode), which features finite electromagnetic field components Hr,Eθ and Hz These components can be used to build other components. At the junction of each layer, essential electromagnetic components must be continuous.

Fig. (1b). With the radial position of core and cladding regions, schematic Diagram of refractive index profile.

Accordingly, in the following computation, Hz and Eθ are the essential electromagnetic components. Thus, the electromagnetic field component given above can also be written as [14], (

𝐻𝑧 𝐴 ) = 𝐷𝑖𝑇𝐸 (𝑟) ( 𝑖 ) 𝑖𝐸𝜃 𝐵𝑖

(15)

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𝐷𝑖𝑇𝐸 (𝑟) ≡ (

𝑇𝐸 𝑑11 𝑇𝐸 𝑑21

𝑇𝐸 𝑑12 𝑇𝐸 ) 𝑑22

where, is the so-called representation matrix (R.M.). In material-free core, each and every R.M. component is shown as; (2) 𝑇𝐸 𝑑11 = 𝐻𝜈 (𝜅𝑐 𝑟),

(16a)

(1) 𝑇𝐸 𝑑12 = 𝐻𝜈 (𝜅𝑐 𝑟), 𝜔𝜇0

(2)′

𝑇𝐸 𝑑21 = −(

) 𝐻𝜈

𝑇𝐸 𝑑22 =(

) 𝐻𝜈

𝜅𝑐 𝑛𝑐2 𝜔𝜀0 𝜅𝑐

(1)′

(16b)

(𝜅𝑐 𝑟),

(16c)

(𝜅𝑐 𝑟).

(16d)

𝜅𝑐 ≡ √(𝑛𝑐 𝑘0 )2 − 𝛽2 ⁡; 0 ≤ 𝑟 ≤ 𝑟𝑐 . (1)

(17)

(2)

Henkel functions, ⁡𝐻𝜈 = 𝐽𝜈 + 𝑖𝑁𝜈 ⁡𝑎𝑛𝑑⁡𝐻𝜈 = 𝐽𝜈 − 𝑖𝑁𝜈 , (where Jν is the Bessel function and Nν is the Neuman function) stick with the characteristics of inward and outward EM waves, respectively. Furthermore, k0 = ω/ c is wavenumber in a vacuum. We may use an asymptotic extension for the Henkel function at large parameters to describe electromagnetic fields in claddings material for larger material free core radius [48]. Thus, the R.M. after assumption is represented as: 2 𝑃𝑇𝐸 𝑄𝑖 (𝑟); 𝜋𝜌 𝑖

𝐷𝑖𝑇𝐸 (𝑟) ≃ √

(18)

where, 𝑃𝑖𝑇𝐸 =

1 √𝜅𝑖

𝑄𝑖𝑇𝐸 (𝜌) = (

(

1 𝑖𝜔𝜇0 /𝜅𝑖

1 ) −𝑖𝜔𝜇0 /𝜅𝑖

exp⁡(−𝑖𝜅𝑖 𝑟) 0 ); 0 exp⁡(𝑖𝜅𝑖 𝑟)

(19a) (19b)

As shown in Fig. (2), the subscript i stands for substrate a, b, and c. κi is the wave vector in various layers. The amplitude pair for substrate 'a' in H-polarisation is connected to each other; 𝑎𝑚+1 𝑋 (𝑏 ) = ( 𝑇𝐸 ∗ 𝑌𝑇𝐸 𝑚+1 𝑖

𝑋𝑇𝐸 = [cos(𝜅𝑏 𝑑𝐿 ) − (

𝜅𝑏

2 𝜅𝑎

𝑖

𝑌𝑇𝐸 = (

𝜅𝑏

2 𝜅𝑎

−

𝜅𝑎 𝜅𝑏

+

𝑌𝑇𝐸 𝑎𝑚 ∗ ) ( 𝑏 ); 𝑋𝑇𝐸 𝑚 𝜅𝑎 𝜅𝑏

) sin(𝜅𝑏 𝑑𝐿 )]exp⁡(−𝑖𝜅𝑎 𝑑𝐻 );

) sin(𝜅𝑏 𝑑𝐿 )]exp⁡(𝑖𝜅𝑎 𝑑𝐻 )

(20) (20a) (20b)

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Each coefficient involved in the above expression accepts the condition |𝑋𝑇𝐸 |2 − |𝑌𝑇𝐸 |2 = 1 the asymptotic approximation. Further, by the Floquet theorem [1 - 3, 48] in concentric structure, the Bragg fiber eigenvalue equation must be formulaised. According to Floquet, if periodicity r + Λ = n(r), the wave travelling through it is represented as: 𝐺𝐾 (𝑟, 𝑧) = 𝐺𝐾 (𝑟)exp⁡(−𝑖𝐾𝑟)exp⁡(−𝑖𝛽𝑧)

(21)

The Bloch wavenumber K is the same for all periodic structures. The Eigenvalue equation may now be deduced from the core-cladding interface boundary condition. As a consequence, 𝐽𝜈′ (𝜅𝑐 𝑟𝑐 ) 𝐽𝜈 (𝜅𝑐 𝑟𝑐 )

+

𝜅𝑐 {[exp(−𝑖𝐾𝑇𝐸 Λ)−𝑋𝑇𝐸 ]−𝑌𝑇𝐸 } 𝑖𝜅𝑎 {[exp(−𝑖𝐾𝑇𝐸 Λ)−𝑋𝑇𝐸 ]+𝑌𝑇𝐸 }

=0

(22)

The above equation can be used to analyse a variety of properties. NUMERICAL RESULTS AND DISCUSSION Both hollow core HRBFW and LRBFW transmission characteristics are investigated in loss free window with a design wavelength of 638.8 nm and 1550 nm. The core of fiber and the outermost layer is air i.e. nc = nout = 1.0 with the core dimension at least rc = 100 (dH + dL) to ignore the losses that occur due to the presence of higher-order modes. The HRBFW is made of high refractive index chalcogenide material (As2Se3) with nH =2.82 and the less R.I. polymer material (PEI) nL =1.66 [18] with thickness values dH = λc/4nH , dL = λc/4nL [47] respectively that must obey the quarter-wave condition. Further, material free core LRBFW high R.I. polymer material (PS) nH =1.581 and less R.I. polymer (PMMA) nL = 1.487 [20] have quarter wave thicknesses. Figs. (2a and b) reveal the change of % transmittance at 1550 nm for both designed structures. The observed PBG for HRBFW is 546 nm (Fig. 2a) due to its strong reflectivity in comparison to the LRBFW 61 nm, (Fig. 2b). These figures reveal that in HRBFW, only some layers of cladding (N=14) are sufficient to provide the perfect bandgap while in LRBFW to obtain a perfect PBG, large cladding layers (N=60) are required. Here, the perfect bandgap is defined as a range of wavelengths with 0% transmittance. Figs. (2c and d) show the transmittance of HRBFW and LRBFW at design wavelength 632.8 nm. Here, the obtained bandgap for HRBFW (225 nm) and LRBFW (25 nm) decreases with a decrease in the design wavelength. At this design wavelength, the perfect bandgap necessitates a lower number of periodicity for HRBFW and a higher number of periodicity for LRBFW. In Table 1, the variation in photonic bandgap width and

Optical Properties

Photonic Materials: Recent Advances and Emerging Applications 223

% transmittance in hollow-core Bragg waveguides with varied numbers of unit cells (N) are also computed and tabulated. Table 1 shows that the observed bandgap is significantly dependent on the contrast of the refractive indices of the materials utilised, and that it decreases as the refractive index contrast lowers; also, the position and size of the bandgap are unaffected by the number of layers.

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Fig. (2). Impact of different cladding layers on PBG at design wavelengths at 1550 nm (a) HRBFW (b) LRBFW and at 632.8 nm (c) HRBFW (d) LRBFW. .(Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier).

14 periodic cells are needful in the HRBFW and 60 periodic cells are needful in the LRBFW waveguide to observe the influence of varied incidence angles of electromagnetic waves on the perfect bandgap. Fig. (3) depicts the spectrum of the photonic bandgap at various incidence angles. The positions of photonic band gaps for HRBFW and LRBFW at loss free window wavelengths 1550 nm and 632.8 nm are shown in Figs. (3a - d) respectively. The locations of the left and right band edges (L.B.E. and R.B.E.) are computed and recorded in Table 2 using these figures. It must be observed from the Table 2 that the incidence angle change from 00 to 750 in HRBFW can shift L.B.E. by 162 nm and 64 nm for

Optical Properties

Photonic Materials: Recent Advances and Emerging Applications 225

window 1550 nm and 632.8 nm respectively, and R.B.E. shifts 234 nm and 94 nm respectively. Similarly, LRBFW’s L.B.E. respective shifts were 341.0 nm and 353.0 nm for window 1550nm and 632.8nm and for R.B.E, these respective shifts were 139.4 nm and 144.1 nm. Finally, as shown in Table 2, when the incidence angle of light increases further, the resulting PBG for both window wavelength diminishes. Additionally, all PBGs get blue-shifted due to the increase in geometrical path of the incoming EM waves. Table 1. Variability in FWHM and % trans in material free Bragg fiber with unit cell values N. (Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier). HRBFW

LRBFW

S. At wavelength 1550 nm At wavelength 632.8nm At wavelength 1550nm At wavelength 632.8nm No FWHM FWHM FWHM FWHM Period %Trans Period %Trans Period %Trans Period %Trans (nm) (nm) (nm) (nm) 1.

4

5.45200 835.00

4

5.45200 341.00

10

70.1300 228.00

10

70.1300 91.80

2.

6

0.66420 698.00

6

0.66420 285.00

20

29.1700 117.00

20

29.1700 47.00

3.

8

0.07930 639.00

8

0.07930 263.20

30

9.6300

92.00

30

9.6300

38.00

4.

10

0.00940 607.00

10

0.00940 246.10

40

2.9480

82.00

40

2.9480

33.50

5.

12

0.00110 588.00

12

0.00110 235.00

50

0.8600

77.00

50

0.8600

31.30

6.

14

0.00010 575.00

14

0.00010 210.00

60

0.2500

73.00

60

0.2500

29.80

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Optical Properties

Photonic Materials: Recent Advances and Emerging Applications 227

Fig. (3). Spectral shift of obtained PBG with incidence angle in material free core HRBFW for (a) 1550nm (b) 632.8nm and in LRBFW at (c) 1550nm (d) 632.8nm. .(Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier). Table 2. The positions variation of L.B.E., R.B.E., and P.B.G. with incidence angle. .(Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier) S. No

HRBFW At wavelength 1550 nm

LRBFW At wavelength 632.8nm

At wavelength 1550nm

At wavelength 632.8nm

θ° L.B.E. R.B.E. P.B.G. L.B.E. R.B.E. P.B.G. L.B.E. R.B.E. P.B.G. L.B.E. R.B.E. P.B.G. (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 (10-9 m) m) m) m) m) m) m) m) m) m) m) m) 1.

0 1328.0 1868.0 540.0

540.0

761.0

221.0 1523.0 1578.0

55.0

622.0

644.0

22.0

2. 15 1317.0 1850.0 533.0

537.0

755.0

218.0 1501.0 1555.0

54.0

613.0

634.0

21.0

3. 30 1285.0 1806.0 521.0

526.0

737.0

211.0 1440.0 1492.0

52.0

588.0

609.0

21.0

4. 45 1244.0 1748.0 504.0

507.0

712.0

205.0 1351.0 1400.0

49.0

551.0

571.0

20.0

5. 60 1199.0 1683.0 484.0

489.0

687.0

198.0 1256.0 1302.0

46.0

513.0

531.0

18.0

6. 75 1166.0 1634.0 468.0

476.0

667.0

191.0 1182.0 1225.0

43.0

482.0

499.0

17.0

The PBG might be employed as a sensing signal in the material free core Bragg fiber proposed for chemical and biosensing applications. The fluctuation in the PBG with core R.I. in both HRBFW and LRBFW at both operating window is shown in Fig. (4) and documented in Table 3 to assess the bio-sensing applicability of these waveguides. The observed variation in FWHM of the PBG spectral shift can be monitored for sensing applications, as shown in Table 3. The

228 Photonic Materials: Recent Advances and Emerging Applications

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obtained sensitivities for HRBFW by calculating the FWHM value of the bandgap region are 314.0 nm/RIU and 136.0 nm/RIU at respective operating windows 1550 nm and 632.8 nm. The sensitivity values for LRBFW are 42.4 nm/RIU and 17.2 nm/RIU at respective operating windows of 1550 nm and 632.8 nm. A high disparity in cladding R.I. and higher operating window will result in greater sensitivity if the FWHM of the photonic bandgap is measured, as shown in the preceding discussion. The sensitivity of the suggested structure may also be determined by tracking the movement of L.B.E. or R.B.E. of the PBG region. Considering R.B.E. of HRBFW, the observed senstivities were 884.0 nm/RIU and 360.6 nm/RIU at respective operating windows of 1550 nm and 632.8 nm. In a similar manner, the LRBFW’s senstivity is 1101.0 nm/RIU and 449.4 nm/RIU for respective operating windows of 1550 nm and 632.8 nm. Sensitivities of 550.0 nm/RIU and 224.8 nm/RIU were achieved by measuring the L.B.E. of HRBFW at respective operating windows 1550 nm and 632.8 nm. Using the L.B.E. of LRBFW, sensitivities reached 1058.6 nm/RIU and 432.2 nm/RIU at operating windows 1550 nm and 632.8 nm, respectively. According to these principles, the maximum sensitivity is achieved in LRBFW, which decreases with the operating window. When comparing the achieved sensitivity of the proposed waveguide to the obtained sensitivity of the L.B.E. and R.B.E. at any operating window, the obtained respective sensitivity of R.B.E. was much more than L.B.E. As a result, monitoring R.B.E. rather than L.B.E. is advised for increased sensitivity. However, the suggested Bragg fiber material free core has been selected to be hollow i.e.nc = 1.0, to study dispersion properties. The two configurations here taken are: High R.I. contrast Bragg fiber having high R.I. cladding (As2Se3) nH = 2.83 and low R.I. cladding being polyether-imide (PEI) nL = 1.66 [18] and for low R.I. contrast polymeric Bragg fiber, high R.I. cladding is (polystyrene) nH=1.581 and low R.I. cladding is poly-methyl-methacrylate (PMMA) nL=1.487 respectively [20]. The corresponding thicknesses of cladding layers are dH and dL for both configuration sustained with quarter-wave condition, nH dH = n LdL=λ0/4, where, λ0 is the operating window. We picked 1550 nm for the design wavelength and 632.8 nm for the He-Ne laser pulse for communication and sensing purposes.

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Photonic Materials: Recent Advances and Emerging Applications 229

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Fig. (4). Variation in the PBG with material free core R.I. in HRBFW at the window (a) 1550 nm, (b) 632.8 nm, and in LRBFW at window (c) 1550 nm (d) 632.8 nm. .(Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier).

Optical Properties

Photonic Materials: Recent Advances and Emerging Applications 231

Table 3. The positional variation of L.B.E., R.B.E. and P.B.G. with core R.I. .(Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier) S. No

HRBFW At window 1550 nm

LRBFW At window 632.8 nm

At window1550nm

At window 632.8nm

Core L.B.E R.B.E P.B.G L.B.E R.B.E P.B.G L.B.E R.B.E P.B.G L.B.E R.B.E P.B.G R.I. (10-9 m) (10-9 m) (10-9 (10-9 (10-9 (10-9 (10-9 m) (10-9 m (10-9 (10-9 (10-9 (10-9 m m m m m m m m 1. 1.00 1153.00 1619.00 466.00 470.70 660.80 190.10 1227.00 1273.00 46.00 500.90 519.70 18.80 2. 1.10 1113.00 1559.00 446.00 454.50 636.40 181.90 1155.00 1198.00 43.00 471.70 489.30 17.60 3. 1.20 1067.00 1489.00 422.00 435.70 607.80 172.10 1072.00 1111.00 39.00 437.50 453.70 16.20 4. 1.30 1014.00 1406.00 392.00 414.00 574.00 160.00 972.30 1008.00 35.70 396.90 411.50 14.60 5. 1.40 952.00 1305.00 353.00 388.60 533.00 144.40 851.50

882.30 30.80 347.60 360.30 12.70

6. 1.50 878.00 1177.00 299.00 358.30 480.50 122.10 697.70

722.50 24.80 284.80 295.00 10.20

The cutoff radius for various TE0μ modes are tabulated below in Table 4 which shows that the cutoff radius for fundamental mode is lower at operating window 632.8 nm than 1550.0 nm. Even, no of modes present is more at 632.8 nm than 1550 nm. Comparatively, more number of modes are present in high R.I. contrast Bragg fiber than low R.I. contrast Bragg fiber. Table 4. Cutoff core radius for various possible guided modes in Bragg waveguide. .(Reprinted from Superlattices and Microstructures, 116, Ritesh Kumar Chourasia,Vivek Singh, Estimation of photonic band gap in the hollow core cylindrical multilayer structure, 191-199., Copyright 2018, with permission from Elsevier). Guided modes @ λ0 = 1550 (nm)

TE0

TE02

TE 03

TE 04

TE 05

TE06

Truncation radius (in 10 m) (for low R.I. Contrast)

0.9410

1.7250

2.5010

3.2770

4.0530

-----

Truncation radius (in 10-6m) (for High R.I. 0.9520 Contrast)

1.7420

2.5140

3.3040

4.0560

4.8460

-6

Modes of guidance @ λ0 TE01TE02 TE03TE04 TE05TE06 TE07TE08 TE09TE010 TE011TE012 TE013TE014 = 632.8 (nm)

TE015

Truncation radius (µm) (for low Contrast)

0.3960 0.7100

1.0240 1.3560

1.6700 1.9850

2.2980 2.6130

2.9270 3.2410

3.5550 3.8870

4.2010 4.5150

----

Truncation radius (µm) (for High Contrast)

0.3950 0.6900

1.0050 1.3400

1.6550 1.9700

2.2840 2.6030

2.9340 3.2490

3.5640 3.8780

4.1930 4.5230

4.8230

232 Photonic Materials: Recent Advances and Emerging Applications

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CONCLUSION The propagation characteristics of material free cored Bragg fiber are investigated and contrasted in both high and low R.I. contrasts of cladding materials. In addition, the propagation properties of suggested Bragg fibers are investigated at two operating windows: one near-infrared and the other visible. The PBG achieved in material free core HRBFW is higher, and only a few periodicities are necessary to achieve a perfect bandgap. The breadth and spectral location of the photonic bandgap in both hollow core HRBFW and LRBFW are greatly dependent on the incidence angle of the light beam. Given the variation in photonic bandgap width with a core refractive index, HRBFW has a high sensitivity, which rises with design wavelength. LRBFW has a higher sensitivity than HRBFW when considering the L.B.E and R.B.E, hence it is suggested for sensing applications. LRBFW's sensitivity can also be improved by raising the design wavelength. Furthermore, it is obvious from the dispersion analysis of the Bragg fiber waveguide structure that HRBFW has a higher number of modes than LRBFW. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

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A.D. Kersey, T.A. Berkoff, and W.W. Morey, "Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry–Perot wavelength filter", Opt. Lett., vol. 18, no. 16, pp. 1370-1372, 1993. [http://dx.doi.org/10.1364/OL.18.001370] [PMID: 19823386]

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M.S. Chen, C.J. Wu, and T.J. Yang, "Narrowband reflection-and-transmission filter in an annular defective photonic crystal containing an ultrathin metallic film", Opt. Commun., vol. 285, no. 13-14, pp. 3143-3149, 2012. [http://dx.doi.org/10.1016/j.optcom.2012.02.087]

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N.K. Chourasia, A. Srivastava, V. Kumar, and R.K. Chourasia, "Doubly electrically tuned cylindrical Bragg fiber waveguide inline optical filter for multiwavelength LASER applications", Mater. Today Commun., vol. 25, 2020.101620 [http://dx.doi.org/10.1016/j.mtcomm.2020.101620]

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R.K. Chourasia, S. Prasad, and V. Singh, "Voltage-tunable pass band in cylindrical multilayered structure containing PMMA and 0.67PMN–0.33PT single crystal as a defect layer", Opt. Quantum Electron., vol. 48, no. 12, p. 539, 2016. [http://dx.doi.org/10.1007/s11082-016-0811-8]

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R.K. Chourasia, and V. Singh, "Cylindrical Bragg waveguide based temperature sensors", 2016 IEEE Uttar Pradesh Section International Conference on Electrical, Computer and Electronics Engineering (UPCON), pp. 87-90, 2016. [http://dx.doi.org/10.1109/UPCON.2016.7894630]

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237

CHAPTER 12

Photonic Nanostructured Bragg Fuel Adulteration Sensor Ritesh Kumar Chourasia1,4,*, Nitesh K. Chourasia2, Ankita Srivastava3,4 and Narendra Bihari1,4 University Department of Physics, Lalit Narayan Mithila University, Darbhanga-846004, India School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, India 3 Department of Physics, Instititute of Science, Banaras Hindu University, Varanasi-221005, India 4 Department of Physics, Samastipur College, Samastipur-848134 (A constituent college of L.N.M.U. Darbhanga-846004, Bihar, India) 1 2

Abstract: The adulteration of liquid fuels has several far-reaching repercussions, including pollution and a rising energy crisis. Around the world, fossil fuels are widely utilized for transportation and energy generation. Fuel adulteration currently threatens a big number of customers. Adulteration of fossil fuels with other recognised hydrocarbons is a common occurrence. Adulterants are added to these base fuels in the form of additional low-cost hydrocarbons with similar compositions, leading the base to be altered and degraded. Adulteration is an unauthorised or illegal introduction of a lower-quality external substance into a higher-quality commodity, causing the latter to lose its original composition and qualities. The Opto-Microfluidics approach is a new field that uses a small sample to identify adulteration in food and fuel, resulting in high-resolution findings. Consumers will benefit from very sensitive detection of dangerous adulteration in any commodity thanks to opto-microfluidic lab-on-chip technologies. Using the metal-polymer nanocomposites’ multilayer cylindrical nanostructure with a microfluidic channel, we develop a real-time and temperaturedependent prototype of the Bragg Opto-microfluidic sensor for effective tracking of contaminated fossil fuels. The purpose of this chapter is to examine the biological motivations for the development of multilayer photonic nanostructures and various types of fuel adulteration detection optical sensors using various sensor-based techniques, as well as to compare the Bragg Metal-Polymer nanocomposites optical sensor with other optical sensors. This chapter is devoted entirely to the use of the theoretical model's Kay, Eykman, Dale-Gladstone, Newton, and Lorentz-Lorenz, as well as Hankel formalism and the transfer matrix method for cylindrical symmetry.

Keywords: Photonic nanostructures, Fossil fuel and energy, Adulteration sensor, Transfer matrix method, Hankel formalism. Corresponding author Ritesh Kumar Chourasia: Department of Physics, Samastipur College, Samastipur-848134 (A constituent college of L.N.M.U. Darbhanga-846004, Bihar, India); E-mails: [email protected], [email protected] *

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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INTRODUCTION The structure, content, and optical presence of biological photonic systems might serve as a source of stimulation for creating innovative artificial photonic elements [1 - 3]. Natural photonics research can occasionally lead to specific optical technology design templates [4 - 9]. After uncovering a hierarchical photonic nanostructure, Margaritaria nobilis fruit’s seed coat, they constructed novel photonic nanostructured fibres. Light interference inside a concentricallylayered architecture found in the individual cells in the seed's outer tissue layers causes the fruit's hue. Regularity of nano-cylindrical symmetry, which leads to selective wavelength dispersion of light in an extensive range of directions; and regularity of nano-cylindrical symmetry, which leads to selective wavelength dispersion of light in an extensive range of directions are two technologically exploited properties for light and colour handling in the natural structure. This lays the groundwork for a new soft, biologically inspired nanostructured photonic fibre with spectrum filtering capabilities and colour brightness similar to a flat Bragg stack, as well as a vast angular spatial range offered by the nanoscale bend. Because of the elastic and transparent synthetic materials, the multilayer interference fibres have high reflectivity that is dynamically changed by a longitudinal mechanical strain. This type of soft photonic fibre is designed and manufactured in a biologically inspired manner, and it marks the transition to new fibre flexible fabrics and photonic materials. Nature's most gaudy hues, extraordinary transparency, brightest whites, or darkest blacks rely on the wavelength of visible light in the order, in a quasi-ordered or disordered structure with scattering element sizes or lattice constants [10 - 14]. By causing diffraction or interference, the wide structural range of biological structures has a significant impact on the spectral composition of transmitted and reflected light, ensuing in remarkable structural colours for many species [15, 16]. The plant Margaritaria nobilis has blue-green fruits, grown in the rain forestry of Middle and South America. The plant relies on birds to disperse the seeds, and the bright display may attract them [17 - 20]. The cells in the blue seed coat of the fruit are lengthy and mostly blue and green in colour. Various cell layers are placed on top of each other, each with a different planar location. A cross-section of a single cell tells that the whole inner volume is occupied by a periodically concentrated morphology. The presence of light on the fruit's surface causes blue light to reflect because the usual structure of each cell is disrupted. Under diffuse light, the colour blue varies as the angle of view increases. Instead, due to the superposition of microscopic curvature, the reflected structural colour over the wide angular

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gamma inside the layers of each seed coat tissue cell, as well as the overall macroscopic curvature of the fruit, is more noticeable in M. Nobilis. This idea is also used to design novel optical technology [6, 7, 9]. The photonic hierarchy in the seed coat of M. nobilis fruit is required for the production of striking blue and green colours. It encourages the use of a combination of nanoscopic regularity and overlaid microscopic curvature to build optically capable artificial photonic fibres. While the artificial system is comparable to its natural counterpart in terms of optical interactions and dimensions, it evades many of the intricacies of natural structure, such as ellipticity and any fine structure inside the regular layers of the fruit cells, for its expedient set of characteristics for precise fiber-optic tuning, as shown below. This material system has previously been chosen in the framework of planar, flexible, multi-layer systems. The emphasis of exertions on a restricted group of materials and architectures that give light guiding in the fibre core over total internal reflection in the transparent region of silica glass has been an unforeseen side consequence of this accomplishment. In the previous ten years, things have changed. Though light-guiding still depends on total internal reflection, nanostructured fibres have been investigated, allowing for a wider range of fibre configurations [21 - 23]. Fibers with two-dimensional (2D) photonic-crystal structures [24] have been proven to direct light via the photonic bandgap [25 - 27] (PBG) effect. The vast range of outcomes achieved by these fibres has been very attractive for research considering the diverse applications [22 - 29]. Traditional materials, such as silica glasses or polymers, are used in these fibres, with the accumulation of air holes that may enclose fluids [30]. Optical transmission and related phenomena are limited by the presence of compressible domains and the usage of electrically insulating materials. The technique of employing a multi-material preform for fibre processing is established first, followed by the assortment criteria for well-matched material combinations. Subsequently, we exhibit a fibre with alternating layers of an electrically insulating polymer and a semiconducting glass of predefined thicknesses that restrict light to a hollow core, resulting in a cylindrical omnidirectional mirror [31 - 35]. Because the index difference between layer materials is strong enough, the electromagnetic field cannot penetrate the solid layers, resulting in a fibre that is significantly more transparent than its basic elements. This structure is unique due to the scalable wavelength, meaning that the structure's period influences the wavelength of light transmitted down the fibre axis. As a result, by merely adjusting the lattice constant of the periodic multilayer structure, fibres that direct ultraviolet (UV), visible, near, or mid-infrared (NIR or MIR), light may be made using the same overall manufacturing process. The drawing of long lengths of fibre results in a huge surface area. This opens the door to the development of

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fibre surface devices. High-efficiency fibre reflectors may be made in extremely vast regions by positioning the omnidirectional mirror assembly near the fibre boundary [36]. In addition, by inserting specified thickness variations in specific layers of the reflecting structure, a radial resonant optical cavity fibre emerges [37 - 40]. The effects of semiconductors, co-drawing metals, and insulators in similar fibre to build thermal and optoelectronic fiber-based devices are discussed next [41 - 44]. Metal-semiconductor–metal (MSM) junctions that perceive thermal or optical stimulation include thin-film [42] and solid core [43]. Multifunctional fibres that incorporate more than one building block into the structure, as well as fibre arrays in which many fibres are combined into significant constructions, are made feasible by these unfunctional building pieces. The same fibre rolling technology that was used to make the fibres has been utilised to make multilayer claddings on rods of macroscopic diameter to make nanostructured planar multilayer stacks. Thermoplastics, such as polystyrene and polypropylene, are other dielectric materials that have been employed in fibre production (methyl methacrylate). Using the rolling approach, thin metal films were integrated into non-stretchable fibres of macroscale widths. Spray coating or blade coating are two industrial processes that are attuned to roll-to-roll manufacturing and can be investigated as feasible options for larger-scale bilayer production. In addition, by inserting specified thickness variations in specific layers of the reflecting structure, a radial resonant optical cavity fibre emerges [37 - 40]. Furthermore, biofuels have attracted increasing worldwide attention in the recent decade due to climate change, environmental challenges, energy balance, and safety, to reduce the use of fossil fuels in the coming times. The Indian government has recently started focussing on certain novel initiatives like Make in India and the Swachh Bharat Abhiyan (loosely translated as “Clean India Campaign”), and research in biofuels would ultimately lead to reducing imports, optimal usage of waste to create wealth, job creation, centralising agricultural production, and would help in the formation of profitable income model for farmers. Furthermore, because it is made up of renewable resources (biodegradable and less harmful to the environment), biodiesel is one of the safest alternative biofuels. Biodiesel results in a reduction of pollutants like carbon monoxide, sulphur dioxide, carbon dioxide, unburned hydrocarbons, sulphates, and other greenhouse gas-emitting compounds [45 - 47]. Aside from this unique feature, biodiesel is compatible with conventional combustion engines (dieselpowered) and hence, requires no engine modifications [48, 49]. Alkali catalyzed transesterification of oils and fats with other appropriate alcohols or methanol produces long-chain fatty acids methyl esters [48], finally resulting in biodiesel fuels. The excellence of the raw material of biodiesel influences the number of

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carbon atoms. Renewable raw materials can be useful for manufacturing biodiesel e.g. oil extracts from materials like coconut, almond, palm, canola, and even soybeans can be used to make mono alcohols and biodiesel [48, 49]. Europe, USA, China, Canada, and India are the countries that produce rapeseed raw material which is used for biodiesel production. According to an agricultural study, rapeseed is planted on around 13% of cropped land in India and hence biofuel produced by it can be used as a substitute for conventional petroleum-based diesel in vehicles [50, 51]. Pure biodiesel can be used in modern engines. Biodiesel is usually blended with conventional diesel to make a (pseudo-binary) mixture in ratios of 2% to 20%. For example, for a B20 mixture, biodiesel can be 20% of the mixture and 80% would be pure diesel [52]. Interestingly, excess biodiesel in the mixture can cause damage to the engine [48, 49], since appropriate lubrication is critical in diesel engines because the injection system's effectiveness is largely reliant on it. If the lubrication is insufficient, the injection mechanism may fail. As a result, the fuel in the pseudo-binary combination needs to be concentrated properly. In addition, the molar concentration of biodiesel fluctuates significantly depending on atmospheric temperature and meteorological conditions [53, 54]. Because all-season cultivars are popular in India and other parts of Asia, it is important to maintain track of the molar biodiesel volume percentage in all weather situations. Because physical properties of a pseudo-binary mixture like viscosity, refractive index (R.I.), and density are temperature-dependent and correlated, one can use the temperature variation to sense the change in the molar concentration of biofuel [53, 54]. There have been recent researches detecting biofuel using photonic crystal sensors [52] that employ electromagnetic and acoustic waves, according to Velusamy and Planiappa's ultrasonic investigation on biodiesel and diesel mixes [55]. In this study, we widen and add the function of temperature change due to changing weather circumstances, and we compute the molar biofuel concentration in pseudo-binary systems. In the present chapter, the sensing platform shall be the Bragg Fiber Waveguide (BFW). The BFW structure is created on a concentric cylinder forming a periodic refractive index arrangement comprising of alternating high and low refractive indexes. The electromagnetic wave propagates through the hollow core through recurrent Bragg reflection via the periodic claddings [55 - 57]. A photonic bandgap (PBG) must also be present in the BFW structure. The PBG is a collection of wavelengths that cannot pass through the BFW structure. BFW has practical applications in sensing [58 - 62]. Biosensors [58], high-temperature sensors [59], wavelength and intensity modulators [60], are some of the additional uses for BFW. In our present chapter, we will look into chalcogenide glass Arsenic tri-selenide and polymer film Poly-ether imide (PEI) based cylindrical

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multi-clad BFW construction with a hollow core. This hollow-core will act as a flow cell for the pseudo-binary mixture throughout the sensing process. Notably, a resonant transmission peak or a narrow transmission bandwidth is quite desirable to increase overall performance. This characteristic may be obtained in the current periodic structure by breaking the cladding periodicity's symmetry and introducing a PEI geometrical defect layer, which is then exploited to create a resonant transmission peak in the area of interest, i.e., the PBG. The BFW design is used to explore the transmission characteristic of electromagnetic waves utilizing the TMM and mathematical Hankel formalism in cylindrical coordinates. MODELLING OF THE BFW PHOTONIC NANOSTRUCTURE Fig. (1a) shows the cladding thickness profile and refractive index of a pseudobinary combination of diesel-filled core BFW structures with a geometrical imperfection. The current waveguide construction is symmetrical around the propagation axis, which is the longitudinal axis. The BFW structure is made up of bilayer chalcogenide, Arsenic tri-Selenide As2Se3 (nHdH) (high refractive index) and polymeric layer poly-ether-imide (PEI) (nL,dL) (low refractive index). We shall now introduce a geometrical defect layer PEI, characterized by (nD,,dD). which breaks the symmetry of the periodicity. For the extreme transmission intensity, the quarter-wave stack condition will be satisfied as nHdH =nLdL= nDdD= λc/4, where λc represents the BFW structure’s design wavelength. In addition, (Fig. 1b) shows the schematic configuration for optimising the temperature-dependent molar volume percentage of biodiesel fuel in a pseudo-binary mixture using a Bragg fibre waveguide sensor. In this schematic setup, the broadband source's electromagnetic wave strikes the intake of the BFW transporting the fuel-filled core. After interacting with the fuel and multilayer Bragg reflectors, it is detected by the optical detector on the BFW's outlet. The optical spectrum analyzer (OSA) and a computer system are used to further evaluate the acquired transmission spectra. Hankel Function Formalism (HFF) and Transfer Matrix Methodology (TMM) in Cylindrical Coordinates The above-mentioned BFW structure has also been theoretically modelled in cylindrical dimensions utilising the TMM and Henkel function. The R.I. profile is as follows:

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Fig. (1a). BFW nanostructure with geometrical defect layer. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier).

Fig. (1b). Fuel adulteration sensor schematic based on BFW photonic nanostructure.

244 Photonic Materials: Recent Advances and Emerging Applications

 nc ,   nH ,  nL ,   :  :   nH , n( r )   n L ,   nD , n ,  H  nL ,   :  :  etc. 

Chourasia et al.

0  r  rc ; rc  r  r1 ; r1  r  r2 ;

rn  2  r  rn  1 ; rn 1  r  rD ; rD  r  rD ' ;

(1)

rD '  r  rn 1 ; rn 1  r  rn  2 ;

The solution to reflection and transmission to the waveguide must be achieved by applying the asymptotically approximated TMM [63]. The Maxwell equations can expressed as [64 - 66] for a material:

  E   jH

(2)

  H  jωεE

(3)

Assuming the temporal element of electromagnetic components is exp(jωt) [33 35], Helmholtz differential equations are written as:   H z  2 1  H z   H z  2 2  r r     r H z  0 (for TE- polarization) r  r   r r       E  1  E z   E z  2 2 r r z   r2      r E z  0 (for TM- polarization) r  r   r r    

r

(4) (5)

The above differential equation must be solved using the variable separation method in mathematics [63, 65]. The all three non-zero electromagnetic components for H-polarized wave must be written as:

H Z (r, )  [ AJ m (kr)  BYm (kr)]e jm 1 H z (r , ) E   j r

(6a) (6b)

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Photonic Materials: Recent Advances and Emerging Applications 245

Er 

m H z (r ,  ) r 

(6c)

where A and B are chosen constants Jm, Ym are Bessel, Neumann functions  

respectively, k   c  n is a usual wave vector. The elements of the transfer matrix, by applying structure theory of Abeles [63, 65, 67] are: T T  Tˆ   11 12  T21 T22 



krc Y ' m krc J m kr   J ' m krc Ym kr  2  T21  j krc pY ' m krc J ' m kr   J ' m krc Y ' m kr  2 T11 

T22 



2

krc J m krc Y ' m kr   Ym krc J ' m kr 

T12   j

where

p

 

 krc 2 p

J m krc Ym kr   Ym krc J m kr 

(7) (8a) (8b) (8c) (8d)

is the medium admittance. Thus the total transfer matrix comes from: V (r f )  V (rc )  ˆ V (rc )  U (r )  T2 N .......... .T2T1    T U (r ) f  U (rc )  c  

(9)

where U(r) and V(r) are the solution part of the Helmholtz differential equation. The aforementioned transfer matrices are similar to Fresnel's equation [63] in planar geometry. Thus, the reflection coefficient can be calculated as follows:

rM 

( 2) (T21'  jp0 c m( 20)T11' )  jp f c mf (T22'  jp0 c m( 20)T12' ) ( 2) (T21'  jp0 c m(10) T11' )  jp f c mf (T22'  jp0 c m(10) T12' )

(10)

' ' ' ' The matrix element T11 , T12 ,T21 and T22 the inverse matrix elements of matrix in Tˆ Eq. (9) and

C

(1, 2 ) ml

H m(1, 2)' (k l rl ) with l  0, f  (1, 2) H m (k l rl )

(11)

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Various Predictive Models The following models explain the relationship between the R.I. of a pseudo-binary mixture (Biodiesel and bare diesel fuel) with temperature-dependent compositional parameters. The following are the prediction abilities of many suggested equations [51]: Lorentz-Lorenz 2 𝑛𝑚𝑖𝑥 −1 2 +2 𝑛𝑚𝑖𝑥

= ∑𝑛𝑖=1 [𝑉𝑖 (

𝑛𝑖2 −1

(12)

)]

𝑛𝑖2 +2

Dale-Gladstone 𝑛𝑚𝑖𝑥 − 1 = ∑𝑛𝑖=1[𝑉𝑖 (𝑛𝑖 − 1)]

(13)

Eykman 2 −1 𝑛𝑚𝑖𝑥

= ∑𝑛𝑖=1 [𝑉𝑖 (

𝑛𝑖2 −1

)]

(14)

2 𝑛𝑚𝑖𝑥 − 1 = ∑𝑛𝑖=1[𝑉𝑖 (𝑛𝑖2 − 1)]

(15)

𝑛𝑚𝑖𝑥 = 𝑉1 𝑛1 + 𝑉2 𝑛2

(16)

2 +0.4 𝑛𝑚𝑖𝑥

𝑛𝑖2 +0.4

Newton

Kay

where nmix is the mixture R.I., ni and Vi are mono R.I. and the volumetric fraction of component i, respectively. We shall use the model of Krisnangkura [54] for assessing the refractive index of the pseudo-binary mixtures considering diverse degrees of blending along with an arbitrary temperature. By using the aforementioned theoretical model, we obtain the functional relationship of the refractive index with the temperature and composition based on the experimental data set of biodiesel [51]. 𝑙𝑛𝑛𝑚𝑖𝑥 = 𝑎 + 𝑏. 𝑉1 + (𝑐 ⁄𝑇) + (𝑑. 𝑉1 ⁄𝑇)

(17)

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Photonic Materials: Recent Advances and Emerging Applications 247

Here, V1 is the percentage in the volume of biodiesel in a pseudo-binary mixture, T is the absolute temperature and a,b,c, and d are correlation constants. The following table shows the anticipated equations for biodiesel made from three kinds of rapeseed oil and pure diesel fuel, along with the root mean square prediction difference (RMSPD): 1

𝑌𝑐𝑎𝑙,𝑖 −𝑌𝑒𝑥𝑝,𝑖

𝑛

𝑌𝑒𝑥𝑝,𝑖

𝑅𝑀𝑆𝑃𝐷 = 100√ ∑𝑛𝑖=1 [

2

(18)

]

where Ycal, and Yexp are the calculated and experimental values, respectively, and ‘n’ is the number of experimental data [51]. Table 1. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier). Components

Expressions

T L-L D-G Eyk New K RMSPD (K) RMSPD % RMSPD % RMSPD % RMSPD % %

Bio-I+Bare Diesel Fuel

𝑙𝑛𝑛𝑚𝑖𝑥 𝑚𝑖𝑥 = 0. 29380 − 0. 00080𝑉1 26. 3030 + 𝑇 1. 89580𝑉1 − 𝑇

298

0.00830

0.00870

0.00780

0.00900

0.00870

303

0.00390

0.00380

0.00420

0.00370

0.00380

313

0.00460

0.00430

0.00490

0.00400

0.00430

323

0.00730

0.00690

0.00770

0.00660

0.00690

𝑙𝑛𝑛𝑚𝑖𝑥 𝑚𝑖𝑥 = 0. 29340 − 0. 00330𝑉1 26. 4240 + 𝑇 1. 27910𝑉1 − 𝑇 𝑙𝑛𝑛𝑚𝑖𝑥 𝑚𝑖𝑥 = 0. 29390 − 0. 00630𝑉1 26. 2600 + 𝑇 0. 57330𝑉1 − 𝑇

298

0.01030

0.01070

0.00970

0.01110

0.01070

303

0.00380

0.00350

0.00430

0.00220

0.00350

313

0.00420

0.00400

0.00450

0.00380

0.00400

323

0.00460

0.00430

0.00510

0.00410

0.00430

298

0.01310

0.01360

0.01250

0.01410

0.01360

303

0.00400

0.00360

0.00450

0.00340

0.00360

313

0.00570

0.00520

0.00630

0.00480

0.00520

323

0.00680

0.00630

0.00740

0.00590

0.00630

Bio-II+Bare Diesel Fuel

Bio-III+Bare Diesel Fuel

The root mean square prediction difference percentage errors for the LorentzLorenz, Dale-Gladstone, Eykman, Newton, and Kay models [51] are shown in Table 1. From Eq. (12-16), we can infer that there are no noteworthy changes in

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prediction accuracy for the refractive index of mixes. Prediction accuracy using Krisnangkura model [51, 54] (Eq. 17) is shown in Table 1. Notably, the greatest inaccuracy recorded was merely 0.0141 percent. There is reasonably good concurrence between the expected and the detected values, which will be useful in theory and practice along with diverse applications. Fuel Energy Adulteration Sensor Performance Parameter The sensitivity, detection accuracy, and overall quality parameter of the recommended biodiesel fuel molar fraction sensor are investigated in this study and defined by the following expressions.

S   res na

or, 𝑆 = ∆𝜆𝑟𝑒𝑠 ⁄∆(𝑉1 ⁄𝑉 )

(19)

where res is the change in resonant sensing peak wavelength (S.P.W.) and n a is the variation in R.I. of an aqueous medium, 𝑉1 is the volume of biodiesel and V is the volume of a pseudo-binary mixture. Further, the detection accuracy is defined as:

D. A.  res 0.5

(20)

where 0.5 is spectral width at 50% of transmittance. Again, the quality parameter is defined as:

Q.P.  S 0.5

(21)

NUMERICAL RESULTS AND DISCUSSION In this chapter, we formulated a theoretical model of a BFW sensor with a periodic defect to optimize the temperature-dependent molar fraction of three kinds of biodiesel fuel (BIO-I, BIO-II, and BIO-III). This sensor is designed at the low loss operating window λc = 1550nm of the EM wave. The core is kept material-free so that it can be utilized as a flow cell. The dia of the material-free core is kept d = 2rc = 330µm. Further, the material-free core is surrounded by many concentric layers of high R.I. material Arsenic tri-selenide (nH = 2.82) and low R.I. material Poly-ether-imide PEI (nl = 1.66) [55 - 57] with their own thicknesses which follow the quarter-wave condition nHdH=nLdL=nDdD=λc/4. Furthermore, a defect layer of low R.I. material (Poly-ether-imide) having a refractive index (nd=1.66) and quarter-wave thickness dD =λc/4nd is introduced after the 8th unit cell of (H/L) bilayers out of N=16 by breaking the symmetry. In our present study, the glass polymer combination has been taken for the ease of

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fabrication of BFW. Polymer PEI shows good thermal stability up to 4000C [68] with temperatures above 220OC which is the transition temperature of glass in the visible and near-infrared region. This is quite important for further investigation of the temperature-dependent transmission spectra. Tables 2,3, and 4 show the results of our present study. Figs. (2 - 6) show these results graphically. At last, Table 5 illustrates the performance of the current device. Table 2. At a variable atmospheric temperature, the transmission peak position of normal diesel fuel. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier) S. No.

T (K)

R.I. of Normal Diesel

T. P. P. (10-9m)

I.

298

1.4650

954.4

II.

303

1.4631

956.8

III.

313

1.4591

961.9

IV.

323

1.4548

967.3

From the results, we can infer that with an increase in temperature, ranging from 25 to 500C, there is a lowering of about 0.01020 in the refractive index of the normal diesel owing to the change in the molar concentration. Hence, the core refractive index varies and a redshift of about 12.9nm is achieved in S.P.W. with full width at half maxima (FWHM) of about 0.1nm. In Table 3, the temperature and molar fraction-dependent refractive index of the biodiesel and normal diesel mixture are shown by means of (7) and Table 1. Table 3. The temperature-dependent molar percentage of different biodiesel fuels in normal diesel causes variations in the R.I. of pseudo-binary mixes (using (7) and Table 1). (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier) S. M. BIO-I R.I. at various BIO-II R.I. at various BIO-III R.I. at various No. F. temperatures and molar fractions temperatures and molar fractions temperatures and molar fractions (V1 /V %)

298 (K)

303 (K)

313 (K)

323 (K)

298 (K)

303 (K)

313 (K)

323 (K)

298 (K)

303 (K)

313 (K)

323 (K)

1

0 1.46500 1.46310 1.45910 1.45480 1.46500 1.46310 1.45910 1.45480 1.46500 1.46310 1.45910 1.45480

2

5 1.46450 1.46260 1.45870 1.45440 1.46450 1.46260 1.45870 1.45440 1.46440 1.46250 1.45860 1.45430

3

10 1.46400 1.46220 1.45820 1.45400 1.46400 1.46210 1.45820 1.45390 1.46390 1.46200 1.45810 1.45380

4

15 1.46350 1.46170 1.45780 1.45350 1.46340 1.46160 1.45770 1.45340 1.46330 1.46140 1.45760 1.45330

5

20 1.46290 1.46110 1.45730 1.45300 1.46280 1.46100 1.45720 1.45280 1.46260 1.46080 1.45700 1.45270

6

25 1.46240 1.46060 1.45680 1.45250 1.46230 1.46050 1.45660 1.45230 1.46210 1.46030 1.45640 1.45210

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Table 4. Using the temperature-dependent molar volume fraction of various biodiesel fuels to detect peak wavelength location for pseudo-binary mixtures at various temperatures. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier). S. M.F. No. (V1 /V%) 1

BIO-I Peak Wavelength Position at Various Temperatures (nm) 298 (K)

303 (K)

313 (K)

323 (K)

BIO-II Peak Wavelength Position at Various Temperatures (nm) 298 (K)

303 (K)

313 (K)

323 (K)

BIO-III Peak Wavelength Position at Various Temperatures (nm) 298 (K)

303 (K)

313 (K)

323 (K)

0

954.40 956.80 961.90 967.30 954.40 956.80 961.90 967.30 954.40 956.80 961.90 967.30

2

5

955.00 957.50 962.40 967.80 955.00 957.5 962.40 967.80 955.20 957.60 962.50 967.90

3

10

955.70 958.00 963.00 968.30 955.70 958.1 963.00 968.40 955.80 958.20 963.20 968.60

4

15

956.30 958.60 963.50 968.90 956.40 958.7 963.70 969.10 956.60 959.00 963.80 969.20

5

20

957.10 959.40 964.20 969.60 957.20 959.5 964.30 969.80 957.50 959.70 964.50 969.90

6 25 957.70 960.00 964.80 970.20 957.80 960.1 965.10 970.40 958.10 960.40 965.30 970.70 The results reveal that for different molar volume fractions at different temperatures, there is a considerable red shift in sensing peak with the least FWHM of 0.1nm which will be used further to optimize the performance parameter of the BFW sensor with the geometrical defect.

Fig. (2). PBG estimation and peak location sensing for normal diesel at various temperatures using transmittance spectra. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier).

Since the role of the temperature has bigger significance in this present study, consequently in Fig. (2), the PBG and resonant S.P.W. position have been optimized over a spectrum of normal diesel at altered temperatures. The achieved result has been tabularized in table 2.

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Fig. (3). Estimation of S.P.W. position for different molar fractions of BIO-I in normal diesel fuel at different temperatures in the spectrum. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier).

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Fig. (4). Estimation of positions of S.P.W. considering various molar fractions of BIO-II in normal diesel fuel taking different temperatures in the spectrum. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier).

We shall now consider the data in Table 3 to optimize the sensing peak position considering the temperature range from 250C to 450C for all three varieties of biodiesels, namely Bio-I, II, III having dissimilar molar volume fractions (V1/V%; 0 to 25%) in conventional diesel. Table 4 shows these results.

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Fig. (5). Estimation of positions of S.P.W. position considering various molar fractions of BIO-III in normal diesel fuel taking different temperatures in spectrum. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier).

The graphs in Fig. (6) indicate the relationship between S.P.W. and different molar concentrations of BIO-I, BIO-II, and BIO-III in the prescribed temperature range (a, b, and c). This study reveals the linear signature of variation in between S.P.W. with different molar fractions at various weather temperatures. The recommended sensor will operate well, as shown in Table 5, because of the constant sensitivity acquired by this linear fluctuation.

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Fig. (6). The linear signature of S.P.W. vs. different molar fractions of BIO-I, BIO-II, and BIO-III at various temperatures. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier).

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Table 5 clearly shows that the greatest sensitivity attained is 1280nm/RIUor 5120nm/(V1/V). We also compare our findings with Sharma et al. [52], who employed a photonic sensor based on phoxonic crystal to detect biodiesel in a pseudo-binary mixture of diesel at a static temperature. In terms of sensitivity, our result of 1280nm/RIU is considerably superior to their result of 142nm/RIU. As a consequence, the BFW sensor might be beneficial for regulating biodiesel's temperature-dependent molar concentration in a pseudo-binary mixture. Furthermore, because the FWHM value of the resonant sensing peak is small enough 0.1nm, other performance metrics such as quality and detection accuracy, which are firmly inversely proportional to the FWHM (Eqs. 20 and 21), may be enhanced. To the best of my knowledge, we are the first to present a sensor with a greater performance parameter than those now available. Table 5. The present BFW sensor's sensitivity estimate is utilized for optimization of the molar fraction which is temperature-dependent considering 3 diverse biodiesel fuels: BIO-I, BIO-II, and BIO-III. (“Reprinted from Fuel, 293, NK Chourasia, A Srivastava, V Kumar, RK Chourasia, Optimizing temperature-dependent molar volume fraction of biodiesel fuel in a pseudo-binary mixture through Bragg fiber waveguide sensor having defect layer, 120489., Copyright 2021, with permission from Elsevier). S. No.

Bio diesel Variety

Sensitivity 298 (K) 10 m/RIU -9

303 (K)

10 m /(V1/V) -9

10 m /RIU -9

313 (K)

10 m /(V1/V) -9

10 m /RIU -9

323 (K)

10 m /(V1/V) -9

10 m /RIU -9

10-9m /(V1/V)

BIO-I

1064.510 4258.040 1032.580 4130.320 1035.710 4142.840 1074.070 4296.280

BIO-II

1259.260 5037.040 1269.230 5076.920 1280.000 5120.000 1240.000 4960.000

I. 1. II. 2. BIO-III 1275.860 5103.440 1285.710 5142.840 1259.260 5037.040 1259.260 5037.040 III. 3.

CONCLUSION This chapter successfully demonstrated the use of a BFW photonic nanostructured adulteration sensor by breaking the periodicity of the structure and thus creating defects for optimizing the temperature-dependent molar fractions of numerous types of biodiesel fuel using pseudo-binary mixture in various weather conditions. Because of the defect in the BFW structure, which is sensitive enough to changes in the core R.I., a modest sensing peak is generated in the observed PBG. The sensing parameters of the suggested sensor are discretely increased when compared to the different sensors due to the shortest FWHM of the sensing peak.

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Photonic Materials: Recent Advances and Emerging Applications 259

The BFW sensor has a maximum sensitivity of 1280nm/RIU, which is greater than the 142nm/RIU reported for a phoxonic crystal-based photonic sensor for simultaneous biodiesel sensing at static temperature. Since additional sensing characteristics, such as quality parameters and detection accuracy, are inversely proportional to the FWHM of resonant transmission peak, the present structure's shortest FWHM of resonant transmission peak improves these sensing parameters concurrently. The present study will lead in eliminating gasoline adulteration, and it must be useful to civilization and industry in a number of ways. CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

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265

CHAPTER 13

Modelling Fabrication Photonic Devices.

Variability

in

Silicon

Mursal Ayub Hamdani1,* and Gausia Qazi1 Department of Electronics and Communication, National Institute of Technology, Srinagar, India 1

Abstract: Silicon photonics allows for high yield and complex integration with large processing, packaging, and testing availability. Using silicon as a material leverages the use of the existing CMOS infrastructure with hybrid and epitaxial layer integration, allowing photonic system-on-chip. Although high refractive index contrast with submicrometer waveguide dimensions allows a dense integration, sensitivity to fabrication variations shows an increased effect. This sensitivity shows a cumulative effect on the optical properties of complex silicon photonic circuits such as lattice filters, and wavelength division multiplexers (WDM). This increases the demand for model fabrication variation at the design stage itself since the fabless users have no insights into the process specifications. As a result, reliability modelling of photonic circuits has shown significant interest in recent years. This is done by using efficient behavioural models at the circuit level and then applying random variations in the model parameters to assess the impact of these variations. In this chapter, different approaches to modelling fabrication variations in photonic integrated circuits, such as Monte Carlo (MC), Stochastic Collocation (SC), and Polynomial Chaos Expansion (PCE) are reviewed. These methods employ random distribution to the varying parameters with the correlation between different parameter sets fixed. Virtual Wafer-based MC (VWMC) allows layout-aware variability analysis, where the placement of circuit components on the layout coordinates is exported to the circuit design for dependence analysis. Using these methods, mitigation strategies to counter the manufacturing variations such as thermal compensation, and tapered designs are quantitatively evaluated by appropriate yield analysis and design for manufacturability.

Keywords: CMOS, Lattice filters, Monte carlo simulation, Silicon photonics, Stochastic collocation, WDM. INTRODUCTION Silicon photonics integrates multiple optical operations on a single chip via CMOS fabrication thereby enabling low cost and high volume manufacturing. Corresponding author Mursal Ayub Hamdani: Department of Electronics and Communication, National Institute of Technology, Srinagar, India; E-mail: [email protected]

*

Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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The high refractive index contrast (Si = 3.4757, SiO2 = 1.5277 at 1550 nm) allows sub-micrometer waveguide size, small bend radius and dense packing on chip [1]. However, the high index contrast imposes restrictions on the dimensional variability (nanometer resolution) due to a small fabrication mismatch. The more complex the photonic circuit, the higher the effects of fabrication variability on its performance [2]. This eventually affects the overall yield of the chip design rendering wafer material to waste. The challenge of capturing the effects of this variability at the design stage to enable a pre-fabrication yield estimate is one of the primary areas of investigation in silicon photonics currently [3 - 7]. Taking the example of a silicon photonic transceiver, yield analysis would entail determining the percentage of chips that have modulators working with bit error rates (BER) below a threshold value [7]. The design for manufacturability (DFM) ensures that the high yield is done at three levels [4, 5]. It starts with the robust design of device elements (like waveguides, and directional couplers) of a photonic system. Secondly, optimization of the photonic circuit, by selecting the robust device components and ensuring proper routing at the schematic level. It should take into account the proper correlation between the device components requiring extensive models continuous in the variational parameter space. These models monitor and map the device variations while conserving its physical properties like passivity, stability, and causality [5]. Finally, the remaining imperfections are compensated using active compensation techniques. In this chapter, we discuss all the three levels of silicon photonic- DFM. In Section II, sources of device variation and their effect, photonic device optimization methods are introduced. In Section III, the evaluation of photonic circuit performance and modelling for correlation is discussed. SC, PCE and layout-aware variability is analyzed for design under test. PHOTONIC DEVICE LEVEL OPTIMIZATION For any photonic circuit, the variation starts with the process conditions of individual elements such as optical waveguides, directional couplers, y-branches, etc. The process conditions include exposure dose, plasma density, slurry composition, mask alignment, and chemical, and mechanical polishing (CMP) for planarization [5]. These process steps are not exactly reproducible from die to die, wafer to wafer and one lot to another [6]. These processing steps affect the pattern density which can lead to geometrical variations such as in waveguide width, thickness, sidewall angle, doping profiles, etc. [7]. The effect of pattern density on the waveguide width with an intra-die correlation coefficient of 0.57 at optimum selected window size resolution of 69 um has already been demonstrated [7]. As

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far as the waveguide thickness is concerned, its variability is mainly attributed to silicon on insulator bonding [7]. To check these geometrical variations, we need measurement methods characterizing these variations. Atomic force microscopy, scanning electron microscope and other indirect methods are some of the common methods employed [5, 8]. Process control monitors with reflection optics are also commonly employed to monitor geometric variations during or after the fabrication. However, most of these methods are costly, time-consuming with the risk of damaging the device permanently [8]. Recently transmission spectrum based methods have been employed to extract the geometry of photonic devices using either the resonance shift method or the curve tracing method [7, 9]. A single circuit with a double MZI circuit has been proposed to monitor and extract multiple parameters of waveguides and directional couplers [7]. Once the exact statistical distribution of geometrical variations is known, the effect on optical properties will follow. For optical waveguides, effective index and group index and for directional couplers, coupling coefficient, splitting ratio vs wavelength are the figures of merit (FOM) [8]. This device FOM then affects the circuit FOM and photonic circuit yield analysis and thus taking the design for manufacturability (DFM) into consideration becomes a possibility. Intuitively, this design of a single device component for a fixed FOM is mostly made by physics-based analytic models, practical know-how and intuition [9]. With complicated geometries required for optimum FOM values, the light matter interaction via numerical electromagnetic simulations gets complex [9]. For an efficient design process, an inverse design assisted by iterative optimization methods and deep neural networks (DNN) is being employed in the state of art designs [10]. The alternative optimization algorithms show satisfactory optimization results, however, the need for parameter sweeps makes it laborious since electromagnetic simulations tend to become time-consuming in that case. The relation between geometry and optical FOMs becomes like that of nondeterministic (NP) hard problems, which are not easy to define explicitly [10]. Iterative gradient free algorithms such as genetic algorithm (GA), particle swarm optimization (PSO) and direct binary search (DBS) have been extensively used to solve such NP hard problems in optical devices [11, 12]. Recently, gradient-based topology optimization (TO) for an irregular design structure based on silicon photonic devices has also been used [13]. Instead of the common iterative algorithm approach of taking key parameters and abstracting them, TO takes the entire design shape space as a bitmap [13]. After selecting the entire design space, all the pixels of this bitmap are iteratively updated towards the direction of gradient descent until the gradient resolves them to a non-assuming value. Very complicated device structures have been possible using this method. However, all the iterative optimization algorithms discussed

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become extremely laborious when similar devices have slightly different target responses such as power splitters with different split ratios or grating couplers at different wavelengths. Lately, DNN (Deep Neural Network) has emerged as a great alternative to iterative algorithms in silicon photonics design. Originally designed as a tool to link features and their uses, it has found great leverage in silicon photonic device design [14 - 17]. Based on mapping directions between geometric parameters and optical response, DNN is trained as a forwarding model and inverse model. In the forward model, geometric parameters of a device are mapped to its optical response. It substitutes EM solvers to accelerate the device performance evaluation in optimizations. The inverse design however maps the desired optical response on the geometrical parameters taking almost no time to generate a device for specific FOM after the model is well trained, thereby eliminating the need for optimization. Iterative Optimisation Algorithms Based on the degrees of freedom (DOF), empirical, QR code and irregular type structure are possible with an inverse design using iterative optimisation algorithms [10 - 13]. By degrees of freedom, we point toward the ways in which the design can shift from the classical structures. Empirical type structures have the least DOF and resemble the classical structure. QR code structures can embed circles, rectangles and other polygon shapes by etching the structure to optimise FOM. The irregular structure takes the whole area as a bitmap and has the most complex structure. Empirical Optimisation Algorithms Parameters such as the radius of waveguides are tuned for optical response targets using the classical structure too. This optimization, however, starts with known structures, grinds through tailoring the key parameters for optimum FOM with a time lag due to parameter set EM simulation sweeps [10]. This relationship between parameters from the empirical structure and the optical response is an NP hard problem which is usually approached via heuristic algorithms such as GA algorithm(used for polarization beam splitter (PBS)) [11] and PSO algorithm wherein the parameters are not updated at each iteration but all parameter steps are optimized towards a global optimised FOM [12]. QR-code Structure Algorithms The designers only define the pitch and the size of the etched holes as well as the footprint of the design area such as in power splitters, polarization rotators, polarization beam splitters, WDM, MDM, crossings and grating couplers [12]. Direct binary search (DBS) algorithm which is the kind of brute force algorithm is

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used with QR structures [12]. During each iteration, one pixel in the design area is reversible and the average transmission efficiency of TE and TM mode is calculated and compared with the FOM of the last iteration. For PBS, 70% efficiency is achieved via PBS algorithm with one dB Bandwidth of 83 nm [12]. To reduce the number of iterations, heuristic algorithms like GA and PSO consider all pixels as individuals and update them together during each iteration [11]. The DBS method for QR-Code structures is shown in Fig. (1).

Fig. (1). DBS Method for QR-Code structures.

Irregular Structure Algorithms While QR code structure pixelates the area of design with a 100-200 nanometre resolution [10], the irregular structure variant has a higher resolution of 10-20 nm [10]. The gradient free algorithms do not work well with such a high resolution structure due to time lag, since a very large number of parameters need to be updated via a large number of simulations. Gradient based Objective First (OF) and Topology Optimisation with the adjoint method are proposed to solve this issue [10 - 12]. In this case, PBS-TO is taken as an example. The first FOM is the transmission of TE mode upper arm and the second FOM is

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the transmission of TM mode lower arm. The design domain Ds needs to be optimised. In this method, the derivative of total FOM to permittivity and permeability in the design domain is calculated by the chain rule. In the design domain, original geometry has E(xs) at xs position and if a small volume at xs has ∂Ɛ permittivity change, an induced dipole moment exists. The EM field variation at xs will occur due to summation effects of this induced dipole moment via green function. We define the adjoint electric field at xs as [12], 𝐷0

𝐸𝐴 (𝑥𝑠 ) = ∫

𝑑3 𝑥0 [ 𝐺𝐸𝑝 (𝑥𝑠 𝑥0 ).

0

−𝜕𝑓 𝜕𝑓 + 𝐺𝐻𝑝 (𝑥𝑠 𝑥0 ). ] 𝜇𝐻(𝑥𝑜 ) 𝜕𝐸(𝑥𝑜 )

(1)

The equation indicates that adjoint electric field EA(xs) is obtained through integration of all induced electric fields which come from EM dipoles amplitudes (G) at different positions x0 in the design objective domain. Through an inverse simulation where light source with amplitude is placed in the objective domain, all the adjoint fields in the design domain can be numerically calculated. With one forward simulation which calculates all the electric fields and one inverse simulation which computes all the adjoint electric fields in the design domain, the derivatives of total FOM to permittivity can be obtained. Finally, the permittivity in the design domain is updated towards the direction of the total gradient as [12]: ∈𝑛𝑒𝑤 =∈𝑜𝑙𝑑 𝑠 𝑠 + 𝛼(

𝛿𝐹 −𝜕𝐹 ′ . ) 𝛿𝜖𝑠 𝜕𝜖𝑠

(2)

where α is the rate of permittivity change due to light source validation. During the updating process, the permittivity ϵs is taken as a continuous variable but discretized for real applications (level set optimization, density optimization, selfbiasing, neighbourhood biasing approach) [13]. Minimum feature size control is another important issue in irregular structures as large DOF can bring features with critical sizes too small to be fabricated [13]. This is done via density filters, penalty functions artificial damping morphological filters or b–spline control method [13]. After discussing the different iterative methods, in brief, we list the comparison between all the different variants as in Table 1.

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Table 1. List the comparison between all variants of Iterative Algorithm based Inverse Silicon Photonic Design Type

Design

Empirical Structure Waveguide Crossing

SWG

GA

Converter

Taper

PSO

PBS

DC

PSO

PR1

QR Code

GA

PR2

QR Code

DBS

MDM

QR Code

DBS

Bends

QR Code

DBS

PBS

QR Code

DBS

Converter

QR Code

DBS

WDM

Irregular

OF

PBS

Irregular

TO

Converter

Irregular

TO

QR code Structure

Irregular Structure

Structural Base Algorithm

Comments ● Not compact. ● Low degrees of freedom. ● Average optimization performance. ● Compact Design. ● Not hard to fabricate. ● Good optimization performance.

● Best DOF available ● Ultra Compact ● Fabrication Issues ● Best Optimization performance

Deep Neural Networks Assisted Silicon Photonics Design If more than one device with a similar structure and optical responses is needed for a specific scenario like arbitrary power splitters, mode converters, wavelength filters grating couplers for different wavelength bands, iterative algorithms need to be performed multiple times for different target responses. Thus we need to find the relationship between optical response and geometric parameters instead of optimising one by one. DNN helps to find very complex and non-linear relationships between input and output data [14]. DNN-based silicon photonic design is shown in Fig. (2). Multilayer Perceptron A forward model takes device geometry and builds optical response via regression (discriminative neural networks), multilayer perceptron (MLP), and convolution neural networks (CNN) for this discriminative problem [14, 15]. MLP is trained to predict the coupling efficiency of grating couplers [15]. The trained MLP has 93.2% prediction accuracy while being 1800 times faster than the FDTD simulation [15]. The trained eight layer Res-Net with 20000 data samples are used to predict the output spectrum of power splitter with a QR code-like structure [15].

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Fig. (2). DNN based Silicon Photonic device Design.

Convolution Neural Network MLP finds it hard to process devices such as irregular structures, QR code structures without the need of reducing data dimensionality [12, 16]. Therefore, CNN has been used as a forward model to predict the output spectra of QR codes like power splitters. The main geometrical structure is separated into pixels. Each pixel is with state 0 or 1 for being non-etched and etched, respectively [12, 16]. This takes the form of the m×m matrix as an input feature of CNN. The input features go through three convolution layers with proper activation and the extracted features from the convolution layer are condensed via the pooling layer and finally flattened into a vector. This vector containing extracted features is then projected to data labels y via fully connected layers with a sigmoid activation function. 15.8% better accuracy than a four layered MLP for the same trained data samples with QR code structure is obtained [12, 16]. Since a specific optical response can have multiple component geometries, it is hard to model DNN as an inverse model using a discriminative approach. The generative approach is used in this case as it is a one-to-many projection case. The underlying distributions of data features (x) and data labels (y) (which are geometry parameters and optical response respectively) need to be modelled with a high number of data samples. To ease the demand for high sample size, semi supervised generative neural networks such as the conditional variational autoencoder (CVAE) and conditional generative adversarial network (CGAN) are proposed for building inverse models [17]. In CVAE, data features are generated through a decoder with sampled normal distribution. Latent vector elements z, which are sampled from Gaussian distributions are described by μ and σ. The sequence of steps for a general CVAE training model is shown in Fig. (3a).

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Fig. (3). The training process for (a) conditional variational autoencoder using a QR-code structure (b) conditional variational autoencoder design steps.

These elements are concatenated with optical response y to work as input to the auto-encoder. After de-convolution, the training sample data set is obtained. The aim of this DNN-based training is to decrease the difference between the original and generated geometry, and decrease Kullback and Leibler (KL) divergence between latent z and the Gaussian prior [17]. The splitting ratio is achieved with 90 per cent efficiency by the PBS design using this method. In CGAN, game theory for training is used [17]. It consists of a generator and a discriminator wherein the former produces the data features (geometry) while the discriminator judges whether the group of generated data features are real or not. The process of training a CGAN as an inverse model has been used to predict the irregular geometry of optical components such as meta gratings, with a particular diffraction angle and wavelength in mind. Efficiencies of the order of 75%, much better than randomly generated Meta gratings (30%) are possible [12, 16, 17]. The sequence of steps for a general CGAN training model is shown in Fig. (3b). Recently, the use of an unsupervised neural network as an inverse model to generate irregular meta-grating geometry was successfully tried. Lately, optical neural networks (ONN) has shown great appeal as an alternate for conventional electronic computation due to the better ability of parallel computation with low power consumption [18]. The layered ONN has a traditional layered feed-forward structure as has been used to design programmable MZI with a unitary matrix design approach [18]. The second is a black box ONN approach with direct mapping of input signals to desired output ports [18].

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PHOTONIC CIRCUIT LEVEL OPTIMIZATION The effects at the device level cumulatively add up to the circuit level and if the circuit elements are distant compared to the size of the circuit, it is challenging to assess the impact on system performance. Considering variations in the effective index of the waveguide, it leads to deviations in optical filter response with increased ripple and channel crosstalk [19]. At the circuit level, the Monte Carlo process with a large number of simulations is mostly used. However, being computationally intensive, it paved the way for the stochastic collocation (SC) method [20] wherein surrogate models replace the simulation models in order to reduce the number of simulation counts. If the circuit model incorporates original model parameters and its distribution moments, the polynomial chaos expansion (PCE) method also becomes an option [21]. In PCE, a more complex model solves the entire circuit simulation in one cycle, while having prerequisite knowhow of probability density function (PDF) of the model parameters. However, both SC and PCE methods consider the stochastic variation of the parameters, thus the information of the design layout cannot be introduced and considered in these methods. However, to get the actual picture of variability, the layout information cannot be neglected completely. To proceed with the inclusion of layout information, distribution of geometry variations in the wafer used is needed along with the layout of the design under consideration (DUC). The latter DUC is superimposed on the former wafer distribution in-order to extract the local distribution of width and thickness. However, this approach requires that the geometrical variables like width and thickness can be used to evaluate the parameters like effective index or loss of elements such as waveguide of DUC. Considering a DUC parameter G affected by the parameter y at position x, the use of perturbation theory leads us to, G(x) = Go +

𝜕𝐺 𝜕𝑦

∆𝑦(𝑥)

(3)

where in is characterized by simulation or measurement. The DUC layout information can then be linked to the circuit models via GDSII layout design or secondly by simply via parametric cell approach as in the IPKISS design framework. In the latter approach, the variability matrix based annotation method is used for variability analysis [22]. The annotation is based on linear, polynomial or custom python functions. The collection of data for each component of the design is performed using the sampling point’s data structure and interpolation approximations. Thus, layout-aware mismatch analysis is done by first positioning the DUC on the virtual wafer, using each sample point to evaluate the local geometrical variations and then using a characterization based sensitivity matrix to update the model parameters for different positions on the wafer. In the

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following subsections, we further discuss SC, PCE and layout-ware methods. Stochastic Collocation Method The SC method is used for both device and circuit level variability analysis [20]. It approximates via the use of interpolation functions in the stochastic space. The cyclic process of solving a non-random problem at predetermined nodes leads to the construction of interpolation. A stochastic process is expressed as [20] 𝑄

𝑌(𝜀) = ∑

𝑌(𝜀𝑖 )𝐿𝑖 (𝜀)

(4)

𝑖=1

where ε denotes the stochastic parameters and Li is the interpolation basis. In photonics, Y corresponds to parameters like the waveguide propagation constants and the coupling coefficients in DC. The stochastic variables ε correspond to device properties affected in a stochastic way by fabrication or other conditions (e.g. width, or temperature change). The number of nodes N for computing an accurate SC model (which is directly related to the efficiency of SC methods) depends upon many factors. The chosen random variable ε, the interpolation scheme adopted, the sampling technique used (efficient sampling limits the number of collocation points), the number of random variables considered is the factor that effects N selection. To increase the efficiency of the SC method, two methods commonly used are the nested sampling method and adaptive the sparse grid method. The effect of variability on the coupling coefficient of the DC is checked using an SC model via Gaussian random variables ε1, ε2.The results are validated through Monte Carlo analysis too. In Fig. (4), we compare the results of Probability Density Function (PDF) and Cumulative Density Function (CDF) obtained from the two methods. At coupling coefficients of 72,540, a sharp rise in CDF with a sharp fall in PDF is observed as shown in Fig. (4). The SC method follows the general MC method while allowing calculations with a reduced simulation cost.

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Fig. (4). PDF vs coupling coefficients at 1.55 μm wavelength via SC – Dark Black and CDF via SC - Grey, DOTS FOR MC method.

Polynomial Chaos Expansion The variable of interest is approximated using polynomial basis functions. It leads to a surrogate model that provides statistical quantification efficiently [21]. Important quantities of interest such as distribution moments and probability density function are computed with zero lag computation approximation. The PCE model expands the output variable in terms of both process variables and design variables. The cost estimate in terms of design constraints is also provided under this estimation leading to good yield quantification. Thus PCE is also useful for design circuit optimization using optimization algorithms such as the iterative ones discussed earlier. In particular, the cost function is a multivariate polynomial because generalized polynomial chaos bases are polynomials and a global polynomial optimization solver can be employed [21]. The sparse combined generalized polynomial chaos model has also been employed recently with good design optimization performance. Let W(X,ε) B be the quantity of interest for any process based random variations, where X is the vector of state/design variables and ε is the random vector for process variation parameters, then the generalized PCE is written as: 𝑊(𝑋, 𝜀) = ∑∝

𝐶∝ ∅∝ (𝑋, 𝜀)

(5)

where Ø∞(X,ε) is the orthogonal polynomial and C∞ (x) is the coefficient with

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nonnegative multi-index ∞ obtained by the Galerkin method or stochastic collocation method. To test the PCE method, we take the example of the 5th order coupled ring resonator filter designed as a standard Butterworth synthesis design. The nominal features include a bandwidth of 15.6 GHz, coupling coefficients of k1=k6=0.3; K2=k5=0.03; k3=k4=0.009, length five ring structures as 230 μm, coupling gap of 0.4 μm and FSR = 200 GHz. For this photonic design, the ε vector includes the effective index perturbation vector, while X declares coupling gap values. Thus the PCE problem comes up during optimization of the coupling gap to get minimized bandwidth deviation. Fig. (5a) shows the filter design and 5(b) makes a comparison between the probability density function (pdf) of the 3-dB bandwidth via Monte Carlo samples and surrogate PCE method with good approximation observed.

Fig. (5). (a) Sketch of 5 ring filter. (b) pdf vs 3-dB bandwidth of the filter under fabrication variations modelled with Monte Carlo samples and through generalized PCE model.

Layout Aware Variational Analysis Variability modelling based on layout aware functionality was first introduced in [22] and it begins from a wafer map showcasing geometrical variations in width and thickness along with the GDSII layout of the circuit under test as already mentioned. Fig. (6) summarizes the sequence of steps to involve layout information into the variability testing scheme. Above the dashed line, circuit design flow is shown, starting with PDK blocks and designing circuits both as schematic and layout. The simulated output for nominal design is plotted. The PDK models are provided with a sensitivity list (either from measurement or

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simulation) to give a wafer distribution of global variables. The Monte Carlo simulations then provide the fabrication variation aware simulated output. The wafer variation is either taken from measurement data samples or using simulation noise generator toolboxes like that provided by Open-Simplex [22].To check the effect, we take an example of a Mach Zehnder Interferometer (MZI) circuit as shown in Fig. (7). The MZI consists of the y-branch acting as a beam splitter and combiner while the path length mismatch leads to a nominal interference pattern with a defined free spectral range (FSR) as shown in Fig. (8). The grating couplers are used to input and output light from optical fiber to integrated chip and vice versa. The nominal response assumes no fabrication variations. However, when such integrated circuits are fabricated, the interference pattern is extremely disturbed even with the best state of art fabrication facilities. This is mainly due to the effect of width and thickness variation in the wafer on which the circuit is imposed and then fabricated [22]. Using layout-aware approach we can model the fabrication variability at the design level itself thereby rendering the designers more flexible in trying different approaches to counter the manufacturing mismatches. We have taken the wafer’s width and thickness wafer distribution map [22] as σwidth= 5.4nm and σthickness= 2.4nm and applied it to our MZI design.

Fig. (6). The methodology of introducing layout aware variability testing. (Reprinted by permission from Springer Nature: Springer Silicon, Evaluating Variability and Improving Tolerance in a Novel and Compact Silicon Photonic Michelson Interferometer, Mursal Ayub Hamdani and Gausia Qazi., Copyright 2022).

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Fig. (7). Layout of photonic integrated Mach Zehnder Interferometer circuit. (Reprinted by permission from Springer Nature: Springer Silicon, Evaluating Variability and Improving Tolerance in a Novel and Compact Silicon Photonic Michelson Interferometer, Mursal Ayub Hamdani and Gausia Qazi., Copyright 2022).

Fig. (8). Nominal transmission response with fixed FSR. (Reprinted by permission from Springer Nature: Springer Silicon, Evaluating Variability and Improving Tolerance in a Novel and Compact Silicon Photonic Michelson Interferometer, Mursal Ayub Hamdani and Gausia Qazi., Copyright 2022).

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This leads to a transmission spectrum as shown in Fig. (9). It is clear from the Fig.(10) that wavelength peaks are not clearly distinguishable thereby rendering the use of MZI for multiple stage MZI based lattice filter or WDM as defunct. Using thermal tuning to tune the variation of neff due to width and thickness leads to a more stable output waveform as shown in Fig. (10).

Fig. (9). Fabrication variation aware transmission response for nominal MZI with σwidth =5.4nm and σthickness= 2.4nm for the input wafer model. (Reprinted by permission from Springer Nature: Springer Silicon, Evaluating Variability and Improving Tolerance in a Novel and Compact Silicon Photonic Michelson Interferometer, Mursal Ayub Hamdani and Gausia Qazi., Copyright 2022).

Fig. (10). Fabrication variation aware transmission response for thermal tuned MZI with σwidth= 5.4nm and σthickness=2.4nm for the input wafer model. (Reprinted by permission from Springer Nature: Springer Silicon, Evaluating Variability and Improving Tolerance in a Novel and Compact Silicon Photonic Michelson Interferometer, Mursal Ayub Hamdani and Gausia Qazi., Copyright 2022).

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By choosing appropriate deviation limits of peak wavelength variation, yield estimation for nominal MZI design and thermal tuned MZI is possible. If 0.3 × FSR variation is acceptable then only 36.2% of nominal MZI designs are acceptable for fabrication, while the same limit renders 76.12% of thermal tuned MZI as acceptable for manufacturing. CONCLUSION Manufacturing variability modelling needs more comprehensive attention in photonic integrated circuits in general and silicon photonics in particular due to high index contrast rending phase mismatch between two paths in interferometric and resonance structures. In order to deal with these variations, we follow either the optimization of device components via iterative path or DNN AI. However, if ordinary device components are used as such, quantifying the variations at the design stage rather than post fabrication stage can add a huge benefit in terms of wafer circuit yield quantification. We have discussed the MZI example to explain the effect of layout design on statistics of variability. Quantitative estimation of layout fabrication yields at the design stage is also done for an intended Fig. of merit (peak wavelength shift). CONSENT FOR PUBLICATION Not applicable. CONFLICT OF INTEREST The author declares no conflict of interest, financial or otherwise. ACKNOWLEDGEMENT Declared none. REFERENCES [1]

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CHAPTER 14

Introduction of Smart Materials: The Art to Outrival Technology Claire Mary Savio1 and Ishu Sharma1,* Department of Engineering and Technology, Amity University, Dubai, UAE Abstract: Smart materials are the name given to materials that can alter their properties on the application of external stimuli. Devices using smart materials might replace more conventional technologies in a variety of fields. Smart materials are attractive due to their lightweight, sensing capability, lower component size, and complexity combined with design flexibility, functionality, and reliability. A smart material is an object which is susceptible to undergoing a material property change and shows a visual and tangible reaction to external stimuli. Proper execution of smart materials will provide a level of environmental robustness that is not easily achieved through conventional technologies as they are susceptible to the influences of nature. One concept which includes the futuristic application of smart materials is the utilization of smart materials in the transportation sector using shape-memory alloys and piezoelectricity. Although the applications of smart materials are far-reaching, a greater dependency on them is prevented by certain drawbacks that need to be addressed if utilization of smart materials is to be accomplished, such as system compatibility, availability, cost, delicateness, decreased performance over time, difficulties with integration and toxicity.

Keywords: Conductors, Insulators, Piezoelectric, Self-Healing, Smart Materials. INTRODUCTION Due to the advancements in the field of material science, the usage of new, highquality, and low-cost materials in different fields of engineering can be observed. In the past century, materials have become more versatile and require the enhancement of various properties and features. Taking past advancements into account, it can be observed that researchers focused on composite materials, and currently, the upcoming revolutionary discovery is that of smart materials. Smart materials or responsive materials are futuristic materials that outperform traditional materials. Smart materials are beneficial due to their lightweight, Correspondence: Corresponding author Ishu Sharma: Department of Engineering and Technology, Amity University, Dubai, UAE; E-mail: [email protected], [email protected], [email protected] Aavishkar Katti and Yogesh Sharma(Eds.) All rights reserved-© 2023 Bentham Science Publishers

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component size, sensing capability, functionality, reliability, and complexity blended with design flexibility [1]. A smart material is an object receptive to undergoing a property change and shows a physically perceptible reaction to an internal or external stimulus [2]. Smart materials have the inherent and acquired ability to acknowledge a stimulus and to operate with respect to those stimuli by undergoing a pre-determined change in property. Pressure, electric or magnetic fields, nuclear radiation, and temperature are a few examples of stimuli that bring about a change in smart materials. The visible changes in physical properties that could potentially occur are stiffness, structural changes, or damping. Material science is used to characterize and determine the functions of various engineering materials, which is then used to develop successful engineering technologies. The quality and functioning of these products rely heavily on the material used for manufacturing the product. In order to make products with exceptional properties, material science engineers turned to smart materials to produce engineering materials. They explored the possibilities of producing conductors and insulators using these smart materials. In recent years, smart materials have been studied extensively for their applications in the aerospace, mechanical, and biomedical fields [3]. Smart materials or responsive materials can be classified as non-living systems which assimilate the functions of logic, sensing, control, and actuation [4]. It refers to any device that can sense a change within its environment, allowing for an optimal response by altering its geometry, mechanical, material, or electromagnetic properties [5, 6]. They may also be defined as structures consisting of distributed actuators, processing networks, and sensors [5]. This idea of smart devices arises from nature due to the response to stimuli capabilities present within all living organisms [6]. The material contains feedback functions in combination with the functions and properties of the material. Active responsive materials are defined as materials that can adjust their material or geometric attributes when under the influence of electric, magnetic, or a thermal field, as a result of which, they acquire an innate capacity for energy transduction [7]. They are capable of adapting based on the external stimulus on the material with an inherent intelligence [8]. These stimuli may refer to temperature, pressure, electric fields, chemicals, magnetic fields, or nuclear radiation and the corresponding physical changes can be shape, viscosity, damping, or stiffness. Piezoelectric materials are known for being used in strain sensors [9], accelerometers, stress waves emitters and receptors [10], actuators [11], pressure transducers [12], and vibration sensors [13, 14]. Table 1 summarizes the types of smart materials, properties, and applications.

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Table 1. Types of smart materials, properties, and applications. Type of Smart Material Piezoelectric Materials Shape Memory Alloys Magneto-restrictive Materials

Stimuli

Property Change

Applied Mechanical Voltage produced Strain Temperature Change

Applications Tracking devices, keyboards, microphones, etc.

Return to original Orthodontic wires, stents, bride shape structures, etc.

Change in Magnetic Field

Give rise to a voltage

Ultrasonic cleaning devices, underwater sonar, etc.

Thermo-chromic Materials

Temperature Change

Colour change

Fashion, home furnishings, food storage, etc.

pH-Sensitive Materials

Change in acidity

Colour change

Drug delivery, cancer imagery, etc.

Photochromic Materials

Light

Colour change

Optical data storage, eyeglasses, etc.

Depending on the temperature, the occurrence of smart materials happens in two stable phases: the material goes from the Austenite phase to Twinned Martensite on cooling, which transforms to Detwinned Martensite when it undergoes applied stress. The material returns to the Austenite phase at high temperatures. During higher temperatures, smart materials exist as Austenite and at low temperatures, they exist as Martensite. Martensite materials are known to have two forms: twinned Martensite and detwinned Martensite. Smart materials remain at a thermodynamically stable phase at a fixed temperature. When temperature varies, smart materials transform between Austenite and Martensite phase, which gives rise to their special properties, like pseudoelasticity and shape memory effect [3]. The material will remain in the Austenite phase under room temperature when the Austenite finish temperature (Af) is extremely low. In this phase range, it can be observed that the material gets converted to a detwinned Martensite phase when the external stress is applied. On the release of externally applied stress, the material goes back to the Austenite phase [3]. The most useful attribute of smart materials is that they possess the ability to carry out significant alteration of their properties in a restrained manner when exposed to stimuli. Built-in sensors, their control mechanism, and actuators are the components of a smart material that give it the ability to identify an external stimulus, fixedly react to that stimulus, and return to its actual state as soon as the external stimulus gets withdrawn [13]. Smart materials can be classified into various categories.

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Piezoelectric materials are a well-known example of smart materials that give rise to a voltage due to applied mechanical strain. Piezoelectric effect can be observed to occur conversely, wherein a voltage that is applied along the material will cause strain in the material. Technological appliances made using this material have the capability to deform, enlarge, and shrink due to an applied voltage or stress. These materials find uses in a large variety of areas, such as tracking devices, keyboards, microphones, high-frequency stereo-speakers, pressure sensors, magnetic heads, transducers, etc [13, 14]. Magneto-restrictive materials, which act like piezoelectric materials, the only difference being that they are only responsive to a change in the magnetic field. These materials are used in motors, in sonar transducers requiring low frequency, and hydraulic actuators, when combined with Nitinol. These materials are deemed to be promising candidates in the achievement of active damping of vibrations [13, 14]. Materials that show a change in colour caused in response to a change in acidity are known as pH-sensitive materials. They may swell or contract or show some form of change based on the change in pH. Depending on their applications, the design of these polymers varies. These materials find application in colour changing paints that indicates the occurrence of corrosion in the underlying metal [13, 14]. pH-sensitive materials can be of two types: those having acidic groups and the ones that have basic groups. The response mechanism for both is the same but the stimuli they react to vary. In the case of acidic or anionic polymers, they show swelling at a pH greater than their acidic strength, whereas basic or cationic polymers show swelling at a pH less than the acidic strength of the polymer. Applications of these materials include drug delivery systems, surface functionalization, biomimetics, separation processes, micromechanical systems, etc. Polymer gels possess a cross linked polymer arrangement that can be enlarged with a solvent. Variations in temperature or pH, give the polymer gel the ability to expand or contract [13, 14]. Polymer gels are found to be an attractive choice due to their unique physicochemical properties. They are one of the first gelation systems and have a wide range of applications in products found at cosmetic stores. It was discovered that these gels change their volume in a discontinuous and reversible manner as a response to external changes in their environment. Current research focuses on preparing polymer structures in the nanoscale to show unique functions or properties such as self-healing, high mechanical strength, and

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rapid response. These gels have a variety of applications, including drug delivery, soft actuators, biosensors, regenerative medicines, etc. Electrostrictive materials are materials that have properties like that of piezoelectric materials, however, the mechanical change taking place in these materials is proportional to the square of the electric field. This characteristic provides electrostrictive materials with the ability to give rise to unidirectional displacements. e.g., Lead Lanthanum Zirconate Titanate (PLZT) [15]. The property of all dielectrics or electrical conductors that brings about a change in shape due to an applied electric field is electrostriction. It is caused due to displacement of ions in the crystal lattice upon exposure to an external electric field. This displacement will collect in the bulk material and result in elongation in the direction of the electric field. Applications of these types of materials include sonar projectors for submarines, actuators to cause small displacements, etc. Optical fibers are used to transmit light between the ends of the fiber. They can be made into excellent sensors to measure temperature, strain, pressure, and other measurable quantities by altering a fiber so that its property to measure regulates intensity, phase, and wavelength [15]. Sensors that change light intensity are the simplest source as only a source and detector are required. Based on the application optical fibers can be used due to their small size or as no electrical power is required at the remote location. They have a wide range of applications in fiber-optic communications, as they can be used to transmit over large distances with higher bandwidths than electrical cables. Fibers are used in place of metal wires as signals travel along optical fibers with less loss and they are not affected by electromagnetic interferences. They have many uses in remote sensing and in certain applications, the sensor itself is an optical fiber. Chromogenic systems display a change in colour as a response to changes in charge, optical changes, or changes in heat. These systems include electrochromic materials, which exhibit a change in colour or opacity when a voltage is applied. Liquid crystal display is a highly common example of electrochromic materials. Similarly, photochromic materials display a change in color after undergoing exposure to light. This phenomenon can be viewed in spectacles that are sensitive to light and darken on exposure to sunlight [16]. Thermo-responsive materials are materials that form different shapes due to a temperature change. They can be of two types, namely, Shape Memory Alloys or Shape Memory Polymers. Their shape can contort, and they tend to go back to the primary shape they exist in nature, because of heating. Upon exposure to high temperatures, this material can regain its original shape. In this procedure, there is

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an actuating force that gets generated. Nitinol, which is an alloy of nickel and titanium having corrosion resistance analogous to that of stainless steel, is a shape memory alloy that is useful in the biomechanical field. These materials can be used in coffee-pot thermostats, super elastic spectacle frames, stents for the treatment of atherosclerosis. These materials typically find use as biodegradable surgical sutures that tighten automatically in order to obtain the correct tension, and in car bodies, can be used for auto-repair as it recovers shape on gentle heating after denting [16]. There are two categories of smart materials, active smart materials, and passive smart materials. Active smart materials can convert energy from one form to another while passive smart materials cannot do so. Responsive materials that hold the potential to undergo an alteration to their properties due to the application of a magnetic, thermal, or electric field, and hence acquire the fundamental ability to convert energy is called active smart materials [7]. A few types of active responsive materials are piezoelectric materials, shape memory alloys, and magnetostrictive. These responsive materials can be operated as actuators or transducers [16]. Smart Materials that lack the innate ability to convert energy are called passive smart materials. An example of passive smart material would be fiber optic materials. These kinds of materials can function as sensors, but they cannot behave as actuators or transducers [16]. PREPARATION METHODS Smart materials can be prepared using a variety of different techniques such as combustion synthesis method, vacuum induction melting, etc. Combustion Synthesis Method This method aims to prepare TiNi SMAs that possess the ability to regain their original shape by combustion synthesis induced due to spark plasma sintering (SPS) and to scrutinize the constraints faced by this method of preparation [17]. Combustion synthesis is the process used to synthesize small particles of oxides at high temperatures, which allows this process to be more energy efficient when compared to other methods of synthesis. Sintering the particles of powder under high density conditions is a technique of rapid synthesis by spark plasma sintering. The mixture of Ti and Ni powders undergoes heating till the temperature at which violent reaction of the powders occurs to form the final compound [17]. Typically, these kinds of reactions take place at a temperature called the eutectic temperature and the reaction occurs at the interface of the two particles. The reason this method is used despite its disadvantages is the short sintering cycle it requires to attain the required properties [18]. This process is advantageous as they steer clear of the problems linked with casting and it has the

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added benefit of being able to control the composition precisely and to easily realize the complex part shapes [19]. The final product formed in this method is extremely porous and constitutes the Ti2Ni phase which is undesirable. To obtain a product with less porosity, the SPS technique is used, but that leads to the formation of Ni4Ti3 and Ni3Ti phases, which are also undesirable [17]. In NiTi alloys, there is a strong relationship between the precipitates of Ni4Ti3 and martensitic phase transformations as seen by observing the characterizations of microstructural changes [20]. Hence, by changing the density and size of precipitates of Ni4Ti3 the temperature for martensitic transformation can be controlled [21]. Precipitates of Ni4Ti3 help to increase the shape memory effect by strengthening the matrix [20]. Preparation of Piezoelectric Materials This method aims to prepare thin film Lead zirconate titanate (PZT). A SiO2 coated silicon surface was taken to be the base material and a mixture of ZrAc and PbAc was prepared in a separate beaker and stirred properly to ensure that the chemicals got completely dissolved. To this mixture, 30ml of acetic acid was added and the beaker was heated on a spinot at 110°C for almost 2 hours, after adding a magnet to the beaker. After the mixture was cooled to room temperature, 2-methoxymethanol was incorporated to dilute the mixture. The diluted mixture was heated again after adding Titanium isopropoxide. The prepared mixture is dropped on the center of the plate using a dropper after the plate is kept on the spinner. The plate is rotated for 30 seconds, following which it is dried for 20 seconds for removal of moisture. The plate is then kept on the spinot at 450°C for 30 seconds. After repeating the process, the plate is kept in an oven at 650°C for 30 minutes. For preparing a thin film coating of PZT, this process was repeated three times. Following this, the plate is kept in the oven for 2 hours. To avoid short circuits and to ensure smooth performance, a tiny portion of the thin film is etched using a solution of hydrofluoric acid. Gold electrodes are deposited on the surface of the prepared thin film using a DC sputtering machine, in the presence of argon gas at 40% power for an hour. After this, the prepared material is tested for piezoelectric and dielectric properties. These properties have a high dependence on the concentration of the compounds used in the experiment [22]. One of the results concluded from the experiment was that the performance of Lead Zirconate Titanate (PZT) is inversely proportional to the operating frequency. As the frequency is increased, the parallel capacitance decreases and an increase in voltage leads to an increase in the parallel capacitance. The performance of Lead Zirconate Titanate is based on Curie temperature. Curie temperature (Tc) varies for different frequencies and it is found that high frequency values have higher Tc. On studying polarization of PZT, it was concluded that on the increase in thickness of the film, the remnant polarization,

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and polarization saturation will decrease. PZT is found to have a dielectric constant that varies from 300 to 3820 [22]. Vacuum Induction Melting Method This method aims to prepare TiNi SMAs, possessing a shape memory effect using graphite crucibles by VIM. VIM generates eddy currents which produce a heating effect in the metal causing it to melt, this process takes place within a vacuum. High density graphite rods were used to fabricate the graphite crucibles used in a vacuum induction melting. The raw materials used for melting the alloys were titanium sponge and nickel plates with high purity. For each melt, the targeted composition was equiatomic NiTi alloy. The titanium sponge and nickel plates were thoroughly cleaned using acetone after which they were dried in a vacuum and charged using the crucible. After charging, melting was conducted in an atmosphere containing argon. In this environment, the liquefied metal was transferred into a steel split mold, that was glazed using graphite and heated before the metal was poured. In order to glaze the mold with graphite, both parts of the mold were coated with a liquid mixture of graphite in water, which got heated in an oven at 150°C for one hour after bolting both parts together and this process was repeated twice. The nickel plates were kept on the bottom and the sides during charging to ensure that direct contact does not occur between the crucible and titanium. The same crucible was used for taking the three melts. The first melt consists of four titanium sponge rods that were wrapped in nickel foils whereas the titanium sponge took the form of a single rod in the consecutive two melts. Homogenization of metal ingot took place at 1000 oC for two hours after which the ingot was cast into the shape of a cylindrical billet. The billet was rolled into a rod at 900°C-950°C. While conducting homogenization, there was no use of a protective atmosphere or coating. The heated rod was involved in wire drawing. The hot rod underwent groove rolling at 1073K until it achieved the dimensions 3mm and 3mm which was then subjected to cold rolling to achieve cross-sectional dimensions of 1mm×1mm. Improvement of cold rolling was ensured by making the rod undergo multiple reductions as well as inter-pass annealing at a temperature of 973K for 10 minutes, using a protective cover made of glass before each cycle, and cleaning the protective glass after each cycle. The diameter of the cold rolled product was decreased even further by cold wire drawing. The obtained wire underwent shape memory heat treatment at a temperature of 748K for 30 min in the presence of argon for evaluation of mechanical and functional properties [23]. Production of high-grade NiTi alloys with a low level of carbon can be achieved using vacuum induction melting on nickel and titanium in a crucible made of graphite. This can be achieved by making a thin layer of titanium carbide on the crucible’s inner layer. If the crucible has never been utilized before, then the loss of titanium and carbon pick

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up increases. Future melting using the same graphite crucible will result in decreased carbon pick up. To reduce the amount of carbon, pick up, it is imperative to ensure no direct contact linking the titanium and the crucible. An alternative to vacuum induction melting is making use of a wash melt having pure titanium and decreasing the amount of NiTi required to attain the desired result to a single melt [23]. Using Molecular Complexes This method aims to prepare hydrogels that display responsiveness towards change in pH using molecular complexes. Since pH and temperature are highly controllable stimuli, hydrogels that are responsive to temperature and pH have found a lot of potential as soft smart materials. Hydrogels that are temperature responsive are prepared using polymers having lower critical solution temperature. Functional groups containing electrolytes are introduced to the polymer chain which ensures that the resultant hydrogel will show abrupt changes in volume as pH is varied [24]. In this experiment to prepare a pH-responsive hydrogel, a divalent phosphate group was used as an electrolyte. A phosmer was used to synthesize hydrogels responsive to pH that experience a volumetric change at two different pHs [25]. Positively charged lysozyme was introduced to the hydrogel showing pH responsiveness by forming a complex with the phosphate groups and the lysozyme released by the hydrogel formed was studied in regard to its response to change in pH [26]. Experiments on the release of lysosomes from hydrogels made using molecular complexes were executed based on pH as complex formation between positive and negative ions is highly influenced by pH. On conduction of the experiment, it could be observed that the hydrogel released lysozyme into the liquid medium at higher pH levels. The release of lysozyme was linked to osmotic inflation of the ion network and it coincided with the expanding behaviour of hydrogels. Due to the complex formation and separation of lysozyme and phosphate groups, an ON-OFF release of the drug is achieved, in response to the pH. Polymer based hydrogels are a promising biomaterial for protein delivery that is intestine-targeted as revealed by the ON-OFF release of lysozyme, in response to pH. This can be accomplished by orally administering the protein drug and due to the retention in the hydrogel depending on the acidity of the stomach. The lysozymes release rapidly when it encounters intestinal environments that are less acidic [24]. Mixed Oxide Technology This method aims to prepare piezoelectric nanoparticles using one of the oldest techniques, mixed oxide technology, and a few precautions to be taken care of. The steps involved in this process are summarized in Fig. (1). An important part

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of the procedure is the mixing of the powders since the degree of homogeneity of the mixture depends on it, as the quantity of each raw oxide is different. Mixing typically occurs in a ball mill, in the presence of a liquid medium, such as an organic substance or distilled water, which do not undergo any reactions with the raw materials used in the process.

Fig. (1). Steps involved in the process of preparation of piezoelectric nanoparticles by mixed oxide technology.

Calcining proves to be important as proper care must be taken when choosing the time and temperature parameters to ensure solid state synthesis occurs between the raw materials and leads to the formation of a new compound. For each composition, the optimum temperature of reaction and reaction time temperature is determined by the experimental method. An important step to acquiring the fine-grained powder necessary to produce a high-quality ceramic is milling the calcined powder. The required morphologic homogeneity is not achieved in the powder if it undergoes too little milling and on the other hand, over milling can cause atomic impurities from the jar to contaminate the powder. In order to overcome the inconvenience caused by over milling, jars lined with plastic having high density media are used such as balls made from alumina. The time required for milling can vary depending on the type of mill that is used, as well as the composition and characteristics of the powder [27]. The mixed oxide technique is the most common way to acquire a fine powder that can be used to make piezoelectric ceramics [28 - 30]. This technique is one of the oldest techniques and it is found to have originated in ancient civilizations [31 33]. This technique can be applied to all combinations of oxides due to its low cost and versatility. The powder formed using this technology has a particle size in the micron range and the piezoelectrics formed by this technique can form a high yield of electrical energy obtained by converting mechanical energy and vice

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versa. This leads to a wide range of applications for piezoelectrics in space industries, electronic industry, medicine among others [27]. SMART MATERIALS IN ELECTRICAL ENGINEERING Material science engineers have been focusing on the use of smart materials to create conductors and insulators. The majority of the time, failure is related to degradation of dielectric material, hence by introducing a smart component into the material so as it achieve continual condition monitoring. While doing this, it is detrimental to ensure that introducing a smart moiety into the dielectric does not have any effect on the mechanical and electrical properties of the bulk material. Research has been done on introducing fluorophores into a dielectric system. Fluorophores were thought to be a good match for such an application as fluorescence is a visible effect even if at low active fluorophore concentrations. The focus was given to optimizing the active fluorophore and determining the best way to introduce it into the insulating system [34]. A new variety of conductive polymers were produced that exhibit conductive behaviour like metals and semiconductors. There are six groups of conducting polymers, i.e., aniline, pyrrole, phenylvinylene, phenylene, thiophene, and acetylene. Due to the simple molecular architecture of poyacetylene, scientists prefer using it to study conduction mechanisms. The electrical conductivity of these polymers is altered by doping, a method similar to that used in semiconductor doping. The process of doping makes these polymers electrically neutral. Given below are applications of insulators and conductors that are enhanced using smart materials. Conductive Inks Conductive inks are the foundation of printed electronics, which is one of the most propitious branches in materials science. They are paints infused with conductive particles and are used to create hand-painted as well as printed electrical traces on paper. Printed electronics provide us with a way to produce cheap, recyclable, and flexible circuits by using paper, conductive ink, and a modified document printer. As of now copper and other conductors are used for traces as the types of conductive inks that are available for use outside laboratories are still too resistive. Conductive ink is a good material and is useful for creating sensors in whichever shape we want as they are painted. Currently, research is being done in order to create variable resistance sensors by making use of paper, conductive ink, and basic electronic components such as resistors and transistors.

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Muscle Wire Muscle wires are made using a shape memory alloy that tends to contract when an electric current is passed through it. Currently, this material is not strong enough to lift heavy objects, such as, rolling up heavy blinds or pulling any significant loads. Once that hurdle is overcome, it can be used to create motion in a smooth and noiseless manner and can be used for several applications in which the usage of motors would be unappealing. Electro-textiles Thread, yarn, and fabrics with electrical properties are what comprise electrotextiles. These fabrics are produced by blending textiles with metallic fibers. Product designers have used conductive fabrics to make electronically enhanced garments. By weaving light emitting diodes and sensors directly into polymers used in textiles, we can create new forms of wearable technology that can be used to monitor health and perform optical communication. The building block of communication and sensor technologies is semiconductor diodes that can detect or emit light. By building these into fabrics, we can unlock a variety of electronic textiles but it has proven difficult to assimilate the function of semiconductor devices with the ability of fibre-based textiles to expand. Light Diffusing Acrylics Acrylics infused with colourless light diffusing particles are called Light Diffusing Acrylics. Regular acrylics only can diffuse light around the edges of the materials, whereas light diffused acrylics can illuminate the material across its entire surface. Current applications of these acrylics are in interior design and for other multi-touch applications. Smart Grids By assimilating smart components into insulator materials, we can ensure higher reliability of smart grids and at the same time, reduce costs. Smart insulators are extremely hydrophobic and environment-friendly. They can lead to a new generation of insulators that can be designed and optimized according to the climate and environment. Due to the availability of such sophisticated software, we can create more customized designs for insulators, that is mechanically and electrically enhanced. Along with the new smart grids, the units that are already in use have to be controlled better in order to improve the reliability of the smart grid. Developing better insulator designs will help guarantee safety and eliminate the need to shut down lines in order to do maintenance.

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Applications in Other Fields Smart materials have a distinct response to different external stimuli because of which they can be applied in a variety of fields. In our daily life, mechatronics, civil engineering, and aerospace are a few different areas where these materials can be used. Finding solutions to problems in engineering that require efficiency levels that are impractical and providing a chance to make products that increase economic growth are also aspects of the application of smart materials [35]. Smart materials and structures incorporate all disciplines of engineering and science, which increases their desirability. The technical utilization of responsive materials takes place in composite materials implanted alongside fiber optics, sensors, Micro- Electromechanical Systems (MEMSs), actuators, vibrational damping, sound and shape control, product life cycle management, cure monitoring, smart system design, active and passive controls, biomimetics, novel sensor systems, magnetic targeting, damping aeroelastic flutter and distribution of stress [36]. Structural Engineering Planting sensors in structures to monitor stress and damage will help decrease maintenance charges and increase lifetime. To prevent failures caused by deterioration, regular inspection, and strong designs are a necessity in buildings, ships, bridges, airplanes, and pipes. Damage preventing designs can reduce performance, while inspections are costly and tedious. In a few modern materials, the occurrences of damages that are serious internally but barely show on the surface can be observed [35]. In this field, assessments of the robustness of engineering structures are conducted using smart materials. Their functions are not restricted to only sensing, at the same time, they also adjust to the encircling environment [36]. Self-Repair A material that could restore its properties is known as a self-healing material [37]. Materials showing self-repair properties are embedded with microcapsules that have a glue-like chemical inside. If the material undergoes damage, the capsules break and the chemical seals the damage caused to the metal. Initially, the restoring abilities of self-healing materials were restricted to healing fractures caused by mechanical stress, but recently their abilities have become more extensive [38]. Self-healing materials can be integrated into metals to prevent corrosion. Self-repair is an important feature for structures in environments that are inaccessible, such as aquatic environments and space [35]. An example of this would be the usage of self-healing coating to reduce marine biofouling [39].

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Defense and Space Materials that can modify the shape of rotor blades of helicopters and subdue vibrations have been produced. Some devices containing shape memory alloys have been developed for airplane wings to provide adaptive control. Research is being done to gauge the feasibility of smart materials to improve performance and decrease the costs of small satellites. New research is underway to develop new designs for the positioning of actuators and sensors as well as new control machinery using smart materials [35]. Structures in Japan and China make use of magnetorheological (MR) fluids to restrain vibrations caused by strong gusts of wind and earthquakes. Astronauts at the International Space Station (ISS) are trying to harness the properties of MR fluids to create responsive materials that contain molecules with the capability to position themselves. Synthetic spider silk made using smart materials is being explored for its feasibility to be used as bulletproof vests since these materials have high elasticity, synthetic spider silk is light, and it is stronger than steel. Nuclear Industries Enhancement of safety, improvement of performance, and reduction of exposure are a few of the opportunities offered by smart technology to the nuclear sector. If the smart materials used in this industry are not carefully tested and chosen, it can bring about new hazards and vulnerabilities to the workers. A limitation faced by smart technology is the radiation environments that are linked with operating technology in nuclear industries and requires cognizance about how these materials react to irradiation [36]. Biomedical Applications Biomedicine is still carrying out investigations and coming up with new ideas for the application of smart materials in this field. Surgeons are certain that synthetic spider silk can be used in surgeries to repair torn ligaments and nerves due to their strength, flexibility, and resistance to tear. Shape memory alloys are used in implants, stents, guide wires for catheters, and filters but even though SMAs are frequently used in medicine and are found to be biocompatible, some people are concerned about how these materials can harm the patient’s health. Recently, experiments have been conducted to discern the feasibility of poly-electrolyte gels as a type of artificial muscle. Smart materials will be useful for drug delivery because of their biodegradability [36].

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Reducing Electronic Waste Electronic waste is becoming an aspect of domestic waste that is uncontrollably increasing all over the world. Before disposal, the recyclable and hazardous components must be removed but manual dismantling can be costly and consumes a lot of time. This is where smart materials come into play; usage of these materials can help to make the process automated. Recently, researchers have come up with the idea of using fasteners made from shape memory alloys that respond to heating by self-releasing, so that the products can be dismantled hierarchically [36]. Reducing Food Waste Foods cultivated for consumption are often thrown away as they cross their expiration date. These expiry dates are approximate, and the food products may have a slightly longer product life. Research has been done on the feasibility of the development of smart labels that will undergo a colour change to show the appearance of a rise in chemical levels or bacteria levels in food, which will indicate the food becoming less fresh [35]. Health Monitoring blood sugar levels and conveying the information to a pump that releases insulin when required are the functions of biosensors constructed using smart materials. Some well-known immobilized enzyme-based biosensors are used for HIV detection, detection of hCG protein in urine in pregnancy tests, regulation of blood glucose levels in diabetic patients. Biosensors made from responsive materials are used to monitor temperature, blood pressure, pulse, heart rate, and blood sugar, but the human body being in a hostile environment can easily damage the sensors. Another application in this field includes replacement joints that relay information when an infection develops or they become loose, hence reducing the requirement for invasive surgery [36]. The Ageing Population Elderly people can be found in all places in the world and there is a rise in the number of new products that will enable them to lead an easier life. A lot of these products can use smart materials to incorporate added operability. Hearing aids have been developed for aged people with hearing disabilities and these aids can sense whether the sound needs to be amplified. Houses that make use of sensors to observe behaviour and guarantee the safety of residents with dementia have been developed [35]. These smart houses send out alerts to relatives or EMTs in case of duress. There are limitations to the application of smart homes as people

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have reported feeling like they are always feeling watched and feel an invasion of their privacy due to all the cameras and monitoring systems. There have also been reports of system failures which could be fatal if it occurs while the aged person needs urgent care. Civil Engineering The aspect of smart materials that has attracted attention from civil engineers is the super-elastic functioning of shape memory alloys. Smart concrete has the potential to sense small structural flaws and it also has more strength than conventional concrete. Smart concrete is used in roads to sense and record road traffic. Smart concrete is also used in highways and airfields to melt ice during winter by passing a current having low voltage through it. Magnetorheological fluids can be incorporated in building materials as MR fluids restrain vibrations and can hence reduce the effect of an earthquake. These alloys are used to retrofit structures to ensure that the structures have designs suitable to withstand earthquakes [40]. Soft Robotics Soft robots are made of distortable matter like gels, elastomers, and fluids that show flexible and rheological characteristics as that of tissues and organs. An emerging trend is the hybridization of artificial and biological matter using tissue engineering that will lead to more biocompatible technologies [41]. Tissue engineering as well as synthetic biology, when applied with soft robotics can result in actuators that are powered using natural muscle tissue that can bend [42] or semi-fluid circuits made of genes, bacteria, and proteins [43]. Like other technologies that use soft matter, these hybrid biomimetic soft robots also do not contain any rigid materials, hence they can be made using soft lithography as well as 3-D printing. To make a robot that is untethered as well as autonomous, an alkaline or lithium-ion battery would need to provide onboard electricity to these actuators [41]. Future Prospects As of now, smart material is a field that is undergoing research in a variety of fields. New and innovative hypotheses are being developed for the use of these materials in different fields and once the barriers that hold back smart materials are overcome; there will be a huge increase in the use of these materials. There is high potential for these materials in the fields of medicine, robotics, and engineering.

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Cardiac Tissue Engineering Engineering cardiac tissues may include biological polymers with adjustable mechanical as well as chemical properties, engineered nanomaterials that recapitulate the inherent nano topography of the extracellular matrix, and bioelectronics that allows for the monitoring or modulation of host-implant assimilation. Technological advancements in 3-D bio-printing have resulted in the augmentation of material-based advances which allows the production of heterogeneous tissues. One of the significant challenges faced by all biomedical technologies including these is biocompatibility and regulatory approval. Nanocomposite scaffolds show a multitude of issues, especially since the outcomes of nanomaterials on the remodelling of extracellular matrix have not been widely researched [44]. However, gold nanomaterials seem to be more promising as they are currently being used for clinical trials in cancer therapy [45, 46]. Despite seeming the most ambitious when compared to the rest of the technological advancements seen in this field, it can be observed that bioelectronic scaffolds show the most promise. Systems like this when implanted in cardiac tissue scaffolds can give rise to an electronic interface that can imitate the communication between the heart and the brain. As the engineering of the heart is a problem requiring new age, interdisciplinary approaches, future advances in this field will generate highly functional tissues that will lead to the opening of new approaches in regenerative medicine [44]. Civil engineering Post-tensioning elements containing shape memory alloys are practical for tensioning curved forms as they strengthen the confinement effect. This is due to the constant increase of recovery force across the length of the post-tensioning element. This can be useful for confining reinforced concrete structures and is called active confinement. Despite their advantages, these elements cannot replace conventional members in bulk structures, but they are suitable for usage in special structures. Engineers can abstain from making edge thickenings to solve stability problems in structures due to these elements [40]. Swarm Robotics Swarm robotics is a field in which, a swarm of robots is used to form a camouflage system that will mimic the adaptive camouflage of animals. This swarm of robots has the functionality to recognize the colour of their surroundings and to develop a pattern based on the recognized pattern that corresponds to their vicinity. One such design uses pattern descriptors to identify patterns, following which an algorithm for a weighted-average consensus (WAC) is applied to cause convergence of the swarm to form a global pattern that matches the background.

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Due to the sensor being overwhelmed at the time the emitter is active, these robots cannot sense colour while simultaneously emitting light. Current research focuses on the reduction of robot size to allow the conduction of experiments requiring high-density deployments using a substantial number of robots [47]. Soft Robotics Soft robotics is a new paradigm in soft-matter engineering with the ability to make far-reaching changes in the field of robotics. Soft robotics allows for safe compatibility for biomechanical contact with humans. Due to their ability to navigate challenging environments and infiltrate confined spaces as natural organisms do, these robots find use in disaster relief. On a small scale, mini soft robots can be used in the field of medicine for drug delivery as well as while conducting a biopsy operating as artificial microbes. As a research field, soft robotics is interdisciplinary and has a lot of challenges that have to be overcome before more scientific exploration can be done, like introducing soft-matter actuators powered electrically and chemically with properties that allow it to imitate the shape and properties of muscle tissue. Innovations in the fields of 3D printing and soft lithography to mass produce inexpensive soft robots are the key to commercial success [41]. Hydrogels Recent developments in nanotechnology combined with the generation of polymeric materials have led to a successful implementation of hydrogels in drug delivery. The most noteworthy results have been shown during the usage of biodegradable synthetic polymers in drug delivery. Directing research on designing drug delivery systems that have minimal restrictions and easy administration routes is the key to refining hydrogel technology. Currently, research is being done on self-assembled and nanostructured hydrogels, as well as on the feasibility of using hydrogels for tumour imaging and targeting [48]. CONCLUSION Smart materials can be produced in a variety of ways and each method of production imparts different properties to the material that enhances the capabilities of the material. This futuristic material finds a few obstacles in its path to commercial success but once those obstacles are overcome, this material will find successful application in all fields, and usage of technology using these materials will propel forward at a rapid pace. The high desirability of these materials is mainly due to their highly adaptive properties. One of the key challenges people face when developing new technology is promoting economic growth while reducing greenhouse emissions. Solutions to this challenge have

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been found in the form of alternate energy sources and the development of energy efficient technologies. Smart materials can create a sustainable environment by reducing energy consumption caused by technology and enhancing the durability and performance of alternative energy devices. Studies in engineering and material science offer us the key to creating a smart city with the help of smart materials. Hence, there is a need for research into innovations that make use of economically suitable and environment friendly smart materials. By its nature, it can be observed that the field of smart materials is highly interdisciplinary. It is applicable in all fields such as the field of basic sciences, applied sciences, and engineering. To attain certain functions, it is imperative to discover a material that is highly flexible in terms of its properties. Smart materials find applications in a variety of fields as they have the potential to achieve maximum requirements of current and future trends in the field of materials. These kinds of materials can perform in a multi-functional manner. Even though the science of smart materials is reaching an advanced level, the application of smart materials is found to have slow progress. Regardless of the slow progress, upcoming technologies that show a lot of promise for more reliability and higher efficiency of a lifetime are those that use smart materials in their construction. The main objective of research into smart materials is to understand and control the structure of new materials on a micro-scale or nanoscale as this is a crucial part of manufacturing high quality smart materials. As time passes, more implementations of smart materials in new and upcoming technologies can be noticed and these smart materials play a huge role in the design of products, be it in medical devices or robot grippers. Once the proper implementation of smart materials is achieved, widespread manufacture and commercial use of these materials will be underway. CONSENT FOR PUBLICATION Not Applicable CONFLICT OF INTEREST The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this chapter. ACKNOWLEDGEMENTS One of the authors would like to express gratitude to Dr. Asha Madhavan and Mrs. Reshmi Nair for their guidance. We would also like to thank Amity University for providing the opportunity to prepare this book chapter.

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306

Photonic Materials: Recent Advances and Emerging Applications, 2023, 306-313

SUBJECT INDEX A Absorption 26, 57, 59, 64, 73, 130, 149, 154, 189, 190 phonon 73 properties 57 Accelerometers 285 Acid 152, 186, 290 acetic 290 deoxyribonucleic 152 hydrofluoric 290 nucleic 152 Air holes 10, 16, 190, 239 elliptical 10 Alkali etching methods 186 Applicability 61, 102, 182, 227 bio-sensing 227 Applications 26, 27, 85, 123, 130, 141, 142, 153, 155, 182, 183, 184, 187, 192, 216, 227, 284, 285, 287, 288, 294, 298 biosensing 227 energy storage 184, 187 gas sensor 182 photothermal 130 photovoltaic 123 telecommunications-based 27 Attenuated total reflection (ATR) 58, 68, 147

metal-polymer 237 opto-microfluidic 237 reflection 3, 11, 26 Bragg fibre waveguide 214, 242 for sensing applications 214 sensor 242

C Cardiac tissue engineering 300 Catalyzed transesterification 240 Cathode, electron pumping 120 Channel, microfluidic 237 Chemical vapour deposition (CVD) 103, 104, 105, 106, 107, 108, 129, 183, 185, 186, 199, 200 Chromosomal abnormalities 153 Coating, photocurable fluoropolymers 95 Convolution neural networks (CNN) 271, 272 Copper indium gallium selenide (CIGS) 86 Corrosion resistance 289 Cosmetic stores 287 Coupling 16, 125 coefficients, impulsive emission 16 weakening intermode 125 CVD technique 185, 186

D

B Bandgap 27, 119, 120, 178 energy 119, 120, 178 telecommunication frequency 27 Biodiesel 240, 241, 242, 248, 249, 250, 258 fuel 240, 242, 248, 249, 250, 258 production 241 Biosensors 132, 187, 215, 241, 288, 298 immobilized enzyme-based 298 Boron nitride 120 Bragg 3, 11, 26, 145, 237 light dispersion 145

DBS method 269 Deep neural networks (DNN) 267, 268, 271, 272, 281 Defects 60, 68, 248, 250 geometrical 250 periodic 248 topological 60, 68 Dense wavelength division multiplexing (DWDM) 2 Density 67, 156, 241, 290 electromagnetic mode 156 Detection 117, 131, 133, 146, 154, 298

Aavishkar Katti and Yogesh Sharma (Eds.) All rights reserved-© 2023 Bentham Science Publishers

Subject Index

Photonic Materials: Recent Advances and Emerging Applications 307

dye molecule luminescence 154 Devices 45, 56, 118, 123, 179, 302 medical 302 optoelectronic 45, 56, 118, 123, 179 Dielectric interfaces 124 Diffraction 11, 27, 143, 145, 238 optical wave propagation 145 Direct binary search (DBS) 267, 268 Dispersion 62, 63, 66, 67, 69, 71, 161 electronic 62 linear energy 63 parabolic energy 66 DNA 152, 153 detection 153 single-stranded 152 Drug delivery systems 287 Dye sensitized solar cells (DSSCs) 96, 101, 106

E Effects, low dielectric 123 Electric field mapping 11, 14, 15 of transverse 11 Electric fields 24, 29, 30, 32, 144, 156, 164, 270, 285, 288, 289 induced 270 Electrochemical 104, 187 capacitors 187 exfoliation 104 Electrodes, transparent conductive 187 Electroluminescence 131 Electromagnetic 11, 12, 17, 23, 31, 44, 46, 57, 58, 116, 142, 145, 218, 221, 239, 241, 242 field 31, 57, 58, 218, 221, 239 waves 11, 12, 17, 23, 44, 46, 116, 142, 145, 241, 242 Electron(s) 2, 58, 60, 63, 64, 68, 69, 70, 75, 91, 92, 97, 101, 106, 107, 117, 120, 121, 128, 142, 143, 144, 145, 149, 151, 164 beam lithography (EBL) 128 energy loss spectroscopy (EELS) 63, 69, 70 function 117

microscopy 143 stationary 120, 121 transitions 75 transmission 2 transport layer (ETL) 97, 101, 106, 107 Electronic(s) 56, 60, 65, 73, 150, 183, 294, 298 nanoscale interactions 150 industry 183, 294 transitions 56, 60, 65, 73 waste 298 Electrostatic force 123, 153 fluorophore-labelled single-stranded DNA 153 Electrostatic repulsion 181 Elements, organic 155 Emission 23, 86, 150, 151, 153, 157, 197, 301 biexcitonic 150 reducing greenhouse 301 single-photon 157 Energy 85, 86, 88, 89, 90, 101, 116, 187, 189, 237, 293, 302 crisis 101, 237 devices 302 electric 90 electrical 189, 293 harvesting 187 photon 88 photovoltaic 116 producing 86 renewable 85 solar 86, 89, 90, 101 Energy resources 85, 86, 98 non-renewable 86 renewable 98 Engineered photon densities 156 Environment 101, 128, 132, 146, 150, 156, 285, 287, 291, 295, 296, 301, 302 liquid 128 nanocrystalline 150 sustainable 302 Erbium-doped fibers (EDFs) 192, 194, 199 Etching 2, 128, 186, 268 dry reactive ion 2 wet chemical 186

308 Photonic Materials: Recent Advances and Emerging Applications

wet electrochemical 2 Exfoliation method 107 External quantum efficiency (EQE) 87, 89

F Fabrication 2, 27, 44, 85, 87, 102, 127, 133, 141 methods 2, 27, 127 process 44, 85, 87, 102, 141 techniques 127, 133 FDTD method 21, 30, 33, 40 Fermi energy 63, 67, 120, 121 Fiber(s) 26, 143, 144, 177, 191, 192, 198, 215, 217, 222, 239, 240, 288, 289, 295, 296 artificial photonic 239 metallic 295 optic communications 288 optics 296 bragg grating (FBG) 26 processing 239 Fiber lasers 177, 178, 179, 187, 190, 192, 193, 194, 195, 197, 198, 200, 202 systems 178, 179, 192, 193, 194, 197, 200, 202 Fields 16, 32, 57, 58, 117, 148, 149, 161, 192, 284, 285, 289, 296, 297, 299, 300, 301, 302 aerospace 161 biomechanical 289 biomedical 285 Films 58, 147, 148, 240, 290 metallic 58, 147 organic 147 silver 147 thin metal 240 Filters 45, 215, 216 narrowband transmission 215, 216 narrow transmission 45 Finite difference time domain method 28 Fluids, magnetorheological 299 Fluorescence-based systems 153 Fluorescent tagging 6 Fluorophores 152, 153, 294

Katti and Sharma

Frequency domain method 29 Fresnel’s equation 245 Fuel 157, 237, 241, 242, 243, 247, 249, 250, 252, 254, 256, 257, 258 adulteration 237 adulteration sensor 243 solar 157 Functions 9, 66, 117, 121, 127, 131, 163, 164, 216, 218, 270, 274, 275, 276, 277, 285, 295, 296, 298 custom python 274 dynamic longitudinal screening 66 energy envelope wave 163 green 270 probability density 274, 275, 276, 277

G Galerkin method 277 Gas 6, 74, 180, 181, 187, 267, 269, 290 argon 290 sensors 74 Glass 239, 248 polymer combination 248 semiconducting 239 Global energy landscape 98 GNR sensors 76 Gold nanomaterials 300 Graphene quantum dots (GQDs) 73, 74, 107 Growth 1, 14, 117, 296, 301 economic 296, 301

H Hankel function formalism (HFF) 242 hCG protein 298 Heated chemical gas-phase reactions 129 Heterojunction silicon solar cell 103 Heuristic algorithms 268, 269 Highest occupied molecular orbital (HOMO) 91 High 70, 123

Subject Index

Photonic Materials: Recent Advances and Emerging Applications 309

resolution electron energy loss spectroscopy (HREELS) 70 resonance frequencies 123 HIV detection 298 Hole transport layer (HTL) 101, 106 Hybrid interfacial architecture 96 Hybridization, interfacial 63 Hydrostatic pressure 43, 44, 45, 47, 48, 49, 50, 51, 52, 53

I Immunoassays 152 Immunohistology 152 Influence light transmission 1 Integration 28, 158 optoelectronic 158 photonic chips 28 International space station (ISS) 297

K Kerker theory 132 Kohn anomalies 69, 70 nonadiabatic 69

L Label-free tomography of surface-absorbed live cells 5 Laser(s) 1, 4, 14, 15, 16, 117, 123, 128, 153, 155, 158, 161, 188, 195, 197 emission 155 induced transfer 128 oscillations 188 vertical-cavity-surfae-emitting 16 Laser systems 155, 177, 178, 179, 190, 197, 199 developing autonomous temperature-based 155 optic-based pulsed 177 pulsed fiber 178, 179, 199 Laser technologies 177, 178

optical fibre 177 pulsed fiber 178 Light absorption 1, 2, 3, 9, 10, 21, 22, 23, 24, 25, 27, 35, 126, 132, 142, 143, 150, 151, 188 activated therapies 151 amplifying 126 broadcast, sensitive 9 diffraction 9 emission 132 energy 10, 25 oscillation 188 propagation 21, 22, 23, 24, 25, 27, 35, 143 transmission 1, 2, 3, 9 Localized surface plasmon resonance (LSPR) 57, 75, 152 Lowest unoccupied molecular orbital (LUMO) 91, 92 LPE 183, 185 method 183 technique 185 Luminescence 142, 149, 154 Lysozyme 292 released 292

M Mach Zehnder interferometer (MZI) 278, 280 Magnetic field 11, 29 density 29 distribution 11 Margaritaria nobilis fruit 238 Materials 2, 154, 161, 162, 215, 222, 288, 295 electrostrictive 288 insulator 295 isolated single-photon emitting 154 nanophotonic 161, 162 photosensitive 2 polymer 215, 222 Maxwell 11, 24, 30, 57, 244 equations 11, 24, 30, 244 Garnett theory 57 Maxwell’s 28, 29, 30, 31, 58, 59, 153 curl equations 30

310 Photonic Materials: Recent Advances and Emerging Applications

electromagnetic theory 58 equations 28, 29, 30, 31, 59, 153 Mechanism 91, 124, 130, 131, 132, 188, 190, 216 biosensing 216 Metals 56, 57, 58, 59, 60, 95, 132, 147, 148, 150, 152, 177, 178, 179, 180, 181, 182, 183, 185, 187, 202, 237, 240, 291, 296 monochalcogenides (MMs) 177, 178, 179, 180, 181, 182, 183, 202 polymer nanocomposites 237 poisonous heavy 95 Micro-electromechanical systems (MEMSs) 296 Microscopy, atomic force 117, 267 Modified hummers method 106 Molybdenum disulfide 120 Monitor stress 296 Monte Carlo 265, 274, 275, 278 analysis 275 process 274 simulations 265, 278 Morphological characterization techniques 117

N Nanoimaging technique 73 Nanolithography 142 Nanoparticle-based colourimetric tests 152 Nanoparticles, metallic 153 Nanophotonic(s) 127, 141, 157 devices 127, 141 hybrid 157 industry 157 Nanoresonators 21, 23, 36, 37, 38, 39, 40, 132 grapheme oxide 39 Nanoscale processes 141 Network 6, 92, 272 conditional generative adversarial 272 nanocavity antenna 6 Neuman function 221 Nominal transmission response 279

Katti and Sharma

O Operation 7, 8, 28, 86, 92, 96, 190, 192, 201, 202 threshold detector 7 Optical 1, 4, 7, 8, 9, 10, 11, 16, 21, 22, 61, 127, 148, 179, 182, 197, 215, 242, 267, 273, 295 communication 7, 127, 215, 295 devices 1, 179, 267 diode development 182 fiber communication 197 field mapping 8 logic gates (OLGs) 1, 4, 7, 8, 16, 21, 22 microscope 148 neural networks (ONN) 273 power beam splitters (OPBS) 4, 9, 10, 11 signal processing 61 spectrum analyzer (OSA) 242 Optical properties 179, 181, 184, 215, 217, 219, 221, 223, 225, 227, 229, 231, 265, 267 layer-dependent 179 Optoelectronic(s) 63, 119, 184, 197, 215 applications 197, 215 properties 63, 119, 184 Opto-microfluidics approach 237 OPV technology 92 Organic solar cells (OSCs) 87, 89, 90, 93, 94, 98, 101, 107 Osmotic inflation 292 Oxides 130, 180, 193, 194, 200, 201, 202, 289, 293 dark-blue color titanium 130 lutetium 193 zinc 201, 202

P Perovskite solar cells (PSCs) 87, 90, 94, 95, 98, 101, 106, 107 Photodetectors 130, 131 industrial 131 traditional commercial 130

Subject Index

Photonic Materials: Recent Advances and Emerging Applications 311

Photoelectric properties 61 Photolithography 118, 128 Photoluminescence 132 Photon emissions 154 Photonic(s) 4, 9, 10, 14, 16, 117, 118, 119, 141, 142, 156, 163, 167, 168, 169, 265, 266, 275, 279, 281 amalgamation 16 circuit technology 141 device optimization methods 266 emerging 167 integrated circuits (PICs) 10, 14, 265, 281 integrated 4, 9 Photonic crystal(s) (PC) 1, 2, 3, 4, 5, 6, 8, 9, 15, 16, 17, 21, 43, 44, 45, 46, 124, 125, 130, 156, 190, 191, 241 fibers (PCFs) 190, 191 sensors 241 Photonic sensor 258, 259 crystal-based 259 Photons 44, 68, 69, 92, 95, 117, 123, 124, 130, 131, 132, 142, 143, 144, 145, 154 microcavity trapping 124 visible 95 Photophysical processes 118 Phototransistors 74, 75, 76 graphene-based hybrid 75, 76 hybrid graphene-based 74 Photovoltaic effect 86, 120, 130, 131 Piezoelectric ceramics 293 Plasma oscillations 57 Plasmon 66, 152 dispersion 66 resonant particles (PRPs) 152 Plasmonic(s) 56, 57, 76, 117, 130, 156 materials, traditional metal 76 metal-based 56 resonance 130 Polarizers, transverse magnetic-pass 14 Polymer(s) 90, 91, 93, 94, 95, 101, 107, 287, 294, 300, 301 biodegradable synthetic 301 biological 300 cationic 287 conducting 294

conjugated 90, 91 fluorinated 95 solar cells 90, 91, 93, 94, 101, 107 Polynomials 274, 276 orthogonal 276 Power conversion efficiency (PCE) 86, 87, 88, 89, 90, 91, 93, 95, 96, 97, 98, 274, 276 Problems, electromagnetic 30 Process 85, 116, 129, 186 greener exfoliation 186 light waves 116 renewable energy transformation 85 solvothermal 129 Production 85, 86, 87, 89, 93, 96, 150, 151, 157, 240, 291, 300 agricultural 240 Properties 120, 125, 126, 152, 187, 285, 290, 294, 295 dielectric 125, 152, 290 electrical 120, 294, 295 electromagnetic 285 metallic 126 sensitive 187

Q Quantum 154, 157, 184 cryptography 154 electronics 184 nanophotonic networking 157 Quantum dot(s) (QDs) 21, 73, 74, 75, 76, 87, 90, 96, 98, 107, 131, 149, 153, 154, 155, 187 photoluminescent 187 semiconductor optical amplifiers (QDSOAs) 21 Quarter-wave 215, 222, 228, 242 condition 215, 222, 228 stack condition 242

R Radiation, nuclear 285 Rain forestry 238

312 Photonic Materials: Recent Advances and Emerging Applications

Raman spectroscopy 69 Reactive ion etching (RIE) 2 Reducing 298 electronic waste 298 food waste 298 Reflection 146, 267 index 146 optics 267 Refractive index (RI) 1, 2, 6, 23, 26, 44, 45, 49, 125, 126, 127, 144, 145, 148, 246, 248, 249 materials 23, 45 Release, lysozymes 292 Resonance shift method 267 Resonant frequency 58, 131, 152 plasmonic 131 Ring resonators (RR) 8, 9 RPA methodology 66, 68, 73

S Screening, dynamic 66 Second harmonic generation (SHG) 125, 182 Semiconductor 27, 145, 149, 295 crystals 145, 149 devices 27, 295 Sensing instruments 5, 6 topologies 6 Sensing process 242 Side mode suppression ratio (SMSR) 16 Silicon 8, 127, 129 carbide nanowires 129 dioxide 127 nanocrystals 8 Solar cells 87, 89, 92, 101, 102 based polymer 92 heterojunction 101 silicon-based 102 technologies 87, 89, 102 thin film 87 Solar cells technology 90 third generation 90 Solar energy conversion 85 Spark plasma sintering (SPS) 289

Katti and Sharma

SPS technique 290 Stresses 123, 141, 287, 296 mechanical 296 physical 123 Surface 5, 6, 121, 146, 147, 152, 153, 157, 191, 192, 290, 295, 296 catalysis 157 coated silicon 290 metallic 152 reflective 191 sensing device 5 Surface plasmon(s) 6, 56, 57, 58, 59, 60, 63, 65, 66, 68, 69, 72, 73, 74, 141, 142, 147, 148, 152 polariton (SPPs) 57, 58, 59, 60, 68, 69 resonance (SPR) 6, 57, 147, 148, 152 Synthesis 86, 127, 128, 129, 183, 186, 289 combustion 289 hydrothermal 129, 186 nanophotonic 128 wet chemical 183 Systems 5, 6, 21, 238, 239, 287 artificial 239 biological photonic 238 microfluidic 5 micromechanical 287 sensing instrument 6 telecommunication 21

T Techniques 183, 237, 266 active compensation 266 electrochemical 183 sensor-based 237 Technology 91, 120, 121, 141, 142, 158, 295, 300 biomedical 142, 300 generation solar cells 91 laser light photonic 121 nano-integrated photonics 141 nanophotonic device 120 solar power 158 wearable 295

Subject Index

Photonic Materials: Recent Advances and Emerging Applications 313

Telecommunications 130, 155, 156, 189 optical 189 Theory 16, 30, 57, 58, 248 electromagnetic 30 quantum-mechanical 57 Therapy 151, 187, 300 cancer 300 photothermal 187 Thermal 60, 149, 181, 240, 280, 281, 289 energy 149 properties 60 Thin film 58, 101, 107, 129, 143, 185, 198, 200, 290 coating 290 Third harmonic generation (THG) 125, 127 Thulium-doped fibers (TDFs) 195, 196, 200, 201 TMDs in fiber laser systems 197 Total internal reflection (TIRs) 22, 146, 147, 160, 161, 162, 168, 174, 215, 239 Transfer matrix 43, 45, 46, 53, 214, 216, 237, 242 approach 216 method (TMM) 43, 45, 46, 53, 214, 216, 237, 242 Transition 64, 150, 167, 238 interband electron 64 photon-induced 167 radiative 150 Transition metal(s) 122, 133, 178, 184, 186, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201 dichalcogenides 122, 133, 178, 184 nitrides 186 oxides (TMOs) 192, 193, 194, 195, 196, 197, 198, 199, 200, 201 Transmission 1, 3, 11, 12, 17, 45, 47, 49, 117, 242, 244, 269, 270 electron microscopes 117 properties 47, 49 Transparent conducting electrode (TCE) 101, 103, 105 Transverse 9, 11, 12, 14, 15, 46, 47, 58, 65, 70 magnetic (TM) 9, 11, 12, 14, 15, 46, 47, 58, 65

plasmons (TP) 70

U UV radiation 128

V Vacuum 24, 28, 60, 87, 147, 165, 166, 221, 291 deposition 147 induction melting method 291 processes 87 Vapour phase reaction 185 Virtual wafer-based MC (VWMC) 265

W Waals 119, 184 forces 184 interactions 119 Waste, domestic 298 Water 129, 186, 291, 293 distilled 293 Wave(s) 12, 24, 32, 46, 47, 142, 152, 189, 215, 216, 217, 221, 225, 241, 248 acoustic 241 propagating 24, 217 stationary 189 travelling electron density 152 Wavelength 1, 16, 34, 44, 49, 73, 89, 125, 126, 143, 145, 158, 160, 162, 166, 168, 174, 225, 227, 241, 265, 268, 280 cubic 16 division multiplexers (WDM) 265, 268, 280 photon 160, 162, 168, 174 Wet chemical synthesis (WCS) 183