Nanostructured Nonlinear Optical Materials: Formation and Characterization (Micro and Nano Technologies) [Illustrated] 0128143037, 9780128143032

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Nanostructured Nonlinear Optical Materials Formation and Characterization

Rashid A. Ganeev Optics and Spectroscopy Department, Voronezh State University, Voronezh, Russia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2018 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-814303-2 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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Preface The motivation in writing this book was related with the specific properties of smallsized particles and structures and their nonlinear response in the field of strong laser pulses, which became the area of interest from the very beginning of my career. The topics presented in this book are mostly concerned with the nonlinear optical characterization of various nanostructured media during my long-lasting collaboration with numerous researchers and could not be realized without their generous efforts. This book describes the analysis of the formation, characterization, and optical nonlinearities of various nanostructures using different methods. The studies include three groups of research fields. First group is related with the modification of the surfaces of materials for the formation of various nanostructures. Those studies include the extended nanoripples formation using a few-cycle pulses, the analysis of the nanoripples produced on the semiconductors possessing different bandgaps, and the formation of nanopores, nanoholes, and nanowires using different conditions of lasermatter interaction. Second group comprises the studies of ablated bulk and nanoparticle targets, the low-order nonlinearities of metal and semiconductor nanoparticles, and the nonlinear refraction and nonlinear absorption of c­ arbon-contained nanoparticles. Finally, third group of studies is related with the high-order harmonic generation in nanoparticle-contained plasmas. These studies of the properties of nanostructured materials could not be realized without the collaboration between various research groups involved in the studies of similar topics. Among numerous colleagues I met and had the privilege to collaborate I would like to thank H. Kuroda, M. Suzuki, T.Q. Jia, M. Baba (Saitama Medical U ­ niversity, Japan), T. Ozaki, L.B. Elouga Bom (Institut National de la Recherche Scientifique, Canada), P.D. Gupta, P.A. Naik, H. Singhal, J.A. Chakera, U. Chakravarty, M. Raghuramaiah, R.A. Joshi, R.K. Bhat (Raja Ramanna Centre for Advanced Technology, India), J.P. Marangos, J.W.G. Tisch, C. Hutchison, T. Witting, D.Y. Lei (­Imperial College, United Kingdom), M. Castillejo, M. Oujja, M. Sanz, I. ­López-Quintás, M. Martín (Instituto de Química Física Rocasolano, Spain), H. ­Zacharias, J. Zheng, M. Wöstmann, H. Witte (Westfäliche Wilhelms-Universität, Germany), T. Usmanov, G.S. Boltaev, I.A. Kulagin, V.I. Redkorechev, V.V. Gorbushin, R.I. Tugushev, S.R. Kamalov (Institute of Ion, Plasma, and Laser Technologies, Uzbekistan), O.V. Ovchinnikov (Voronezh State University, Russia), E. Fiordilino, D. Cricchio, P.P. Corso (­Università degli Studi Palermo, Italy), M.K. Kodirov, P.V. Redkin (Samarkand State University, Uzbekistan), A.L. Stepanov (Physics Technical Institute, Kazan, Russia), N.V. Kamanina (Vavilov State Optical Institute, Saint Petersburg, Russia), and A.A. Ishchenko (Institute of Organic Chemistry, Kiev, Ukraine) for their activity in the development of this interesting field of nanoscience. Finally, as husband, father, and grandfather, I feel the privilege to underline the role of my family in most of my endeavors.

 

Rashid A. Ganeev Voronezh State University, Optics and Spectroscopy Department, 1 University Square, Voronezh, Russia

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The area of studies related with the small-sized objects is associated with the science of nanotechnology, which commonly dubbed as nanoscience, and is an extremely developing field attracted the attention of enormous groups of researchers. It is hard to imagine sizes of the book, which can describe all aspects of nanoscience. That is why the researchers in their monographs dedicated to this area of studies try to restrict the consideration of a few topics of the nanoscience. The frames of the reviews of previous studies overlap, from time to time, in these books, while providing a descriptive character of the studies in the limited areas of interests of the authors. Present book follows those principles and describes a few groups of studies, which for a long time attracted the attention of this author. So we shortened the frames of consideration of this extremely large field of nanoscience and tried to concentrate on some specific topics, which did not previously find the attention from the point of view of nanoparticles characterization. As it was mentioned in preface, this book is dedicated to the analysis of the formation, characterization, and, mainly, optical nonlinearities of various nanostructures using different methods. The discussed studies include three groups of research fields. First group is related with the modification of the surfaces of materials for the formation of various nanostructures including the extended nanoripples formation using a few-cycle pulses, the analysis of the nanoripples produced on the semiconductors possessing different bandgaps, and the formation of nanopores, nanoholes, and nanowires using different conditions of laser-matter interaction. Second group comprises the studies of ablated bulk and nanoparticle targets, the low-order nonlinearities of metal and semiconductor nanoparticles, and the nonlinear refraction and nonlinear absorption of carbon-contained nanoparticles. Third group of studies is related with the high-order harmonic generation of ultrashort laser pulses in nanoparticle-containing plasmas. The formation of parallel fringes with characteristic spacing in the order of the wavelength of the acting light (so-called “nanoripples”) is an interesting effect of the interaction of ultrashort laser pulses with semiconductors and other materials. It was primarily reported that the spacing between the fringes lay in the range of the wavelength of the used femtosecond lasers, that is, it was about 800 nm or less for most cases when Ti:sapphire lasers were employed. The interest in these periodic surface structures was related to fundamental problems of interaction of ultrashort pulses with different materials and to their possible practical applications. The appearance of these nanostructures was ascribed to the formation of diffraction gratings with submicron spacing between the fringes; the interactions between laser light and plasmon–polariton waves, which occur due to the initial heterogeneity of the surface, the Bose condensation, and so on.

 

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At the same time, the information about nanostructures with periods notably smaller than the wavelength of the light (∼200 nm [1,2]) have aroused additional interest connected, in particular, with the uncertain mechanism of the formation of these structures. Several theoretical models have been proposed to explain the appearance of these short-periodic structures based on the principles used for interpreting of nanostructures with long periods [3,4]. Note that a common feature of the latter structures was approximate correspondence between the laser light wavelength and the interfringe spacing. At the same time, the observed short-period structures, which occur on semiconductor surfaces show not only much shorter distances between the fringes but also demonstrate some other specific features, among which are the effects of the laser light polarization, bandgap width of the semiconductor, and angle of incidence on the structural properties of short-period nanostructures. The elucidation of the mechanisms of formation of short-period nanostructures requires additional study because none of the mechanisms proposed thus far can explain, in full measure, all characteristic features of this process. Direct femtosecond laser surface nano- and micro-structuring is a versatile method to tailor material surface morphologies, which enhance diverse interesting physical properties. These include the ability to permanently modify the surface absorption spectrum or change appearing colors of metals and semiconductors without any addition of pigments, the possibility to fabricate super-hydrophobic and selfcleaning surfaces, and so on. In this context, the formation of laser-induced periodic surface structures in the form of subwavelength ripples is extensively studied. By carefully adjusting laser parameters like wavelength, polarization, pulse duration, pulse number or pulse fluence, the shape, size, and orientation of the structures created can be controlled, as shown by numerous empirical studies. Because this method of structuring is an easy-to-implement, low-cost, top-down method, which is able to fabricate periodic structures on the nanoscale without the sophisticated multistep processes and ultrashort wavelengths needed for nanolithography, high spatial frequency periodic structures has gained increasing attraction for practical applications. Previous experimental studies have shown that the periodic nanostructure formation develops through the bonding structure change, the near-field ablation, and the excitation of surface plasmon polaritons in the thin layer on the target surface. It has also been demonstrated that use of multiple shots of low-fluence femtosecond pulses is important for suppressing undesirable thermal processes in the nanostructures formation through the ablation. The purpose of those studies was to analyze specific features of the formation of nanostructures and short period nanostructures on the surfaces of different semiconductors under variable experimental conditions. The studies in this field include the analysis of different aspects of nanoripples formation ([5–12] to mention few of them). In this book, we analyze the effect of the aforementioned and some other experimental parameters (light polarization, duration of the acting femtosecond pulses, bandgap width of the semiconductor, angle of incidence of the laser beam upon the surface, characteristics of the surrounding medium, and so on) on the pattern of the nanostructure, including the cases when the regimes of formation of the long- and

 Introduction 

short-period structures could be distinguished. We also show, in some cases, the formation of nanoholes and nanodots on the surface of different semiconductors. The third-order nonlinear optical susceptibilities of various media change in a broad range. Nonlinearities responsible for variations in the refractive and absorbing properties are especially important because they strongly affect the propagation of intense radiation in media. Numerous investigations have been performed in this field due to increasing interest in applications of nonlinear optical effects in optoelectronics, various nonlinear optical devices, optical switching, and so on. Interest in nonlinearities of different nanostructure media is also caused by their strong nonlinear optical response attributed to the quantum size effect. The application of nanostructures in the aforementioned fields and optical computers, storage devices, nonlinear spectroscopy, and so on can lead to considerable developments of new methods of the studies of matter. The studies in this field [13–15] were dedicated to different aspects of applications and developments of new devices based on the advanced nonlinear optical properties of nanoparticles. In this book, we present the studies of the low-order nonlinear optical parameters of different nanoparticles. Some of these nanostructured media are analyzed from the point of view of their application as optical limiters of the laser radiation intensity. The results of measurements of the nonlinear refractive indices, nonlinear absorption coefficients, and third-order nonlinear susceptibilities of these media are presented and discussed. Coherent short-wavelength radiation is of increasing importance for a broad spectrum of basic and applied research in various fields of physical, chemical, and life sciences. Among them, femtosecond time-resolved coherent diffractive imaging and photo-induced processes on surfaces and nanoparticles, as well as lithography, plasma diagnostics, and materials processing and diagnostics are of foremost interest. Highorder harmonic generation from femtosecond visible laser pulses allows producing coherent radiation in the extreme ultraviolet spectral range. Table-top lasers render these processes possible with the prospect of widespread scientific applications. So far, however, only low conversion efficiencies for high-order harmonic ­generation have been obtained, despite the enormous efforts. Many interesting experiments can be performed by harmonic generation based on laboratory scale femtosecond lasers. These sources easily cover the spectral range between the 10- and 100-eV photon energy of the harmonics, and with few-cycle laser systems, even up to several hundred eV. For practical applications of high-order harmonic sources, higher conversion efficiency and, thus, an increase in the photon flux and the maximum photon energy of the harmonic radiation would be beneficial. High-order harmonic generation itself can be used as a spectroscopic tool for analysis of the optical, nonlinear optical, and structural properties of the harmonic generation emitters, presently comprising a few noble gases. The generation of higher harmonics in laser-produced plasmas from various solid-state targets, being for this purpose a relatively new and largely unexplored medium, promises to yield these advances. Most interestingly, the studies have shown that enhanced higher harmonics can also be generated from gas clusters

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and ablated nanoparticles, which opens the door to applications of local field enhancement­ [­16–22]. Thus, the above approach may be useful for producing an efficient source of short-wavelength ultrashort pulses for various applications and studies of the properties of harmonic emitters. Laser ablation-induced high-order harmonic generation spectroscopy is a new method for materials science and can be considered as one of the most important applications of harmonic generation. In this book, we also discuss the realization of new ideas that have emerged over the last few years, which led to further improved high-order harmonic generation conversion efficiency through harmonic generation in specially prepared nanoparticle-containing plasmas and allowed the spectral and structural studies of matter through plasma harmonic spectroscopy. Nanoparticle research has demonstrated the feasibility of increasing the efficiency of harmonic generation with the use of cluster media. We note that the majority of previous works on harmonic generation involving nanoparticles was limited to the analysis of rather exotic clusters (Ar,Xe) that were formed during rapid cooling resulting from the adiabatic expansion of gases issuing from jet sources under high pressure. The interest for nonlinear optical characterization of materials is closely related to the new field of nanostructures formation during laser-matter interaction. The formation of nanoripples, nanogrooves, nanoparticles, nanowires, and other exotic nanostructures during laser ablation of different materials, formation of plasma plumes containing clusters, characterization of their structural, morphological, optical, and nonlinear optical properties, use of nanostructures for various applications, such as, for example, optical limiting and high-order harmonic generation, and many other features of newly developed and commercially available nanoparticles open the doors for their use in photonics, electronic industry, medicine and other scientific and industrial applications. The content of book shows that the advantages of nanomaterials for different applications (in present case, nonlinear optical research) may further push this field of morphology and high-order nonlinearities studies toward both fundamental and practical needs thanks to better understanding of the processes involving in the lasernanostructure interactions. The formation of various nanostructures using different methods is a broad field of studies, which shows specific methods, as well as advantages and disadvantages of each approach. As main method of nanostructures formation in this book is, to some extent, a laser ablation, one can expect consideration of other methods as well. Obviously, this book cannot be considered as a completed ­review of those methods, as some of them are too sophisticated and are out of the scope of this book. Meanwhile, it can provide some bridge between the existing methods of nanoparticles and nanostructures formation. One can assume that single authorship can provide more homogeneity in the different topics and grant a better explanation from the basics. Being an author of this book, I tried to combine different aspects of nanostructuring, nanoformation, characterization, and application of small-sized species. These efforts obviously resulted in broadening of the topics of this book. Combined by reasonable logic, various topics can offer better overview and understanding of the main goals of this monograph.

 Introduction 

There are several books on nanomaterials. However, neither of them were dedicated to their nonlinear properties and laser processing or laser characterization in general. Often these publications are the multiauthor books that do not give enough introductory information on the nonlinear optical properties of small sized species. Indeed, there are not so much competitive books in this relatively new field. Most of them combine different issues of nanomaterials studies. Meanwhile, the chapters of those books are not closely related with each other. Obviously, different authors are trying to underline some specific topics, which resulted in some incoherency in presentation of a whole book. I, for myself, was an author of some chapters in such books and can evaluate both drawbacks and advances in such sort of presentation. The necessity in this book is related with the frequent requests of researchers from different fields to the author to provide the information obtained during his numerous studies of the nanomaterials. Currently, these studies are disseminated in different journals. The collection of these studies in a single book may help the reader to learn about the interconnection of the morphological, optical, and nonlinear optical features of these species. The primary groups of the readers of this book comprise the researchers in the fields of laser physics, nanomaterial studies, plasma physics, and nonlinear optics. The book would be of interest for both academics and professionals in the applied fields. Meantime, the book can be served as the source of information in different fields of nanoscience for the high education students and postdocs, who are interested in further development of their career in the fields of laser-matter interaction, coherent extreme ultraviolet sources, laser-produced plasmas, and nonlinear optics of nanomaterials. Obviously, it is an extreme task to combine in a single book the topics, which attract different groups of readers. I tried to balance between pedagogical introductory parts and more advanced topics so that the readership can include students as well as more experienced researchers. The processing and characterization of nanomaterials are certainly an important topic in the science of condensed matter. This book is not only of interest to advanced students in physics, but also in chemistry and engineering. Thus the book can represent a reference textbook for advanced graduate courses. The proposed field could be useful to Ph.D. students especially after one to two years of experience. Based on my experience, as more advanced the paper (or book) as more students will be interested in it. Obviously, it is not a text book. However, the young people trying to advance in nanoscience have to make some efforts to reach the level, which allows the reading of such sort of research studies. Concluding, though this book is mostly addressed to the experiences auditory, the PhD and postdoc students can find many new and interesting information in this collection of different topics of a specific field of nanoscience. This book is organized as follows. To emphasize previous studies of periodic structures, we discuss in Chapter 1 various methods of nanoripples formation on the semiconductors possessing different bandgaps. In Chapter 2, the principles of the formation of nanoparticles, nanopores, nanoholes, and nanowires using different conditions of laser-matter interaction are analyzed. Chapter 3 is dedicated to

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the analysis of laser ablated bulk and nanoparticle targets and different modes of nanostructured materials formation. The low-order nonlinear optical properties of metal nanoparticles is discussed in Chapter 4. Chapter 5 considers nonlinear absorption and refraction in semiconductor and carbon-containing nanoparticles. In Chapter 6, frequency conversion of ultrashort pulses in fullerenes is analyzed. Chapter 7 is dedicated to the studies of the high-order harmonic generation in carbon-containing nanoparticles. Harmonic generation in metal and semiconductor nanoparticles is discussed in Chapter 8. In Chapter 9 we analyze the peculiarities of high-order harmonic generation in nanoparticles. Finally, we summarize the discussion of the properties of nanostructured nonlinear optical materials.

REFERENCES [1] A. Borowiec, H.K. Haugen, Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses, Appl. Phys. Lett. 82 (25) (2003) 4462–4464. [2] N. Yasumaru, K. Miyazaki, J. Kuichi, Fluence dependence of femtosecond-laser-induced nanostructure formed on TiN and CrN, Appl. Phys. A 81 (4) (2005) 933–937. [3] T. Kondo, S. Matsuo, S. Juodkazis, V. Mizeikis, H. Misawa, Multiphoton fabrication of periodic structures by multibeam interference of femtosecond pulses, Appl. Phys. Lett. 82 (17) (2003) 2758–2760. [4] N.D. Lai, W.P. Liang, J.H. Lin, C.C. Hsu, C.H. Lin, Fabrication of two- and three-dimensional periodic structures by multi-exposure of two-beam interference technique, Opt. Express 13 (23) (2005) 9605–9611. [5] M. Beresna, M. Gecevičius, P.G. Kazansky, Ultrafast laser direct writing and nanostructuring in transparent materials, Adv. Opt. Photon. 6 (2) (2014) 293–339. [6] S. He, J.J.J. Nivas, A. Vecchione, M. Hu, S. Amoruso, On the generation of grooves on crystalline silicon irradiated by femtosecond laser pulses, Opt. Express 24 (4) (2016) 3238–3247. [7] S. Höhm, A. Rosenfeld, J. Krüger, J. Bonse, Laser-induced periodic surface structures on titanium upon single- and two-color femtosecond double-pulse irradiation, Opt. Express 23 (20) (2015) 25959–25971. [8] S.K. Das, H. Messaoudi, A. Debroy, E. McGlynn, R. Grunwald, Multiphoton excitation of surface plasmon polaritons and scaling of nanoripple formation in large bandgap ­materials, Opt. Mater. Express 3 (1) (2013) 1707–1715. [9] G. Miyaji, K. Miyazaki, Fabrication of 50-nm period gratings on GaN in air through plasmonic near-field ablation induced by ultraviolet femtosecond laser pulses, Opt. Express 24 (5) (2016) 4648–4653. [10] Y. Nakajima, N. Nedyalkov, A. Takami, M. Terakawa, Formation of periodic metal nanowire grating on silica substrate by femtosecond laser irradiation, Appl. Phys. A 119 (5) (2015) 1215–1221. [11] K.C. Phillips, H.H. Gandhi, E. Mazur, S.K. Sundaram, Ultrafast laser processing of ­materials: a review, Adv. Opt. Photon. 7 (4) (2015) 684–712.

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[12] O. Varlamova, C. Martens, M. Ratzke, J. Reif, Genesis of femtosecond-induced nanostructures on solid surfaces, Appl. Opt 53 (1) (2014) l10–l14. [13] H. Zhang, S. Virally, Q. Bao, L.K. Ping, S. Massar, N. Godbout, P. Kockaert, Z-scan measurement of the nonlinear refractive index of graphene, Opt. Lett. 37 (11) (2012) 1856–1858. [14] L.A. Gómez, C.B. de Araújo, L.M. Rossi, S.H. Masunaga, R.F. Jardim, Third-order nonlinearity of nickel oxide nanoparticles in toluene, Opt. Lett. 32 (11) (2007) 1435–1437. [15] O. Sánchez-Dena, P. Mota-Santiago, L. Tamayo-Rivera, E.V. García-Ramírez, A. ­Crespo-Sosa, A. Oliver, J.-A. Reyes-Esqueda, Size-and shape-dependent nonlinear ­optical response of Au nanoparticles embedded in sapphire, Opt. Mater. Express 4 (1) (2014) 92–100. [16] M. Aladi, R. Bolla, P. Rácz, I.B. Földes, Noble gas clusters and nanoplasmas in high harmonic generation, Nucl. Instrum. Meth. Phys. Res. B 369 (1) (2016) 68–73. [17] H. Park, Z. Wang, H. Xiong, S.B. Schoun, J. Xu, P. Agostini, L.F. DiMauro, Size-dependent high-order harmonic generation in rare-gas clusters, Phys. Rev. Lett. 113 (26) (2014) 263401. [18] M. Oujja, I. Lopez-Quintas, A. Benítez-Canete, R. de Nalda, M. Castillejo, Harmonic generation by atomic and nanoparticle precursors in a ZnS laser ablation plasma, Appl. Surf. Sci. 392 (3) (2017) 572–580. [19] C. Vozzi, M. Nisoli, J.-P. Caumes, G. Sansone, S. Stagira, S. De Silvestri, M. Vecchiocattivi, D. Bassi, M. Pascolini, L. Poletto, P. Villoresi, G. Tondello, Cluster effects in highorder harmonics generated by ultrashort light pulses, Appl. Phys. Lett. 86 (11) (2005) 111–121. [20] T. Shaaran, M.F. Ciappina, R. Guichard, J.A. Perez-Hernandez, L. Roso, M. Arnold, T. Siegel, A. Zaır, M. Lewenstein, High-order-harmonic generation by enhanced plasmonic near-fields in metal nanoparticles, Phys. Rev. A 87 (4) (2013) 041402 (R). [21] H. Singhal, P.A. Naik, M. Kumar, J.A. Chakera, P.D. Gupta, Enhanced coherent extreme ultraviolet emission through high order harmonic generation from plasma plumes containing nanoparticles, J. Appl. Phys. 115 (3) (2014) 033104. [22] D.F. Zaretsky, P. Korneev, W. Becker, High-order harmonic generation in clusters ­irradiated by an infrared laser field of moderate intensity, J. Phys. B 43 (10) (2010) 105402.

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Periodic nanoripples formation on the semiconductors possessing different bandgaps

1

1.1  NANORIPPLE FORMATION ON DIFFERENT BANDGAP SEMICONDUCTOR SURFACES USING FEMTOSECOND PULSES Surface structuring with lasers is a highly competitive method due to its capability to implement changes in the structure design. Compared to the picosecond and the nanosecond laser pulses, the energy in a femtosecond pulse can be precisely and rapidly deposited in a solid material with fewer thermal effects. Therefore the femtosecond lasers are widely used for microfabrication in transparent materials, metals, and semiconductors for applications such as modifying the optical properties of the surface [1], improvement of photovoltaic devices [2] by increasing their effective surface area, and in turn increasing the response and conversion efficiency. The other applications include modification inside refractive index of bulk materials for fabricating photonic devices, optical data storage, and biophotonic components [3–5]. Mid-infrared femtosecond laser radiation is generally used for the generation of surface ripples, also known as laser-induced periodic surface structures (LIPSS). Various mechanisms have been proposed to explain the nanoripple formation. Initially, it was thought to be the result of the interference between the incident laser light and the scattered light from the surface roughness [6]. However, if this mechanism was to be true, nanoripple width should always be of the order of the incident laser light wavelength, whereas, in many experiments, very narrow ripple formation has been observed. For example, Ozkan et al. [7] found that ripples resulting from 248 nm femtosecond laser irradiation of thin diamond films had a period varying between 50 and 100 nm. Yasumaru et al. [8] reported formation of ripple patterns with mean periods of 100–125 and 30–40 nm on TiN and diamond-like carbon after irradiation with 800 and 267 nm femtosecond pulses, respectively. To explain the nanoripple formation in the case of metals, the surface plasmon created during initial random surface heterogeneities was considered [9]. In dielectrics, the coupling of the electron plasma wave and incident laser explains the observed periodic structure in glass [10]. Other proposed mechanisms for ripple formation are self-organization [11], Coulomb explosion [12], influence of second harmonic generation [13], anisotropic local field enhancement invoking nanoplasmonics [14], etc. Nanostructured Nonlinear Optical Materials. http://dx.doi.org/10.1016/B978-0-12-814303-2.00001-5 Copyright © 2018 Elsevier Inc. All rights reserved.

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CHAPTER 1  Periodic nanoripples formation on the semiconductors

Below, we analyze surface nanoripple formation on different bandgap semiconductors [15]. A study of the ripple formation with different laser parameters and ambient media is presented with the objective of identifying conditions of forming narrow period ripples. Nanoripples were formed using a Ti–sapphire laser with 8 mJ energy, 45 fs pulse duration, and 800 nm wavelength (1.56 eV) at a fluence in the range of 100 mJ cm−2–1 J cm−2. The effects of the number of laser shots, the angle of incidence, polarization of the laser, fluence, incident laser wavelength (λ), bandgap, and ambient medium were studied. Depending upon the experimental parameters, the nanoripple sizes varied in the range of λ/9–λ. Narrow nanoripples were formed on the wide bandgap (WBG) semiconductors. The width of the nanoripples decreased with the laser wavelength and the laser fluence. The observation of high and low spatial frequency ripples in different conditions is explained considering the transient metallic nature of the semiconductor surface on irradiation with intense femtosecond pulses. The surface eventually supports the surface plasmon excitation, which interferes with incident laser light for ripple formation. The very delicate dependence of ripple period with incident laser parameters is attributed to the critical role of electron density. The finding helps identifying suitable bandgap materials and laser parameters for obtaining nanoripple period considerably small compared to the incident laser wavelength.

1.1.1  EXPERIMENTAL ARRANGEMENTS AND RESULTS For studying the nanoripple formation from ultrashort laser pulse irradiation of semiconductor materials of different bandgaps, the following semiconductor materials of narrow (1.5 eV) bandgap materials such as GaP (2.3 eV), GaN (3.4 eV), SiC (3.37 eV), and ZnSe (2.82 eV) were used. Multiple laser shots from a Ti–sapphire laser with 45 fs pulse duration, and 800 nm wavelength were focused in air on the semiconductor wafers at a fluence in the range of 100 mJ cm−2–1 J cm−2, that is, around the plasma formation threshold. Using a beta barium borate (BBO) crystal, the second harmonic (400 nm) of the 800 nm laser beam was also used to study the influence of the laser wavelength on the ripple formation while keeping the other parameters the same. Fig. 1.1 shows the surface morphologies resulting from the irradiation of a GaP wafer by femtosecond pulses with two different polarizations and in two different spatial regions. Fig. 1.1A shows the formation of spherical nanoparticles (∼100 nm) with circularly polarized laser light also reported by several groups [16]. Fig. 1.1B shows narrow nanoripples with ∼200 nm spacing at the peripheral regions of the laser irradiated spot with linearly polarized laser pulses. Fig. 1.1C shows wider nanoripples with about 600 nm spacing near the central hot region of the laser-irradiated spot. As expected, for linearly polarized light, the nanoripple orientation was always orthogonal to the laser polarization [17,18]. Nanoripple formation was observed with 10–100 shots fired on the semiconductors. It was also observed that number of shots fired on the semiconductor did not have any effect on the ripple period.

1.1 Nanoripple formation

FIGURE 1.1  GaP Irradiated by 800 nm Ultrashort Laser Pulses, (A) with circularly polarized beam, and (B) and (C) with linearly polarized beam in two different regions; (B) is around the periphery of the irradiated spot (low fluence); and (C) is in the central region (high fluence). [The length of the horizontal bar is 1 µm in (A), (B), and 2 µm in (C).]. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

Fig. 1.2 shows the nanoripple formation using linearly polarized laser pulses. In the narrow bandgap semiconductors such as GaAs and InP, the scanning electron microscope (SEM) pictures of the irradiated spot show the spacing to be of the order of 500–600 nm. On the contrary, in WBG materials such as GaN and SiC, the spacing is of the order of 170–270 nm. Thus it was observed that, in general, the narrow nanoripples are formed from material with a WBG. This observation of narrow ripple formation in WBG semiconductor is consistent with previous reported results by Borowiec et al. [19], but no explanation of the physical processes involved was given in those experiments. Fig. 1.3 shows different microstructure formations using different fluences and ambient conditions for narrow bandgap semiconductors such as GaAs. The SEM picture of the irradiated spot, as seen in Fig. 1.3A, shows that, at a high fluence in air, no nanoripple formation takes place, and only random heterogeneities of a few microns order appear. Fig. 1.3B shows formation of nanoripples of a typical size of 600 nm at low fluence (in air). Interestingly, Fig. 1.3C shows formation of nanoholes at high fluence in water. Fig. 1.3D shows narrow nanoripples of 150 nm size in GaAs at low fluence in water. This shows the crucial role of the ambient medium in formation of the nanoripples. Very narrow nanoripples were observed in narrow bandgap materials such as GaAs in water, whereas in air, the corresponding nanoripple period was of the order of the laser wavelength. Even for a WBG material such as GaP, which

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CHAPTER 1  Periodic nanoripples formation on the semiconductors

FIGURE 1.2  Nanoripple Formation Using 800 nm Pulses in Narrow Bandgap Semiconductors: (A) GaAs and (B) InP; and in wide bandgap semiconductors: (C) GaN and (D) SiC [The length of the horizontal bar is 1 µm in (A), (B), (C), and (D).]. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

FIGURE 1.3  Nanoripple Formation Using 800 nm Pulses in GaAs: (A) at high fluence in air, (B) at low fluence in air, (C) at high fluence in water, and (D) at low fluence in water. [The lengths of the horizontal bars correspond to 5 µm in (A), (B), and (C), and 1 µm in (D).]. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

1.1 Nanoripple formation

shows narrow nanoripples of period ∼200 nm in air, a narrower period of 150 nm was observed during irradiation in water. Fig. 1.4 shows the angular dependence of nanostructure size obtained from narrow (GaAs) and wide (GaP) bandgap materials irradiated by 800 nm pulses in water. It was observed that, for the narrow bandgap (GaAs) semiconductor, the ripple size increased with increasing angle of incidence, as shown in Fig. 1.4A. In the case of the WBG (GaP) material with increasing angle of incidence, the ripple period first increases and becomes constant at higher angles, as shown in Fig. 1.4B. It was mentioned earlier in the context of Fig. 1.3C that, at high fluence, nanoholes are formed in GaAs. Fig. 1.4C shows that the size of the holes increases with increasing the angle of incidence of the incident p-polarized light. In many photonic applications, for example, two-dimensional photonic crystals and micro-diffraction elements, the nano/micro holes on the surface of bulk materials are required as the building blocks. Although the mechanism of formation of such structures is still not clear, this process was achieved during irradiation in water medium, thus emphasizing the role of dense surrounding medium in formation of such novel hole structures. It is commonly accepted that the nanoripple width is dependent on the wavelength of the incident light. Therefore to obtain narrow nanoripples, second harmonic of the fundamental laser radiation is a better choice. Fig. 1.5 shows the influence of laser wavelength on nanoripple formation. Fig. 1.5A and B show that, in the case of SiC, the use of 400 nm wavelength reduces the ripple period by more than a factor of two as compared to the nanoripples produced by the 800 nm pulses (from 190 nm to about 90 nm). Similarly, for a narrow bandgap material such as InP, there is a reduction of ripple period from 620 nm to about 280 nm, as shown in Fig. 1.5C and D. It may be noted that, for GaP, the ripple period using 400 nm pulse becomes larger than that formed using 800 nm pulses (from 180 nm to about 300 nm).

1.1.2  DISCUSSION OF EXPERIMENTAL RESULTS AND DESCRIPTION OF THE MODEL The consistent observations regarding the nanoripple formation are as follows: (1) the ripple period is larger at higher fluence, (2) the ripple period is large for the materials having bandgap narrower than the incident photon energy compared to that for materials having a wider bandgap, and (3) a denser ambient medium results in the formation of narrower ripple period. The plausible explanation for such observations is consistent with the theory that ripple formation is due to excitation of the surface plasmon at the semiconductor surface by the incident laser light with the rough target surface. The surface plasmon is a well-known phenomenon in the context of the metals, which have free electrons for excitation of this longitudinal charge density oscillation. However, in the case of semiconductors, the origin of free electrons is because of the excitation of free electrons due to multiphoton ionization by the laser. The typical intensity of 2 × 1012 W cm−2 is just sufficient to start plasma formation for an ultrashort (45 fs) laser pulse, wherein multiphoton ionization is the dominant process. These laser-generated free electrons give a transient metallic character to

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CHAPTER 1  Periodic nanoripples formation on the semiconductors

FIGURE 1.4  Angular Dependences of Formation of Various Nanostructures Using 800 nm Pulses in Water. (A) Nanoripple formation in GaAs, (B) Nanoripple formation in GaP, and (C) Nanohole formation in GaAs. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

1.1 Nanoripple formation

FIGURE 1.5  Nanoripple Formation Using 800 nm Pulses and 400 nm Pulses Respectively in Wide Bandgap Semiconductors: SiC (A) and (B), GaP (E) and (F), and in narrow bandgap semiconductor InP (C) and (D). [The length of the horizontal bar is 2 µ m in (A), 500 nm in (B), 5 µ m in (C), 1 µm in (D), 1 µm in (E), and 2 µm in (F).]. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

the molten surface, which can now support the surface plasmon. During the initial few shots, the surface roughness is enhanced due to irradiation by the high-intensity pulses and formation of random nanostructures. The subsequent shots fired on the roughened surface lead to more efficient excitation of surface plasmon. The molten material assumes the shape of a grating, which satisfies the relation between wave vectors of the incident laser (which depends on its wavelength, the angle of incidence, and the refractive index of the ambient medium) and the surface plasmon (which depends on the electron density of the molten surface). This grating gets frozen once the material cools down as soon as the laser pulse ends giving rise to the observed nanoripples. The wave vector of this grating satisfies the following relation, as per the momentum conservation [18] G = k i − ks , (1.1)

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CHAPTER 1  Periodic nanoripples formation on the semiconductors

where ki is the wave vector of incident laser and ks is the wave vector of surface plasmon. By substituting the expression for k's in terms of corresponding wavelengths, and |G| as 2π/d (where d is the ripple period), one gets the expression for d as [18] λL (1.2) d= λL ± µ sin θ λS

Here λL, λS, θ, and µ are the incident laser wavelength (in vacuum), the surface plasmon wavelength, the incident angle, and the refractive index of the ambient medium, respectively. It is clear from Eq. (1.2) that for normal incidence (i.e., θ = 0) d = λS. Thus at normal incidence the ripple period d is equal to the surface plasmon wavelength, which is obtained from its dispersion relation as λL λS = (1.3) εε m ε + εm

Here ε is dielectric constant of the surface plasma created by the intense femtosecond pulse, and εm is the dielectric constant of the ambient medium (i.e., εm = µ2; 1 for air and 1.76 for water). Because the multiphoton ionized surface has free electrons, its dielectric constant can be taken similar to a plasma, that is, (neglecting the collision term frequency) n ε = 1− e (1.4) nc

where ne is the surface plasma free electron density, and nc is the critical density corresponding to the laser wavelength. Putting the expression of ε in Eq. (1.3) and then using Eq. (1.2), one gets for the case of normal incidence λL (1.5) d= n ε m 1− e n c ne ε m + 1− n c

(

(

)

)

From Eq. (1.5), it is clearly seen that the ripple period is related to the incident laser wavelength, electron density of the surface plasma, and the dielectric constant of the ambient medium. It is also evident from the expression that real values of d will be obtained only if ne < nc or ne > ( ε m + 1) nc (1.6) When this condition given by Eq. (1.6) is not satisfied, the nanoripples are not formed. Fig. 1.6A shows the variation of nanoripple period in air and water with electron density plotted using Eq. (1.5) for 800 nm wavelength. Fig. 1.6B shows the same curve for 400 nm wavelength.

1.1 Nanoripple formation

FIGURE 1.6  Nanoripple Period as a Function of the Electron Density for Air (Dotted Line) and Water (Continuous Line), using (A) 800 nm pulses and (B) 400 nm pulses. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

A few conclusions can be made from these figures. Two branches of solutions exist for the nanoripples; one is the super-wavelength nanoripple, and the other is the subwavelength nanoripple. The super wavelength can be formed if the electron density is below the critical density, but such structures are not observed primarily due to two reasons; the first one is that being a low electron density process the plasmon excitation is very weak, and the second one is that the long period structure will have lower groove depth, so that any short period structure may overshadow the long period one. The third possible reason could be that at such low fluence, the material simply does not melt to reorganize, thereby precluding any possibility of ripple formation. The subwavelength nanoripples are formed if the electron density is greater than (εm + 1)nc. The nanoripple period is equal to laser wavelength if the electron density is high, and a very sharp decrease in the

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ripple period is observed when the electron density starts approaching (εm + 1)nc. If water is the ambient medium, then the nanoripple period is smaller for the same electron density. Further, the graph also brings out the critical role of electron density in formation of large and narrow ripple formation. Because the electron density generating surface during femtosecond laser pulse irradiation depends on the laser parameters such as fluence and wavelength, a drastic change in the nanoripple period is expected on a small variation of these parameters. Now let us explain the experimental observations in terms of the aforementioned formulation. The observation from Fig. 1.1 regarding narrow ripple of 200 nm width at the periphery of the focal spot and wider 600 nm ripple width in the central hot spot region can be explained as follows. Because the electron density of the laser irradiated surface is proportional to the incident laser fluence, higher intensity of irradiation in the central region of the focal spot will generate a higher electron density leading to formation of wider nanoripples of 600 nm spacing, whereas, at the region of the edges of focal spot, a lower fluence generates a lower electron density, which leads to a narrower (∼200 nm) ripple formation. It is also seen from Fig. 1.6, that a slight change in the electron density can cause a drastic change of the ripple period, and this explains the observation of the large difference in the ripple period in two spatial regions. The abrupt change in the ripple period is because, as explained earlier, it is very critically dependent on the electron density (see Fig. 1.6). At high intensity, ripples slightly smaller than the laser wavelength are formed, and at mid-range intensity or lower intensity, the ripple period can become very narrow depending on the free electron density generated on the surface. At very low intensities, no rippling takes place, as explained earlier. Of course, the semiconductor material chosen, its bandgap and material breakdown property, and irradiation intensity decide the electron density generated. The main observation that narrow LIPSS are formed on WBG material can also be explained in a similar way. When a femtosecond laser irradiates a narrow bandgap material semiconductor surface, more free electrons are likely to be available in comparison to the case if the WBG material is irradiated by the same laser. This is because in addition to the multiphoton excitation of electrons, the single photon absorption process will also allow the excitation of free electrons, provided the incident photon energy is larger than the bandgap. Therefore, the absorption of the ultrashort pulses in narrow bandgap semiconductor material is through a combination of linear and nonlinear absorption processes leading to a larger availability of free electrons in the conduction band. Therefore as observed from Fig. 1.2, the ripple period is large (∼600 nm) in the case of narrow bandgap materials, as a higher electron density [ne >> (εm + 1)nc] leads to formation of larger width nanoripples. On the other hand, as the generated electron density for WBG material is low as single photon absorption is not possible, narrow ripples are formed in this case (provided the incident laser intensity is low).

1.1 Nanoripple formation

Next, we discuss the nanoripple formation in GaAs in air and water. The ambient medium has substantial effect on the nanoripple formation as expected from Eq. (1.2). This was experimentally observed in Ref. [20] for methanol as the surrounding medium. The discussed experiments in air and water were carried out at two fluences. At higher fluence, no nanoripple formation takes place. As seen in Fig. 1.3A, only random heterogeneities of few microns order appear. At low fluence, subwavelength nanoripples of 600 nm period are formed (Fig. 1.3B. Such micron-order microstructures have also been observed in Ref. [21]. They observed that, as the laser fluence increases, the ripples become disordered due to the enhancement of the thermal effects. Also, as the laser fluence becomes high, the surface morphology evolves and the degree of ripple irregularity becomes large, resulting in the dominant shift of spatial scales of the laser-induced structures from a few hundred nanometers to a few micrometers. Next, in water, a drastic reduction of the nanoripple period is recorded from 600 nm in air to about 150 nm in water. Such a reduction in nanoripple period is expected from Fig. 1.6 as already discussed. An interesting observation is that at high fluence, irradiation of GaAs in water leads to nanohole formation. The reason of nanohole formation is still not understood, although it seems to be like a self-organization process due to surface tension relaxation of strained molten GaAs against another liquid (in this case, water). The experiment was repeated at low fluence for GaP (wide bandwidth material), which shows 200 nm nanoripple period in air and 150 nm in water. Many groups have done LIPSS width study with different incident angles of the laser [22,23]. There are contradictory observations regarding this, as some have reported increase of period with increase in angle of incidence [22], whereas, some have shown decrease in the ripple period with angle of incidence [23]. In the discussed experiment, an overall increase in ripple period with the angle of incidence for GaAs in water was recorded as shown in Fig. 1.4A. GaP in water also shows an increase of ripple period from 150 nm to 300 nm, as the angle is increased from normal incidence to an angle of 30 degree. However, the ripple period saturates at larger angles as seen from Fig. 1.4B. These trends can be partially explained from Eq. (1.2) if one takes the “minus” sign. In that case, the denominator decreases in magnitude as the angle of incidence in increased, leading to increase in the ripple period. Although this explanation holds true for GaAs which show a continuous increase of ripple period with increasing angle, the same is not correct for GaP where ripple period shows saturation at larger angles. This shows that it is important to consider other factors that contribute to the variation of ripple period with angle of incidence. The most crucial among them is perhaps the electron density of the surface plasma created after irradiation of the semiconductor. The electron density generated, in turn, depends on the laser irradiation conditions (fluence, laser wavelength, etc.), material properties (bandgap, melting point, etc.), and the absorption of laser energy in the material through linear (single photon absorption) and nonlinear processes (multiphoton absorption). As the angle of

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CHAPTER 1  Periodic nanoripples formation on the semiconductors

incidence is increased, the circular focal spot gets elongated and becomes elliptical, leading to a decrease in the laser fluence. For example, at an incident angle of 45 degree, the fluence becomes about 70% of the fluence at normal incidence. Therefore as the angle is increased, the fluence decreases, resulting in the decrease of electron density of the surface plasma. This should lead to a decrease in the ripple period. However, as the angle of incidence is increased, the laser energy absorption also increases (up to the Brewster angle). The increase in the absorption is because the reflectivity of the p-polarized light keeps decreasing as the angle of incidence increases and approaches the Brewster angle. The range of angles in discussed studies was below the Brewster angle of the semiconductor (the Brewster angle of semiconductors is as high as 70–80 degree). This decreasing reflectivity leads to an enhanced absorption and, hence, a higher electron density, which should result in generation of larger period ripple. Therefore resultant ripple period is governed by the aforementioned two competing processes of electron generation, one being fluence and the other being absorption. On increasing the angle of incidence, the fluence reduces and less electron generation is expected, but on the other hand, on increasing the angle, more laser light absorption will occur resulting in more electron generation. The relative dominance of the electron generation process, namely, laser energy absorption or laser fluence, determines whether as a function of angle the electron density decreases or increases. One can now explain the observation that in the case of GaP the LIPSS period initially increases and then becomes nearly constant at larger angles (Fig. 1.4B). It seems that in the used experimental conditions, the laser energy absorption and the laser fluence compete against each other equally. The generated electron density is such that the ripple period becomes constant at higher angles for GaP. The monotonic increase of ripple period with the angle of incidence in the case of GaAs (Fig. 1.4A) implies that the electron density in this case is continuously increasing on increasing the angle. This means that electron generation is dominated by absorption process and the fluence has comparatively less influence in the case of GaAs. Had fluence been the more important parameter, the electron density would be reduced on increasing angle (due to the decrease of fluence), and the nanoripple period would have decreased. Further, it may be noted that for s-polarized light, the period should be independent of the angle of incidence, as the magnitude of the laser k vector in the direction of the surface plasmon remains unchanged in this case. However, as the angle increases, due to increasing reflectivity (due to s-polarization), the absorption of the laser light will keep decreasing with angle, leading to lower electron density and corresponding decrease in ripple period. Interestingly, the nanoholes formed on GaAs (at high laser fluence, in water) also showed increase for the hole diameter for higher angles of incidence (Fig. 1.4C). Further experiments are needed to understand generation mechanism of nanoholes. The role of the laser wavelength in changing the LIPSS period is seen from Fig. 1.5. As follows from Eq. (1.5), the ripple period is expected to decrease by a factor of two as one goes from fundamental to the second harmonic laser beam, while other conditions remain the same. This is precisely what is seen in Fig. 1.5A for SiC

1.1 Nanoripple formation

[a WBG (3.37 eV) material for both wavelengths] and Fig. 1.5B for InP [a narrow bandgap (1.35 eV) material for both wavelengths]. In both cases, there is a reduction in ripple period by a factor of two (190 nm to 90 nm and 620 nm to 280 nm). The observation in the case of GaP is just the opposite. Here the ripple period is observed to double (180–300 nm) instead of becoming half. This is because GaP has a bandgap energy of 2.3 eV, which is wide for 800 nm radiation (1.56 eV), but narrow for 400 nm radiation (3.12 eV). So, as a WBG material, GaP shows a narrow ripple period (180 nm) for the fundamental (800 nm), and as a narrow bandgap material, it shows larger ripple period (300 nm) for the second harmonic radiation (400 nm). The aforementioned correlation of the bandgap with ripple period becomes more obvious in Fig. 1.7. The vertical dotted line in Fig. 1.7A is the photon energy of the

FIGURE 1.7  Observed Nanoripple Period as a Function of Bandgap. The dotted line indicates the incident laser photon energy for (A) 800 nm pulses and (B) 400 nm pulses. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

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CHAPTER 1  Periodic nanoripples formation on the semiconductors

800 nm laser, and for Fig. 1.6B, the dotted line represents the photon energy of the 400 nm laser radiation. From these two graphs, it is clear that if the bandgap of the semiconductor material is narrower than the incident photon energy, the ripple width formed is slightly less than, but of the order of the laser wavelength. For the materials having a bandgap larger than the energy of the laser photon, the ripple period is much smaller (typically of the order of 1/4th laser wavelength). The reason for this difference has been already explained earlier in terms of single photon and multiple photon ionization of the molten surface. Although most of the observations of LIPSS formation could be explained from the aforementioned formulation, there could be other parameters, which are responsible for finer variations of different nanoripple formation of different materials. We have neglected the material parameters such as melting point, conductivity of semiconductors, charge mobility, surface tension, and viscosity of the molten material of the surface, which may also be very critical in deciding the resultant structure. For example, melting is one of the primary requirements of restructuring the surface. To see if the melting point plays a deciding role in ripple period, the ripple period has been plotted in Fig. 1.8 as a function of melting point for various materials, at a constant laser fluence. There is clear trend that shows materials having a high melting point forming narrow ripples. Fig. 1.8, seen in isolation, would give an impression that melting point decided the ripple period. However, when seen with Fig. 1.7B for the ripples formed with second harmonic laser beam, it becomes clear that looking at GaP ripple period, it is the bandgap (relative to the laser photon energy) that decided the ripple period, and not the melting point, although melting is crucial in nanoripple formation.

FIGURE 1.8  Observed Nanoripple Period as a Function of Melting Point for Various Semiconductors. Reproduced with permission from U. Chakravarty, R.A. Ganeev, P.A. Naik, J.A. Chakera, M. Babu, P.D. Gupta, Nanoripple formation on different band-gap semiconductor surfaces using femtosecond pulses, J. Appl. Phys. 109 (8) (2011) 084347, AIP Publishing.

1.2 Nanosecond laser-induced periodic surface structures formation

1.2  NANOSECOND LASER-INDUCED PERIODIC SURFACE STRUCTURES FORMATION ON WIDE BANDGAP SEMICONDUCTORS USING NANOSECOND ULTRAVIOLET PULSES As the early observation of LIPSS on semiconductors [24], this kind of nanostructures has been imprinted on almost all kinds of materials and has been extensively investigated using low-power cw and pulsed laser sources of nanosecond (ns) and femtosecond (fs) duration [25–28]. In general, the ripples have a period d dependent on laser wavelength λ, on the angle of incidence of the radiation θ, and on index of refraction n and can be described by the relation d = λ/(n − sinθ) [29]. After exposure of a smooth solid to a linearly polarized radiation at normal incidence, often the lateral period of the fabricated LIPSS is very close to the wavelength of the incident radiation. It has been proposed that this type of ripples arises from optical interference effects due to the superposition of the incident radiation with a surface electromagnetic wave, which is created at the material–medium interface during irradiation together with a feedback mechanism. LIPSS resulting from femtosecond laser irradiation of solids have received considerable attention in attempts to determine their formation mechanism (see Section 1.1.1 and Refs. [30–33]). Irradiation of surfaces at normal incidence usually leads to the formation of low spatial frequency LIPSS (LSFL) with period comparable to the laser wavelength. This type of structures is explained in reference to the previously mentioned interference mechanism. In the case of metals, semiconductors, and dielectrics, the formation of ripple structures with subwavelength periods has also been observed. These high spatial frequency LIPSS (HSFL) have been obtained using femtosecond laser pulses of different duration, wavelength, fluence, and number of pulses [34–37]. Several mechanisms have been proposed as the origin of HSFL, as was mentioned in the previous section. WBG semiconductors have expanded the scope of applications beyond those of silicon. The developing list of such materials for use in device production is remarkable and continues to provide new design possibilities. The inherent properties of WBG make them ideal candidates for high-power, high-temperature electronic devices, power amplifiers, switches, and short wavelength light sources. Therefore modification of structure and properties of WBGs at the nanometer scale attracts great interest [38]. In the studies reviewed in the forthcoming paragrphs [39], LIPSS were imprinted on the surface of WBG semiconductor wafers of indium phosphide (InP), gallium arsenide (GaAs), gallium phosphide (GaP), and silicon carbide (SiC) by irradiating in air with linearly polarized, 266 nm, 6 ns laser pulses. The period and amplitude of the LIPSS were characterized by atomic force microscopy (AFM) as a function of the laser fluence and number of pulses. It was observed that, as the bandgap of the semiconductor material increases, higher fluence or number of pulses are needed for LIPSS formation, whereas the amplitude of the ripples is related with the optical and

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Table 1.1  Initial Roughness (Ra), Energy Bandgap [41], Melting Temperature [41] (Tm), Specific Heat [41] (c), Thermal Conductivity [41] (k), Density [41] (ρ), and Linear Absorption Coefficient at 266 nm [40] (α) of Semiconductor Wafers. Material

Bandgap Ra (nm) (eV) Tm (°C)

c k (J kg−1 K) (W mK−1)

ρ (kg m−3) α (cm−1)

InP GaAs GaP SiC

0.30 0.28 0.62 0.65

310 330 430 690

4810 5316 4138 3210

1.35 1.42 2.30 3.37

1060 1240 1457 2730

68 55 110 370

1.47 × 106 1.72 × 106 1.21 × 106 2.44 × 106

Reprinted from M. Sanz, E. Rebollar, R.A. Ganeev, M. Castillejo, Nanosecond laser-induced periodic surface structures on wide band-gap semiconductors, Appl. Surf. Sci. 278 (2) (2013) 325–329, with permission from Elsevier.

thermal penetration depth. Estimations of surface temperature increase are discussed with reference to the WBG semiconductor's electrical, optical, and thermal properties.

1.2.1  EXPERIMENTAL SETUP For studying ripple formation, multiple pulse laser irradiation of undoped semiconductor wafers of InP, GaAs, GaP, and SiC was carried out in ambient air at normal incidence. The electrical, optical, and thermal properties of those materials [40,41] are summarized in Table 1.1. For irradiating the samples, the linearly polarized fourth harmonic output of a Q-switched Nd:YAG laser (266 nm, λ = 6 ns, 10 Hz repetition rate) was used. This irradiation wavelength corresponds to an energy of 4.67 eV, well above the bandgap energies of these semiconducting materials (Table 1.1). The ­central (4 mm diameter) most uniform part of the beam spot was selected for irradiation by using a diaphragm. The laser beam was focused on the substrate surface with a spherical lens of 15 cm focal length. The irradiation fluence was below the ablation threshold (Fth) for a single pulse of each material. The fluence was determined as the ratio of the laser pulse energy, measured in front of the sample with a joulemeter, and the area of the irradiated spot. Ablation thresholds of samples were determined by measuring the minimum single pulse energy necessary to yield a surface change as detected by optical microscopy using a 160× microscope objective equipped with a charge coupled device (CCD) camera. The obtained values were 190, 380, 470, and 950 mJ cm−2 for InP, GaAs, GaP, and SiC, respectively. The morphology of the lasertreated semiconductor surfaces was characterized using AFM. The AFM measurements were performed in 5 different positions of each sample to check the uniformity of the fabricated nanostructures. The pristine substrates, of around 300 µm thick, present a flat surface, with mean roughness (Ra) values 300

125 150 125 –

200 200 300 –

248 ± 6 253 ± 7 263 ± 10 No LIPSS observed

5 ± 1 9 ± 2 15 ± 5

Reprinted from M. Sanz, E. Rebollar, R.A. Ganeev, M. Castillejo, Nanosecond laser-induced periodic surface structures on wide band-gap semiconductors, Appl. Surf. Sci. 278 (2) (2013) 325–329, with permission from Elsevier.

1.2.2  NANORIPPLE FORMATION BY NANOSECOND PULSES Irradiation of the semiconductor wafers was performed at different fluences and number of pulses to find the conditions for obtaining the most uniform ripples in terms of period and amplitude. The minimum fluence (Fm) needed for LIPPS fabrication is displayed in Table 1.2 together with the experimental conditions for the optimum LIPSS fabrication for each semiconductor wafer. Fig. 1.9 shows AFM height images and corresponding cross-section of the LIPSS obtained in InP, GaAs, and GaP. Fig. 1.9A displays the ripples fabricated in InP with 200 pulses of 125 mJ cm−2. Ripples were perpendicular to the laser polarization direction, and the measured period was estimated to be 248 nm with amplitude of 5 nm. The structures were observed at fluences of 100–150 mJ cm−2 with 100–300 pulses. In the case of GaAs, the optimum conditions for LIPSS formation of InP (125 mJ cm−2, 200 pulses) resulted in isolated rounded nanostructures (Fig. 1.10A). When increasing the number of pulses, the ripples started to form and align (Fig. 1.10B), whereas the increase of fluence finally results in LIPSS formation (Figs. 1.9B and 1.10C). Further fluence increase induced the destruction of the nanostructures (Fig. 1.10D). The best ripples were obtained with 200 pulses at 150 mJ cm−2. Under these conditions, the estimated period was 253 nm and the amplitude 9 nm. For GaP, a number of pulses higher than that used for GaAs are necessary to fabricate uniform LIPSS. The best ripples, with a period of 263 nm and amplitude of 15 nm, were obtained at 125 mJ cm−2 and 300 pulses (Fig. 1.9C). Increasing the fluence and/or the number of pulses caused the disappearance of the uniform ripples. In the case of SiC, no LIPSS were obtained for fluences as large as 300 mJ cm−2 and for a large number of pulses (up to 600).

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FIGURE 1.9 AFM height images (2 × 2 µm2 size) (left) and corresponding cross-sections (right) of LIPSS fabricated using 266 nm radiation in (A) InP with 200 pulses at a fluence of 125 mJ cm−2, (B) GaAs with 200 pulses at 150 mJ cm−2, and (C) GaP with 300 pulses at 125 mJ cm−2. The ripples are perpendicular to the laser polarization direction. Reprinted from M. Sanz, E. Rebollar, R.A. Ganeev, M. Castillejo, Nanosecond laser-induced periodic surface structures on wide band-gap semiconductors, Appl. Surf. Sci. 278 (2) (2013) 325–329, with permission from Elsevier.

1.2 Nanosecond laser-induced periodic surface structures formation

FIGURE 1.10  AFM Height Images (3 × 3 µm2 size) of LIPSS Fabricated Using 266 nm Radiation in GaAs at the Indicated Conditions. The ripples are perpendicular to the laser polarization direction. Reprinted from M. Sanz, E. Rebollar, R.A. Ganeev, M. Castillejo, Nanosecond laser-induced periodic surface structures on wide band-gap semiconductors, Appl. Surf. Sci. 278 (2) (2013) 325–329, with permission from Elsevier.

1.2.3  DISCUSSION OF NANOSTRUCTURE FORMATION As mentioned, the mechanism of LIPSS formation can be explained as the result of the optical interference effects due to the superposition of the incident radiation with a surface electromagnetic wave, which is created and scattered along the irradiated surface [42]. This results in a modulated distribution of energy on the surface, which consequently induces a similarly modulated heating. The thermal penetration depth of the irradiated zone can be calculated by dth = (Dτ)0.5 with D = k/ρc being the thermal diffusivity, k the thermal conductivity, ρ the density, c the specific heat of the material (Table 1.1), and τ the pulse duration (6 ns). For the materials under study, dth is below 1 µm, thus much smaller than the diameter of the irradiated area. Therefore for estimating the temperature increase of the irradiated substrate, one can assume for simplicity that the temperature distribution is associated with depth x and time t. It is also assumed that, during the irradiation time interval, the material parameters and the irradiation intensity are constant. Fig. 1.11 shows for the materials studied herein the temporal evolution of the estimated surface temperature upon irradiation with a single laser pulse of 266 nm at 125 mJ cm−2. The maxima of the curves correspond to the maximal temperature attained in the surface of the different samples.

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FIGURE 1.11  Time Dependence of the Temperature Reached on the Semiconductor Surface Under Irradiation With a Single Pulse of 266 nm at 125 mJ cm−2, for the Different Materials, as Indicated. Reprinted from M. Sanz, E. Rebollar, R.A. Ganeev, M. Castillejo, Nanosecond laser-induced periodic surface structures on wide band-gap semiconductors, Appl. Surf. Sci. 278 (2) (2013) 325–329, with permission from Elsevier.

The optimum conditions for LIPSS formation can be related with the surface temperature reached upon irradiation and with the semiconductor melting point. For InP and GaAs, the temperature attained at the surface is slightly below the corresponding melting point (Table 1.1). In the case of GaP, the temperature of the irradiated surface is clearly below the melting point. For SiC, and due to the high values of specific heat c and thermal conductivity k of this material, higher fluences than those explored in discussed study are expected to be required to melt the outer sample layer. Those results indicate that to obtain LIPSS a minimum fluence is necessary to assure that the surface temperature is high enough for allowing melting and material rearrangement. On the other hand, material evaporation produces emission of atoms from the semiconductor surface. Evaporation is the origin of microdefects that raise the surface roughness and enhance the absorption. This in turn increases the surface heterogeneities, which facilitate the feedback mechanism necessary for the ripple formation. The relative increase of fluence and number of pulses needed for imprinting optimum LIPSS on the semiconductors are observed to rise as the energy bandgap increases. This is related to the fact that higher melting temperatures correspond to wider bandgaps (Table 1.1) and thus higher temperatures should be reached upon irradiation to allow melting and rearrangement of material. Measured ripple periods are of the order of the irradiation wavelength for all analyzed semiconductors, whereas a moderate increase in amplitude is observed as the optical and thermal penetration depths of the material increase. The optical absorption depth for a single pulse, calculated as the inverse of the linear optical absorption coefficient (Table 1.1), is larger for GaP than for InP and GaAs. The thermal penetration depth dth is also higher for GaP (610 nm) than for InP and GaAs (around 450 nm)

1.3 Fabrication of two-dimensional periodic nanostructures

for the given irradiation conditions. The larger optical and thermal penetration depths induce a deeper effect close to the surface in GaP as compared to InP and GaAs, in good agreement with the observed ripple amplitude values: the ripple amplitude for GaP is 15 nm, whereas 5 nm amplitude ripples are obtained in the case of InP.

1.3  FABRICATION OF TWO-DIMENSIONAL PERIODIC NANOSTRUCTURES BY TWO-BEAM INTERFERENCE OF FEMTOSECOND PULSES 1.3.1  2D AND 3D STRUCTURES There has been considerable interest in the fabrication of two- and three-dimensional (2D and 3D) PCs, which consist of artificial periodic structures [43–48]. Various techniques were used to fabricate these periodic structures such as self-assembly of colloidal particles, direct laser writing, and holographic lithography (HL), etc. HL is a highly useful technique to fabricate periodic patterns in photosensitive materials by the interference of several coherent laser beams. One can fabricate various 2D and 3D periodic structures by changing the number of laser beams (usually >3) and their arrangements. The periods were larger or nearly equal to the laser wavelength; therefore it is still a challenge to improve the resolution of laser fabrication to obtain PCs with bandgap in visible spectrum. Periodic structures induced by single laser beam have been studied intensively in the last four decades [49]. As already mentioned, it was found that the periods were usually close to the laser wavelength when long pulse (nanosecond and picosecond) laser or cw laser were used. Meantime, nanoripples with periods much less than the laser wavelengths have been fabricated in semiconductors and dielectrics after irradiation by femtosecond laser pulses [14,19,35]. LIPSS with periods of 40–500 nm have been fabricated by using lasers at wavelengths of 267–2000 nm. Further studies of the formation of regular short periodic nanogratings on the surface of semiconductor crystals, particularly ZnO irradiated by two collinear laser beams, were described in Ref. [50], which suggested that the short periodic nanostructures have potential applications in optical recording and PCs fabrication. ZnO is an important compound semiconductor with a WBG of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. These properties make it a very useful material in optoelectronics. Several groups have reported the fabrication and the applications of various types of ZnO nanostructures such as nanoparticles, nanowires, nanobelts, and nanofilms [51,52]. Below we discuss the fabrication of 2D periodic nanostructures formed on the surface of ZnO crystals applying a method of two-beam interference of femtosecond pulses [53]. This method is based on the two aspects discussed previously: one is the long periodic structures determined by the two-beam interference pattern, another is the short periodic nanostructures induced by single femtosecond laser beam. Furthermore, the formation mechanism is discussed.

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FIGURE 1.12  Experimental Setup of Two-Beam Interference Used for Fabrication of 2D Periodic Structures. BS, 50% beam splitter; GZ, Glan polarizer along z axis; HF, half-wave plate; L, lens; M, mirror. Reproduced with permission from T. Jia, M. Baba, M. Suzuki, R.A. Ganeev, H. Kuroda, J. Qiu, X. Wang, R. Li, Z. Xu, Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses, Opt. Exp. 16 (3) (2008) 1874–1878, Optical Society of America.

1.3.2  EXPERIMENTS AND DISCUSSION Fig. 1.12 shows the experimental setup for fabrication of 2D periodic structures. Laser pulses at the wavelength of 790 nm with pulse duration of 120 fs were delivered from a commercial Ti:sapphire regenerative amplifier operated at 10 Hz repetition rate. The laser beam was propagated through a half-wave plate and Glan polarizer, and then was split into two beams via a 50% beam splitter. Both of the two beams polarized along z direction. One beam went through a delay line and reached the sample surface at the exact same time with the other one. Zero temporal point was determined by the signal of double frequency via a BBO crystal. The angle between the two laser beams was denoted as 2θ and could be easily changed by rotation of two mirrors of M1 and M2. ZnO crystal plate was 1.0 mm thick, and its surface was normal to the x axis and optically polished. The sample was mounted on a XYZ-translation stage. After laser pulses irradiation, the sample was dipped in ethanol and pure water, and cleaned for 5 min with ultrasonic cleaner, respectively. The periodic structures formed on the sample surface were observed by using SEM. The laser beams were focused with lenses of 250 mm focal length. The sample was transmitted to the position of 0.5 mm in front of the focal plane to enlarge the focus to 110 µm in diameter. SEM images of the periodic structures are shown in Fig. 1.13. The spatial overlapping length was 36 µm. Only in the central part of overlapping area, two laser beams interfered thoroughly. Therefore, the periodic structures were observed on the ablation area of 30 × 100 µm2 (see Fig. 1.13A). Fig. 1.13B and C show the SEM images in the white square marked in Fig. 1.13A and B at higher magnifications, respectively. These photos represented 2D periodic structures. Only one long-dimensional periodic structure was obtained by two-beam

1.3 Fabrication of two-dimensional periodic nanostructures

FIGURE 1.13  SEM Images of 2D Periodic Nanostructures. The total pulse energy and irradiation time were 127 µJ and 6 s in (A–C), 154 µJ and 4 s in (D), and 250 µJ and 10 s in (E), respectively. Reproduced with permission from T. Jia, M. Baba, M. Suzuki, R.A. Ganeev, H. Kuroda, J. Qiu, X. Wang, R. Li, Z. Xu, Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses, Opt. Exp. 16 (3) (2008) 1874–1878, Optical Society of America.

interference method, and the period was determined by the interference pattern described by relation Λ = λ/2sinθ. The laser wavelength λ was 790 nm, and the angle between the two laser beams was 2θ = 35.2 degree, thus the period was estimated to be 1.31 µm. The long period was 1.33 µm (see Fig. 1.13B and C), which was close to the theoretical value. Therefore the long periodic structures are determined by the usual interference pattern of the two laser beams. Besides the long periodic structures, there were short periodic nanostructures (nanogratings) embedding in the long ones, similar to those discussed in the two previous sections. The nanograting was of 1.14 µm wide, and its period was only 250 nm. If the total pulse energy increased to 154 µJ and the irradiation time decreased to 4 s, the nanograting width decreased to 1 µm, and its period increased to 270 nm (shown in Fig. 1.13D). Once the laser conditions adjusted, it was found that the width of the short periodic structure changed in the range of 0.4–1.15 µm while the long period was kept as ∼1.33 µm. If the pulse energy was higher than 240 µJ, the nanogratings were ablated and disappeared. However, on the interval line between each nanogratings, periodic nanospots of 0.5 µm long and 0.25 µm wide appeared (Fig. 1.13E). In one square centimeter in the sample surface, there was about 3 × 108 nanoripples or nanospots, and the short period was only 250 nm. The 2D periodic nanostructures have great potential applications in ultrahighdensity optical record and PCs in ultraviolet and visible light range. The evolution of short periodic nanostrucutures with the increase of laser intensity was studied, and the results are shown in Fig. 1.14. At the edge of ablation area denoted as strip 1, the sample surface was melted slightly. Some small ripples emerged randomly on strips 2–4 with the increase of laser intensity. In strip 5, several groups of nanoripples

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FIGURE 1.14  The Evolution of Short Periodic Nanoripples With the Increase of Laser Intensity in Each Strip Denoted as 1, 2,…9. The total pulse energy is 153 µJ, and laser irradiation time is 6 s. Reproduced with permission from T. Jia, M. Baba, M. Suzuki, R.A. Ganeev, H. Kuroda, J. Qiu, X. Wang, R. Li, Z. Xu, Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses, Opt. Exp. 16 (3) (2008) 1874–1878, Optical Society of America.

began to emerge. There were two or three parallel nanoripples in one group, and all of these nanoripples were perpendicular to the laser polarization. With further increase of laser intensity, these groups of nanoripples extended and connected each other (see strips 6 and 7). Regular short periodic nanoripples formed in strips 8 and 9. The evolution processes with laser irradiation time were also studied. The two processes were similar to each other. The experiments were conducted to study the formation of nanoripples in ZnO crystal surface induced by single beam of 790 nm laser and found the periods were in the range of 200–240 nm. The results, as shown in Fig. 1.13, were very close to this value. The laser polarization was rotated by 45 degree, and it was found that the orientation of nanogratings embedding in the long periodic structures rotated by 45 degree, too. The above-discussed formation processes were also similar to the cases of single femtosecond laser beam. These three aspects indicated that the formation mechanism of the short periodic nanostructures in the 2D periodic patterns was similar to that induced by single femtosecond laser beam. The interference method of two femtosecond laser beams is rather complicated, and it is not convenient to be used in 3D microfabrication. To overcome this problem, the laser beam was enlarged to more than 40 mm in diameter via a couple of positive and negative lenses, and then a double iris was used to select two laser beams of the same profile, same polarization, and same intensity. These two laser beams were exactly focused on the sample surface. Fig. 1.15 represented the results induced by a Ti:sapphire femtosecond laser operated at 1 kHz repetition rate. Several ablation spots were observed on the sample surface. They were distributed periodically

1.4 Extended homogeneous nanoripple formation

FIGURE 1.15  SEM Images of Periodic Nanostructures Induced by 1 kHz, 130 fs Laser. The pulse energies are 145 µJ in (A–C), and 130 µJ in (D). The laser irradiation time is 1 s in (A). The sample is transmitted at a speed of 0.2 mm s−1 in (B–D). Reproduced with permission from T. Jia, M. Baba, M. Suzuki, R.A. Ganeev, H. Kuroda, J. Qiu, X. Wang, R. Li, Z. Xu, Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses, Opt. Exp. 16 (3) (2008) 1874–1878, Optical Society of America.

and symmetrically, and the period was equal to that expected by λ/2sinθ. Therefore these periodic ablation spots were determined by the interference between two laser beams. On each spot, there were many nanoripples with periods of 230 nm. The nanoripples were in 45 degree direction for the laser polarization rotated by 45 degree (Fig. 1.15C). The sample was transmitted at a speed of 0.2 mm s−1, which allowed obtaining regular 2D periodic structures with periods of 3.3 µm and 250 nm, respectively (Fig. 1.15D). More details on the extended LIPSS formation are presented in the following section.

1.4  EXTENDED HOMOGENEOUS NANORIPPLE FORMATION DURING INTERACTION OF HIGH-INTENSITY FEW-CYCLE PULSES WITH A MOVING SILICON WAFER Surface patterning has a variety of applications [54]. Surface roughening can be beneficial, for instance, when an increased surface area is desired. Such is the case for improving adhesion of other materials and applications where high optical absorption is required [55]. Pointed structures can be used as field emission sources [56]. Texturing can change the hydrophobic properties of surfaces and affect cell attachment in biomedical applications [57]. Femtosecond laser milling can provide various advantages in prototyping and material manufacturing as an alternative to photolithography [58]. Long-range ordering is relevant to the use of nanostructures in electronic materials applications [59]. As was discussed in previous sections, periodic structure formation through the interaction of laser pulses with semiconductors is a precise method of surface

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modification and has been an important topic of laser-matter studies during the last few decades. In particular, the fabrication of two- and three-dimensional PCs, which consisted of artificial periodic structures, has been reported. Another approach leading to 3D PC structures has been described in Refs. [60,61]. The interest in extended structures has led to studies of different approaches to the modification of semiconductor surfaces (multibeam interference of femtosecond pulses, HL, multiexposure of two-beam interference technique, etc.). An alternative approach is an extension of the area of periodic structure using a moving target technique [62–64]. Among various semiconductors used as the targets for LIPSS formation, silicon has attracted a special attention due to its frequent use in the optoelectronic industry. Most femtosecond laser modification experiments to date have been performed with a photon energy greater than the bandgap energy of silicon (1.11 eV) corresponding to a wavelength of 1120 nm. Most experimental observations of nanoripples were based on the study of structures created during focusing of short laser pulses on the same spot of the semiconductor surface. By the meaning of “short pulse” we assume the commonly used multicycle femtosecond pulses (of 50–200 fs duration). Meanwhile, it would be interesting to analyze application of considerably shorter pulse duration (i.e., of a few optical cycles), which may considerably change the dynamics of ripple formation and maintenance. The area of interest of most previous studies was restricted by the sizes of the beam waist of the focused radiation. In most cases, the LIPSS appeared in the ablated area at the center of the focal spot, with limited nanorippling seen at the edges of the ablated area. Another pattern dominated at stronger ablation levels, when the ripples appeared at the edges of the focal spot, while at the center of the ablated area, the ripples were destroyed, and the chaotically distributed irregular nanostructures were observed. This heterogeneous distribution of nanoripples across the ablated area was induced by the heterogeneous distribution of laser fluence over the focal spot. In most of the experiments, the number of shots on the ablating spot, the intensity and fluence of the laser radiation, and its wavelength were the main variable parameters, which allowed the optimal conditions of LIPSS formation to be determined. It has been shown that short laser pulses are favorable for the creation of nanoripple structures, which has led to the application of laser pulses as short as 25 fs [65]. Even at this short pulse duration, all these observations showed the heterogeneity of LIPSS across the focal spot. This heterogeneity has frequently appeared in the case of longer pulses; however, even in the case of the shortest pulses used, there are no reports showing homogeneous ripple structures across the whole ablated area. Another feature of reported studies was a very small area of observed nanoripples, because almost all previous experiments were carried out using fixed (stationary) ablating samples or fixed position of the ablating spot on the semiconductors surface. However, one of the primary goals of such studies is the formation of extended nanostructures along the whole surface of the material. For this reason, one has to considerably increase the area of the ablating spot, which has typically been in the range of hundreds of micrometers in diameter.

1.4 Extended homogeneous nanoripple formation

All of the aforementioned problems in the formation of extended homogeneous LIPSS have motivated the researchers to search for new approaches in their formation. Below we review LIPSS experiments using extremely short, few-cycle laser pulses (3.5 fs), which interact with the continuously moving silicon sample along the focal plane. We analyze the results of those studies, which have shown a way to produce extended homogeneous nanoripple structures over entire length of the target surface [66].

1.4.1  EXPERIMENTAL ARRANGEMENTS The Ti:sapphire laser, which was used in those studies, provided after first compression the carrier-envelope phase-stabilized pulses of 25 fs duration and energies of up to 0.8 mJ at a repetition rate of 1 kHz. The amplified pulses were focused into a 1 m long differentially pumped hollow core fiber filled with neon (250 µm inner core diameter) [67]. The spectrally broadened pulses at the output of the fiber system were compressed in the second compressor by 10 bounces of double-angle technology chirped mirrors. A pair of fused silica wedges was used to fine tune the pulse compression and compensate for the dispersion of the air and focusing optics. It was confirmed by analyzing the high-order harmonic generation in gas jets, when highest harmonic cutoff was achieved by introducing the glass of the same thickness as the one of focusing lens for nanoripple formation and changing the thickness of fused silica wedges in the second compressor. High-intensity few-cycle pulses (E = 0.2 mJ, τ = 3.5 fs, λ = 760 nm, 1 kHz pulse repetition rate) were typically obtained at the output of this laser [68]. The compressed pulses were characterized with a spatially encoded arrangement for direct electric field reconstruction by spectral shearing interferometry [69]. Part of this radiation was focused using a 400 mm focal length lens to a spot of 200 µm onto the surface of a silicon wafer at normal incidence. The Si wafers with 〈111〉 surface orientation were used for LIPSS formation. The wafers were cleaned in acetone and ethanol before ablation. The intensity and fluence of the laser beam on the ablated surface were maintained at the level of 8 × 1013 W cm−2 and 0.3 J cm−2. The sample was moved perpendicular to the focused beam along the X axis with different velocities (0.1–5 mm s−1) using a motorized translation stage (Fig. 1.16A). The variation of the speed of sample movement allowed to vary the number of overlapping shots on the same spot. The experiments were performed in ambient air. The ablated samples were rinsed in deionized water and cleaned in an ultrasonic bath. The morphology of ablated surfaces at different conditions of laser-surface interaction was analyzed using a SEM.

1.4.2  RESULTS AND DISCUSSION The LIPSS orientation in these studies was always found to be orthogonal to the laser polarization. At a fixed position of the target, nanoripples were formed after 100–500 shots fired on the same spot of the semiconductor, depending on the fluence

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FIGURE 1.16 (A) Experimental scheme. AP, Ablating pulse; FL, focusing lens; SW, silicon wafer; TS, translation stage. (B–D) SEM images of the ablation of silicon wafer by 3.5 fs, 760 nm, 1 kHz pulse repetition rate Ti:sapphire laser carried out at slow velocity of the translating stage (0.2 mm s−1) and at the fluence of 0.3 J cm−2. Reproduced with permission from R.A. Ganeev, D.Y. Lei, C. Hutchison, T. Witting, F. Frank, W.A. Okell, T.R. Roschuk, S.A. Maier, J.W.G. Tisch, J.P. Marangos, Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer, Appl. Phys. A 112 (2) (2013) 457–462, Springer.

and intensity of the laser radiation on the surface of the silicon wafer. It was also observed that the number of shots fired on the semiconductor did not have any effect on the ripple period. Most of the previous LIPSS studies were carried out using 50–200 fs pulses. As already mentioned, the ripple formation in those experiments was observed either at the central part of ablated spot or on the periphery of the ablated region. The surface of ablated semiconductors displayed two areas: one, which contained LIPSS, and another, which contained weakly ablated material (in the case of weak ablation of the edges of the irradiated spot) or overablated irregular structures (in the case of strong ablation of the central part of irradiated spot). In the latter case, the LIPSS being formed during initial shots disintegrated into irregular structures, such as nanodots, nanoholes, and nanowires. The silicon wafer was moved at different velocities orthogonally to the axis of the focusing beam propagation. This approach allowed to determine the conditions for homogeneous ripple formation in one set of ablation measurements. Moreover, during those studies, it was possible to produce extended ripples over the entire length of the sample (5 mm), which considerably increased the area of homogeneously formed periodic structures. Fig. 1.16 shows SEM images of an ablated Si wafer when the target was moved at a velocity of 0.2 mm s−1. When the target was moved continuously along the X axis, the number of overlapping shots on the same spot is given by (π/2)1/2 × (w0f/v) [70], where f is the pulse repetition rate (1 kHz in our case), v is the

1.4 Extended homogeneous nanoripple formation

translation speed (0.2 mm s−1 in the case presented in Fig. 1.16), and w0 is the spot radius. We note that ablation of moving targets by longer (150 fs) pulses [70] showed blunt irregular conical structures on the silicon surface. Meanwhile, in the discussed studies, the number of overlapping laser shots on the same spot was calculated to be ∼300, based on an ablated area of ∼0.1–0.2 mm in diameter. At the beginning of irradiation before the silicon wafer started moving, the ablated spot displayed the irregular structures due to multiple shots (see Fig. 1.16B, the large spot at the right side). Then the target motion started. In the case presented in Fig. 1.16, the relatively small target velocity led to “over-ablation” of the target surface caused by multiple shots on the same spot (intensity and fluence were 8 × 1013 W cm−2 and 0.3 J cm−2, respectively). Nanoripples formed during first few tens shots in the central part of the ablation spot were transformed by subsequent shots on the same area into randomly shaped nanodots (Fig. 1.16C). The ripples remained intact only at the periphery of ablation (Fig. 1.16D). The period of those ripples was 500 ± 40 nm. In that case the structures dominating along the whole line of ablation were randomly formed dots of ∼1  µm in size, which were irregularly distributed in the ablation area (Fig. 1.17). The translation of the target allowed the creation of such a line pattern along the whole sample length. This structure can be formed along the entire area of the silicon wafer by moving the sample along both X and Y axes. By adjusting the translation speed, a considerable improvement of the quality of the LIPSS on the Si wafer was achieved. The increase of speed from 0.2 to 1 mm s−1 led to the formation of an almost homogeneous structure along the entire ablation area (i.e., at both the periphery and central area). This homogeneity was observed

FIGURE 1.17  Chaotic Nanodots Formation at the Central Part of Focal Spot During OverHeating of Target Surface at the Velocity of the Translation Stage of 0.2 mm s−1 and at a Laser Fluence of 0.3 J cm−2. Reproduced with permission from R.A. Ganeev, D.Y. Lei, C. Hutchison, T. Witting, F. Frank, W.A. Okell, T.R. Roschuk, S.A. Maier, J.W.G. Tisch, J.P. Marangos, Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer, Appl. Phys. A 112 (2) (2013) 457–462, Springer.

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FIGURE 1.18  Ablation of Silicon Wafer Using 3.5 fs, 760 nm, 1 kHz Pulses at Optimal Speed of the Target (1 mm s−1) at a Laser Fluence of 0.3 J cm−2. Reproduced with permission from R.A. Ganeev, D.Y. Lei, C. Hutchison, T. Witting, F. Frank, W.A. Okell, T.R. Roschuk, S.A. Maier, J.W.G. Tisch, J.P. Marangos, Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer, Appl. Phys. A 112 (2) (2013) 457–462, Springer.

both along the X and Y directions of the ablated surface. Fig. 1.18 shows an example of such ablation carried out along the whole length of a sample. As in the previous case, the ablation started with the static target (see the large ablated spot on the right side of Fig. 1.18A, but once the silicon wafer started moving at a speed of 1 mm s−1, the formation of extended ripples over the entire area of the focused beam was observed (Fig. 1.18B). The homogeneous shape of the ripples was maintained both in the central area of the focused radiation (Fig. 1.18C) and at the peripheries of ablation. These SEMs show well-defined, almost rectangular, ∼350-nm-thick ripples with sharp edges separated from each other by ∼150 nm. The period of LIPSS was ∼500 nm (Fig. 1.18D). The enlarged picture of these sharp-edged ripples is shown in Fig. 1.19A. The number of shots on the same spot at these conditions of target movement was ∼60. The multicycle 40 fs, 780 nm, 2 mJ pulses from another Ti:sapphire laser operating at 1 kHz pulse repetition rate were also used as the ablation source, maintaining approximately the same fluence on the target surface (0.27 J cm−2). The intensity of these laser pulses on the moving target surface was almost ten times smaller (7 × 1012 W cm−2) compared with few-cycle pulse case, while the number of shots on the same spot was in the range of 50–100. The extended LIPSS using the same technique of target movement were produced (Fig. 1.19B). However, the sharpness and homogeneity of the ripples were inferior to those produced using 3.5 fs pulses.

1.4 Extended homogeneous nanoripple formation

FIGURE 1.19 (A) Surface pattern containing regular nanoripples with sharp edges along the entire area of ablation (5 mm) at the conditions presented in Fig. 1.18. (B) LIPSS in the case of ablation using 40 fs pulses at approximately the same fluence as in the case of 3.5 fs pulses. Reproduced with permission from R.A. Ganeev, D.Y. Lei, C. Hutchison, T. Witting, F. Frank, W.A. Okell, T.R. Roschuk, S.A. Maier, J.W.G. Tisch, J.P. Marangos, Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer, Appl. Phys. A 112 (2) (2013) 457–462, Springer.

The increase in the number of overlapping shots on the same spot led to degradation of ripple structure, with its further disintegration. There is no exact threshold in breaking of ripples in the case of 3.5 fs pulses; however, one can consider the highest speed of 2 mm s−1 as a starting point in extended homogeneous ripples formation and maintenance at used experimental conditions. Also, at the speed of 4 mm s−1 the ripples do not appear due to small amount of overlapping shots on the same spot. The lowest speed at which the ripples start to break is approximately 0.4 mm s−1. We would like to reiterate that all these parameters are relevant with current experimental conditions, when very short pulses are applied for ripple formation. The images of nanoripples at different speeds of target movement are shown in Fig. 1.20. One can clearly see the dynamics of the beginning of nanoripples formation (upper panel, v = 5 mm s−1), extended ripples formation (middle panel, v = 1 mm s−1), and the beginning of their disintegration (bottom panel, v = 0.2 mm s−1). Using an optimized speed of target, the area of extended homogeneous LIPSS in the case of 3.5 fs ablating pulses can be considerably enlarged by further movement of the ablating surface along both the X and Y axes, thus covering a large area of Si wafer with such ripples. No degradation of the LIPSS has been observed in the months after the ablation. The use of cylindrical focusing optics [71] could further increase the patterning speed. The formation of these structures is attributed to the specific features of ablation using few-cycle pulses. As already mentioned, the extended LIPSS were previously formed by moving the target with regard to the focal spot of multicycle pulse. However, none of those studies have demonstrated homogeneous LIPSS along the entire ablation area. The extremely short period of ablation in the case of few-cycle

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FIGURE 1.20 Dynamics of the beginning of nanoripples formation (upper panel, v = 5 mm s−1), extended ripples formation (middle panel, v = 1 mm s−1), and the beginning of their disintegration (bottom panel, v = 0.2 mm s−1). Reproduced with permission from R.A. Ganeev, D.Y. Lei, C. Hutchison, T. Witting, F. Frank, W.A. Okell, T.R. Roschuk, S.A. Maier, J.W.G. Tisch, J.P. Marangos, Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer, Appl. Phys. A 112 (2) (2013) 457–462, Springer.

1.5 Concluding comments

pulses is expected to allow the maintenance of LIPSS formation conditions, with only the central part (∼100  µm; see the size of ablated lines on Figs. 1.16B and 1.18A) of the focal spot (200 µm) ablating the surface to create the conditions of a metal-like plasma. In that case, the optimum number of overlapping shots on the same area of the Si wafer was determined by changing the speed of the translation stage, thus allowing the maintenance of a clean pattern of LIPSS, without debris and irregularities in both the central part and peripheries of the ablated area. The observed pattern of LIPSS formation can be explained by considering the transient metallic nature of the semiconductor surface during irradiation by intense femtosecond pulses [10]. Such irradiation eventually supports surface plasmon polariton excitation [72,73]. LIPSS formation can be considered to be the result of the interference of the laser radiation with the surface plasmon polariton generated at the silicon surface [9]. This consideration is consistent with the used experimental conditions where the surface of the semiconductor is irradiated during an extremely short period, which allows the formation of transient metal-like conditions. Formation of surface plasmon polariton waves is a well-known phenomenon in the context of metals, which have free electrons for excitation of this longitudinal charge density oscillation [9]. However, in the case of semiconductors, the origin of free electrons is the transient excitation of free electrons due to tunnel or multiphoton ionization. Though the threshold of surface breakdown increases with decreasing ablating pulse duration, the typical intensity of those experiments (8 × 1013 W cm−2) was well above the ablation threshold of silicon wafers. At these conditions, tunnel ionization is the dominant process, which causes the formation of a considerable amount of free electrons above the target surface. As the electron density generated on the surface after femtosecond pulse irradiation depends on various laser parameters, including the pulse duration, it is not surprising that significant changes in LIPSS formation are observed using some of the shortest high-power laser pulses currently available.

1.5  CONCLUDING COMMENTS We discussed different methods of periodic nanoripples formation. Particularly, we analyzed nanoripple formation on different bandgap semiconductor surfaces using femtosecond pulses, nanosecond LIPSS on WBG semiconductors, nanostructuring of semiconductor surfaces under the action of femtosecond pulses, fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses, and extended homogeneous nanoripple formation during interaction of highintensity few-cycle pulses with a moving silicon wafer. LIPSS formation from ultrashort laser pulse irradiation of semiconductors of different bandgaps has been studied using a Ti:sapphire laser with 8 mJ energy, 45 fs pulse duration, and 800 nm wavelength (1.5 eV) at a fluence in the range of ∼100 mJ cm−2–1 J cm−2. The effects of the number of laser shots, angle of incidence, laser polarization, fluence, incident laser wavelength, bandgap, and ambient medium

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on the ripple period, have been studied. Depending upon the experimental parameters, nanoripple sizes varied in the range of λ/9–λ. The studies clearly show that narrower nanoripples are formed from WBG semiconductors. In addition, the width of the nanoripples decreases with the laser wavelength and fluence. The observed results were explained considering the transient metallic nature of the semiconductor surface on irradiation with intense femtosecond pulse, which excites surface plasmon leading to the LIPSS formation. The nanoripples get frozen due to fast cooling once the laser pulse is over. The critical role of the surface plasma electron density in deciding ripple period is identified, which helps in generation of narrow subwavelength nanoripples. LIPSS perpendicular to the laser polarization direction and with period of the order of the irradiation wavelength were obtained on the surface of WBG semiconductor wafers of InP, GaAs, GaP, and SiC upon repetitive irradiation at 266 nm with pulses of 6 ns. The ripples were fabricated at fluences below the ablation threshold for a single pulse irradiation and are produced mainly by optical interference effects due to the superposition of the incident radiation with a surface electromagnetic wave created on the wafer surface. Calculation of the temperature increase induced by laser irradiation indicated that a minimum fluence is necessary to assure that the surface temperature is high enough for allowing melting and material rearrangement. The increased surface heterogeneity caused by microdefects created by laser irradiation facilitates the feedback mechanism generating the ripple formation. The higher fluence and number of pulses needed for LIPSS formation as the bandgap increases is related to the higher melting temperatures corresponding to larger bandgaps. It was observed that the amplitude of the ripples increases with optical and thermal penetration depths. The nanostructures inspected by AFM are produced upon multiple pulse irradiation at fluences near the ablation threshold. It was observed that the accumulative effect of both fluence and number of pulses needed for LIPSS formation increased with the material bandgap energy. Those results, together with estimations of the growth of surface temperature, were discussed with reference to the semiconductor electrical, optical, and thermal properties. Two-dimensional periodic nanostructures on ZnO crystal surface were fabricated by two-beam interference of 790 nm femtosecond laser. The long period was, as usually reported, determined by the interference pattern of two laser beams. There were other short periodic nanostructures with periods of 220–270 nm embedding in the long periodic structures. The periods, orientation, and the evolution of the short periodic nanostructures were studied, and found them analogous to the self-organized nanostructures induced by single femtosecond laser beam. Finally, the study of extended nanoripple structures formed during the interaction of high-intensity 3.5 fs pulses with a moving silicon wafer was discussed. The optimization of laser intensity and sample moving velocity allowed the formation of long strips (∼5 mm) of homogeneous nanoripples along the whole area of laser ablation. The comparison of nanoripples produced on the silicon surfaces by few- and multicycle pulses was discussed. It was found that few-cycle pulses produce sharp and homogenous structures compared with multicycle pulses. Those images appeared at the speeds of 0.7–2 mm s−1, with the best observed results at 1 mm s−1 at the used

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CHAPTER

Formation of nanoparticles, nanoholes, nanoripples, and nanowires using different conditions of laser–matter interaction

2

2.1  FORMATION OF DIFFERENT PERIODIC NANOSTRUCTURES ON SEMICONDUCTORS In previous chapter, we have shown that formation of parallel fringes with characteristic spacing on the order of the wavelength of the acting light is an interesting effect of the interaction of ultrashort laser pulses with semiconductors and other materials. The appearance of these nanostructures (NSs) was ascribed, particularly, to the formation of diffraction gratings with submicron spacing between the fringes and the interactions between laser light and plasmon-polariton waves, which occur due to the initial heterogeneity of the surface [1]. We also mentioned that the reports about NSs with periods are much smaller than the wavelength of the light (∼200 nm [2–4]) have aroused additional interest. The elucidation of the mechanisms of formation of short-period nanostructures (SNSs) requires additional study because none of the mechanisms proposed thus far can explain, in full measure, all characteristic features of this process. The purpose of the discussion of reported studies in this field is to analyze specific features of the formation of NSs and SNSs on the surfaces of different semiconductors under variable experimental conditions. In this section, we analyze the effect of the aforementioned and some other experimental parameters on the pattern of the nanostructure, including the cases when the regimes of formation of the long- and short-period structures could be distinguished [5]. We also demonstrate the formation of nanoholes and nanodots on the surface of different semiconductors [6].

2.1.1  PECULIARITIES OF NANOSTRUCTURES FORMATION UNDER THE ACTION OF SHORT PULSES To obtain an NS on the surface of a semiconductor, the part of radiation of a Ti:sapphire laser (wavelength 790 nm, pulse duration 120 fs, pulse energy 0.5 mJ) was used. In some cases, shorter optical pulses (800 nm, 35 fs, 1 mJ) were also used. This radiation was focused by a spherical (or, in some cases, cylindrical) lens with a Nanostructured Nonlinear Optical Materials. http://dx.doi.org/10.1016/B978-0-12-814303-2.00002-7 Copyright © 2018 Elsevier Inc. All rights reserved.

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CHAPTER 2  Formation of nanoparticles

focal length of 150 mm, under conditions of normal incidence onto polished semiconducting plates of GaAs, InAs, Si, Ge, SiC, ZnSe, GaN, and ZnO. The irradiation was mainly performed in the air, but sometimes it was also performed in a vacuum or on samples immersed into ethanol. The light power density on the surface of semiconductors was varied in the range of 0.1–0.3 J cm−2. For a pulse repetition rate of 10 Hz and a pulse energy of 0.05 mJ, the surface was irradiated for 0.2–2 min. In some cases, the formation of the NS was analyzed for irradiation of the sample by 2–10 pulses. The size of the irradiated area was varied in the range of 100–200 µm. The appearance of the NS for an angle of incidence of 76° and for the laser light interacting with the samples surrounded by different media (air, liquid, or vacuum) were also analyzed. The structure of the arising NS was analyzed using a JEOL JSN5600 scanning electron microscope. In those studies, different types of nanoripples were observed. For a laser light intensity of about 1012 W cm−2 on the surface of the samples, the distances between the fringes was, in most cases, close to (or slightly smaller than) the laser light wavelength (790 nm). At this intensity, the NS appeared mainly near the edges of the irradiated regions [5]. The central part of the irradiated area was, to a considerable extent, destroyed for intensities above the damage threshold after multiple shots into the same spot and appeared to be an irregular structure (Fig. 2.1A), whereas the edges of the laser craters revealed nanofringes (Fig. 2.1B). A comparison of

FIGURE 2.1 Structures (A) of the central part of the irradiated surface of GaAs and (B) of peripheral region of the same spot. Nanostructures (NSs) obtained on the surface of silicon for the case of (C) linear and (D) circular polarizations of the laser beam. Reproduced from R.A. Ganeev, M. Baba, T. Ozaki, H. Kuroda, Long- and short-period nanostructure formation on semiconductor surfaces at different ambient conditions, J. Opt. Soc. Am. B 27 (5) (2010) 1077–1082 with permission from the Optical Society of America.

2.1 Formation of different periodic nanostructures

FIGURE 2.2  Variations in the Direction of Ripple Fringes Obtained for Different Directions of Polarization of the Laser Beam Acting on a Surface of Silicon. Polarization Directions are Indicated by Arrows. Reproduced from R.A. Ganeev, Formation of different periodic NSs on semiconductors, Opt. Spectrosc. 106 (1) (2009) 142–146 with permission of Springer.

the structures for the cases of linear and circular polarizations of the laser light is shown in Fig. 2.1C and D. A typical structure of the ripples obtained for linearly polarized femtosecond pulses with the fringes aligned across the direction of the polarization (Fig. 2.1C) was replaced by chaotic or dotted structures for the case of circularly polarized light (Fig. 2.1D). The size of these nanodots varied within a range of 100–130 nm. In this case, chaotically oriented nanoripples were also observed. To study the dependence of the alignment direction of the surface structures on the direction of the light polarization, the plane of polarization of the electromagnetic wave was rotated using a half-wave plate. In this case, the ripples rotated in such a way that they always remained normal to the polarization direction (Fig. 2.2). This feature was also observed in most of the previous studies ([7–9], see also Chapter 1), while in some publications [10,11], it was reported that the direction of the ripples coincided with the electric field vector of the electromagnetic wave.

2.1.2  LONG- AND SHORT-PERIOD NANOSTRUCTURE FORMATION ON SEMICONDUCTOR SURFACES In some cases, the simultaneous appearance of two types of NSs with different spacing between the fringes, that is, SNSs and normal long-period NSs (Fig. 2.3), was observed. How the ripple structure separates under identical conditions of irradiation at almost same place of irradiation is unclear. These mixed structures were obtained with the use of spherical focusing. The appearance of an SNS with characteristic spacing between the ripples ∼λ/5–λ/3 contrasted both with the broader-spaced structures observed in the discussed study and with most of the earlier observed NS generated by femtosecond pulses (starting with the work of Birnbaum [12] who first observed these structures with spacing between the fringes on the order of the laser light wavelength). As was noted above, the appearance of these SNSs obtained under different experimental conditions was already observed by different experimental groups. There exist a number of proposals concerning the application of these SNSs for different purposes, such as the manufacturing of diffraction gratings with small inter-fringe spacing, enlargement of the surface area of the objects to increase rate of

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FIGURE 2.3 (A–C) Three SEM images of the ripples on the SiC surface showing different periods (730 and 180 nm). (D) High resolution SEM of the subwavelength LIPSS produced on the SiC surface. Reproduced from R.A. Ganeev, M. Baba, T. Ozaki, H. Kuroda, Long- and short-period nanostructure formation on semiconductor surfaces at different ambient conditions, J. Opt. Soc. Am. B 27 (5) (2010) 1077–1082 with permission from the Optical Society of America.

catalytic reactions in the vicinity of such structures, recording information with the aid of the SNS, and applications in spectroscopy and nonlinear optics. In the case of laser light interacting with surfaces at large angles of incidence (in particular, in the vicinity of Brewster's angles lying for these semiconductors in the range of 70°–80°), the formation of a number of exotic structures in the shape of extended successions of nanoholes, nanocracks, etc. was observed. The period of these structures depended on the angle of incidence φ and polarization of the laser light. For the p-polarized wave, this period was determined by the relation dp = λ/ (1 + sinφ), while for the s-polarized light, this dependence is given by the relation ds = λ/cosφ. Laser ablation of a number of materials (Si, SiC, and GaAs) was performed for the angle of incidence φ = 76° and found that the distances between the fringes (400–500 nm) were close to the expected result for the case of p-polarized wave. An analysis of the nanostructuring of different semiconductors has shown that the arrangement, as well as the appearance of laser-induced periodic surface structure (LIPSS), also depend on the ratio of the bandgap width of the samples and the photon energy. It was reported previously that structures with characteristic periods equal to fractions of the light wavelength are formed when the photon energy (Eph) appears to be smaller than the bandgap width (Ebg) of the semiconductor [13]. This assertion is SiC = 3.9 eV) and confirmed in the discussed experiments with SiC ( Eph = 1.56 eV, Ebg

2.1 Formation of different periodic nanostructures

si = 1.12 eV), where SNSs were observed, while, upon the ablation of GaAs ZnO ( Ebg GaAs si InAs Ge ( Ebg = 1.43eV), Si ( Ebg = 1.12 eV), InAs ( Ebg = 0.35 eV), and Ge ( Ebg = 0.66 eV), long-period structures and a chaotic arrangement of the fringes under the same experimental conditions were observed. At the same time, surface ablation of ZnSe ( EbgZnSe = 2.35eV) did not give rise to the appearance of SNSs, despite the fact that, according to the above rule, these structures should arise in this semiconductor. The quality of NSs formed on the surfaces of GaN, InAs, and Ge was worse than those on SiC, GaAs, ZnO, and Si. Ablation of ZnSe demonstrated appearance of chaotic structure both for linear and circular polarization of the laser light, with some traces of ripples in the former case. The ablation of SiC resulted in appearance of SNS with characteristic spacing between the fringes of about 180–230 nm. For the case of ZnO, the appearance of ripples with a period of 300–400 nm was observed. In the ablation of ZnO, in some cases, the simultaneous appearance of the two types of NSs with different periods aligned perpendicular to one another was observed. In some cases, a cylindrical lens for focusing femtosecond pulses to analyze the possibility of generating extended LIPSS was used. Fig. 2.4A shows extended rows of nanoholes obtained with cylindrical focusing of the beam onto the surface of silicon. Another exotic structure was observed during ablation of GaAs (Fig. 2.4B); in that case, the distance between the fringes was about 2 µm.

2.1.3  ROLE OF SURROUNDING MEDIUM DURING NANORIPPLES FORMATION The NSs shown above were obtained upon ablation by 120 fs pulses in the air. The formation of NSs on semiconductors placed in a vacuum or a liquid was also analyzed. The laser ablation of semiconductors in liquids considerably changed the structure formed on their surface as compared with ablation in the air. This behavior was also reported in Katayama et al. [14]. The modification of the semiconductor surface was analyzed for the case of its treatment by 120 fs pulses in ethanol and air under identical experimental conditions. The NSs shown in Fig. 2.5A and C were obtained on an irradiated surface of silicon for these two cases. As the damage threshold for the silicon surface under similar conditions (800 nm and 130 fs) is 0.26 J cm−2 [15], the power density in the discussed experiments was several times higher. When the semiconductor was immersed in liquid, the structure of the irradiated surface showed nanodots 400–600 nm in size. This structure appeared on the edges of the damaged area, while, in the central part of the irradiated surface, the dominated pattern of ablation was chaotic (Fig. 2.5B). At the same time, in air, a periodic structure similar to those described above was observed. A different picture was observed upon the ablation of silicon by shorter (∼35 fs) pulses. In the air, the NS pattern was similar to the one obtained with 120 fs pulses; at the same time, the nanofringes in ethanol were more pronounced. The ablation of the sample in a vacuum chamber in the absence of ambient gas or liquid gave rise to the appearance of either dotted islands with sizes of approximately a few microns or a completely chaotic structure for the case of 35- or 120-fs pulses acting on the surface.

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FIGURE 2.4 (A) Nanoholes generated on a surface of silicon upon focusing femtosecond pulses using a cylindrical lens. (B) The ripples on the surface of GaAs were obtained through the cylindrical focusing of the beam. Reproduced from R.A. Ganeev, M. Baba, T. Ozaki, H. Kuroda, Long- and short-period nanostructure formation on semiconductor surfaces at different ambient conditions, J. Opt. Soc. Am. B 27 (5) (2010) 1077–1082 with permission from the Optical Society of America.

The comparative properties of nanoripples were analyzed by irradiating the silicon strips placed in methanol and a vacuum chamber. In the former case, four different residual air pressures were used during LIPSS formation (10−1, 1, 100, and 760 torr). Fig. 2.6 shows different patterns of nanoripples formed at equal conditions of laser irradiation (35 fs, 0 Hz pulse repetition rate, 200 shots). In the case of dense surrounding medium (methanol), the high-quality ripples with flat tops were obtained (Fig. 2.6A and B). In the case of atmospheric air conditions (760 torr) in a vacuum chamber, the ripples were also well defined, though less pronounced (Fig. 2.6C). With decrease of surrounding gas pressure (100 torr), the ripples became less configured (Fig. 2.6D), while at moderate vacuum conditions (1 and 10−1 torr), the LIPSS formation almost stopped (Fig. 2.6E and F). One can assume that surrounding

2.1 Formation of different periodic nanostructures

FIGURE 2.5 (A) Formation of nanodots 400–600 nm in size on the edge of the spot irradiated by 120 fs pulses; (B) chaotic pattern in the central region of irradiated spot for silicon inserted into a cell with ethanol; (C) the pattern of NS on the surface of silicon for the case of laser ablation with 120 fs pulses in the air. Reproduced from R.A. Ganeev, Formation of different periodic NSs on semiconductors, Opt. Spectrosc. 106 (1) (2009) 142–146 with permission of Springer.

medium prevents the material's chaotic movement during laser ablation. It prevents the chaotic movement of ablated material during first initial shots and serves for fixation of surface waves. It is clearly seen in Fig. 2.6A and B that the flat, top waves became “frozen” at the conditions when a dense medium (methanol) surrounds the area of laser ablation. This distinction between the structures of the irradiated surfaces indicates the decisive role of the dense ambient medium in the formation of the NS under

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FIGURE 2.6  SEM Images of the NSs Formed on the Silicon Wafer at Different Conditions of Surrounding Medium. (A) Methanol; (B) the same pattern at higher resolution; (C) air, 760 torr; (D) air, 100 torr; (E) air, 1 torr; and (F) vacuum, 10−1 torr. Reproduced from R.A. Ganeev, M. Baba, T. Ozaki, H. Kuroda, Long- and short-period nanostructure formation on semiconductor surfaces at different ambient conditions, J. Opt. Soc. Am. B 27 (5) (2010) 1077–1082 with permission from the Optical Society of America.

conditions of laser ablation by ultrashort pulses of different duration. The appearance of ripples during multiple irradiation by laser pulses implies the presence of some “memory,” which causes the accumulation of the initial destruction of the surface. How this memory works is not clear and was proposed by some researchers only as a hypothesis. For the ablation in liquids, this process operates in different ways for the pulses of different duration. For relatively extended femtosecond pulses, the memory mechanism does not work and the surface pattern becomes chaotic. For shortest pulses, the opposite is true, and the presence of a viscous medium improves

2.1 Formation of different periodic nanostructures

the structuring of the surface as compared with generation of the NS in the air. The processes leading to this difference in structuring of the semiconductor surfaces upon ablation by pulses of different duration have thus far remained unclear and need further investigation.

2.1.4 DISCUSSION When femtosecond pulses are used, thermal and mechanical forces have no effect because those forces associated with heat flow or material viscosity are unimportant over distances of the order of wavelength or the absorption depth, which are typically of the order of micrometers. Upon femtosecond laser irradiation, the high electronic excitation leads to a nonthermal, ultrafast phase change, which occurs within less than 1 ps. The periodic structures then exhibit typical signature of pressure-induced transformation. The cooling time is an important parameter, which influences the ripple-like pattern formation on the surfaces of semiconductors. In semiconductors, this parameter is of order of 10−10 to 10−9 s [16]. The typical thermal conductivity of semiconductors is of the order of 0.5(T/300) W cm−1 K−1 [17]. In the meantime, the energy relaxation time in semiconductors is 10−12 to 10−11 s [18]. This difference just shows that ablation time, which involves the displacement of particles, is considerably longer than laser interaction and energy relaxation after absorption. As for thermal conductivity for the ablation process, for example in methanol as a surrounding medium, its role seems insignificant taking into account the pulse duration of laser radiation. The sample quality (defect density and surface roughness) and specific processing conditions (pulse fluence and number of pulses) can change the pattern of LIPSS. As the intensity of surface plasmons shows its maximum at the surface of material, the quality of ripples depends on the roughness of the surface. In the case of ablation in liquid, the roughness of the surface becomes less important due to the pressure of ambient medium. So, at equal conditions, the ablation in a dense medium allows one to achieve a better quality ripple. A CCD camera and optical microscope were used for checking the quality and roughness of semiconductors before and after femtosecond laser ablation. The ablated area was illuminated by white light and checked for the reflectivity of the area of ablation. The initial CCD images of nonablated surfaces were smooth, and the inclined incident light had a high reflection ratio, especially on the surfaces of high-refractive-index semiconductors. The ablated areas have shown another pattern. The central parts of ablated spots, which were ablated at higher peak intensity of femtosecond laser, had a decreased reflection due to increased scattering from the crater walls. The edges of ablated spots showed an increased reflectivity due to the creation of nanoripples. The color of these parts was yellow and red due to preferential reflection of these parts of the white-light spectrum from the ripples. It is known that surface roughness causes an increase in the modulus of the surface plasmon wave vector [19], and this will correspond to an increase in the real part of the refractive index. An increased real part of the refractive index for propagating surface plasmons will cause a reduced LIPSS period.

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In some previous reports, the period between nanoripples in subwavelength LIPSS was associated with the generation of a second harmonic on the surface [20,21]. In those reports, the period was defined by the relation of d = λ/2n. Here n is the refractive index of the semiconductor. In the discussed studies, this parameter was equal to 120 and 200 nm for GaAs and ZnO, respectively. At the same time, the experimentally observed measurements of this parameter were 700 and 380 nm. Lack of coincidence between the calculated and observed periods of ripples points out the difficulties of this hypothesis for the description of discussed observations of both the ordinary and subwavelength LIPSS. Previously, fluid confinement has been reported to produce substantially subwavelength structures for femtosecond irradiation of semiconductors. In particular, in air a 700 nm ripple period was observed in [22] with λ = 800 nm pulses, while a periodicity of 100 nm was formed in silicon in water. As the laser ablation is conducted in air, the ablated plume cannot expand as rapidly as those in a vacuum chamber, which will induce an instantaneous high-energy and high-pressure region in the laser focus [23]. Therefore, the nanoripples grow very rapidly. The same can be said about the ablation in more dense media (i.e., liquids). The extremely short pulse duration of femtosecond lasers allows one to deposit a high-density energy with minimal thermal effect into a restricted volume of the target material.

2.2  NANOPARTICLE FORMATION DURING LASER ABLATION OF METALS AT DIFFERENT PRESSURES OF SURROUNDING NOBLE GASES 2.2.1  INTRODUCTION TO NANOPARTICLE FORMATION USING LASER–MATTER INTERACTION The structural, optical, and nonlinear optical parameters of nanoparticles are known to differ from those of the bulk materials due to the quantum confinement effect. Silver, copper, and gold are among the most useful metals suited for nanoparticle preparation for optoelectronics and nonlinear optics. Nanoparticle formation in vacuum and liquids during laser ablation of solid-state targets using short laser pulses is a well-developed technique [24–26]. It is known that metal ablation in air is significantly less efficient than that in vacuum due to redeposition of the ablated material. The ablation rates in vacuum can be calculated using a thermal model, which also allows estimating the ablation rates for other metals from basic optical and thermal properties. A comparison of the morphology of the deposition sites after nanosecond and picosecond ablation shows unequivocally the advantages of short-pulse ablation for the preparation of nanoparticles. Pulsed laser deposition in vacuum using short pulses has gained considerable interest because of a number of advantages over other processes, such as the possibility of producing materials with a complex stoichiometry and a narrowed distribution of nanoparticle sizes with reduced porosity. When laser deposition is carried out in

2.2 Nanoparticle formation during laser ablation of metals

an ambient gas, the latter quenches the ablated plume. Some previously reported studies [27] suggest that nanoparticles are generated as a result of some relaxation processes of the extreme material state reached by the irradiated target surface. The morphology of ablated nanoparticles after their laser-induced deposition on various substrates was analyzed in Ganeev et al. [28]. Note that nanoparticle formation and deposition on nearby substrates can also be accomplished by laser ablation of the nanoparticles already presented on the target surface. The influence of surrounding gas on the conditions of cluster formation during laser ablation of the metals by short laser pulses has previously been analyzed only at two conditions, when the target was placed in vacuum or ambient air. It is of interest to analyze the influence of the concentration of surrounding gas on nanoparticle formation. One can study this process using noble gases of different atomic numbers (Z) at the pressures varying between 10−2 torr (i.e., vacuum conditions) and 760 torr (i.e., atmospheric pressure). It would be interesting to analyze whether there is a threshold in gas pressure scale above which the conditions of nanoparticle formation get spoiled. In this section, we analyze the dynamics of nanoparticle formation during laser ablation of silver at different concentrations of surrounding noble gases (helium and xenon) [29]. We compare the formation of silver nanoparticles at these conditions, with those in the case of vacuum and atmospheric air. It was found that nanoparticle formation stops at the pressures of used gases above ≈12–33 torr. The experiments were carried out using the Nd:YAG laser. To create the ablation, a single pulse from the pulse train of oscillator was amplified (λ = 1064 nm, t = 38 ps, E = 10 mJ, 2 Hz pulse repetition rate) and focused on a target placed in a vacuum chamber. The laser pulses were focused on the bulk targets (Ag or Cu plates) at two regimes of focusing. In the first case (referred to as tight focusing), the intensity of laser radiation was in the range of (0.5–2) × 1012 W cm−2, and in the second case (referred to as weak focusing), the intensity was considerably lower (∼4 × 1010 W cm−2). The laser fluence during ablation at tight focusing conditions was in the range of a few tens of joules per square centimeter. The chamber was maintained at the vacuum pressure of 8 × 10−4 torr. The pressure was varied by adding different noble gases up to p = 300 torr. The debris from the plasma plume was deposited on a nearby placed BK7 glass plates. The distance between the target and BK7 plates was 40 mm. The presence of nanoparticles was inferred by analyzing the spatial characteristics of the deposited material and the spectral absorption of the deposited material. The structure of the deposited material was analyzed by an atomic force microscope. The absorption spectra of the deposited films were analyzed by a fiber spectrometer.

2.2.2  ANALYSIS OF FORMED NANOPARTICLES The formation of silver and copper nanoparticles of Ag in vacuum and noble gases was completed using the longer (38 ps) laser pulses. In those experiments, the morphology and spectral characteristics of silver and copper films deposited on the

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surrounding glass plates were compared at vacuum and atmospheric conditions during laser ablation of bulk targets. The targets were ablated during 30 min using the picosecond pulses at 2 Hz pulse repetition rate. Most of these studies were carried out at tight focusing conditions. In the case of vacuum deposition, relatively thick (∼0.6–0.9 µm) Ag and Cu films were produced. The deposition in air conditions at analogous parameters of experiments showed considerably thinner layers of deposited material (∼0.03–0.06 µm). The nanoparticle formation was mostly described as a process of short excitation of electronic gas and transfer of this energy to the atomic cell with further aggregation processes, which continued during evaporation of the material [30]. Atomic force microscopy (AFM) measurements of nanosized structures were carried out in noncontact mode under an ambient environment. The analysis of deposited films has shown a considerable difference in their morphology. Fig. 2.7 shows the AFM images of the Ag and Cu films deposited in vacuum at tight focusing conditions. Here were also shown the histograms of nanoparticle size distribution corresponding to the AFM images of these films. The mean sizes of nanoparticles were 20 nm (in the case of Ag ablation) and 60 nm (in the case of Cu ablation). The sharp images of nanoparticles indicate that concentration of the deposited atomic layer containing single particles is insignificant. To define the relative concentrations of nano- and monoparticles, one has to use the time-of-flight spectroscopy, which was not available in the discussed studies. However, the analysis of AFM images has clearly indicated a difference in the “sharpness” of nanoparticle images in the case of vacuum (p  5 × 1011 W cm−2). No nanoparticles were observed on the substrates at moderate intensities of laser radiation on the bulk target surface (5 × 109–7 × 1010 W cm−2). This was confirmed by the analysis of appearance of the SPR on the absorption spectra of deposited films. The light (He, Z = 4) and heavy (Xe, Z = 131) noble gases were used to define the influence of weight characteristics of the surrounding particles on the nanoparticle formation during laser ablation of silver target at different gas pressures. Fig. 2.9 shows the absorption spectra of deposited Ag films in the case of different pressures of xenon. One can see that SPR appears up to 12 torr. Above this pressure, only monotonic growth of absorption toward blue side was observed. The same measurements in the case of helium (Fig. 2.10) showed analogous tendency,

FIGURE 2.9  Absorption Spectra of the Silver Films Deposited at Vacuum Conditions, 12, 26, 101, and 285 torr of Xenon. Reproduced from R.A. Ganeev, G.S. Boltaev, R.I. Tugushev, T. Usmanov, Nanoparticle formation during laser ablation of metals at different pressures of surrounding noble gases, Appl. Phys. A 100 (1) (2010) 119–123 with permission of Springer.

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FIGURE 2.10  Absorption Spectra of the Silver Films Deposited at Vacuum Conditions, 11, 33, 87, and 136 torr of Helium. Reproduced from R.A. Ganeev, G.S. Boltaev, R.I. Tugushev, T. Usmanov, Nanoparticle formation during laser ablation of metals at different pressures of surrounding noble gases, Appl. Phys. A 100 (1) (2010) 119–123 with permission of Springer.

when the SPR of silver nanoparticles disappeared from the absorption spectra of the deposited films obtained at gas pressure above 33 torr. Note a shift of SPR toward the longer wavelengths with increase of gas pressure. The difference in threshold pressures for these two gases can be explained as follows. With the growth of surrounding particles, the formation of nanoparticles becomes suspended due to some impeding processes. One can assume that, above threshold pressure, these particles start to play a negative role in the aggregation of silver particles during the heating of ablated targets by short pulses. Depending on atomic weight, they can diminish the probability of accumulation of Ag particles in the clusters at different pressures of surrounding gas. Heavy xenon particles more strongly affect the cluster formation compared with lighter helium particles. This follows with smaller threshold pressure in the former case, above which the nanoparticle formation almost stops.

2.3 Deposition of nanoparticles

The ablation-induced nanoparticle formation in laser plumes has carefully been documented in multiple experiments (for example [32–39]). In the case of bulk target ablation, the attention is taken for the creation of the conditions when laser energy is accumulated for a short period at a small area to maintain the conditions of nonequilibrium heating. In that case, extremely heterogeneous conditions help creating the nanoparticles in the small areas of heated samples. One can maintain the conditions when the aggregated atoms do not disintegrate during evaporation from the surface. The analysis of aggregation state of evaporated particles was studied by different techniques. Among them the time-resolved emission spectroscopy, CCD camera imaging of the plasma plume, Rayleigh scattering, and laser-induced fluorescence have shown the ability of defining the nanoparticle formation. The approach developed in discussed paper allows the definition of nanoparticles in the deposited films through the analysis of absorption spectra. It is well accepted that when a solid target is ablated by laser radiation the material evaporates in the form of atoms, ions, and clusters. These atoms and clusters tend to aggregate during the laser pulse or soon afterward leading to the formation of larger clusters. Heterogeneous decomposition, liquid phase ejection and fragmentation, homogeneous nucleation, and decomposition are among those processes that can lead to nanoparticle formation [40]. Short laser pulses heat a solid to a higher temperature and pressure than do longer pulses of comparable fluence as the energy is delivered before significant thermal conduction can take place. The discussed studies have shown a role of the weight characteristics of surrounding particles on the suppression of nanoparticle formation. Even small concentration of surrounding gas (5 × 1017 cm−3) led to considerable decrease of the probability of cluster formation during laser–matter interaction.

2.3  DEPOSITION OF NANOPARTICLES DURING LASER ABLATION OF NANOPARTICLE-CONTAINING TARGETS 2.3.1 INTRODUCTION We have shown in previous section that nanoparticle formation during laser ablation of solid-state targets using short laser pulses is a well-developed technique. Together with formation of the ripples of wavelength range sizes this technique gives the opportunity of creating the exotic structures with variable physical and chemical properties. The increased surface area of the nanostructured materials allows for the enhancement of the velocity of catalytic reaction, provides the opportunity of application of such structures for information writing, manufacturing of lubricants, semiconductor technologies, etc. [41–43]. There are few mechanisms describing NSs formation during laser–matter interaction. While the nanoripple formation is considered using different physical models (see Chapter 1), the nanoparticle formation was mostly described as a process of short excitation of electronic gas and transfer of this energy to the atomic cell with further aggregation processes, which continue during evaporation of the material [30,44].

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In the case of bulk target ablation, the attention is taken for the creation of the conditions when laser energy is accumulated for a short period at a small area to maintain the conditions of nonequilibrium heating. In that case, extremely heterogeneous conditions help in creating the nanoparticles in the small areas of heated samples. One can maintain the conditions when the aggregated atoms do not disintegrate during evaporation from the surface. The analysis of aggregation state of evaporated particles was studied by different techniques. In the meantime, a few studies have been reported when nanoparticles already exist at the surface of ablated targets. Currently, the nanoparticles with different sizes and shapes are commercially available from various manufacturers and can be attached to the surfaces. In that case one has to carefully define the optimum laser intensity and fluence, pulse duration, and focusing conditions for heating of the nanoparticles until they evaporate from the targets. In the meantime, the history of ablated nanoparticles in that case can be difficult to analyze using the above techniques due to some restrictions and difficulties in identification of the nanoparticles at moderate heating of the targets. In that case, the comparison of the size characteristics of initial nanoparticles and deposited debris becomes a versatile approach for a definition of the changes of nanoparticle morphology during laser ablation. The maintenance of the original properties of nanoparticles allows one to analyze the optical and nonlinear optical properties of nanoparticlecontaining laser plasma at well-defined conditions. Below we analyze the experimental studies of the structural modifications during laser ablation of nanoparticle-containing targets [28]. We discuss the morphology of ablated nanoparticles by analyzing the debris deposited on nearby substrates. This technique allowed the optimization of laser ablation parameters for maintaining the nanoparticles in the laser plumes. These plumes can be further used as the nonlinear media for studies of the high-order harmonic generation in nanoparticle-containing media.

2.3.2  RESULTS AND DISCUSSION The experiments were carried out using the 10 Hz, 10 TW Ti:sapphire laser. To create the ablation, part of the uncompressed radiation from the Ti:sapphire laser (λ = 800 nm, t = 210 ps) was focused on a target placed in the vacuum chamber. The spot size of this radiation on the target surface was maintained in the range of 0.5–0.8 mm. The intensity of this radiation (I) on the target surface was varied in the range of 2 × 109–5 × 1010 W cm−2, while the laser fluence during ablation was adjusted in the range of 0.4–1 J cm−2. The chamber was maintained at the vacuum pressure of 8 × 10−4 Pa. The debris from the plasma plume was deposited on a nearby placed Si wafer and an Al foil. The distance between the target and the substrates was 40–70 mm. The structure of deposited material was analyzed by the scanning electron microscope. The targets containing nanoparticles were ablated. The commercially available nanoparticles (Cr2O3, In2O3, Ag, Sn, Au, and Cu) were glued on the glass or silver substrates by mixing with the drop of superglue. The bulk materials of the same origin as nanoparticle powder were also used to compare the ablation from monoparticle-containing targets.

2.3 Deposition of nanoparticles

Initially, the size distribution of the nanoparticles used in those studies was defined using SEM. The sizes of nanoparticles were varied in the range of 30–300 nm. Fig. 2.11 presents the SEM images of some nanoparticles (silver, tin, and gold) before the ablation. The presence of nanoparticles in the plumes was confirmed by analyzing the morphology of the ablated material deposited on the substrates placed

FIGURE 2.11  SEM Images of Various Nanoparticle Powders Measured Before the Ablation. (A) Ag, (B) Sn, and (C) Au. Reproduced from R.A. Ganeev, L.B. Elouga Bom, T. Ozaki, Deposition of nanoparticles during laser ablation of nanoparticle-containing targets, Appl. Phys. B 96 (2) (2009) 491–498 with permission of Springer.

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at the distance of 40 mm from the targets. It was shown that, at optimal excitation conditions, the nanoparticles remain almost intact in the plasma plume. The material directly surrounding nanoparticles is the glue, which has a lower ablation threshold than the nanoparticle materials. Therefore, the glue starts to ablate carrying the nanoparticles at relatively low intensities of laser radiation. This feature allowed for easier creation of the optimum plasma conditions, which resulted in the presence of the nanoparticles with defined size characteristics in the laser plumes. The laser ablation was carried out at different laser intensities at the surfaces of targets. The laser intensity was maintained at the level when the size characteristics of deposited material remained intact with regard to the initial morphology of nanoparticles. The intensity of subnanosecond pulse at which these conditions were satisfied was in the range between 3 × 109 and 1 × 1010 W cm−2. At these conditions, the sizes of the nanoparticles deposited on the substrates during laser ablation were close to those glued on the surface of targets. One can note that the duration of deposition plays important role in the aggregation of the debris on the substrates. The deposition of silver nanoparticles over 5 min at I = 1 × 1010 W cm−2 and 10 Hz pulse repetition rate led to aggregation of deposited debris and creation of a film containing large grains (Fig. 2.12A). Furthermore, the increase of laser intensity above certain level (I ∼ 3 × 1010 W cm−2, laser fluence 4 J cm−2) led to considerable disintegration and aggregation of nanoparticles on the target surface. This followed by the appearance of chaotic groups of silver aggregates on the substrate surface (Fig. 2.12B). Another pattern was observed in the case of ablation of the bulk targets. The ablation of bulk silver led to appearance of deposited nanoparticles (∼15 nm) on the substrates (Fig. 2.12C). This process was observed at considerably higher laser intensity (I ∼ 6 × 1010 W cm−2). No nanoparticle formation was observed on the substrates at moderate intensities of laser radiation on the bulk target surface (5 × 109–2 × 1010 W cm−2). The SEM images of deposited nanoparticles in most cases, when the optimal excitation of nanoparticle-containing targets was maintained, revealed that they remain approximately same as the initial powders. The sizes of deposited Cr2O3 clusters were in the range of 30–150 nm. The same can be said about the Au (40–180 nm), Cu (60–200 nm), and other deposited nanoparticles. The SEM images of initial and deposited In2O3 and Cr2O3 nanoparticle debris are presented in Fig. 2.13. The images of deposited Sn and Au nanoparticles are shown in Fig. 2.14. The intensity of laser pulse at the nanoparticle-containing targets was kept during these studies in the range of 3 × 109–8 × 109 W cm−2. The elemental consistence of ablated material and debris on the surface of deposited substrates was confirmed using the energy-dispersive X-ray spectroscopy (EDX) spectroscopy. The size distribution of tin nanoparticles on the target and after the ablation and deposition on the Si substrate at the optimal conditions of ablation is presented in Fig. 2.15. One can see that, at optimal conditions of laser ablation (I = 8 × 109 W cm−2), the morphology of ablated powder basically retained the same as before the ablation. At the same time, broader wings of size distribution point out the appearance of both small and large nanoparticles due to partial aggregation and disintegration of some

2.3 Deposition of nanoparticles

FIGURE 2.12 SEM images of the (A) appearance of the grained silver film created on the Si substrate during 5 min ablation (I = 1 × 1010 W cm−2) of the nanoparticle-containing target, (B) chaotic deposition of silver debris deposited on the Si substrate at the intensities exceeding threshold of disintegration of nanoparticles on the target surface (I ≥2 × 1010 W cm−2), and (C) silver nanoparticles produced during laser ablation of silver bulk target at I = 6 × 1010 W cm−2. Reproduced from R.A. Ganeev, L.B. Elouga Bom, T. Ozaki, Deposition of nanoparticles during laser ablation of nanoparticle-containing targets, Appl. Phys. B 96 (2) (2009) 491–498 with permission of Springer.

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FIGURE 2.13 SEM images of the initial (A) In2O3 and (B) Cr2O3 nanopowder and of the debris of (C) In2O3, and (D) Cr2O3 nanoparticles deposited on the Al substrate during laser ablation of nanopowder-containing targets. Reproduced from R.A. Ganeev, L.B. Elouga Bom, T. Ozaki, Deposition of nanoparticles during laser ablation of nanoparticle-containing targets, Appl. Phys. B 96 (2) (2009) 491–498 with permission of Springer.

nanoparticles (Fig. 2.5B). Note that the mean sizes of nanoparticles before and after irradiation remained almost unchanged (130 and 150 nm, respectively). The important parameter of laser-produced plasma is the nanoparticle concentration, as well as relative concentration of the nanoparticles and monoparticles of the same origin. How to exactly define the concentration in these cases is a difficult task. To estimate the relative concentration one can analyze the deposition of the debris from the used samples (i.e., both bulk targets and nanoparticle-containing targets) on the Si wafer and Al foil placed close to the ablation area by maintaining the same conditions of ablation. The thickness of evaporated material was studied during these studies and it was found to be approximately the same in both these cases. The approximately equal thickness of deposited material can be interpreted as roughly equal concentration of particles in the plasma area. The analysis and calculations of silver particle concentration in laser plume in the case of bulk silver targets using the code HYADES gave the concentration in the range of few units of 1017 cm−3 [45]. Unfortunately, this simulation technique could not be used in the case of ablated nanoparticle-containing targets due to the lack of information of the absorbance of these materials. Those studies have shown that application of commercially available nanopowders allowed for precisely defining the dimensions and structure of these nanoparticles in the plume. The proposed technique makes possible the predictable manipulation of plasma consistence, which leads to the creation of the laser plumes containing the nanoparticles with known spatial structure. The latter allows the application of such

2.4 Application of ion implantation

FIGURE 2.14 SEM images of deposited debris from the (A) Au and (B) Sn nanoparticle-containing targets. Reproduced from R.A. Ganeev, L.B. Elouga Bom, T. Ozaki, Deposition of nanoparticles during laser ablation of nanoparticle-containing targets, Appl. Phys. B 96 (2) (2009) 491–498 with permission of Springer.

plumes in nonlinear optics, X-ray emission of clusters, deposition of nanoparticles with fixed parameters on the substrates for semiconductor industry, production of nanostructured and nanocomposite films, etc.

2.4  APPLICATION OF ION IMPLANTATION FOR SYNTHESIS OF COPPER NANOPARTICLES IN A ZINC OXIDE MATRIX FOR OBTAINING NEW NONLINEAR OPTICAL MATERIALS In recent years, much effort has been devoted to the research and development of new composites based on wide-bandgap semiconductors and insulators containing metal nanoparticles, which are promising materials for optoelectronics and nonlinear optics. The phenomenon of collective excitation of conduction electrons in such nanoparticles

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FIGURE 2.15 Histograms of the tin nanoparticle size distribution (A) before and (B) after the ablation of the Sn nanopowder-containing target. Reproduced from R.A. Ganeev, L.B. Elouga Bom, T. Ozaki, Deposition of nanoparticles during laser ablation of nanoparticle-containing targets, Appl. Phys. B 96 (2) (2009) 491–498 with permission of Springer.

under the action of electromagnetic waves, called the SPR, accounts for a selective optical absorption and gives rise to nonlinear optical effects in the same spectral range [46]. As is known [47], materials with a high concentration of metal nanoparticles synthesized, for example, by ion implantation, possess pronounced nonlinear optical properties. For this reason, such composites can be successfully used in integrated optoelectronic devices, for example, in waveguides with nonlinear optical switching providing for signal conversion at short (pico- or femtosecond) laser pulse durations. Previously, metal nanoparticles have been successfully synthesized in the matrix of zinc oxide (ZnO), a wide-bandgap semiconductor characterized by an optical bandgap width of 3.8 eV (corresponding to 326.3 nm). The first study in this direction [48] was devoted to the synthesis of gold nanoparticles in a ZnO matrix.

2.4 Application of ion implantation

Table 2.1  Metal Nanoparticles in a ZnO Matrix: Types and Methods of Synthesis and Diagnostics Metala

Method of Synthesis

Co

Au

Ion implantation and thermal annealing RF sputtering and thermal annealing Ion implantation and thermal annealing Chemical deposition from solution RF sputtering and thermal annealing Electrodeposition from solution Chemical deposition from solution Magnetron sputtering

Au

Laser ablation

Au

Sol–gel coating and thermal annealing RF sputtering and thermal annealing

Cu Cu Ru Pt Au Au

Au

Method of Diagnosticsb

Particle Size (nm)

References

XRD and SQUID OS, XRD, and TEM OS

3.5

Norton et al. [57]

2–17 –

Varquer-Cuchillo et al. [50], Pal et al. [51] Kono et al. [52]

OS, XPS, and SEM OS, XRD, and TEM OS and TEM

2

Bozlee et al. [53]

1–15

Pal et al. [51]

5–50

Yoshino et al. [48]

OS, XRD, and SEM OS, XRD, and TEM OS, XRD, and SEM OS, XRD, and SEM OS, XRD, and TEM

5–40

Bozlee et al. [53]

20–70

Liao et al. [54]

2–6

Tiwari et al. [55]

50–100

Wang et al. [56]

1–27

Pal et al. [51]

a

Metals are listed in order according to the Periodic Table. Methods of diagnostics: OS, Optical spectroscopy; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy; SQUID, superconducting quantum interference device magnetometry. The referred papers correspond to the references [48–57] of this chapter. Reproduced from D.P. Norton, M.E. Overberg, S.J. Pearton, K. Pruessner, J.D. Budai, L.A. Boatner, M.F. Chisholm, S. Lee, Z.G. Khim, Y.D. Park, A. Wilson, Ferromagnetism in cobalt-implanted ZnO, Appl. Phys. Lett. 83 (26) (2003) 5488–5490 with permission of Springer. b

Presently available data on the synthesis of metal nanoparticles in ZnO [48–57], with indication of a particle size and the methods of synthesis (including ion implantation) and characterization, are summarized in the Table 2.1. However, the ion implantation synthesis of cobalt nanoparticles described in [57] requires additional (postimplantation) thermal treatments, which complicates the technology of such composite materials. On the other hand, copper nanoparticles have been formed directly by low-energy ion implantation, but the particle size was so small that a postimplantation annealing was still required to enlarge them. It should be noted that there is a large number of other publications devoted to the implantation of various metal ions into ZnO, but the ion doses were so small that even subsequent high-temperature treatments did not lead to the formation of metal nanoparticles.

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From the standpoint of effective manifestation of nonlinear optical properties, the most promising metals are those with a high density of free conduction electrons, in particular, copper [47]. The study analyzed below was aimed at experimental verification of the possibility of obtaining a new nonlinear optical material directly by the synthesis of copper nanoparticles in a ZnO substrate by means of ion implantation, without any postimplantation treatment [58]. As seen from literature indicated in Table 2.1, nonlinear optical properties of ZnO with dispersed metal nanoparticles have not been studied so far. A composite material was obtained using single crystal ZnO substrates optically transparent in a broad spectral range (∼500–1100 nm). This matrix was implanted with 160-keV Cu+ ions at a dose of 1 × 1016 or 1 × 1017 cm−2 at a high ion beam current density (∼20–50 mA cm−2). The ion implantation was performed at room temperature in a vacuum of 10–6 torr on an EATON 3204 implanter. The optical density spectra were measured using a Perkin-Elmer Lambda 19 spectrophotometer. The spectra of the absorption cross section were also modeled, within the framework of the classical theory of interaction between electromagnetic waves and a spherical particle (Mie's theory), using a method described in [59]. Fig. 2.16 shows the experimental optical absorption spectra of the ZnO samples implanted with Cu+ ions at different ion doses. For the sample implanted with copper to a lower dose, the spectrum is virtually identical to that of unirradiated ZnO, except

FIGURE 2.16  Experimental Spectra of the Optical Density η of ZnO Implanted with Copper to Various Doses, in Comparison to the Model Spectrum of the Absorption Cross Section σ Calculated Using the Mie Theory (Curve 1) for a Single 10 nm Spherical Particle in a ZnO Matrix. Reproduced from D.P. Norton, M.E. Overberg, S.J. Pearton, K. Pruessner, J.D. Budai, L.A. Boatner, M.F. Chisholm, S. Lee, Z.G. Khim, Y.D. Park, A. Wilson, Ferromagnetism in cobalt-implanted ZnO, Appl. Phys. Lett. 83 (26) (2003) 5488–5490 with permission of Springer.

2.5 Pulsed laser deposition

for a somewhat increased absorption in a short-wavelength region (2 nm) implanted copper particles capable of producing significant optical absorption due to the SPR. In contrast, a wide selective absorption band with a maximum at ∼600 nm observed in the spectrum of the sample implanted at a higher dose is direct evidence of the formation of such copper nanoparticles exhibiting SPR. For comparison, Fig. 2.16 also shows a model spectrum of the absorption cross section calculated for a single 10-nm spherical particle in a ZnO medium, which is characterized by maximum optical absorption at the same wavelength. Thus, the calculation confirmed the possibility of formation of the copper particles in a ZnO matrix. The difference between shapes of the model and experimental spectra is probably explained by the distribution of the synthesized metal nanoparticles with respect to size, which usually takes place during the ion implantation of dielectrics. One can assume that the main factor favoring the formation of larger nanoparticles of copper in ZnO, in the discussed case, in comparison to the results obtained in [52], is the higher ion beam current density used in the experiments. The high ion beam power ensures significant local heating in the irradiated ZnO layer, increases the diffusion mobility of implanted copper, and, hence, favors the effective nucleation and growth of metal nanoparticles.

2.5  PULSED LASER DEPOSITION OF METAL FILMS AND NANOPARTICLES IN VACUUM USING SUBNANOSECOND LASER PULSES 2.5.1 INTRODUCTION The structural, optical, and nonlinear optical parameters of nanoparticles are known to differ from those of the bulk materials due to the quantum confinement effect. Further search of prospective materials in nanoparticle form, their preparation, and application are of considerable importance today. Past studies on nanoparticles prepared using metal vapor deposition [60], reduction of some salts by alkalides [61], and the solution dispersion method [62] have revealed many interesting structural and optical properties of those materials. In previous sections, we have shown that laser ablation of metals in vacuum and liquids is among the perspective techniques, which can also be successfully applied for the preparation of nanoparticle-containing media. Particularly, the application of laser ablation in liquids for the preparation of semiconductor [63,64] and metal [31] colloids has been demonstrated. It is known that metal ablation in air is significantly less efficient than that in vacuum due to redeposition of the ablated material. To prove the generality of the vacuum deposition method and its potential use for preparing nanoparticles, one can consider various metals and analyze this process at different focusing conditions of the laser radiation. The most interesting and new features of laser ablation and

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nanoparticle formation during irradiation of the solid targets have been observed in the case of short laser pulses (100 fs–1 ps). In this section, we analyze the formation of nanoparticles of silver, chromium, stainless steel, and indium in vacuum, using much longer (subnanosecond) laser pulses [65]. Here, the structural properties of the nanoparticles deposited on different substrates are discussed. The effect of focusing conditions of the laser (tight or weak focusing) on the films prepared by the laser ablation of bulk targets in the vacuum is analyzed. Those results have shown that the nanoparticles can be successfully formed using long laser pulses in tight-focusing conditions.

2.5.2  EXPERIMENTAL DETAILS The ablation of various materials was carried out in vacuum using uncompressed pulses (of 300 ps duration) from a chirped-pulse amplification-based Ti:sapphire laser system. The samples were placed inside a vacuum chamber. Uncompressed pulses from the Ti:sapphire laser (λ = 793 nm, t = 300 ps, E = 30 mJ, 10 Hz pulse repetition rate) were focused on a bulk target at two regimes of focusing. In the first case (referred to as tight focusing), the intensity of laser radiation was in the range of 2 × 1012 W cm−2, and in the second case (referred to as weak focusing), the intensity was considerably lower (4 × 1010 W cm−2). Ag, In, stainless steel, and chromium were used as targets. Float glass, silicon wafer, and various metal strips (Ag, Cu, and aluminum) were used as the substrates, and were placed at a distance of 50 mm from the targets. The deposition was carried out in oil-free vacuum (∼1 × 10-4 mbar) at room temperature. The structure of the deposited films of ablated metals was analyzed using different techniques. For this purpose, the nanoparticle-containing films were deposited on different substrates. The nature of the nanoparticles is governed by the thermodynamic conditions at the target surface. The presence of nanoparticles was inferred by analyzing the spatial characteristics of the deposited material and the spectral absorption of the deposited material. The absorption spectra of the deposited films were analyzed using a fiber-optic spectrograph (USB2000). The analysis of the sizes of deposited nanoparticles was carried out using total reflection X-ray fluorescence (TXRF). Details of the TXRF are described in [66]. The structural properties of the deposited films were analyzed using the SEM, AFM, and transmission electron microscope (TEM).

2.5.3  RESULTS AND DISCUSSION The absorption spectra of the materials deposited on transparent substrates (float glass) were used to determine the presence or absence of nanoparticles. The presence of nanoparticles was inferred from the appearance of strong absorption bands associated with SPR. Fig. 2.17 presents the absorption spectra of Ag films deposited on float glass substrates. Numerous studies (e.g., [31]) have shown that the SPR of spherical silver nanoparticles induces a strong absorption in the range from 410 to

2.5 Pulsed laser deposition

FIGURE 2.17  Absorption Spectra (Curves 1–3) of the Ag Films Deposited at Different TightFocusing Conditions and the Absorption Spectrum of Ag Nanoparticles (Curve 4) Implanted Inside the Silica Glass Plate by Ion Bombardment. Reproduced from R.A. Ganeev, A.I. Ryasnyansky, Influence of laser ablation parameters on optical and nonlinear optical characteristics of semiconductor solutions, Opt. Commun. 246 (2) (2005) 163–171 with permission from the Optical Society of America.

480 nm, depending on the preparation technique. In the discussed studies, a variation of the position of the absorption of Ag deposition was observed, which depended on the conditions of excitation and evaporation of bulk targets by the 300 ps pulses interacting with the surface at different tight focusing conditions. However, in all these cases, the peaks of SPR were centered in the range between 440 and 490 nm (Fig. 2.17, curves 1–3). In the case of the deposition of Ag film at the weak-focusing conditions, no absorption peaks were observed in this region, thus indicating the absence of nanoparticles in the deposited material. Fig. 2.17, curve 4 shows analogous measurements made on a sample of Ag nanoparticles embedded in silica glasses using the ion bombardment, reported in [67]. In that work, the thickness of the implanted layer was 60 nm, and the size of Ag nanoparticles was reported to vary from 4 to 8 nm. It was observed that the absorption curve of this sample is quite similar to those of Ag deposited on the glass surfaces during discussed studies (Fig. 2.17, curves 1–3). The only difference is that the position of the peak of SPR (∼415 nm) was on the shorter wavelength side (Fig. 2.17, curve 4). Much attention has been devoted during the past years to precisely determining the spatial arrangement in 2D and 3D structures of metals. However, the ordering and use of the nanomaterials necessitate synthesis of monodispersed individual nanoparticles, for which no general method is presently available. Here, we analyze the synthesized nanoparticles produced by laser ablation of bulk targets at two different conditions of ablation. The structure of the ablated Ag was analyzed by studying the films deposited on silicon substrates. One of the aims of this study was to investigate whether the

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plumes contain nanoparticles. The presence of the latter could be responsible for the enhancement of nonlinear optical characteristics. In particular, high-order harmonic conversion efficiency in plasma medium may be affected due to the quantum confinement during propagation of a femtosecond laser pulse through a nanoparticlecontaining plasma. Total reflection X-ray fluorescence measurements were performed using an inhouse-developed TXRF [68,69] for the analysis of the structural properties of the deposited material. The angular dependence of the fluorescence intensity in the total reflection region [70] can be successfully used to identify the presence of nanoparticles on a flat surface. The structure of the nanoparticles prepared by the laser ablation was analyzed using TXRF, which identified the presence of nanoparticles. This was done for the deposition in tight-focusing conditions of the laser. In the case of weak focusing, it showed a thick filmlike deposition of metal, without any inclusion of nanoparticles (see the TXRF image for the case of the ablation of indium at the weak-focusing conditions, Fig. 2.18). It can be seen from this figure that the fluorescence intensity of In-Lα decreased abruptly below the critical angle for In (∼0.36° at 8.5 keV). For incident angles larger than the critical angle, In-Lα fluorescence intensity increased monotonously. It showed behavior like that for a thick film of atomic In deposited on the substrate. The fluorescence measurements were carried out using a Peltier-cooled solidstate detector (Eurisus-Mesures EPXR 10-300), a spectroscopy amplifier AMP 6300,

FIGURE 2.18  Recorded X-Ray Fluorescence Profile for an In Deposition Prepared by Ablation at Weak-Focusing Conditions. The profile shows that the In is deposited in the form of a continuous monoatomic film instead of nanoparticles. The solid curve shows a fitted profile. The critical angle of In is ∼0.36° for 8.50 keV X-ray energy. Reproduced from R.A. Ganeev, A.I. Ryasnyansky, Influence of laser ablation parameters on optical and nonlinear optical characteristics of semiconductor solutions, Opt. Commun. 246 (2) (2005) 163–171 with permission from the Optical Society of America.

2.5 Pulsed laser deposition

FIGURE 2.19  Recorded X-ray Fluorescence Profile of Ag Nanoparticles Prepared by the Laser Ablation in Vacuum and Deposited on a Float Glass Substrate. The dots show experimental data, while the solid curve shows a fitted profile. Reproduced from R.A. Ganeev, A.I. Ryasnyansky, Influence of laser ablation parameters on optical and nonlinear optical characteristics of semiconductor solutions, Opt. Commun. 246 (2) (2005) 163–171 with permission from the Optical Society of America.

and a multichannel pulse height analyzer card installed in a personal computer. The solid-state detector had an energy resolution of 250 eV at 5.9 keV. A well-collimated primary beam, from a line focus Cu X-ray tube, was used as an excitation source. Fig. 2.19 shows the X-ray fluorescence trace recorded in the case of Ag nanoparticles deposited on a glass substrate at tight-focusing conditions. It can be seen from this figure that the angle-dependent fluorescence profile of the Ag film shows the presence of nanostructure on the flat surface, as well as indicates a monoatomic layer. The solid curve presents the best theoretical fit, from which the average size of the nanoparticles was estimated to be 60 nm, while the thickness of the layer of monoatomic Ag particles was estimated to be 30 nm. The TEM measurements also confirmed the presence of Ag nanoparticles in these deposited films. The SEM studies of the structural properties of deposited films showed that, in tight-focusing conditions, these films contain a lot of nanoparticles with variable sizes. In weak-focusing conditions, the concentration of nanoparticles was considerably smaller compared to the tight-focusing condition. Fig. 2.20A shows the SEM images of the deposited chromium nanoparticles on the surface of a silicon wafer. In the case of weak focusing, the deposited film was almost homogeneous with a few nanoparticles appearing in the SEM images (see Fig. 2.20A showing the deposition of chromium), while in tight-focusing conditions, plenty of nanoparticles ranging from 30 to 100 nm appeared in the SEM images (see Fig. 2.20B showing the deposition of stainless steel). The average size of these NSs was estimated to be 60 nm. An enlarged SEM image of the Ag nanoparticles prepared in tight-focusing conditions is presented in Fig. 2.20C. The average size of these spherical nanoparticles was also

69

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FIGURE 2.20  SEM Images of the Chromium, Stainless Steel, and Ag Nanoparticles Deposited on Silicon Wafer as Substrate. These images were obtained at (A) weak-focusing (chromium deposition), (B) tightfocusing (stainless-steel deposition), and (C) tight-focusing (Ag deposition) conditions. (Note that different magnifications were used). The average size of the spherical clusters in all three cases was measured to be 60 nm. Reproduced from R.A. Ganeev, A.I. Ryasnyansky, Influence of laser ablation parameters on optical and nonlinear optical characteristics of semiconductor solutions, Opt. Commun. 246 (2) (2005) 163–171 with permission from the Optical Society of America.

measured to be 60 nm. The same behavior was observed in the case of other targets. Those studies have shown that the material of the target does not play a significant role in the formation of nanoparticles in the case of laser ablation using 300 ps laser pulses in tight-focusing conditions. Hence, further study on the effects of the substrate material on nanoparticle deposition was carried out using Ag targets. Fig. 2.21 shows the SEM images of Ag deposited on a Cu substrate. One can see a considerable difference in the concentrations of nanoparticles in the cases of weak and tight focusing. It may be pointed out that there is special interest in Ag nanoparticles due to their potential applications. In general, highly dispersed metals have a much higher surface area for a given volume and hence they can be useful for efficient catalytic conversion. Ag nanoparticles are widely used for surface-enhanced

2.5 Pulsed laser deposition

FIGURE 2.21 SEM images of the Ag nanoparticles deposited on a Cu substrate at (A) weak-focusing conditions and (B) tight-focusing conditions. Reproduced from R.A. Ganeev, A.I. Ryasnyansky, Influence of laser ablation parameters on optical and nonlinear optical characteristics of semiconductor solutions, Opt. Commun. 246 (2) (2005) 163–171 with permission from the Optical Society of America.

Raman scattering. Ag nanoparticles have an advantage over the other metal nanoparticles (e.g., gold and Cu) from the point of view of the position of the SPR energy of Ag, which is far from the interband transition energy. This enables one to investigate the optical and nonlinear optical effects in the Ag nanoparticles by focusing on the surface plasmon contribution. Further studies on the characteristics of nanosized structures of the deposited materials were carried out using atomic force microscopy. AFM measurements were carried out in noncontact mode under an ambient environment. Silicon cantilever tips of resonant frequency ∼180 kHz and spring constant 5.5 N/m were employed. Fig. 2.22A shows the AFM image of the Ag nanoparticles deposited on the surface of a copper strip. The average size of Ag nanoparticles was 65 nm. In contrast to this, the AFM images obtained from the deposited films prepared at weak focusing conditions showed a considerably smaller number of nanoparticles. Fig. 2.22B shows an AFM picture of Ag deposition prepared at these conditions. The image indicates the presence of very few nanoparticles. The same difference in AFM pictures was observed in the case of In deposition under the two focusing conditions. To characterize the ablation process, the temporal and spectral characteristics of the light emitted by the plume were also studied. The oscilloscope traces showed a considerable increase in the duration of the plasma emission in the case of tight focusing that could be expected considering the excitation conditions. A combined analysis of the spectra and oscilloscope traces in the cases of two different regimes of

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FIGURE 2.22 AFM image of (A) the Ag nanoparticles deposited on a Cu strip in tight-focusing conditions and (B) the Ag nanoparticles deposited on an aluminum strip in weak-focusing conditions. Very few nanoparticles are seen in the case of (B) weak-focusing conditions compared to the case of (A) tight-focusing conditions. Reproduced from R.A. Ganeev, A.I. Ryasnyansky, Influence of laser ablation parameters on optical and nonlinear optical characteristics of semiconductor solutions, Opt. Commun. 246 (2) (2005) 163–171 with permission from the Optical Society of America.

the excitation of surface plasma revealed that the structureless continuum appearing in the spectrum in the case of tight focusing is due to the emission from hot nanoparticles produced during laser ablation. Such hot nanoparticles behave like black-body radiators emitting for a longer time, until they get cooled down. It is well accepted that when a solid target is ablated by laser radiation, the ablating material cotains a mixture of atoms, ions (and electrons), clusters, and nanoparticles. The atoms and multiparticle species tend to aggregate during the laser pulse or soon afterward leading to the formation of larger clusters. The reported results

2.6 Concluding comments

(see, e.g., [71]) also indicate that the ablation processes in the picosecond and femtosecond time scales are very different compared with the nanosecond one. In addition to the early experimental observations, several theoretical studies have suggested that rapid expansion and cooling of the solid-density matter heated by short laser pulses may result in nanoparticle synthesis via different mechanisms. Heterogeneous decomposition, liquid phase ejection and fragmentation, homogeneous nucleation and decomposition, and photomechanical ejection are among those processes that can lead to nanoparticle production [30,40,44]. Short pulses, contrary to the nanosecond pulses, do not interact with the ejected material, thus avoiding complicated secondary laser interactions. Further, short pulses heat a solid to a higher temperature and pressure than do longer pulses of comparable fluence as the energy is delivered before significant thermal conduction can take place. The model developed in [72] for aluminum predicts that, for short laser pulses at intensities in the range from 1012 to 1013 W cm−2, the adiabatic cooling drives the system into a metastable region of its phase diagram, resulting in the production of a relatively large fraction of nanoparticles. At larger intensities (≥1014 W cm−2), the system can never reach the metastable region, resulting in an almost atomized plume. The laser deposition using short pulses has gained interest because of a number of advantages over other processes, such as the possibility of producing materials with a complex stoichiometry and a narrowed distribution of nanoparticle sizes with reduced porosity. Typically, laser deposition is carried out in an ambient gas, which quenches the ablated plume, thus controlling the mean particle size [73]. However, some previously reported studies, as well as the discussed study, suggest that nanoparticles are generated as a result of some relaxation processes of the extreme material state reached by the irradiated target surface. This stands in stark contrast to the formation of nanoparticles during nanosecond laser ablation in a background gas, where vapor condensation is considered to be an important mechanism.

2.6  CONCLUDING COMMENTS We analyzed the effect of the polarization of femtosecond pulses, the bandgap width of semiconductors, the angle of incidence of laser light onto the surface of a sample, and the surrounding medium on the morphology of LIPSS formed under the action of short laser pulses upon the surfaces of different semiconductors. We have discussed the regimes of formation of short- and long-period structures and demonstrated the appearance of a number of exotic nanofeatures (nanoholes and nanodots). As a whole, the studied process of LIPSS formation may serve as an example of self-organization in a system that does not initially reveal any structural properties. Note that some of the above results, such as the distinction in the LIPSS formation in semiconductors immersed into a liquid under the action of the 35 and 120 fs pulses, the effect of the angle of incidence on dimensional characteristics of the ripples, and the simultaneous appearance of long- and short-period

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structures, do not result in a preference for any one of the hypotheses of ripples formation proposed by different authors. We discussed systematic studies of the influence of ambient conditions on LIPSS formation and their quality. It was suggested that, for a dense surrounding medium, the formed nanoripples show better shape and stability. We have also analyzed the formation of two sets of nanoripples at similar conditions. In particular, we analyzed the studies of the influence of polarization of the femtosecond radiation, bandgap of semiconductors, angle of interaction of the laser radiation with the surface, etc., on the variation of nanoripple pattern, when one can distinguish the regimes of the formation of ordinary and subwavelength LIPSS, as well as some exotic patterns (nanoholes and nanodots). We discussed a unique type of laser-induced periodic surface structure when simultaneous appearance of regular structures of different periods was observed. The self-organization was clearly shown in the case of a dense surrounding medium (methanol), while in the case of insufficient amount of surrounding material (i.e., at different air pressure conditions), the quality of ripples was considerably decreased. These observations point out the role of optimal ambient conditions at which one can achieve high-quality LIPSS formation. We analyzed the nanoparticles formation on the surface of target during laser ablation of metals by short (of a few tens of picosecond) laser pulses, which strongly depended on the concentration of surrounding gas. While, at vacuum conditions, nanoparticle formation shows very “sharp” AFM images of aggregated nanoparticles, following with appearance of plasmon resonance on the absorption spectra of deposited films, an addition of gas particles starts to decrease the probability of multiparticle species formation. This process has shown a threshold for both helium (33 torr) and xenon (12 torr) above which no SPR and correspondingly no observable nanoparticles in the deposited films were detected. The destruction of nanoparticle formation was attributed to the negative influence of surrounding gas particles on the ablated particles aggregation. We discussed possibility of formation of the NSs on the surfaces of semiconductors with different bandgap widths with the dimensions of those features being much smaller than the light wavelength (790 nm). It was shown that the technique of restoration of two-dimensional NSs makes it possible to modify the surface depending on the material of the object, angle of incidence of the light, light polarization, and the angle between the two crossing beams. A characteristic property of the structural objects demonstrated in this study is the appearance of NSs with the dimensions of the order of 110–250 nm. Finally, a study of silver, chromium, stainless-steel, and indium thin films prepared by subnanosecond laser deposition in vacuum was discussed. We compared the laser ablation in vacuum at the weak- and tight-focusing conditions of a Ti:sapphire laser beam and analyzed the nanoparticles synthesized in the latter case using absorption spectroscopy, X-ray fluorescence, atomic force microscopy, and scanning electron microscopy. Those results have shown that the nanoparticle formation can be accomplished using long laser pulses under tight-focusing conditions.

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CHAPTER

Methods of nanostructured materials characterization

3

3.1  MORPHOLOGY OF LASER-PRODUCED CARBON NANOPARTICLE PLASMAS 3.1.1  INVOLVEMENT OF SMALL SPECIES IN THE HIGH-ORDER HARMONIC GENERATION Various approaches have been explored for the enhancement of high-order harmonic generation of laser radiation in the extreme ultraviolet range. One of these approaches, which has been explored to increase the ratio of the energies of the single harmonic pulse in the plateau range and the driving pulse (i.e., conversion efficiency, η) of high-order harmonic generation, is the application of clusters as the nonlinear optical media. Successful attempts to enhance harmonic yield were made using gaseous clusters [1,2]. Meanwhile, the comparative studies of High-order harmonic generation (HHG) in the laser-produced plasma plumes consisting of metal clusters and monomers have shown that, at equal experimental conditions, the former emitters provide stronger harmonic yield, thus pointing out the specific advanced properties of the clustered emitters of harmonics in the metal nanoparticle-containing plasmas [3,4]. With regard to this, clustered carbon species provide a variety of morphological shapes (such as spherical nanoparticles and fullerenes, as well as extended nanofibers and nanotubes), contrary to the less variable shapes of spherical metal nanoparticles. The use of former clusters in high-order harmonic generation studies has long attracted the attention of researchers. The high-order harmonic generation spectra of carbon nanotubes (CNTs) have been theoretically studied in [5,6]. It has been suggested that there are many reasons for using the nanotubes to generate high-order harmonics. The CNT targets lack a number of serious drawbacks, which are characteristic of the molecular ones. The chemical bonding between carbon atoms in nanotubes and carbon nanofibers (CNF) is rather strong, which means that their deformation and dissociation in intense fields take place on longer time scales than those of aromatic molecules. The lack of light hydrogen atoms in nanotubes and nanofibers contributes to the same stability effect. The spectra of harmonics may provide useful information on the dynamics of electron motion in such extended structures. Finally, the pronounced electronic nonlinearity in CNTs and CNFs with the two-dimensional-confined distribution on their surfaces shows some potential for the efficient generation of high-order harmonics and could Nanostructured Nonlinear Optical Materials. http://dx.doi.org/10.1016/B978-0-12-814303-2.00003-9 Copyright © 2018 Elsevier Inc. All rights reserved.

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thus represent a useful mechanism for producing coherent ultrashort Extreme ultraviolet (XUV) pulses. The advanced properties of CNTs were revealed during the first experimental demonstration of their high-order nonlinear optical properties [7], which has shown harmonic yields similar to or exceeding those obtained using other ablated species. Similarly, various peculiarities have been identified in the theoretical studies of high-order harmonic generation from the C60 fullerenes [8,9]. Compared to laser– atom interaction, in big molecules or clusters, such as C60, additional degrees of freedom are introduced, particularly electronic degrees of freedom, including collective effects such as the formation of plasmons, vibrational degrees of freedom, or fragmentation. The importance of 20 eV giant broadband resonance of C60 for the simultaneous enhancement of a few neighboring harmonics within the bandwidth of this resonance was confirmed during the first experimental demonstration of the fullerene-induced high-order harmonic generation [10]. On the other hand, it has been suggested that nanoparticle formation during ablation of bulk graphite was the main reason for the observed enhanced yield of harmonic radiation from the plasmas containing carbon species [11]. It has been proven that the physical characteristics of those plasmas, such as the presence of clusters, directly affect the harmonic yield. It is unclear whether all those nanoparticles with the observed sizes in the range of a few nanometers to a few tens of nanometers were involved in the process of efficient harmonic generation. Thus, to select the optimal plasma conditions for HHG, a detailed analysis of the plasmas containing different carbon clusters, as well as monomers, is required, which will allow further enhancement of the harmonics. The analysis of high-order harmonic generation in nanostructures will be discussed in details in the following sections. Here, we analyze plasma morphology at conditions of laser ablation of the nanoparticle targets suitable for the efficient high-order harmonic generation in carbon cluster-containing plasmas [12]. The timeof-flight mass spectrometry and harmonic generation studies of laser plasma suggest that the origin of the observed growth of η from such clustered plasmas is probably related to the consideration of lower-sized nanoparticles, rather than higher-sized ones, as the emitters of efficient harmonics. Based on this analysis, the efficient high-order harmonic generation of Ti:sapphire laser pulses in the range of 15–33 eV was demonstrated using optimally prepared 5 mm long carbon-based cluster plasma plumes.

3.1.2  EXPERIMENTAL ARRANGEMENTS AND RESULTS The uncompressed radiation of a Ti:sapphire laser was used as a heating pulse (central wavelength λ = 804 nm, pulse duration 370 ps, pulse energy up to Ehp = 4 mJ) to ablate the targets for extended plasma formation. The heating pulse was focused using the 200 mm focal length cylindrical lens inside the vacuum chamber containing an ablating material to create the extended plasma plume above the target surface. The intensity of heating pulses on a plain target surface was varied in the

3.1 Morphology of laser-produced carbon nanoparticle plasmas

range of 1 × 109–3  × 109 Wcm−2. The compressed driving pulse from the same laser with energy of up to Edp = 4 mJ and 64 fs pulse duration was used, 45 ns from the beginning of ablation, for harmonic generation in the plasma plumes. The HHG experiments will be discussed in the following chapters. Here we concentrate on the analysis of the properties of laser-produced plasmas. CNT powder, CNF powder, diamond nanoparticle (DN) powder, C60 powder (all Sigma-Aldrich), and 5 mm long bulk graphite were studied as the ablating targets. The powders of the above materials were glued onto the 5 mm long glass plates and were installed on the translating stage in the vacuum chamber for further ablation. The morphology of materials was analyzed using the transmission electron microscopy of original clusters and plasma debris. The content of the plasmas was analyzed using time-of-flight mass spectroscopy. Fig. 3.1 shows the Transmission electron microscopy (TEM) images of the assupplied clusters of CNT, CNF, C60, and DN. The sizes of DN were in the range of

FIGURE 3.1 TEM images of clustered targets: (A) fullerene agglomerates, (B) diamond nanoparticles, (C) carbon nanotubes, (D) carbon nanofibers. Black markers correspond to (1)–(3) 20 nm and (4) 200 nm. Reproduced with permission from R.A. Ganeev, M. Baba, M. Suzuki, H. Kuroda, Morphology of laser-produced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media, J. Phys. B 47 (13) (2014) 135401 [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

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3–8 nm. The diameter of CNTs was ∼15 nm with the lengths varied in a wide range (500 nm–10 µm). CNFs were presented with a variety of fibers with diameters of 30–400 nm and the lengths varied from hundreds of nanometers to a few micrometers. Fullerene powder was presented in the shape of the aggregated C60. The plasma debris collected on the substrates placed near to the ablation area were also analyzed at the optimal and nonoptimal conditions of clustered plasma formation. These terms refer to the strong and weak harmonic yields from those plasmas. The analysis of postablation conditions of the deposited debris can provide some information about the plasma components, despite the difference between the composition of the plasma at the early stage of formation and the deposited material, which can be modified due to aggregation on the substrate [13]. To compare the dependence of the properties of the deposits under different ablating conditions, the clusters were ablated using the 370 ps pulses of the Ti:sapphire laser at a repetition rate of 10 Hz and variable fluence. Fig. 3.2 shows the TEM images of the material ejected upon ablation of CNT, CNF, C60, and DN. One can see that the characteristic sizes of the deposited material were close to those of the initial cluster targets (Fig. 3.1). In those studies, the moderate laser ablation intensities (up to 3 × 109 Wcm−2) corresponding in each case to the conditions of efficient HHG were used. Over-excitation of the nanoparticle-containing targets, by using higher intensities and fluences, did not yield similar nanostructured deposits, but rather the groups of chaotically formed aggregates. The deposits generated upon laser ablation of the bulk graphite were analyzed at conditions that ensured the efficient harmonic generation in the plasma plume. Under these conditions of using the moderate fluences of heating pulses the collected deposits showed evidence of ∼10–20 nm nanoparticles. Figs. 3.3–3.5 show the mass-resolved spectra of the clustered plasmas using the TOFMS. The 3 ns pulses were used to ablate the species at the fluences corresponding to both efficient and inefficient HHG. Those studies have revealed that most of the analyzed laser plumes contained C5–C25 clusters. To ascertain the presence of neutral species in the ablated plasma, an additional source of ionization should be used. The TOFMS measurements did not reveal the presence of neutral clusters in the plasmas, as there were no opportunity in the postionization of the plasma plume. However, previous studies (see for example [14]) have shown the presence of neutral carbon clusters at similar conditions of ablation. Ablation of C60 at a weak fluence revealed the appearance of fullerene ions, while a strong signal from C26 clusters pointed out the preferable disintegration of the C60 molecule rather than its ionization (Fig. 3.3A). The growth of heating pulse fluence on the target surface has considerably modified this spectral pattern (Fig. 3.3B). While C26 still remained in the mass spectrum, a very strong signal from C60 ion dominated over the whole spectrum, together with the appearance of some lighter clusters (C56 and C58). Moreover, two additional groups of clusters appeared in the high-mass/charge range with the masses ranged between C108 and C118 and between C164 and C176 (Fig. 3.3c). The presence of sodium and potassium lines was attributed to the substrate containing ablating species.

FIGURE 3.2 TEM images of deposited debris: (A) carbon nanotubes, (B) fullerene agglomerates, (C) diamond nanoparticles. Black markers correspond to (a) 100 nm, (b) 20 nm and (c) 50 nm. Reproduced with permission from R.A. Ganeev, M. Baba, M. Suzuki, H. Kuroda, Morphology of laser-produced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media, J. Phys. B 47 (13) (2014) 135401 [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

FIGURE 3.3 Time-of-flight mass spectra of ablated fullerene powder at (A) weak and (B), (C) strong excitation of targets. Bottom plot (C) shows the spectra of higher mass fullerenes. Reproduced with permission from R.A. Ganeev, M. Baba, M. Suzuki, H. Kuroda, Morphology of laser-produced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media, J. Phys. B 47 (13) (2014) 135401 [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

3.1 Morphology of laser-produced carbon nanoparticle plasmas

FIGURE 3.4  Mass Spectra of the Low-Sized Diamond Nanoparticles at Weak (Upper Panel) and Strong (Bottom Panel) Excitation of Targets. Reproduced with permission from R.A. Ganeev, M. Baba, M. Suzuki, H. Kuroda, Morphology of laser-produced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media, J. Phys. B 47 (13) (2014) 135401 [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

The mass spectra of ablated DNs in the cases of weak and strong excitation are shown in Fig. 3.4. In both cases the appearance of a group of light clusters was reported. At each of these conditions the stronger signals came from the clusters obeying the rule of magic numbers 4n + 3. The clusters with numbers of carbon atoms of 7, 11, 15, and 19 dominated those spectra. No other ionic clusters were observed in those studies while searching up to a few tens of thousands of the mass/ change units. CNTs and CNFs showed similar spectra as the DNs, though these targets were completely different from each other by the initial morphology. As in the previous case, no ions higher than low-sized carbon clusters were observed in the CNT mass spectra up to 5000 mass/charge units (Fig. 3.5A). Those small sized particles almost

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FIGURE 3.5 Mass spectra of (A) carbon nanotube ablation and (B) graphite ablation. Reproduced with permission from R.A. Ganeev, M. Baba, M. Suzuki, H. Kuroda, Morphology of laserproduced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media, J. Phys. B 47 (13) (2014) 135401 [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

completely repeated the structure of the ionic components of DN plasma (compare Fig. 3.4, bottom panel, and the inset of Fig. 3.5A. The ablation of the graphite target also showed the appearance of the same group of clusters (Fig. 3.5B). Those studies have revealed a similarity in the properties of almost all studied plasmas from the point of view of the presence of lower-mass

3.2 Synthesis and photoluminescence properties of silver nanowires

carbon clusters. The ease in formation of these nanoparticles from the clustered media may lead to their stronger influence on the HHG. The time-of-flight mass spectra of most ablated carbon clusters (at conditions of maximum harmonic yield) resembled those obtained by many researchers [15–22]. As mentioned, the (4n + 3) clusters had a greater presence compared with the neighboring ones. Those clusters (C7, C11, C15, C19) could probably be considered as the most effective emitters of harmonics among all the clusters. This assumption can be generally attributed to a lower ionization potential of these clusters rather than to their exceptional structural stability. At optimal plasma conditions, when the highest harmonic conversion efficiency from the carbon cluster ablation was obtained, the TEM images reveal the presence of nanoparticles in the deposited debris with sizes similar to the initial material (in particular, ∼5–10 nm diamond nanoparticles and ∼20 × 2000 nm2 nanotubes).

3.2  SYNTHESIS AND PHOTOLUMINESCENCE PROPERTIES OF SILVER NANOWIRES 3.2.1  THE PRINCIPLES OF SILVER NANOWIRE FORMATION The development of electronic devices at the atomic scale highlights the importance of the study of silver nanowires. Those can be used as interconnects in microelectronic and magnetic devices [23–26]. Several synthetic procedures have been developed for the formation of metal nanoparticles, nanowires, nanoprisms, both within the templating membranes and in solutions [23–34]. A universal template-confinement, step-electrochemical technique to fabricate Ag-based nanowires is developed by Liang et al. [27]. Apart from the optical and electronic properties, the special shape of Ag nanoparticles makes them the ideal choice for applications in modern nanotechnology of integrated circuits [28–32]. Therefore, it could be highly useful to develop an effective preparation method of Ag nanoparticles with well-controlled shape and size. Theoretically [33–35] it has been predicted that, for silver and gold nanoparticles, the number and position of surface plasmon resonance peaks, as well as the effective spectral range for surface-enhanced Raman scattering, are strongly dependent on the particle shape. Ditlbacher et al. [36] showed that a well-defined crystal shape and surface structure can minimize surface plasmon damping due to scattering at roughness, domain boundaries, or defects. With a phenomenological model Boyd et al. [37] argued that the light emission from the radiative recombination of sp-band electrons with d-band holes is enhanced by the local field association with the particle plasmon oscillation. This would imply possibility of an accelerated radiative process. In most of the related studies reported previously, characterization of the SPR of silver nanoparticles and silver nanowires have always been given the maximum importance because of the fact that it has been the first optical response of a nanoscale metal in the visible range of the spectrum. In spite of the technological significance, precise spectroscopic studies to characterize the thermal stability of these optical nanomaterials are still only successful to a limited extent. However, the

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exact mechanism about how the photoluminescence (PL) at different excited wavelengths work is still unclear. Also no detailed study had been carried out so far for determining the variations of the PL spectra of silver nanowires for wide variations of excitation wavelengths. Here we discuss the synthesis of the silver nanowires and silver nanoparticles in prismatic and hexagonal shapes through chemical processes [38]. The synthesis is carried out through two processes called as cases I and II. The methanol is employed in the case I as a solvent, which resulted in the formation of silver nanowires with diameter lying in 200–400 nm range at 70°C temperature. In the process II, using ethylene glycol (EG) as solvent resulted in the synthesis of silver nanowires of 50–190 nm in diameters and of lengths lying between 40 and 1000 µm along with smaller nanoparticles of 60–200 nm diameters in pyramidal and octahedral shapes. It is found that the use of solvent and reaction temperature played an important role in the formation of thinner silver nanowires and nanoparticles. The synthesized silver nanowires and nanoparticles are characterized by using X-ray diffraction (XRD), scanning electron microscopy, and UV–visible absorption spectroscopy. Photoluminescence emissions from the synthesized samples are recorded at room temperature at different excitation wavelengths varying widely in between 300 and 414 nm. With excitation by the radiation in the range of SPR (420 nm) for the nanowires prepared in the case II and with EG as solvent, the absorption of radiation does not favor for the radiative energy transfer to the emitting particles. The PL studies using a wide range of excitation wavelengths showed interesting visible luminescence emissions from the synthesized samples.

3.2.2  EXPERIMENTAL ARRANGEMENTS Chemical method has been employed for the synthesis of polyvinyl pyrrolidone (PVP) capped metallic silver nanowires and nanoparticles. The primary reaction of the used synthesis process involves the reduction of an inorganic salt at an elevated temperature. PVP is added as a stabilizer to prevent agglomeration of the colloidal particles. The versatility of this synthesis includes the ability of polyol to dissolve precursor salts (and ions), its highly temperature-dependent reducing power and its relatively high boiling point (for EG, it is about 196°C) [39]. In particular, the temperature-dependent reducing power of polyol helps synthesizing the colloidal particles over a broad range of sizes, as it gives the ability to control the nucleation and growth processes through careful regulation of reaction temperature. This wellknown method was adopted for the preparation of the silver metal colloids employing EG as the reducing agent. In the first process (case I) the silver nanowires were synthesized by a soft template liquid phase method. The solution (10 mL) of 0.001 M AgNO3 (Merck) in methanol and the solution (5 mL) of 0.002 M PVP (Merck) in methanol were taken in a 50 mL beaker. AgNO3 solution was prepared by sonication for 15 min to get a clear solution. The solution of PVP in methanol was then added to it slowly, at a rate of 0.2 mL per minute and the whole solution immediately turned into pale yellow. A

3.2 Synthesis and photoluminescence properties of silver nanowires

further addition of PVP solution turned the mixture into red. The reaction is continued at 70°C for 60 min. In the second process (case II), silver nanoparticles, silver nanowires, and nanorods were prepared through a polymer mediated polyol process. In this synthesis process, EG (10 ml) was first refluxed in a three-neck round-bottom flask, which was heated up to 210°C for 2 h. Then a solution of 0.025 M AgNO3 (99.98% pure) in EG (20 ml) and a solution of 0.050 M PVP in EG (20 ml) were simultaneously injected into the refluxing solvent, by using two syringes drop-wise at a rate of 0.2 mL per minute. Once the solution of AgNO3 and PVP in EG were added, the refluxing solution immediately turned pale yellow and gradually red. With further injection of the reactants, the solution became gradually gray–white, implying the formation of Ag nanoparticles and nanowires. The reaction was continued for 90 min at 210°C. After the completion of reaction the supernatant liquid was poured away and the gray precipitate remained inside the flask. The precipitation was done in an ice bath (0°C) for effective precipitation of the Ag nanowires. The collected aggregates were used for their characterization. The samples for the analysis with X-ray diffractometer (Philips-PANalytical) were prepared in a glass slide. The samples for the scanning electron microscope (SEM, Hitachi, S-3000 N) measurement were also prepared in the glass slides. The absorbance spectroscopy was carried out using a double-beam spectrophotometer (Hitachi, U-3010) and the PL spectra of these samples were studied by a spectrofluorimeter (F-2500 FL Spectrophotometer, Hitachi) at room temperature.

3.2.3  RESULTS AND DISCUSSION The formula of PVP repeated unit, with carbon atom numbering is shown in the inset of Fig. 3.6. The lone pair of PVP (oxygen and nitrogen) may be donated to Ag+ ions to construct a linear coordinative bond. Such bonding can effectively decrease chemical potential and enable the capping of PVP-bound Ag+ ions more easily. In case I, it was found that the color of the solution changed gradually from pale yellow to dark red. Fig. 3.6 illustrates a typical SEM image of as-separated Ag nanowires with diameter lying in 200–400 nm range and about 100 µm in length. Cohesive forces acting between the neighboring nanoparticles finally led to the formation of single crystalline nanowire. The longer Ag nanowire normally shows a curved shape, as observed in the SEM images. The reflux procedure at 70°C in methanol without stirring of the solution allowed a slow growth of silver nanowire in the presence of PVP. This could ultimately lead to the thicker and longer nanowires formation in the absence of any disturbance in the solution. The carboxyl oxygen atom (C@O) of PVP and Ag+ ions showed strong interaction resulting in the PVP capped Ag nanowires, which formed a complicated network depending on the nature of the solvent. In case II, the formation of thinner silver nanowires were visually observed through a continuous color change from pale yellow to red and finally to gray white upon the additions of the AgNO3 solution and PVP. The SEM image of the prepared samples through case is shown in Fig. 3.7A. It was found that at the early stages the

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FIGURE 3.6  SEM Image of the Ag Nanowires Prepared by the Case I and With Methanol as Solvent. Reprinted with permission from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

FIGURE 3.7 (A) The SEM image of the prepared samples through case II. The circles show the existences of the (B) prismatic and (C) hexagonal shapes of the prepared silver nanoparticles. Reprinted with permission from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

3.2 Synthesis and photoluminescence properties of silver nanowires

small Ag particles formed when the solution is allowed to boil for several minutes at 210°C, well above the boiling point of EG. The prepared nanowires exhibited the lengths ranging between 40 and 1000 µm and diameters in the range of 50–190 nm. In process II, the Ag nanoparticles with special morphologies, such as prismatic and hexagonal shapes (shown in the insets B and C of Fig. 3.7) were observed in addition to the thinner and longer Ag nanowires. In case I, at the initial stage a large number of silver ions were formed which acts as seed. Finally through slow reaction long and thicker nanowires were formed in relatively low viscous liquid like methanol at low temperature. On the other hand in the process II, when there is insufficient PVP and relatively high viscous liquid like EG, higher temperature played the important role in the formation of relatively thinner and longer nanowires and nanoparticles with different sizes and shapes. Fig. 3.8 shows the representative Energy dispersive X-ray spectroscopy (EDX) spectra of prepared samples. The crystallite phase and purity of as-synthesized nanowires were ascertained using X-ray diffraction method and the corresponding XRD pattern is shown in Fig. 3.9. The XRD pattern shows that the face-centered cubic phase and hexagonal phase of Ag coexist. The peaks appeared at 38degree, 44 degree and 65 degree were due to face-centered cubic phase corresponding to (1 1 1), (2 0 0), and (2 2 0) planes and other two peaks appeared at 45 degree and 51 degree due to hexagonal phase of silver corresponding to (1 0 3) and (1 0 4) planes. Fig. 3.10 shows the UV–visible absorption spectra of the samples prepared through cases I and II. For collecting the UV–visible absorption spectra, the samples were kept in a quartz cell of path length 1 cm. It was observed that the peak in the absorption spectra appeared at around 413 and 448 nm, respectively, when dissolved in methanol and EG, due to the SPR [40]. In general, the value of SPR wavelength is dependent on the shape and size of the nanoparticles as well as on the host characteristics. The red shift of the SPR wavelength of silver nanoclusters dissolved in EG

FIGURE 3.8  Energy Dispersive X-Ray Spectrum of the Prepared Silver Nanowire. Reprinted with permission from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

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FIGURE 3.9  X-Ray Diffraction Pattern of the As-Prepared Ag Nanowire. Reprinted from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

FIGURE 3.10  UV–Visible Absorption Spectra of the Prepared Samples. Curve marked as 1 corresponds to the samples prepared through case I and dissolved in methanol. Curves marked as 2 and 3 correspond to the samples prepared through case II and dissolved in EG and methanol, respectively. Reprinted with permission from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

3.2 Synthesis and photoluminescence properties of silver nanowires

is taken place due to the higher refractive index of EG compared to that of methanol and such effect is reported previously by Gomez et al. [41]. Photoluminescence emissions from Ag nanoparticles are studied by several authors [42–47] and reported the changes in luminescence spectra with conditions of preparation and annealing of the Ag clusters. However, no detailed study had been carried out so far for determining the variations of the PL spectra of silver nanowires due to the excitation by radiation close to the SPR of these clusters [48]. It may be relevant to note here that the position of the SPR peak of silver clusters strongly depends on the size of the nanoparticles. Thus the size of the dispersed silver nanoparticles in the solutions is a crucial factor for the PL behavior. In Figs. 3.11 and 3.12, the variations of PL emission from the prepared samples through case II and dispersed in methanol and in EG, respectively, under different excitation wavelengths ranging between 300 and 414 nm are shown. Curves marked as 1 (dotted) and 2 (solid) in Fig. 3.11 are for excitation wavelength of 390 and 400 nm, respectively. Also curves of Fig. 3.12 marked as 1 (solid), 2 (dotted), and 3 (dashed) are for excitation wavelength of 300, 390, and 414 nm, respectively. Blue– green PL emissions from the samples were observed under excitation wavelengths varying between 300 and 414 nm, which could be attributed to Ag+ and Ag+–Ag+ [49,50]. In analogy with PL spectra of other noble metals, these PL peaks in visible can be assigned to radiative recombination of Fermi level electrons and sp- or d-band holes [51]. It is seen from Figs. 3.11 and 3.12 that the position of PL peaks in the discussed experiments shifted toward the red in a regular manner with the increasing excitation wavelength. Basak et al. [52] had also shown the shift in the PL peak position toward longer wavelengths for Ag nanoparticles with increasing excitation

FIGURE 3.11  Room-Temperature Photoluminescence Spectra of the Sample Prepared Through Case II and Dispersed in Methanol. Curves marked as 1 and 2 are for excitation wavelengths of 390 and 400 nm, respectively. Reprinted with permission from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

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FIGURE 3.12  Room-Temperature Photoluminescence Emission Spectra of the Sample Prepared by Case II and Dispersed in Ethylene Glycol for Different Excitation Wavelengths. Curves marked as 1, 2, and 3 for excitation wavelength of 300, 390, and 414 nm, respectively. Reprinted with permission from R. Sarkar, P. Kumbhakar, A.K. Mitra, R.A. Ganeev, Synthesis and photoluminescence properties of silver nanowires, Current Appl. Phys. 10 (7) (2010) 853–857 [38], with permission from Elsevier.

wavelength in the range of 370–550 nm and speculated this phenomenon due to resonance between the luminescence transition and silver plasmons. With excitation by the radiation in the range of SPR (420 nm) for the nanowires prepared by case II, the absorption of radiation did not favor the radiative energy transfer to the emitting particles. This may be due to the reason of decrease of PL intensity in the SPR range. Probably these results show that the structure of silver nanowires can be modified in the dissolved state, and they partially coalesce in the form of spherical assemblies, which show the SPR peaks close to the conventionally observed SPR of the Ag nanoparticles.

3.3  OPTICAL PROPERTIES AND LUMINESCENCE OF COPPER NANOCLUSTERS IN ZnO 3.3.1  OVERVIEW OF FORMATION OF NANOPARTICLES IN ZnO Zinc oxide (ZnO) is an interesting wide and direct band gap II–VI semiconductor, which has a high exciton binding energy of 60 meV and the material is valuable as it has high radiation resistance. It has therefore been considered a promising candidate for the development of light-emitting structures and lasers of the blue and ultraviolet [53]. Moreover, ZnO is commonly used in various optoelectronic displays as a lowvoltage cathodo-luminophore with green luminescence [54]. Inclusion of nanoparticles in the material offers further optical responses including nonlinear effects linked

3.3 Optical properties and luminescence of copper nanoclusters in ZnO

to absorption and SPR [55]. To probe possible changes from inclusion of nanoparticles the discussed study [56] has included some preliminary studies of Cu nanoparticle formation in ZnO. The host material is both transparent and semiconducting and somewhat more stable than other wide band gap semiconductors such as GaN [57]. Prior to implantation ZnO is visibly transparent with a band gap at room temperature of ∼3.4 eV, and this increases at lower temperatures. ZnO is considered to be relatively radiation resistant but in the discussed work the injection of high concentrations of Cu to form the nanoparticles will inevitably result in considerable lattice damage. At the dopant concentrations required for nanoparticle production there may be for example displaced host metal (to offer Zn particles), formation of new alloy compounds, or precipitates of CuOx or CuZnO compounds, as well as the Cu particles. As all changes occur within  0) and, on the contrary, if the transmission maximum first appears and then minimum, the medium has self-defocusing properties (γ 3 × 1010 Wcm−2) did nanoparticles appear on the Si substrate, as it is displayed in Fig. 9.5A, C, and D for the different bulk metallic targets. For ablation with 10 ns pulses, the deposits appear as flat, smooth layers on the Si substrate (Fig. 9.5B) in the entire fluence range. It is important to note that the high-fluence

9.1 Ablation of nanoparticles and efficient harmonic generation

FIGURE 9.4 SEM images of the material deposited on Si substrates by ablation of targets containing (A, B)Ag, (C, D)Al, and (E, F) Cu nanoparticles ablated with (A, C, E) 160 ps (1 kHz) and (B, D, F) 10 ns (10 Hz) pulses. Reproduced with permission from R.A. Ganeev, C. Hutchison, M. Castillejo, I. Lopez-Quintas, F. McGrath, D.Y. Lei, J.P. Marangos, Ablation of nanoparticles and efficient harmonic generation using 1 kHz laser, Phys. Rev. A 88 (3) (2013) 033803 [6] with permission from American Physical Society.

conditions that ensured deposition of nanoparticles from bulk targets resulted in extremely inefficient HHG. As emphasized earlier, production of nanoparticles by laser ablation of metallic targets is a well-studied phenomenon (see, for example, [9,10]). However, the use of high ablation fluences results in the generation of large free-electron densities, which is a detrimental factor in HHG due to the electron contribution to the phase vector mismatch between the driving and harmonic waves. This explains why, under ablation conditions leading to nanoparticle production in the plume of bulk metallic targets, the HHG signals are extremely weak; in this case,

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FIGURE 9.5  SEM Images of Material Deposited on Si Substrates From Ablation at High Laser Intensity of Targets Constituted by Bulk (A, B) Ag, (C) Al, and (D) Cu Ablated With (A, C, D) 160 ps and (B) 10 ns Pulses. Reproduced with permission from R.A. Ganeev, C. Hutchison, M. Castillejo, I. Lopez-Quintas, F. McGrath, D.Y. Lei, J.P. Marangos, Ablation of nanoparticles and efficient harmonic generation using 1 kHz laser, Phys. Rev. A 88 (3) (2013) 033803 [6] with permission from American Physical Society.

the presence of nanoparticles in the plasma does not compensate for the deteriorated phase mismatch conditions caused by over-ionization and production of large electron densities. Notice another extreme case when, by ablation of silver at an intensity of 1013 Wcm−2, HHG has previously been attributed to newly generated Ag nanoparticles ([11], see also Chapter 8). The morphological characterization of ablation deposits was complemented by measurements of their absorption spectra. Those measurements, performed using the plasma plume debris collected on glass substrates, are illustrated in Fig. 9.6 for the case of silver. Fig. 9.6A shows the appearance of a strong absorption band in the vicinity of the 400–500 nm region for deposits obtained by ablation of 25 nm silver nanoparticles using 160 ps and 10 ns pulses. The absorption band in each case is ultimately associated with the SPR of this metal and its position and width are related with the shape, size, structure, and assembly of the nanoparticles [12]. In fact, for this metal, one can note that the SPR band appearing on the deposits obtained by ablation with 160 ps pulses is related with the presence of small-sized silver clusters (3–5 nm). The TEM images of the as-supplied nanoparticle powder shown in Fig. 9.2 indicate the presence of nanoparticles with smaller sizes than the nominal 25 nm. Thus the SPR band of these deposits reveals both the presence of

9.1 Ablation of nanoparticles and efficient harmonic generation

FIGURE 9.6 Absorption spectra of deposits generated from (A) an Ag nanoparticle-containing target and (B) bulk Ag for ablation by 160 ps (thick curves) and 10 ns (thin curves) pulses. Reproduced with permission from R.A. Ganeev, C. Hutchison, M. Castillejo, I. Lopez-Quintas, F. McGrath, D.Y. Lei, J.P. Marangos, Ablation of nanoparticles and efficient harmonic generation using 1 kHz laser, Phys. Rev. A 88 (3) (2013) 033803 [6] with permission from American Physical Society.

nanoparticles directly ejected from the target and of those generated by disintegration of larger ones. The shape of the SPR band for deposits produced by ablation with 10 ns pulses is broader and redshifted with respect to that of the 160 ps pulse-induced deposits, indicating the larger characteristic sizes of deposits grown with ablation using longer laser pulses. In the spectral region explored (i.e., above 350 nm), the deposits corresponding to ablation of Al nanoparticles did not show any characteristic absorption feature. This was most probably related with the sizes of the deposited nanostructures that, for values below ≈50 nm, should present a SPR band at shorter wavelengths (20 nm) and broadening of the size distribution of nanoparticles (bottom panel). The average size of nanoparticles increased up to ∼7 nm. The ablation of Ag nanoparticles at these three conditions caused a similar effect. At 0.4 J cm−2, the ∼15 nm nanoparticles dominated in the deposited surfaces (Fig. 9.16B, upper

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FIGURE 9.17 Mass spectra of ablated (A) bulk silver and (B) silver nanoparticles at different fluencies of heating pulses (see text). Reproduced with permission from R.A. Ganeev, Influence of a few-atomic silver molecules on the high-order harmonic generation in the laser-produced plasmas, J. Nonlin. Opt. Phys. Mater. 26 (1) (2017) 1750010 [46] with permission from World Scientific Publishing.

panel), while at higher fluence (0.7 J cm−2), the aggregates of larger-sized nanoparticles were frequently seen in the TEM images (bottom panel). The content of the plasmas produced during ablation of nanoparticles and bulk silver was analyzed using the ToFMS. The mass spectra of ablated bulk Ag are presented in Fig. 9.17A. The 3 ns pulses used for ablation during measurements of mass spectra were focused on the target surfaces to maintain the same fluence of heating radiation, which was used in the studies for the observation of efficient harmonic generation along a longer wavelength range of XUV. These mass spectra were dominated with the singly charged Ag1 ions (upper curve), while containing some amount of singly charged Ag2 molecules. The growth of heating pulse fluence above the optimal value (0.4–0.6 J cm−2) led to significant changes in the mass spectra. At 1.3 J cm−2, the presence of doubly charged Ag1 ions became significantly larger (bottom curve), while some additional low-mass species appeared in the spectra. The Ag2 molecules at these conditions nearly disappeared in the mass spectra. One can remind that at these conditions of plasma formation, the harmonic generation was significantly suppressed due to large amount of free electrons.

9.3 Influence of a few-atomic silver molecules

The mass spectrum of ablated Ag nanoparticles at 0.6 J cm−2 contained the lowmass ionized molecules (Ag1 to Ag3; Fig. 9.17B, upper curve) together with some low-mass and higher-mass components. The growth of heating fluence (1.2 J cm−2) caused significant changes in the mass spectra (bottom curve). Similar to bulk Ag, the dominant components became the singly charged and doubly charged Ag1 without the presence of larger clusters.

9.3.3 DISCUSSION Those studies were focused on the definition of the physical reasons of why application of silver plasma demonstrates it as an effective medium for the HHG. This medium has already been well explored in numerous publications. In particular, several studies were devoted to HHG in plasma plumes of silver or silver nanoparticles [49–54] Some studies explored HHG in silver plasmas under different conditions, like HHG from plasma multijets [55,56] and HHG by two-color laser pulses in plasma [57]. Thus, the interest to the silver-contained plasmas is quite high. The silver-containing plasma distinguishes from other metal plasmas. It allows generation of the strongest harmonics in the shorter wavelength region of XUV [49]. This prevalence of silver over other metals was not clearly explained during initial HHG studies, as no analysis of plasma components arriving in the area of interaction with femtosecond pulses was performed. Later, we analyze the results of HHG, TEM, and ToFMS studies of the ablated species in the case of bulk Ag and Ag nanoparticle targets. In discussed studies, the ablation of nanoparticle-containing targets by subnanosecond pulses showed the advanced properties of such plasmas. The application of single-color and two-color pumps at these conditions resulted in strong odd and even harmonic generation. The free electrons already existing in plasma and appearing during ionization by driving pulses may decrease the coherence length of higherorder harmonics. This process stops the accumulation of harmonic yield along the propagation of extended plasma. The density characteristics of the laser-produced plasma at the delays of 25–45 ns were estimated using the hydrodynamic code HYADES [58]. For the heating pulse intensity of 2 × 109 Wcm−2 (i.e., at a fluence of 0.8 J cm−2), the ionization level and the ion density of the plasma produced on the bulk silver were estimated to be 0.3 and 2 × 1017 cm−3, respectively. Thus, the influence of the free electrons appearing during plasma formation on the HHG conversion efficiency could not be underestimated. The increase of HHG conversion efficiency in previous studies of harmonic generation in the nanoparticle-containing plasmas was attributed to the change of the cross-section of these species compared with smaller-sized particles. The cross-section of recombination of the accelerated electron with the parent particle in the case of nanoparticles is higher compared with atoms. Still debated, this enhancement of harmonics was frequently observed in the ablation experiments using nanoparticle targets. The influence of the delay between pump and fundamental pulses on the efficiency of harmonic generation was analyzed in the case of bulk Ag and Ag nanoparticle

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plasmas. Initially, with a growth of the delay (5–25 ns), the intensity of harmonic radiation was increased. Further growth of the delay led to saturation (at 30–60 ns delay) and gradual decrease of harmonic intensity (at 100 ns delay). As was mentioned in previous chapters, the optimal delay depends on the target material. In the case of both the used silver-containing plasmas, the maximal harmonic yield was observed at 45 ns. The HHG at much longer delays (i.e., of the order of a few tens of µs) was not analyzed to observe the influence of large nanoparticles on the harmonic yield, as was reported in recent studies of the HHG in the ablated ZnS [59]. Later, we address the time dynamics of the movement of the small and large agglomerates of atoms during laser ablation. To better understand the physics behind intense HHG from LPP, the first step would be the identification of the silver species that are responsible for the HHG. This is especially important in the case of ablated nanoparticles where the composition of the plume is complex, typically involving not just nanoparticles, but also large molecules, clusters, atoms, and ions. As expected from the three-step model of HHG, clusters can increase the efficiency of HHG in laser plasmas due to the larger cross-section of recombination of the accelerated electron with the parent particle. The laser pulses can be used to influence the outcome of ablation and ionization of nanomaterials either by acting on the laser/material interaction dynamics [60,61] or by controlling the phase state of the ablation plasma [62,63]. At these conditions, the separation of small molecules containing a few atoms of silver from the whole nanoparticle may lead to the appearance of the former species above the ablating surface much earlier compared with heavy nanoparticles, which has been shown in the discussed mass spectroscopy studies of the ablated nanoparticles. The expansion of laser-produced nanoparticle plumes in vacuum was studied in [64]. The time-resolved CCD images have distinguished two features of component expansion generated by plasma and nanoparticle plumes separated in time. These images showed that the emission from atomic plume persisted at ∼400 ns, while the nanoparticle-containing plumes persisted for a much longer time. The estimations of expansion velocity showed that the nanoparticle plume moved at a significantly lower velocity compared to the plasma plume dominantly containing single particles and the expansion velocities differed by ∼25 times. One can assume the absence of large (3–20 nm) nanoparticles in the area of ∼0.3 mm above the ablated surface shortly (30–50 ns) after the beginning of ablation. The only multiparticle species here were the few atoms containing species. Correspondingly, their presence and participation in light-matter interaction allowed the achievement of the high yield of harmonics. As it was mentioned, there are three different population species with specific velocities in the expanding plasma [65]. The ion population is the fastest (8 × 104 m s−1) and is attributed to Coulomb explosion. A neutral population follows adiabatic expansion with a similar or smaller velocity. Finally, the nanoparticle population has the slowest velocity. Depending on the sizes of nanoparticles, there is a wide range of the velocities of these species. Lightest clusters (i.e., those containing a small amount of atoms) possess only a few times slower velocity compared with the

9.3 Influence of a few-atomic silver molecules

single atoms and ions. Contrary to them, large nanoparticles, which contain 104–106 particles, arrive in the area of interaction (∼0.3 mm above the target surface) a few tens of microseconds from the beginning of laser ablation. Thus, only small clusters can be involved in the process of HHG, which demonstrated a notable enhancement of harmonic yield. One can assume from this consideration that large nanoparticles do not participate in the process of HHG, but rather become the source of the formation of small clusters. The partial disintegration of large nanoparticles during heating, melting, and evaporation causes the formation of the notably tiny clusters containing small amount of atoms, which became the main emitters of harmonics. We assume that the discussed results may not be crucial for direct demonstration of the role of small molecules or small clusters for achieving the enhancement of HHG in silver plasmas. To prove that, a systematic investigation of the role of sample sizes in HHG yield should be studied at the same experimental conditions. Furthermore, the results obtained in different targets containing dispersion of nanoparticles with different average sizes and different cluster densities should be compared. Even in that case, the direct confirmation of the crucial role of small molecules or small clusters in the enhancement of harmonics should be followed with simultaneous experiments including both HHG and ToFMS in the same chamber and at the conditions exactly corresponding to the highest conversion efficiency of harmonics. Moreover, ToFMS measurements should provide information about the presence of neutral clusters in the plasma, which is a nontrivial task. One should remind that time-of-flight method allows observation of mass/charge distribution of ionized species. To retrieve the data on the presence of neutrals, one has to use additional, preferably strong UV source for ionization of species in the volume of the ToFMS chamber. Only in that case, one may draw a conclusive picture of the exceptional role of a few atoms of larger species in the HHG in silver plasma plumes, especially compared with large nanoparticles. Unfortunately, the complexity of such studies did not allow to carry out such experiments in a single set of measurements. Moreover, one has to understand that both the analysis of nanoparticles deposited on substrates near the plume and the mass spectra reported in discussed work does not provide enough information about phenomena occurring during the laser-plasma interaction, as they can only attest the final state of the debris after this interaction occurred, as well as characterize the presence of different ionized species in the plasma, which is not exactly similar to the one used in the HHG experiments containing dominantly neutral components. Anyway, the qualitative coincidence of the highest conversion efficiency and appearance of small ionized molecules may draw the conclusion on the certain influence of these components of plasma on overall yield of harmonics. ToFMS diagnostics was applied up to 10,000 mass/charge units. It was sufficient for detection of clusters containing up to 100 atoms of silver. The 5 nm clusters contain ∼27,000 atoms. Obviously, it was not possible to analyze such a large species using the ToFMS equipment. However, there is no need to register such large nanoparticles, as, as was underlined, they do not participate in HHG due to their

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CHAPTER 9  Peculiarities of high-order harmonic generation

absence in the plasma plume at the moment of femtosecond pulse propagation. Only a few-atoms containing species can be involved in the HHG due to aforementioned reasons. TEM data just indicate the presence of large nanoparticles in the plasmas during the entire stage of plasma spreading out of target surface. These data cannot help in indicating of the involvement of small-sized species in the process of HHG. The density of ablated material was estimated to be ∼6 × 1017 cm−3 in the case of bulk silver. One can assume that the ionization state of plasma at the moment of HHG was similar to the one defined using the hydrodynamic code HYADES, as during the first few tens of nanoseconds no significant changes in the charge state could be expected at the used conditions of ablation. The increased conversion to harmonics cannot be explained by the increased total density of the ablated material in the case of nanoparticle-coated targets because in that case a weaker ablation was used. The ablation of nanoparticle-containing target could be maintained at smaller intensity of heating pulses compared with the case of bulk silver. Thus, the optimal ablation conditions, at which the highest harmonic yield was obtained in the case of nanoparticlecoated target, were achieved at rather weaker intensity of heating pulse. The velocity ∼4 × 103 m s−1 corresponds to the fast components responsible for the optimal harmonic conversion, and not to the large clusters and nanoparticles. ToFMS diagnostics showed Ag2 ions and in some cases Ag3 ionized molecules and no larger clusters, though those can be presented in the spreading plasmas in the form of neutrals. Actually, the observed components are two- and three-atomic species, which can hardly be called low-mass clusters. So, one can assume that these molecules, to some extent, could be the sources of increased conversion. Some more diagnostics need to be done to better understand the role of different plasma species in the HHG. Especially in case of proving the statements about the effect of clusters (or molecules) the density of the ablated material must be measured at the time of the generation of harmonics, either, for example, by interferometry or by light scattering. Even when knowing the bulk density, the size of the clusters will be difficult to estimate. It has to be cleared as well, whether the interaction occurs in plasmas or in a cloud containing neutral atoms, molecules and fragments. Further studies are needed to quantitatively prove the assumptions drawn from the coincidence of two effects, that is, the presence of small ionized molecules of silver and the enhancement of harmonic yield.

9.4  RESONANCE-ENHANCED HARMONIC GENERATION IN NANOPARTICLE-CONTAINING PLASMAS 9.4.1  ROLE OF RESONANCES IN HARMONIC ENHANCEMENT As it was seen, the excellent nonlinear optical properties of nanoparticles may cause improvement of high-order harmonic efficiency. Initially, studies of nanoparticleinduced harmonic generation were limited to exotic clusters of noble gases. The

9.4 Resonance-enhanced harmonic generation

physical origin of this process in the gas clusters was mostly related to standard atomic harmonic generation, modified by the fact that the atoms in nanoparticles are close to each other. The mechanisms of HHG from gas nanoparticles have been analyzed in a few studies and the ionization and recombination to the same ion, to neighboring ions and to the whole nanoparticle have been compared. Meanwhile, the experiments with gas nanoparticles have revealed some difficulties in disentangling the harmonics produced by different species, such as monomers and nanoparticles of different sizes. As it was seen from previous sections of this chapter, similar studies of HHG in laser-produced plasmas consisting of nanoparticles or monomers showed that, in equivalent experimental conditions, the former species provide a considerably stronger lower-order harmonic yield, thus indicating the advantages of the application of ablated nanoparticles for harmonic generation in the longer-wavelength range of extreme ultraviolet [66]. Currently, it is possible to maintain laser-produced plasmas containing different nanoparticles due to the availability of various metal-based nanoparticle powders. The enhancement of the harmonics in nanoparticle-containing plasmas compared to monomer-containing plasmas has been studied in various laboratories. Meanwhile, an additional option for the amendment of HHG could be the resonance enhancement of a single harmonic in nanoparticle-containing plasmas. There are no fundamental restrictions in the resonance enhancement independent of the size of the harmonic emitters. To achieve the resonance-induced growth of harmonics from large particles one has to find the conditions of optimal plasma formation during ablation of the nanoparticle-containing targets. In this case one can simultaneously observe two mechanisms of the growth of harmonic yield (i.e., nanoparticleinduced and resonance-induced enhancement of harmonics). In this section, we discuss the results of the research on three nanoparticlecontaining plasmas (In2O3, Sn, and Mn2O3) allowing the efficient generation of lower-order harmonics and the resonance enhancement of single (13th, 17th, and 33rd) harmonics of Ti:sapphire laser [67]. The comparison of the harmonic spectra obtained using the plasmas produced on the bulk and nanoparticle materials of the same origin has shown that the enhancement of single harmonics in nanoparticlecontaining plasmas occurs predominantly in the case of strong excitation, while in the case of bulk targets this enhancement can be obtained using both weak and strong fluences of heating pulses. The experimental scheme was similar to the one described in [68] and previous sections. 802 nm, 370 ps, 10 Hz repetition rate uncompressed pulses of a Ti:sapphire laser were used to ablate the targets (Fig. 9.18). In2O3, Mn2O3, and Sn nanoparticles were used as the ablating targets. The powders of these nanoparticles were glued on the 5 mm long glass plates and then installed in the vacuum chamber. The harmonic spectra also studied from the ablation of 5 mm long In, Mn, and Sn bulk targets. The manufacturer stated that the sizes of the Mn2O3 nanoparticles were in the range of 40–60 nm, the Sn nanoparticles had sizes