Dissertations in Forestry and Natural Sciences
DISSERTATIONS | DMITRII KLIUKIN | FEMTOSECOND LASER MODIFICATION OF... | No 318
DMITRII KLIUKIN
FEMTOSECOND LASER MODIFICATION OF PHOTO-THERMO- REFRACTIVE GLASS WITH VOLUME BRAGG GRATINGS
PUBLICATIONS OFTHE UNIVERSITY OF EASTERN FINLAND
uef.fi
PUBLICATIONS OF
THE UNIVERSITY OF EASTERN FINLAND Dissertations in Forestry and Natural Sciences
ISBN 978-952-61-2911-2 ISSN 1798-5668
DMITRII KLIUKIN
PUBLICATIONS OF THE UNIVERSITY OF EASTERN FINLAND DISSERTATIONS IN FORESTRY AND NATURAL SCIENCES
N:o 318
Dmitrii Kliukin
FEMTOSECOND LASER MODIFICATION OF PHOTO-THERMO-REFRACTIVE
GLASS WITH VOLUME BRAGG GRATINGS
ACADEMIC DISSERTATION
To be presented by the permission of the Faculty of Science and Forestry for public examination in the Auditorium M103 in Metria Building at the University of Eastern Finland, Joensuu, on October 5th, 2018, at 12 o’clock.
University of Eastern Finland Department of Physics and Mathematics
Joensuu 2018
Grano Oy Jyväskylä, 2018
Editors: Pertti Pasanen, Matti Tedre, Jukka Tuomela, and Matti Vornanen
Distribution:
University of Eastern Finland Library / Sales of publications julkaisumyynti@uef.fi
http://www.uef.fi/kirjasto
ISBN: 978-952-61-2911-2 (print) ISSNL: 1798-5668
ISSN: 1798-5668 ISBN: 978-952-61-2912-9 (pdf)
ISSNL: 1798-5668 ISSN: 1798-5668
Author’s address: University of Eastern Finland
Department of Physics and Mathematics P.O. Box 111
80101 Joensuu Finland
email: dmitrik@uef.fi
Supervisors: Professor Nikolay Nikonorov ITMO University
Photonics and Optoinformatics 199034 St. Petersburg
Russia
email: nikonorov@oi.ifmo.ru Professor Yuri Svirko
University of Eastern Finland
Department of Physics and Mathematics P.O. Box 111
80101 Joensuu Finland
email: yuri.svirko@uef.fi
Reviewers: Professor Amin Abdolvand
University of Dundee
School of Science and Engineering DD1 4HN Dundee
United Kingdom
email: a.abdolvand@dundee.ac.uk Professor Valentina Zhurikhina Saint Petersburg Academic University 194021 St.Petersburg
Russia
email: zhurikhina@spbau.ru
Opponent: Professor Christian Rüssel
Friedrich Schiller University Jena 07743 Jena
Germany
email: Christian.Ruessel@uni-jena.de
Dmitrii Kliukin
Femtosecond laser modification of photo-thermo-refractive glass with volume Bragg gratings
Joensuu: University of Eastern Finland, 2018 Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences No: 318
ABSTRACT
This work is dedicated to femtosecond laser modification of chloride photo-thermo- refractive glass, which contains silver nanoparticles at high concentration. This com- posite materials was developed to record high diffraction efficiency volume Bragg gratings. Femtosecond laser processing allows one to destroy the nanoparticles and increase transmission of the chloride photo-thermo-refractive glass in visible and near IR spectral range.
The most important result obtained in this Thesis is transformation of the ampli- tude -phase volume Bragg grating into pure phase one. It is shown experimentally that bleaching of silver nanoparticles leads to a slight decrease of the index mod- ulation in the volume grating accompanied by a considerable suppression of the absorption coefficient. That is the femtosecond laser processing results in drastic decrease of the absorption losses, while diffraction efficiency remains nearly the same. It is also demonstrated that the bleaching of silver nanoparticles in chloride photo-thermo-refractive glass results in formation of silver clusters in the irradi- ated area. These clusters contain only few atoms and show fluorescence in a wide spectral range that spans from visible to near IR.
In order to estimate heating of the Volume Bragg gratings recorded in chloride photo-thermo-refractive glass, we irradiate the grating with a high-power near IR laser diode beam. This study revealed that before bleaching temperature of the irradiated grating can be as high as 350oC. Such a high temperature significantly changes grating parameters and results in decrease of the diffraction efficiency.
However, after irradiation of the bleached grating the temperature of the grating did not exceed 50oC at the same light intensity. This experimental finding paves the way towards using the bleached grating for intense laser beams.
Universal Decimal Classification:535.3, 535.4, 535.8, 544.1, 538.9 Keywords:Femtosecond laser, volume Bragg gratings, bleaching, glass
ACKNOWLEDGEMENTS
I would like to thank my supervisors Nikolay Nikonorov and Yuri Svirko for pa- tience and support during my study. It was pleasure for me working with you.
Many thanks to Martti Silvennoinen for countless hours spent in the lab rebuild- ing optical setup and discussing different aspects of optics, laser physics and pro- gramming. I am also thankful to Sergey Ivanov for fruitful discussions regarding mysteries of holography. I am grateful to Alexander Sidorov for motivation and special perspective on science. Thanks to PhD students Sergey Malykhin, Igor Re- duto, Semen Chervinsky, Maxim Stolyarchuk, Evgeny Sgibnev and Viktor Dubrovin for sharing experience and great fun. Thanks to my family and friend for support and respect of my choices. And finally, I want to express my deepest and warmest gratitude to my lovely fiancee Alisa for care and tenderness.
Joensuu, February 27, 2018 Dmitrii Kliukin
LIST OF PUBLICATIONS
This Thesis consists of the present review of the author’s work in the field of optical phenomena and the following selection of the author’s publications:
I Klyukin, D., Silvennoinen, M., Krykova, V., Svirko, Y., Sidorov, A., Nikonorov, N. (2017). Fluorescent clusters in chloride photo-thermo-refractive glass by femtosecond laser bleaching of Ag nanoparticles. Optics Express, 25(11), 12944.
II Klyukin, D., Krykova, V., Ivanov, S., Obraztsov, P., Silvennoinen, M., Nikonorov, N. (2017). Volume Bragg gratings in chloride photo-thermo-refractive glass af- ter femtosecond laser bleaching. Optical Materials Express, 7(11), 4131.
III Klyukin, D., Ivanov, S., Krykova, V., Silvennoinen, M., Svirko, Y., Nikonorov, N. (2018). Thermal stability of volume Bragg gratings in chloride photo- thermo-refractive glass after femtosecond laser bleaching. Optics Letters, 43(5), 5–8.
IV Klyukin, D., Dubrovin, V., Pshenova, A., Putilin, S., Shakhverdov, T., Tsyp- kin, A., Nikonorov, N., Sidorov, A., Nikonorov, N. (2016). Formation of lu- minescent and non-luminescent silver nanoparticles in silicate glasses by NIR femtosecond laser pulses and subsequent thermal treatment: the role of halo- genides. Optical Engineering, 55(6), 067101.
Throughout the overview, these papers will be referred to by Roman numerals. In addition, the author has the following peer-reviewed journal papers
• Pshenova, A. S., Klyukin, D. A., Nashchekin, A. V., Sidorov, A. I. (2017). Migra- tion of silver on the nanoporous glasses surface under the action of an electric field. Applied Optics, 56(10), 2821–2825.
• Klyukin, D. A., Khmelev, A. Y., Pshenova, A. S., Sidorov, A. I., Fedorov, Y. K.
(2016). Multicolour laser recording of optical information in silicate glasses with europium, silver and cerium ions. Quantum Electronics, 46(10), 930–934.
• Klyukin, D. A., Sidorov, A. I., Ignatiev, D. A., Ignatiev, A. I., Nikonorov, N. V.
(2014). Influence of mechanical deformation and heat treatment on the state of silver nanoparticles in photo-thermo-refractive glasses. Materials Physics and Mechanics, 22(1), 39–43.
• Egorov, V. I., Zvyagin, I. V, Klyukin, D. A., Sidorov, A. I. (2014). The formation of silver nanoparticles on the surface of silver-containing glasses when they are irradiated with nanosecond laser pulses. Journal of Optical Technology, 81, 270–274.
• Klyukin, D. A., Sidorov, A. I., Ignatiev, A. I., Nikonorov, N. V. (2014). Lumines- cence quenching and recovering in photo-thermo-refractive silver-ion doped glasses. Optical Materials, 38, 233–237.
• Klyukin, D. A., Sidorov, A. I., Nikonorov, N. V., Silvennoinen, M., Svirko, Y. P., Ignatiev, A. I. (2015). Formation of luminescence centers and nonlinear optical effects in silver-containing glasses under femtosecond laser pulses. Optics and Spectroscopy, 119(3), 456–459.
• Ignatiev, A. I., Klyukin, D. A., Leontieva, V. S., Nikonorov, N. V., Shakhverdov, T. A., Sidorov, A. I. (2015). Formation of luminescent centers in photo-thermo- refractive silicate glasses under the action of UV laser nanosecond pulses. Op- tical Materials Express, 5(7), 1635.
• Pshenova, A. S., Klyukin, D. A., Sidorov, A. I., Andreeva, O. V. (2016). Porous glasses with silver nanoparticles as the sensitive material for sensors to mea- sure the index of refraction of analytes. Journal of Optical Technology, 83(7), 438–440.
and International conference proceedings
• Klyukin, D. A., Pshenova, A. S., Sidorov, A. I., Stolyarchuk, M. V. (2016). X-ray- induced fluorescent centers formation in zinc- phosphate glasses doped with Ag and Cu ions. Journal of Physics: Conference Series, 741, 012125.
• Ignatiev, A. I., Ignatiev, D. A., Klyukin, D. A., Nikonorov, N. V., Nuryev, R.
K., Sidorov, A. I. (2016). Influence of 532 and 355 nm nanosecond laser pulses on photodestruction of silver nanoparticles in photo-thermo-refractive glasses.
In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (pp. 241–245).
related to this PhD Thesis.
AUTHOR’S CONTRIBUTION
The author carried out femtosecond laser processing of glass samples, did program- ming of SLM and linear stages, prepared manuscript for publication in articlesI-IV.
In articlesI,III andIVauthor did characterization of the sample using optical and fluorescent spectroscopy. In articleIII author improved optical setup for studying of thermo-optical properties of PTR glass. Author proposed idea of articlesI and III.
TABLE OF CONTENTS
1 Introduction 1
2 Volume Bragg gratings 5
2.1 Surface diffraction gratings... 5
2.2 Optical properties of VBG... 7
2.3 Materials for recording of VBG... 8
2.4 Applications of VBG... 11
2.5 Coupled wave theory... 14
3 Laser bleaching of silver nanoparticles 21 3.1 Optical properties of metal nanoparticles... 21
3.2 AgNP laser modification... 24
4 Experimental 29 4.1 PTR glass synthesis... 29
4.2 VBG recording... 29
4.3 VBG bleaching... 30
4.4 VBG characterization ... 30
5 Results and Discussions 33 5.1 Bleaching of chloride PTR glass... 33
5.2 Fluorescence of silver clusters in PTR glass... 36
5.3 Bleaching of VBGs... 40
5.4 Heating of VBG by high-power laser diode... 40
6 Conclusions 45
BIBLIOGRAPHY 47
1 Introduction
Over the last decades laser diodes (LD) have become sufficiently stable and effi- cient to be involved in material processing, pumping of solid-state lasers, Raman spectroscopy and operation of various telecommunication devices. Correspondingly during last decades, a variety of approaches has been developed for increasing laser output power, improving optical and spectral quality of laser beam and reducing the cost.
Pumping of alkali-vapor laser usually is performed by high-power LD operating at visual or near-IR spectral range. However, since alkali-vapor absorption line is very narrow the pumping efficiency is rather low because the width of the LD emis- sion band exceeds several nanometers. Increasing the pumping efficiency requires narrowing of the LD emission spectrum.
A conventional technique for narrowing LD emission spectrum is the stabiliza- tion when an external mirror selecting a particular cavity mode serves as cavity output allowing coupling energy from other modes. Moreover, stabilized LD pos- sess less drift of output wavelength caused by heating. Recently, reflective volume Bragg gratings (VBG) have been proposed as external mirror of diode, fiber and disk laser cavities [1]. Thanks to unique optical properties reflective VBGs can serve as dielectric mirror with narrow reflection spectrum.
On the other hand, output power of contemporary LD, fiber and disk lasers is limited to tens of several kilowatts, which is far from industry demands. Combining several laser beams is promising way to achieve up to tens of kilowatt of laser power.
An efficient way to combine multiple laser beam was demonstrated with transmis- sive and reflective VBGs recorded in PTR glass [2]. Spectral beam combining allows to sum up power of two laser beams with carefully chosen wavelengths using sin- gle VBGs. The number of combined laser is scalable, but the complexity of system grows dramatically with lasers number because each grating requires temperature stabilization. A great benefit of VBGs recorded in PTR glass is that several gratings can be recorded in the same substrate, which greatly simplifies design and align- ment of optical system a lot (Figure 1.2) [3]. However, beams combining provides output beam with relatively broad spectrum of partially coherent light. Another ap- proach is coherent beam combining utilize VBGs to reflect light from two different active media toward mutual output mirror. So, two laser resonators become coupled and output beam is almost fully coherent.
Trying to fulfill high demand for VBG with excellent optical quality and high diffraction efficiency a lot of efforts were taken during last 50 years in order to develop optical material for VBG recording. The main drawbacks of conventional holographic media based on photosensitive polymers limited ray stability due to low damage threshold and poor chemical stability. In contrast, silicate glass does not have these problems and allows to achieve much higher intensity. Approximately 30 year ago new type of photosensitive silicate glass were developed, where VBG recording carried out via photo-thermo-induced crystallization process in the glass volume. This class of photosensitive glass was called photo-thermo-refractive (PTR) glass [4]. Classical PTR glass possessed outstanding optical properties, which is
Figure 1.1: Spectral beam combining using four multiplexed gratings for different laser wavelength with 2 nm separation [3].
crucial for VBG recording with up to 100 % diffraction efficiency and high power operation.
Figure 1.2: Example of data storage using AgNPs laser bleaching with controlled polarization. (a) Optical image of all regions. (b)–(d) Cross polarized images with different orientation: (b) rotated by 20o with respect to the sample; (c) 45o, and (d) 70o. [5].
The key feature of classical PTR glass is NaF crystal phase, which is formed in glass volume under laser irradiation. The crystals of average size 100 nm are formed in irradiated area of glass. However, recently the family of PTR glass was extended. Several new types of PTR glass were synthesized and the photo-thermo- induced crystallization of NaCl-AgCl, AgBr and CaF2was shown [6–8]. Moreover,
the recording of VBG with high diffraction efficiency was demonstrated for PTR glass with halide crystal phase [9].
One of the main disadvantages of PTR glass is residual absorption of silver nanoparticles (AgNPs) remaining in the glass volume after VBG recording [10].
They are formed during thermal treatment of glass in irradiated area and serve as centers of crystallization of crystal phase responsible for the grating formation [6].
However, AgNPs absorption coefficient in visible spectral range varies from 1 to 100 cm−1 depending on type and composition of the PTR glass. Especially high absorption of AgNPs in chloride PTR glass limits their application for high power laser stabilization and beam combining. The tail of AgNPs absorption band causes considerable heating of VBG even for near IR light. Thus, it is highly desirable to reduce of AgNPs concentration in the glass after VBG recording.
The process of metal particles modification and photodestruction in glass ma- trix by pulsed lasers was studied extensively during last two decades [11]. Ultra- short laser pulses allow one to reshape AgNPs thus changing optical properties of nanocomposite glass. Such a reshaping (e.g. elongation) can be used for data storage, where information is encoded into orientation of elongated particles with respect to the beam polarization (Figure 1.2). Moreover, the irradiated nanocompos- ite glass slabs can serve as broad band polarizers [11].
Since the AgNPs modification under irradiation with femtodecond laser pulses is well understood and can be described in the framework of the three temperature model [11], this approach can be applied for bleaching of VBG in PTR glass. Pre- viously there were only few attempts to study laser bleaching of PTR glass with AgNPs in volume. The measurements of kinetics of AgNPs bleaching using green nanosecond laser pulses in conventional PTR glass showed thermo-optical nature of 2 particle photodestruction process [12].
In this Thesis, we examine optical properties of VBGs in chloride PTR glass, in which AgNPs dominate light absorption. By using ultrashort laser micromachining of the PTR glass we control modulation of the absorption coefficient and refractive index in the grating via photodestruction of AgNPs. By studying effect of the light- induced modification of the VBG by absorption spectroscopy, luminescence mea- surements and x-ray diffraction we propose a model of AgNPs photodestruction in chloride PTR glass.
2 Volume Bragg gratings
In this chapter, we discuss the optical properties of VBGs, which are capable to effi- ciently control angular and frequency spectra of laser radiation. High angular selec- tivity of VBGs enables spatial filtering providing high laser beam quality. Moreover, VBGs possess a high frequency selectivity, which can be used for laser stabiliza- tion and combining of laser beams. Outstanding optical, chemical and mechanical properties of photo-thermo-refractive glass have attracted a particular interest for fabrication VBGs.
2.1 SURFACE DIFFRACTION GRATINGS
A diffraction grating comprises of a collection of reflecting (or transmitting) ele- ments separated by a distance comparable to the wavelength. The fundamental characteristics of diffraction grating is spatial amplitude or phase modulation of the incident electromagnetic filed. It results in several diffraction orders of light after the interaction with the grating. The diffraction grating performance depends on its period, light wavelength and angle of incidence, which determine the diffrac- tion pattern. The diffraction orders of the grating can be found from the grating equation [13]:
d(sin(θm)±sin(θi)) =mλ (2.1) wheredis period of the grating, m is the order of diffraction,θiandθmare angles of the incidence andmthdiffraction order, respectively,λis wavelength of the incident electromagnetic wave. In Eq. 2.1, ”+” and ”-” signs correspond to transmissive and refractive gratings, respectively. θiis positive andθmis negative if the diffracted and incident beams are on different sides of grating surface normal.
Figure 2.1 shows different types of diffraction gratings used in photonics. Diffrac- tion gratings are conventionally divided on reflective and transmissive ones depend- ing on which side of the grating diffracted beam comes out. Specifically, in reflective gratings, the incident and diffracted beams are from same side, while in transmis- sion gratings, the incident and diffracted beams propagate at opposite sides of the grating. The periodic structure can be fabricated either on the surface of the grating or within its volume.
Surface relief diffraction gratings are the most common type of gratings and include ruled (blazed) and holographic gratings. Ruled gratings possess a high diffraction efficiency and are optimized for a specific wavelength. However, they suffer from periodicity errors, such as ghosting. Moreover, sensitive measurements can be affected by considerable amount of scattered light. On the other hand, holo- graphic gratings allow one to decrease ghosting effect by sacrificing diffraction effi- ciency. One of the key advantages of surface relief diffraction gratings is that they are relatively easy to fabricate.
Master ruled gratings can be fabricated by diamond tool scanning along the metal or dielectric surface. Then master grating can be replicated many times by imprinting it onto different substrates. The profile of the grating can vary signifi-
Figure 2.1: Classification of diffraction gratings commonly used in pulse compressor systems [14].
cantly depending on the application. The most common shape of cross-section is saw-tooth with different angle of teeth tilt.
A photolithography is used for fabrication of holographic gratings, in which periodic structure is produced by exposure of interference pattern in photosensitive layer on the substrate surface and following etching of the developed fringes. The most common profile of the holographic diffraction grating is sinusoidal. However, photolithography allows fabrication of virtually arbitrary grating profile.
Blazed diffraction gratings have many configurations for different applications.
One of them is Littrow configuration (Fig. 2.2), where the angle of incidence is equal to the angle of diffraction in particular order, which is equal to the blaze angle at the design wavelength. The diffraction efficiency of that blazed grating is maximal for Littrow configuration. Such diffraction grating found application for laser stabilization and laser tuning [13, 15]. However, for high power laser diodes such diffraction gratings have relatively low laser induced damage threshold and the external optical setup is rather bulky. One of possible solutions for this problem is to employ volume diffraction gratings.
Figure 2.2: Littrow configuration of laser diode stabilization. [15]
2.2 OPTICAL PROPERTIES OF VBG
VBG can be written inside bulk transparent optical materials by creating harmonic modulation of the complex refractive index. Usually, the required amplitude of refractive index modulation is of the order of 10−3at the optical slab thickness of >1 mm.
Figure 2.3: (a) Transmissive and (b) reflective VBG.
There are transmissive and reflective VBGs. With transmissive volume Bragg gratings (TVBG) (Fig.2.3a), incident and diffracted light beams propagate at opposite surfaces of the glass slab. Light passes through the grating and is partially coupled to non-zero diffraction order. In contrast, with reflective volume Bragg gratings (RVBG) (Fig. 2.3b), the incident and diffracted beams propagate at the same surface of the glass. That is RVBG resemble a dielectric mirror yet having much larger number of layers with lower refractive index modulation and narrower reflection spectrum.
Fig. 2.4 and 2.5 summarizes key optical properties of RVBG and TVBGs. RVBGs have narrower spectral detuning with typical value 200 pm and can be as small as 20 pm. In contrast, a typical value of bandwidth for TVBG is on the range of sev-
Figure 2.4: Typical (a) spectral and (b) angular selectivity of reflective VBG. [16]
Figure 2.5: Typical (a) spectral and (b) angular selectivity of transmissive VBG. [16]
eral nanometers. However, TVBGs possess very high spatial selectivity, which is typically in the range 0.1-10 mrad. In opposite, the angular selectivity of RVBGs is limited by 10-100 mrad, which again makes it closer two dielectric mirrors. Theoret- ical evaluation of the VBGs main parameters will be considered in the next section.
2.3 MATERIALS FOR RECORDING OF VBG
The main requirement for optical materials to be used for VBG recording is pho- tosensitivity, which can be quantified by the amount of power required to produce desired modulation of refractive index and/or absorption coefficient of the mate- rial. Photochemical properties also determine spatial resolution and dynamic range of recorded volume holograms. Moreover, photosensitive materials are required to possess excellent optical quality, high homogeneity and chemical stability, which are usually in high demand in high precision and high power application. There are a few materials suitable for the recording of thick VBGs with large diffraction efficiency.
VBGs recorded in lithium niobate possess refractive index modulation as high as 0.002 [17]. Together with high transparency range spanning from 0.4-5.0 µm it
(a) (b)
Figure 2.6: (a) Scanning technique using phase mask with ultrafast laser pulses for VBGs recording [24]. (b) Schematics of experimental setup for Gauss-Bessel beam generation and its downsizing by a telescope in volume of fused silica. [25]
allows one to produce VBGs with diffraction efficiency up to 99 % [18]. However, lifetime of such gratings is limited by several years. After that period the grating’s optical properties weaken considerably. Several attempts to overcome the problem were made using dopants and different thermal treatment regimes [19]. Unfortu- nately, diffraction efficiency of such gratings does not exceed 77 % [20]. It worth noting that lifetime for VBGs recorded in PTR glass under study is virtually unlim- ited, because the crystal phase grown in glass volume is not affected by any chemical or mechanical changes.
Another material for VBGs recording is calcium fluoride (CaF2). The main ad- vantage of VBGs recorded in CaF2is that its transparency region spans from 0.15 to 9.5µm. Photosensitivity of CaF2 crystals can be achieved via thermal treatment in reduction atmosphere [21] that creates color centers (F-centers) with a sharp ab- sorption resonance in the visible range (550-620 nm). After holograms recording at 150-200oC temperature there has been observed both absorption (8 cm−1) and refractive index modulation (10−5), which provides maximum diffraction efficiency as low as 20% [21].
Porous glass was widely considered as a promising material for VBGs recording in 1980th. In porous glasses, silica matrix is embedded with connected voids and channels, which size varies from 5 nm up to tens nanometers. Silica matrix has wide transparency from 0.17 to 2.0 µm, possess excellent thermal resistance and is chemically stable. Pores in bulk glass can be filled with various materials (gelatin, AgCl, photopolymers etc.) in order to enhance photosensitive properties for holo- gram recording with high diffraction efficiency [22, 23]. However, surface porosity ultimately leads to adsorption of organic molecules and water from surrounding atmosphere resulting in deterioration of glass surface and appearance of new ab- sorption bands. Moreover, nanoporous glass possesses a noticeable Mie scattering even for near IR light. Unfortunately, it limits application of porous glass as holo- graphic media.
Over last few years rapid development of femtosecond micromachining of trans- parent materials has made it possible to modify fused silica and some soft glasses with micron and even sub-micron spatial resolution [26]. In particular, one can in- duce permanent positive refractive index change of the order of 10−4−10−2by fo- cusing intense femtosecond pulses inside bulk silica. This allows one to record VBG in glass volume by direct scanning of focused Gaussian [27–31] or Bessel [25, 32, 33]
Figure 2.7: TEM image of classical PTR glass with NaF nanocrystals
beams, scanning of the interference pattern produced by phase mask [24, 34, 35], scanning of lines [36] and filamentation (Fig. 2.6). Such approach allows to fabricate VBGs with large area and customized period distribution. However, the period of the grating usually exceeds 1µm and the profile of refractive index modulation is far from harmonic one. Nevertheless, VBGs recorded in fused silica possess diffrac- tion efficiency above 80%, excellent chemical and thermal stability inherent to the material. The thickness of the grating is limited by the focal distance of objective lens in use, which is critical for high diffraction efficiency reflective VBG fabrication.
There are a number of applications that still require high optical and chemical quality, large thickness and long lifetime of the hologram. The research and devel- opment of photo-thermo-refractive (PTR) glasses stared seven decades ago by S.D.
Stookey who tried to optimize glass composition in order to enhance modulation of refractive index in irradiated area. In 1980th, a profound research of holographic properties of PTR glass was launched in State Optical Institute (USSR) by L. Gle- bov and N. Nikonorov. They have proposed and implemented new approach of fabrication of VBGs using UV holography and thermal treatment of PTR glass.
Nowadays this type of glass is conventionally used to fabricate VBGs for visible and near IR spectral range. Having been composed of 70% SiO2 it possesses ex- cellent chemical properties and allows one to obtain surface of high optical quality.
The damage threshold of PTR glass is as high as 40 J/cm2 for pulsed sources and 100 kW/cm2 for CW laser, i.e. it outperforms all polymer materials. The modu- lation of refractive index in PTR glass vary from 10−4−10−3 depending on glass
Figure 2.8: Beam combining using multiplexed RVBGs. [65]
composition.
Currently, there are several types of PTR glass with slightly different glass com- positions and optical properties. The glasses differ by a type of crystal phase pre- cipitated in volume during thermal treatment. In most common PTR glass, which were extensively studied over the last decades [4, 37, 37, 38, 38–54], NaF nanocrystals are needle-like having a length of up to 100 nm (Fig. 2.7). Such PTR glass is man- ufactured by several international companies all around the world and nowadays is the best material for high-power VBGs recording.
Chloride and bromide PTR glasses that have NaCl-AgCl or AgBr nanocrystals in volume have been developed only several years ago at ITMO University [6, 7]. Their optical properties have been also thoroughly studied [6, 7, 9, 55–61]. The fabrication of chloride PTR glass will be discussed in section 4. VBGs recorded in chloride PTR glass possess similar diffraction efficiency as those fabricated in classical PTR glass and positive sign of refractive index modulation. However, it suffers from extremely high absorption in visible and near IR [46]. Recently, new type of PTR glass with CaF2crystal phase was discovered at Jena University [8,62–64]. Currently, it is known that this glass has similar optical properties as conventional PTR glass, however, no data on refractive index modulation and VBGs recording is available.
2.4 APPLICATIONS OF VBG
Both TVBGs and RVBGs are widely used in photonics and lasing thanks to their exclusive properties. One of the most important applications of VBG is combining of several beams produced by diode, fiber or disk lasers. One approach for beam combining is spectral beam combining (SBC). In order to combine two laser beams the first one can directly pass through the grating while the second beam should un- dergo Bragg reflection [65, 66]. The direction of the first beam is chosen so that it is co-linear with the first diffraction order of the grating. In this configuration the out- put beam is not coherent, because input beams are produced by independent laser sources. Moreover, the spectral width of output beam is proportional to number of combined lasers. The wavelength of each laser source should be calculated taking into account the reflection spectrum of VBG in use. More than two laser beams can be combined using several separate substrates with single VBG or by multiplexing of several VBGs fabricated in the same volume of the PTR glass [3, 16, 65, 67, 68].
Figure 2.9: Multi-channel fiber laser coherent locking with VBG [2]
Specifically, each grating is recorded for a particular Bragg angle in such a way that all diffracted beams propagated in the same direction. VBGs can be also used for coherent beam combining (CBC) [2]. In this approach, laser beams from two or more laser sources are reflected from multiplexed VBGs recorded in PTR glass to mutual output resonator mirror for both laser sources. The output power from the mirror is coherent. The wavelength of each laser should be the same providing narrow spec- tral width of output radiation. Both approaches allow one to generate high quality laser beams with power of the order of tens of kilowatts [65].
In past decades for high-power CW and pulsed sources for pumping solid-states lasers needed for laser cutting, drilling, soldering etc are of high demand. Fiber, thin-disk, diode and CO2 lasers take the largest part of the market of the high- power materials processing. Laser diodes, which are widely used for the pumping of solid state lasers, have spectral width of several nanometers. It causes extra heat- ing of active media due to photon-phonon coupling. That is the spectral width can be significantly reduced by laser stabilization with VBG as output mirror of exter- nal diode laser resonator [70–74]. Whereas leaking longitudinal mode has narrow bandwidth the rest of the modes are accumulated in the resonator and eventually couple into the leaking mode. Thus, the output power drop can be as small as 10%
from initial value. In similar manner RVBGs have been demonstrated to stabilize fiber, hybrid and bulk crystal laser in CW and pulsed regimes allowing one to to achieve a single mode operation with line width as narrow as several pm [2, 75–88].
The use of VBG also allow to make these lasers tunable in wide range up to 50 nm
Figure 2.10: Two BragGrateTMNotch filters (BNFs) and thin film notch filter (blue) optical density spectra. The right inset shows magnified spectral region of a 785 nm BNF. [69]
by rotation of the grating which changes Bragg condition [75, 82, 89–96].
Narrow and tunable spectrum of stabilized laser makes it possible to combine the laser beams in coherent or non-coherent way as it was described before [2,67,68,97].
Stabilized with VBGs laser diode bars are used for zero-phonon line pumping of thin-disk laser with following decrease of active medium heating up to 32% [98–100].
Another important feature of laser stabilization is that it allows to reduce a shift of central wavelength caused by heating of diode laser at higher current. The use of VBG as external mirror prevents wavelength drift because Bragg condition is determined strictly by the design of the grating.
VBG found an application in beam quality improvement as a spatial filter. The grating can be placed both to the laser resonator for selection of low index trans- verse modes or outside the laser resonator [101]. The mode selection is achieved due to high angular selectivity of transmissive VBGs, which diffracts only beams propagating in very narrow cone of angles [50]. At the same time the diffraction efficiency can be up to 90-95% and loses induced by the grating are small enough due to low absorption and scattering and high homogeneity.
Reflective VBGs are also in wide use for Raman spectroscopy [69]. Raman signal frequency is usually very close to the laser line and has relatively low intensity. The laser line has to be suppressed by several orders of magnitude. Notch-filters based on RVBGs allow to detect Raman signal with 3-4 cm−1with 10-40 dB attenuation of laser line (Fig. 2.10). Moreover, a detection of anti-Stocks shift became possible as well with RVBGs because it works selectively for a particular wavelength.
Refractive VBGs are used for chirping of ultrashort pulses instead of well-known chirped pulse amplification (CPA) [75,102]. A grating with gradually varying period can stretch and compress ultrashort pulses depending on direction of propagation.
Instead of two collimated beams interference one can use interference of diverging and converging beams with equal angles to achieve linear change of grating period.
Each wavelength of transfer-limited pulse reflects on the grating with certain delay.
chirped VBGs recorded in PTR glass can be used in the range 0.8 to 2.5 um with diffraction efficiency up to 90 %. Nowadays femtosecond pulse can be stretched up to 1 ns duration and compressed back to 200 fs with energies and average power 1
Figure 2.11: Scheme of RVBG (a) and chirped VBG (b) [102].
mJ and 250 W respectively. The main drawback of such chirping approach is losses induced by the grating during the pass which can be as high as 20% per double pass.
2.5 COUPLED WAVE THEORY
Coupled wave theory was developed in late 1960th by H. Kogelnik at Bell lab [103].
It describes an interaction of light with a medium possessing sinusoidal modula- tion of refractive index and absorption coefficient. The theory rigorously describes operation of thick VBGs (the grating thickness should be much larger than light wavelength) with diffraction efficiency larger than 50%. The outlined below allows one to describe diffraction of the light wave polarized perpendicular to the incidence plane. It can be slightly modified to describe diffraction of laser beams of arbitrary polarization.
Figure 2.12 schematically shows VBG with arbitrary orientated fringes with pe- riod d in the volume having a thickness T. The dielectric permittivity changes ac- cording to sine law along grating vector K with the length of the vector|~K|=2π/d.
The grating fringes make an angleφ0with the grating surfaces. Incident light~vwith polarization laying in XZ plane hits the surface and enters the grating along vector
~ρ. It makes an angleψwith Z axis. Diffracted wave propagates along vector~σ.
Propagation of light wave through such a thick hologram can be described us- ing Maxwell equations for non-magnetic media with magnetic permeabilityµ=1.
These equations connect electric field vector~E, magnetic field vector H~ and electric displacement fieldD~ as the following:
curl~E=−µ0d~H
dt (2.2)
Figure 2.12: Scheme of transmissive VBG. [103]
curlH~ =e0ed~E
dt +σ~E (2.3)
div~D=0 (2.4)
divH~ =0 (2.5)
Taking curl of Eq. 2.2, substituting it into Eq. 2.3 and taking into account that~Eand
~Kare orthogonal we arrive at the second-order differential equation
∇2~E−µ0σd~E
dt −µ0e0ed2~E
dt2 =0 (2.6)
We can consider light wave linearly polarized along X axis of the laboratory Carte- sian frame. The x-component of the electric field can be presented in the following form
E(y,z,t) =Rena(y,z)exp(iωt)o (2.7) where a(y,z) is a complex amplitude and ω is the light angular frequency. The spatial evolution of the complex amplitude is described by the following equation:
∇2a−µ0σda
dt −µ0e0ed2a
dt2 =0 (2.8)
Thus, the phase of incident light can be determine at any point inside the glass using expression
2π~r~n
d =~K~r (2.9)
where~r~n=constis a plane of constant phase,~r=~ix+~jy+~kzis radius vector,~nis normal to theemodulation. The dielectric permittivityeand conductivityσcan be expressed as
e=e+e1cos(~K~r)
σ=σ+σ1cos(~K~r) (2.10) If one substitutes Eq. 2.10 into Eq. 2.8 and does simple algebra transformation the wave equation can be obtained
∇2a+q2a=0 (2.11)
where
q2=β2−2iαβ+2χβn
exp(i~K~r) +exp(i~K~r)o (2.12)
β=ke1/2 (2.13)
α= ωµ0σ
2ke1/2 (2.14)
χ= 1 2
k e1
2e1/2−iα1
(2.15) α1= ωµ0σ1
2ke1/2 (2.16)
Hereα1is a absorption index modulation amplitude (AIMA). It is more convenient in optics to characterize optical properties of media using refractive indexn = √
e instead of dielectric permittivity. Assuming that permittvity modulation is weak, e1e, the refraction index can be presented in the following form:
n=n+n1cos(~K~r) (2.17) wherenis average refractive index andn1is refractive index modulation amplitude (RIMA), which can be found as
n1= e1
2n (2.18)
Both RIMA and AIMA are of great importance for comparison of VBGs at the fab- rication step. They can be combined in a single complex coefficient
χ= πn1 λa
− iα1
2 (2.19)
Assuming that incident angle is close to Bragg conditions we can take into ac- count only two waves: incident and diffracted. Complex amplitude of the light wave can be presented in the following form:
a=R(z)exp(−i~ρ~r) +S(z)exp(−i~σ~r) (2.20) where first and second term in the right-hand side represent amplitude of the inci- dent and diffractive waves. Figure 2.13 shows relation between vectors~ρ,~β,~σ and
~Kfor a case when angleψ6=0. Under Bragg conditions following expression must be hold
~σ=~ρ−~K (2.21)
Figure 2.13: Vector diagram for Bragg condition case
By substituting Eq. 2.20 to Eq.2.11 and taking into account Eq. 2.21 we obtain cRR0+αR=−iχS
cSS0+ (α+iΓ)S=−iχR (2.22) HerecR = ρz
β = ρcos(ψ)
β = cosψ,cS = σz
β, Γ = βδsin(2θ0), whereδ is deviation from Bragg angleθ0.ρzandσz are projections of incident and diffracted beams to Z axis. The solutions of Eq. 2.22 can be found as
R(z) =R1exp(γ1z) +R2exp(γ2z)
S(z) =S1exp(γ1z) +S2exp(γ2z) (2.23) where
γ1,2=−1 2
α cR+ α
cS +iΓ cS
±1 2
h α cR − α
cS −iΓ cS
2
− 4χ
2
cRcS i1/2
(2.24) Index 1 corresponds to plus sign at±, and index 2 corresponds to minus sign there.
Further we continue discuss only transmissive VBGs because all experiments where performed with this type of gratings. However, this theory gives satisfactory results for reflective VBGs as well.
For transmissive VBGs we need to determine coefficients R1, R2, S1 and S2 from Eq. 2.23.
R1=−χ
2
cS(γ1−γ2)(cRγ1+α) R2= χ
2
cS(γ1−γ2)(cRγ2+α) S1=−S2=− iχ
cS(γ1−γ2)
(2.25)
wherecR=cS =cosθ0in case of transmissive VBGs with fringes orthogonal to the grating surface (ψ = π/2). If one substitutes Eq.2.24 and 2.25 into Eq. 2.23 the transmitted and reflected wavesRandSafter the grating can be found as
R= χ
2
cS(γ1−γ2)
hexp(γ2d)
cRγ2+α − γ1d cRγ1+α
i
(2.26)
S=i χ
cS(γ1−γ2) h
exp(γ2d)−exp(γ1d)i (2.27)
Figure 2.14: Diffraction efficiencyηdof transmissive VBGs depending onn1anda1.
Finally, from Eq. 2.26 and Eq. 2.27 we can find diffraction efficiency for trans- mitted (ηt) and diffracted (ηd) beams depending on
ηt(θ) =RR∗= exp
− 2αd cosθ
z0
"
Γ2 4 +z0
4
2cosh√
z0dcos(ψ0/2) cosθ
−Γ
2
4 −z0 4
2cos√
z0dcos(ψ0/2) cosθ
−Γ√
z0sin(ψ0/2)sinh√
z0dcos(ψ0/2) cosθ
−Γ√
z0cos(ψ0/2)sin√
z0dcos(ψ0/2) cosθ
#
(2.28) ηd(θ) =SS∗=
exp
− 2αd cosθ
z0
χ21+χ22 z0
"
2cosh√
z0dcos(ψ0/2) cosθ
−2cos√
z0dsin(ψ0/2) cosθ
# (2.29)
where
z0=h(Γ2+4(χ21−χ22))2+ (8χ1χ2)2i1/2 (2.30)
ψ0=arccos−[Γ2+4(χ21−χ22)]
z0 (2.31)
Γ=δKsin(ψ−θ0)−∆λK2λa/4πn (2.32) where ∆λ is deviation from central wavelength. Equations 2.28 and 2.29 are the main results from Couple wave theory. They will be used extensively during dis- cussion of experimental results to estimate RIMA and AIMA values for mixed VBGs in photo-thermo-refractive glass. It is worth noting that almost 100% diffraction effi- ciency can be achieved for very small RIMA values of phase VBGs (aboutn1=10−4) by increasing the thickness of the grating. Fig. 2.14 shows that diffraction efficiency almost sinusoidally depends on RIMA values. So, presentation of diffraction effi- ciency as a main characterization of the grating is controversially and I will try to keep using RIMA and AIMA values for this purpose.
3 Laser bleaching of silver nanoparticles
High absorption at wavelength range that spans from visible to near IR dominates the optical properties chloride PTR glass. This broad and strong absorption band is due to high concentration of the silver nanoparticles (AgNPs) in the glass matrix. In this chapter we consider a mechanism of the light absorption in chloride PTR glass and its dependence of the irradiation with ultrashort laser pulses.
3.1 OPTICAL PROPERTIES OF METAL NANOPARTICLES
The interaction of light wave with spherical metal particles with the size much smaller than the wavelength was first considered by Gustav Mie in 1908 [104]. He has shown that at optical frequencies, the optical properties of the metal particle in dielectric matrix can be described by introducing the NP dipole moment [105]
p=ehδE, (3.1)
whereEis a vector of electric field,ehis host permittivity, andδis polarizability of the particle
δ=4πr3 ep−eh
ep+2eh (3.2)
Extinction cross-section of spherical metal particle embedded in dielectric host can be expressed as
σ(ω) =σa(ω) +σs(ω) =kIm(δ) + k
4
6π|δ|2=12πr3ω ce3/2h
e00p(ω)
(e0p(ω) +2eh)2+e00p(ω)2 (3.3) where r is radius of metal particle,e0pande00pare its real and imaginary permittivity, ωis angular frequency of incident light, c is speed of light. Extinction cross-section determines the area of equivalent particle which would scatter and absorb the same portion of light as it does the particle under consideration. For example, a dielectric particle has extinction cross-section equal to its actual areaπR2. However, a metallic particle of the same size can have 10 times larger extinction cross-section at resonant frequency. Eqs. 3.1-3.3 can be used to find an effective radius of spherical particle in dielectric host by fitting measured extinction spectrum. The intensity of light passing through the composite material can be expressed as
I(x) =I0(x)e−Cσx (3.4) where C is the number of particles per unit volume.
Figure 3.1 shows diversity of resonant frequencies and extinction cross-section for different metals. One can notice that Au particle possesses remarkably larger extinction cross-section in comparison with Ag particles.
It has been recently shown that in chloride PTR glass, AgNPs have NaCl-AgCl shells, which are actually responsible for refractive index modulation [6, 9]. Thus, it
Figure 3.1: Extinction cross section and resonant wavelength of metal NPs with 10nm size in air. [106]
is necessary to take into account the influence of the dielectric shell on LSPR spectral position. Spherical particle with dielectric shell can be described by eq. 3.3 as well, but with polarizability [107]
δ=4πr3s esea−eheb
esea+2eheb (3.5)
where
ea=ec(3−2P) +2esP (3.6)
eb =ec+es(3−P) (3.7)
P=1−rc rs
3
(3.8) rc and rs are core and shell radii, correspondingly. In the case of ellipsoidal nanoparticle with a shell the polarizability should be determined for each axis sep- arately as following:
δi= v((e2−em)[e2+ (e1−e2)(L(1)i −f L(2)i )] +fe2(e1−e2)) [e2+ (e1−e2)(L(1)i − f L(2)i )][em+ (e2−em)L(2)i ] +f L(2)i e2(e1−e2)
(3.9)
where v = 4π
3 a2b2c2is a volume of the nanoparticle, a1,b1,c1 are core semiaxises, a2,b2,c2 are shell semiaxises, f = a1b1c1
a2b2c2 is a portion of the core in whole particle, L(1,2)i is a geometrical factor for core (1) and shell (2) [107].
Figure 3.2: Electron plasma oscillation in metal NPs during interaction with light.
[106]
In order to evaluate optical properties of glass composites with embedded metal nanoparticles we can use Drude theory to describe movement of conduction elec- trons in metal nanoparticles in presence of the oscillating electric filed of the light wave (Fig. 3.2). The dielectric permittivity of metal particle core can be found as
ec(ω) =eb+1− ω
2p
ω2+iγω. (3.10)
Here eb is the complex electric permittivity related with interband transitions of the core electrons in the atom,γis damping constant of electron oscillations, ωpis plasma frequency, which is determined as
ωp= s
Ne2
m∗e0 (3.11)
whereNis electron density,m∗is effective electron mass,eis electron charge.
Below plasma frequency the dielectric constant is negative, i.e. refractive index is purely imaginary giving rise to the high reflectivity of bulk metal. At ω > ωp
metal dielectric constant is positive, i.e. the Drude metal behaves similarly to or- dinary dielectrics. However, when one can substitute eq. 3.10 into eq. 3.11 and find a condition for a phenomenon known as Localized Surface Plasmon Resonance (LSPR) (eq. 3.12). This resonance corresponds to the collective oscillations of con- duction electrons in the metal nanoparticle that manifests itself in drastic increase of electric filed strength at the nanoparticle surface. The excitation of the localize plasmon causes a sharp absorption peak at the SPR frequency, which depends on material, size and shape of the metal; core, presence of the shell of particle’s core, and thickness and material of the shell [11, 106].
ec=−2eh (3.12)
Once the dielectric permittivity of metal nanoparticles embedded in dielectric matrix is determined it is convenient to evaluate refractive index and absorption co- efficient of the composite material. Using Maxwell-Garnet effective medium theory the dielectric permittivity of composite material can be expresses as
ee f f =eh(ec(ω) +2eh) +2f(ec(ω)−eh))
(ec(ω) +2eh) + f(ec(ω)−eh) (3.13)
whereeiis dielectric permittivity of metal particle from eq. 3.10, f =Vparticles/Vglass
is filling factor of metal particles in glass volume [108]. Thus, taking into account this equation the refractive index and absorption coefficient of composite material can be determined as following:
n=Re(pee f f) (3.14)
α=2ω
cIm(ee f f) (3.15)
3.2 AGNP LASER MODIFICATION
Interaction of intense laser pulses with metal nanoparticles embedded in dielectric material induces collective coherent oscillation of particle electrons. If the wave- length of incident light is within absorption band of AgNPs the incident pulse energy is partially absorbed and transferred to the metal lattice and surrounding medium. The cooling of the electron plasma in nanoparticles is often described in the framework of the two temperatures model (2TM) illustrated in Fig. 3.3. Electrons excited by incident light start relaxation after several dozens of femtoseconds (Fig.
3.3a). Then during hundreds of femtoseconds energy is transferred to other elec- trons through electron-electron and electron-phonon scattering (Fig. 3.3) [109, 110].
The occupied electronic states follows a Fermi-Dirac distribution for particular tem- perature. At this state the temperature of electrons is much higher than the lattice one. The electron-phonon interaction causes an increase of lattice temperature after tens of picoseconds, i.e. the absorbed light energy transferred to the particle deter- mines temperature of the particle. However, 2TM model does not take into account energy transfer to surrounding medium.
Figure 3.3: (a) The electronic distribution is at an equilibrium temperature Teq. (b) Electron–electron scatterings results in the electrons thermalization to a hot Fermi distribution (Te Teq) during several hundred femtoseconds. (c) Electron–phonon coupling processes, reaching a temperature which is equal to the lattice temperature Te = T1> Teq. [11]
When metal particles are embedded in dielectric material it is necessary to take into account temperature of a medium surrounding the particle. This can be done by introducing, besides electron and lattice temperaturesTeandTl, the matrix temper- atureTm. Such a three temperature model (3TM) can be described by the following equations:
Ce(Te)∂Te(t)
∂t =−G(Te)(Te−Tl) +S(t) (3.16) Cl(Tl)∂Tl(t)
∂t =G(Te)(Te−Tl) (3.17)
∂Tm(r,t)
∂t = χ r
∂2[rTm(r,t)]
∂r2 (3.18)
whereχis the thermal diffusivity of the glass medium.
According to 3TM model the temperature of particle and surrounding medium strongly depends on the electron-phonon coupling coefficients. As one can see from eq. 3.17 and 3.18 heat capacity and electron-phonon coupling coefficients depend on electron and lattice temperature. However, heat capacity of lattice changes with 1000 K increase insignificantly, by 20% [11]. On the other hand electron heat ca- pacity changes linearly for electron temperature less than 2500 K. Whereas, in case of femtosecond laser excitation electron temperature can easily achieve 8000 K and heat capacity should be described by
Ce(Te) = Z +∞
−∞
∂f(e,µ,Te)
∂Te g(e)ede (3.19)
Furthermore, in regime of strong perturbation electron-phonon coupling is no longer a constant. So, it requires similar consideration as heat capacity depending on electron temperature.
G(Te) = π¯hkBλhω2i g(eF)
Z +∞
−∞ g2(e)−∂f
∂e
de (3.20)
whereλdenotes the electron–phonon coupling constant, and the value ofλhω2iis 22.5 for silver.
In the strong excitation regime, the enhanced electric field in the vicinity of the particle may be responsible for photoemission of electrons and ions into surround- ing medium [111, 112]. As a result a reshaping or complete dissolution of particle can take place. At even stronger excitation when electron temperature achieve 7000- 10000 K Coulomb explosion mechanism starts dominating.
Reshaping and photodestruction of metal particles were studied extensively for past 50 years. Silver and gold nanoparticles (AgNPs and AuNPs) of different shape where embedded or obtained into different liquids and solid materials.
Starting from late 1970th photochromic glass doped with silver and chloride ions was able to be darkened by UV sunlight and become transparent under white light.
The mechanism for this process is well studied: (i) under UV light with wavelength within absorption band of silver halide nanocrystal electron-hole pair is formed, (ii) electron is neutralized by Ag+ ion in the vicinity of the crystal, (iii) the hole is trapped by Cu+ ion, (iii) neutral atoms form metal shell on the halide nanocrys- tal [113, 114]. Formed metallic silver is responsible for broadband absorption due to LSPR. Following exposure of the sample to visible light results in optical bleach- ing of glass [114, 115]. The mechanism proposed to explain that phenomenon is following: (i) incident light excites surface plasmon in metal particle which decay via single electron excitation [116], (ii) if the electron has sufficient energy it escapes from metal particle to conduction band of halide crystal [114]. This mechanism is
Figure 3.4: Laser-induced shape modification of the metal nanospheres in the case of linearly polarized laser pulses irradiation. [11]
slightly different from one used to describe photodestruction process of AgNPs in glass and solutions. It is mostly related with presence of silver halide nanocrystal which play a key role in the process.
Unique optical, chemical and mechanical properties of glass nanocomposites with AgNPs have been extensively studied during the last decade [106]. A consid- erable part of studies were done with ion-exchanged soda-lime glass, where sodium ions are exchanged by silver ions at a bath with mixture of salts (NaNO3-AgNO3) at temperature above 300 C. Such composite material can have 20-50 nm AgNPs in surface layer deepened at 30-100µm with gradient decrease in concentration. A book devoted to this topic was published recently and covers most of the research done in this field during last 15 years [11]. It has been shown how shape of AgNPs depends on pulse energy, polarization, duration, wavelength, doze and in which ap- plication this technology can be used [5, 117–121]. The mechanism of ultrafast laser
Figure 3.5: TEM of AgNPs in soda-lime glass after irradiation (a) with relatively low intensity I2= 2 TW/cm2, (b) at considerably higher intensity I>I2[11]
reshaping described in [11] and showed at Fig. 3.4 (a-g) in case of linear polariza- tion with power density 0.2-2 TW/cm2is following: (i) due photoelectron emission from the poles of the particle along the polarization direction accelerated electrons are trapped in the surrounding matrix and create defects; (ii) positively charged particle emit Ag ions in random directions to the glass matrix after several picosec- onds; (iii) recombination of trapped electrons and Ag ions results in formation of Ag atoms along light polarization; (iv) Ag atoms attach to the particle poles and transform it to a prolate spheroid. Fig. 3.5 shows the TEM images of the AgNPs after different intensity laser action. One can clearly see that at intensity higher than 2 TW/cm2the form of nanoparticles changes dramatically. In case of circular polar- ization only oblate particles can be obtained because trapped electrons created in all directions perpendicular to the light propagation [122].
Other studies of optical bleaching of metal particles embedded in dielectric ma- terials confirmed that proposed bleaching model is applicable to other glass. In [123]
authors produced amplitude grating using periodic bleaching of AgNPs in silicate glass by second harmonic of YAG:Nd nanosecond laser. Other authors showed that in thin film of SiO2with AgNPs they can be bleached by fourth harmonic of YAG:Nd laser and recovered by following thermal treatment [124]. Similar results were observed in phosphate glass doped with silver ions using second harmonic of YAG:Nd laser [125, 126].
There were several studies on intense laser influence on optical properties of classical PTR glass with AgNPs. Group of L. Glebov studied in details interaction of classical PTR glass with second harmonic of YAG:Nd nanosecond laser. Firstly, they showed that such laser can be used for fabrication of VBG via bleaching of AgNPs by interference pattern of the laser. The achieved diffraction efficiency was lower that 14% [127]. Later, they studied kinetics of the process of AgNPs bleaching in classic PTR glass under similar laser conditions [47]. First group, which studied interaction of IR femtosecond laser pulses with classic PTR glass was the group of N.
Nikonorov [128]. It was shown that ultrashort laser pulses can bleach AgNPs even far from resonant interaction. These discoveries opened a way to study of optical bleaching of VBGs in chloride PTR glass with femtosecond laser. Thus, to the best
of our knowledge there was no study of VBGs operation after laser bleaching.
4 Experimental
4.1 PTR GLASS SYNTHESIS
In the experiment, we use PTR glass that belongs to the sodium-alumina-silicate system, Na2O - Al2O3- ZnO - SiO2- NaF - NaCl and was activated by CeO2(0.007 mol.%), Sb2O3(0.04 mol.%), and Ag2O (0.12 mol.%). Glass was synthesized in fused silica crucibles at 1500◦C in the air environment. Stirring with a Pt thimble was used to homogenize the liquid. After melting during 10 hours, the glass was cooled down to 500◦C. Then it was annealed at glass transition temperature (Tg=494◦C) for 1 hour to get rid of stress, and cooled down to room temperature with a rate of 0.15◦C/min. After the synthesis glass is transparent and colorless without bubbles and other contaminations. The samples were prepared as 1.5 mm thick polished plates. For the precipitation of AgNPs in the bulk PTR glass samples were irradi- ated by mercury lamp EFOS Novacure N2001 (Artisian) and thermally treatment in programmable muffle furnaces (Neibotherm) at 560◦Cat 90 min. The average size of AgNPs after development of the grating is 5 nm according to previous transmission electron microscopy study (not published).
Figure 4.1: Optical setup for VBGs recording. 1 - He-Cd laser, 2 - beam expander and spatial filtering, 3 - iris, 4 - beam-splitter, 5 - mirrors on rotation stages, 6 - linear stage, 7 - glass sample. [61]
4.2 VBG RECORDING
For VBGs recording we used CW single-mode (TEM00) He-Cd laser (Kimmon, IK3501R- G) operating at wavelength of 325 nm with output power of 50 mW. Collimated laser beam was spatially filtered using UV objective lens and microdiaphragm with 30µm diameter in order to improve the beam quality. Then the beam was expanded with a telescopic system of lenses. Lloyd holographic setup was used for interference
