Study of photoinduced anisotropy in chalcogenide Ge-As-S thin films

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Study of Photoinduced Anisotropy in

Chalcogenide Ge-As-S Thin Films

Thèse

Kristine Palanjyan

Doctorat en Physique

Philosophiae Doctor (Ph.D.)

Québec, Canada

© Kristine Palanjyan, 2015

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Résumé

Étude de l'anisotropie photo-induite dans les

couches minces vitreuses de chalcogénures

Cette thèse porte sur l'étude expérimentale de la photosensibilité de verres de chalcogénures (ChG) sous la forme de couches minces. Plus particulièrement, elle est dédiée à l’étude des modifications photoinduites de leurs propriétés optiques ainsi qu’aux changements structuraux qui y sont liés au niveau atomique. Une étude systématique des propriétés des ChG sélectionnés dans le système vitreux Ge-As-S a été réalisée en fonction de la concentration relative des éléments Ge, As et S, de l’épaisseur des couches minces déposées ainsi que des différentes conditions expérimentales d’exposition au faisceau laser. Tout d’abord nous nous sommes intéressés au band gap optique du matériau, au décalage du bord d'absorption et au changement de sa pente qui sont les résultats d’arrangements atomiques complexes dans le réseau désordonné du ChG.

Ensuite, les résultats expérimentaux ont démontré que la composition vitreuse Ge25As30S45 possède la plus forte photosensibilité et notamment la valeur la plus élevée de biréfringence photo-induite (PIB) parmi les verres des systèmes Ge-As-S et As-S. La conversion de liaisons homopolaires (Ge-Ge, As-As) à hétéropolaires (Ge-S, As-S) a de plus été mise en évidence pour expliquer ce phénomène. En outre, la modélisation théorique simple que nous avons proposée avec une certaine approximation, montre que la valeur locale du PIB peut être d’un ordre de grandeur plus élevée que sa valeur moyenne. Les changements dynamiques d’absorption photo-induite étudiés pour différentes conditions expérimentales sont caractérisés par de forts changements asymétriques et non-monotones durant l'excitation et la relaxation. Ces changements ont été décrits par un modèle phénoménologique unipolaire que nous avons proposé et qui est basé sur certaines conversions séquentielles de liaisons se produisant après le franchissement d’une barrière énergétique donnée (estimée sur la base de nos mesures). Puis cette photosensibilité élevée des couches minces Ge-As-S a été utilisée pour l'enregistrement d’un réseau polarisé et pour la fabrication d’une lentille à gradient d’indice (GRIN) sur la surface, obtenus par irradiation laser à une longueur d’onde correspondante à la valeur de son band gap optique. La variation des efficacités de diffraction maximale obtenues pour les hologrammes scalaires et vectoriels a été discutée en considérant les différentes unités structurales identifiées et le rôle des transitions électroniques directes et indirectes dans ces deux types de réseaux. La stabilité thermique des hologrammes vectoriels a été montrée expérimentalement grâce à l’ajout de l’élément germanium Ge dans la composition de la couche mince. Enfin, les forces optiques des lentilles obtenues ainsi que les distorsions de front d'onde et l’effet de vieillissement ont été caractérisés à l’aide de capteurs Shack Hartmann.

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Summary

Study of photoinduced anisotropy in chalcogenide vitreous thin films

This PhD thesis refers to the experimental study of photosensitivity of chalcogenide glassy (ChG) thin films and their induced structural changes at the atomic level. A systematic study of the ChG properties is presented as a function of the elemental composition in the selected Ge-As-S system and the film thickness. More particularly, the goals of this work were to evidence and characterize the photoinduced birefringence and dichroism effects, to investigate the mechanisms involved and to correlate experimental observations with theoretical modeling.

The first part of the work was dedicated to the study of the optical properties, specifically the optical band gap of the prepared composition within the Ge-As-S vitreous system to reveal the most appropriate composition for further photoinduced effects examination. The shift and slope change observed for the absorption edge (associated with the optical band gap) according to the film thickness resulted from complex atomic (re)arrangements in the ChG network. The experiments carried out for the photoinduced effects have permitted to determine the best composition to be Ge25As30S45 among the Ge-As-S and Ge-As-S glasses in terms of higher photosensitivity and higher value of photoinduced birefringence (PIB) produced by the conversion from homopolar (Ge-Ge, As-As) to heteropolar (Ge-S, As-S) bonds. Moreover, the simple theoretical model proposed herein showed, with some approximation, that the local value of the PIB in these ChG thin films may be one order of magnitude higher than its average value. Then, the dynamic study of the photoinduced absorption revealed a strong asymmetric and non-monotonic behavior as a function of the irradiation laser power. To account for this specific behavior, a new unipolar phenomenological model is proposed based on sequential bond conversions occurring beyond an estimated energetic barrier.

The photoinduced anisotropy of these ChG Ge-As-S thin films was then used to record polarization gratings and gradient index lenses (GRIN). The maximum diffraction efficiencies achieved between scalar and vector holograms was discussed by means of involved structural units and the role played by indirect and direct electronic transitions. In addition, an improved thermal stability of the recorded vector holograms was experimentally shown after incorporation of germanium Ge into the material composition. The optical performance of the obtained lenses as well as the wave front distortions, aging effect and so on were studied by means of Shack Hartmann sensor.

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List of Tables

Table 1.1: Classification of the main photoinduced modifications ... 20 Table 2.1: Elemental analyses of the bulk and thin film of composition Ge25As30S45. The experimental error of the measurement is estimated to be around 5 At.%. ... 47 Table 3.1: Material analyses of the composition Ge25As30S45. The experimental error of this measurement is estimated to be of the order of 3%-5%. ... 57 Table 5.1: Depolarization ratio calculated from polarized and depolarized Raman spectra before and after laser exposure at 514.5 nm... 100 Table 5.2: Bond energies (in kJ/mol) in Ge-As-S ChG, from [180]. ... 104

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List of Figures

Figure 1.1: Periodic table showing the elements (highlighted in blue) usually combined with chalcogen elements (highlighted in orange) to fabricate ChG. ... 3 Figure 1.2: Photographs of ChG fabricated in the research group of Prof. Younès Messaddeq, COPL, Laval University (source: http://www.cercp.ca). From left to right: gallium germanium sulfide, arsenic sulfide and arsenic selenide glasses. ... 4 Figure 1.3: Optical transmission of the three families of chalcogenide glass, compared to silica and fluoride glass, from [9]. The glass thickness is 2mm... 5 Figure 1.4: Schematic representation of the electronic band structure in amorphous semiconducting materials. Arrows A and B, C show the optical electronic transitions in the Weak Tail Absorption (WTA)/Urbach and Tauc regimes, respectively. ... 6 Figure 1.5: Typical spectral dependence of the optical absorption coefficient in amorphous semiconductors. In the A and B regions, the optical absorption is controlled by optical transitions between tail and tail, and tail and extended states, respectively. In the C region, the optical absorption is dominated by transitions from extended to extended states. In the domain B, the optical coefficient follows Urbach rule. In the region C, the optical absorption coefficient follows the Tauc’s relation, from[13]. ... 7 Figure 1.6: Compositional variation of the refractive index and optical band gap of chalcogenide glass thin films [17]. ... 9 Figure 1.7: Ternary Ge-As-S system diagram presenting: (i) the GeS2-As2S3 and Ge2S3-As2S3 vitreous stoichiometric tie lines (red dot lines); (ii) the GeS-As4S4 compound tie line (brown dot line); the vitreous domain reported by Musgraves et al. [23] and the S-rich and S-poor (blue region) vitreous compositions ranges. ... 14 Figure 1.8: Structure models of (a) three-dimensional continuous random network like fused silica SiO2 or chalcogenide glass GeS2 and (b) two-dimensional distorted layers in As2S3 chalcogenide glass, from [27]. ... 15 Figure 1.9: As2Se3 thin film sample with photo-darkened spots indicated by the arrows, from [84]. ... 25 Figure 1.10: Reversible change in the density of state of the valence band of amorphous As2Se3 thin film, from [90]. ... 26 Figure 1.11: Photo-darkening E in ChG as a function of where Ti is the irradiation temperature and Tg is the glass transition temperature. Measurements of E were performed at temperatures below Ti, from [91]. ... 26 Figure 1.12: Observation of the photodarkening effect in amorphous As2S3 thin film and its partial recovery after thermal annealing. The black solid line is the band edge of the as-deposited film, the blue line is the band edge of the irradiated film and the red one is the band edge of the irradiated film after thermal

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annealing. The numbers 1 to 7 represent the successive cycles of irradiation/annealing applied to the

As2S3 thin film, from [100]. ... 28

Figure 1.13: Photoinduced anisotropy in a pnictogen–chalcogenide system before (a) and after (b) excitation. Open circles represent pnictogen atoms and solid circles chalcogens ones, from [49]. ... 33

Figure 1.14: The dielectric tensor  after irradiation by a linear polarized light (a) and an unpolarized light (b), from [121]. ... 34

Figure 1.15: The projection of the As4S4 molecule in the parallel plane of the As-As bond, from [122]. ... 35

Figure 2.1 Images in transmission of: a) Polished slice of home-made Ge25As30S45 glass sample (16 mm diameter); b) Commercial chalcogenide As2S3 glass window of 25 mm diameter placed between two polarizers parallel to each other. ... 40

Figure 2.2 Ge25As30S45 glassy thin film (of 3 µm thickness and 2.5 x 5 cm dimensions) placed between two parallel polarizers. ... 41

Figure 2.3: Photograph of a typical Ge25As30S45 glass rod and polished slice. ... 46

Figure 2.4: Ge25As30S45 glass thin films prepared with different thicknesses. ... 47

Figure 2.5: Ge25As30S45 optical band gap as a function of the film thickness. ... 48

Figure 2.6: Absorption coefficient spectra of Ge25As30S45 for different film thicknesses. ... 49

Figure 2.7: Normalized Raman spectra of Ge25As30S45 thin films of different thicknesses. ... 50

Figure 3.1: Typical transmission spectra of obtained thin ChG films. ... 58

Figure 3.2: Absorption coefficients of the Ge25As30S45 as function of probe’s energy obtained for photoexposition intensity of 8W/cm2_{for 60 min. The solid curve corresponds to the unexposed case;} the dashed curve corresponds to the photoexposed case. ... 59

Figure 3.3: The experimental setup used for the study of PIB: P-polarizer, M–mirror, - half wave plate, S-sample; A-analyzer; F1 and F2-filters, d-diaphragm, D-detector. ... 60

Figure 3.4: Typical cycle of excitation and partial relaxation of the PIB in the Ge25As30S45 film. The solid curve shows the experimental result and the dashed one (behind the experimental curve) represents the fitted curve. The thickness of the film was 1.5 µm and the excitation intensity was 8W/cm2_{. ... 61}

Figure 3.5: Dependence of the PIB upon the amount of As in the film of Ge-As-S. ... 62

Figure 3.6: The dependence of the established (saturated) value of PIB upon the excitation intensity for the composition Ge25As30S45. ... 63

Figure 3.7: Normalized Raman spectra of thin Ge-As-S films for different compositions: Ge25As10S65 (dotted black line), Ge25As20S55 (short dash dotted red line), Ge25As30S45 (dashed green line), Ge25As35S40 (short dotted blue line), Ge25As40S35 (solid cyan line). ... 65

Figure 3.8: Normalized Raman spectra of Ge25As30S45 bulk glass (dotted black line) and thin films unexposed (short dash dotted red line) and exposed at 2.14 W/cm2_{(dashed green line), 4.24 W/cm}2 _{(short dotted} blue line) and 7.87 W/cm2_{(solid cyan line) for 60 min. ... 68}

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Figure 4.1: Transmission spectra (in non-polarized light) of non-exposed (black, dotted curve) and exposed (red, solid curve) Ge25As30S45 thin films of 3 µm thickness. Samples were irradiated at 514 nm for 30 min. ... 75 Figure 4.2: Experimental setup used for the study of PIB: P-polarizer, M–mirror, - half-wave plate,

S-sample; A-analyzer; F1 and F2-filters, d-diaphragm, D-detector. ... 76 Figure 4.3: Transmitted intensity of the probe beam (3 m thick sample is used versus time for pump intensity of 8W/cm2_{. The points 1 and 1’ represent the established values of excitation and 2 and 2’} represent the established values of relaxation corresponding to the probe transmission without (drawn by triangles in the fig.) and with analyzer (drawn by squares in the fig.), respectively. ... 79 Figure 4.4: Dependence of the established values of PIB (under CW excitation) upon the excitation intensity for the 3 m Ge25As30S45 thin film. The line is used to guide eyes only. ... 80 Figure 4.5: Experimentally measured dependence of the established output probe intensity upon the input pump intensity for the 3 m Ge25As30S45 thin film. The line is used to guide eyes only. ... 81 Figure 4.6: Average (solid curve) and local maximum (at the input front of the ChG film, dotted curve) values of the established PIB as a function of pump intensity for the 3 m Ge25As30S45 thin film. Solid and dashed lines are used to guide eyes only. ... 82 Figure 5.1: Experimental setup used for the PID study: M1 and M2 - mirrors; /4 - quarter wave plate (placed on the path of the probe beam), S – ChG sample; W- Wollaston prism; BS1 and BS2 - polarization insensitive beam splitters, P1 and P2 – polarizers, D1-4 – photo detectors. Note: BS1 allows using the Ar+ laser (514.5 nm) as probe and pump beams simultaneously. In a different experiment, the BS1 is removed to use the He-Ne laser (632.8nm) as probe and the Ar+_{laser (514.5 nm) as pump. ... 89} Figure 5.2: (a) Elemental chemical composition measured by SEM-EDAX analyses at 9 distinct points on a Ge25As30S45 thin film of 7µm thickness. The horizontal (black dashed) lines correspond to the nominal values (b) Raman spectra recorded at 9 distinct points on the same Ge25As30S45 thin film (numbers 1-9 correspond to their locations, as depicted in the inset) and normalized at 215 cm-1_{. ... 90} Figure 5.3: Transmission spectrum of a 3 µm thick Ge25As30S45 thin film. Vertical arrows show the position of the band gap and sub band gap lights used in the present work for excitation (at 514.5 nm) and probing (at 632.8 nm), respectively. ... 91 Figure 5.4: Transmission (right vertical axis, in red) and reflection (left vertical axis, in black) of horizontal IHT, IHR (solid lines) and vertical IVT, IVR (dashed lines) components for the sample of 3 µm thickness. Pump beam (at 514.5 nm) is vertically polarized, Ip= 10 W/cm2_{; probe beam was obtained from a} He-Ne laser (at 632.8 nm), Ipr = 3.5 mW/cm2_{. ... 92} Figure 5.5: Transmission (right vertical axis, in red) and reflection (left vertical axis, in black) of horizontal IHT, IHR (solid lines) and vertical IVT, IVR (dashed lines) components for the 3 µm thick sample. Pump beam is vertically polarized (at 514.5 nm), Ip = 10 W/cm2_{; probe beam is also obtained from the same} Ar+ laser (at 514 nm), Ipr = 1mW/cm2_{. ... 93}

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Figure 5.6: Kinetics of the photoinduced dichroism PID (measured at 514.5 nm) in a 3 µm thick Ge25As30S45 thin film annealed at 350°C. ... 94 Figure 5.7: Kinetics of PID as evidenced by the sum of transmitted and reflected beam powers for two orthogonal polarization components (normalized to their initial value). (a) probe He-Ne laser, at 632.8 nm; (b) probe Argon-ion laser, at 514.5 nm. The excitation was achieved with a vertically polarized pump at 514.5 nm. Ip = 10 W/cm2, d = 3 µm. Letters h and v correspond to horizontal and vertical polarization components of the probe in the established excitation state. Letters h’ and v’ show the values of same components in the partial relaxation state. ... 95 Figure 5.8: Averaged amplitudes of the decrease of the summed (and normalized) transmission and reflection intensities (T+R) measured from its initial (maximum) value (1-(T+R)) up to the steady state of excitation (a) and relaxation (b) for two orthogonal components; horizontal (black solid curve) and vertical (dotted red curve), as a function of pump intensity. The excitation and probing were performed at 514.5 nm. The polarization of the pump is vertical. The thickness of the film of Ge25As30S45 is 3 µm. ... 96 Figure 5.9: Polarized and depolarized Raman spectra recorded at 632.8 nm before (a) and after (b) vertical

polarized laser exposition at 514.5 nm (0.3W/cm2_{) during 45 min on the Ge}

25As30S45 thin film. ... 98 Figure 5.10: Measured temperature dependence of the Ge25As30S45 thin films surface as a function of the pump intensity. Squares represent the experimental data and circles represent the theoretical estimation results using the equation (5.3). ... 102 Figure 5.11: Photoinduced darkening (PD) of the Ge25As30S45 thin film at different oven temperatures for two orthogonal components (horizontal: black solid curve and, vertical: dot red curve): a- excitation, b-relaxation. Intensity of the pump was 3 W/cm2_{and the thickness of the film was 3 µm. ... 103} Figure 5.12: Qualitative reproduction of the dynamics of absorption changes during the relaxation of a

4-level system with consecutive conversion between bonds. ... 105 Figure 6.1: Experimental set-up for the vector hologram study: pump – Ar-ion laser; probe - He-Ne laser;

/2- half-wave plate; /4- quarter-wave plate; WP-wollaston prism; S-sample; D₁_{, D}₂, D₃-detectors. 112 Figure 6.2: Normalized diffraction efficiencies of the vector holograms recorded in Ge25As30S45 (black) and As2S3 (red) thin films as a function of the temperature. Lines are guide to the eye. ... 113 Figure 6.3: Diffraction efficiency (%) of vector (solid line) and scalar (dashed line) holograms recorded in the same Ge25As30S45 thin film as a function of pump intensity. The vector hologram was recorded by (RCP+LCP) polarizations beams while the scalar hologram was recorded with two linearly s-polarised beams. Lines are guide to the eye. ... 114 Figure 6.4: Dynamic diffraction efficiency (%) of vector (sold line) and scalar (dashed line) holograms recorded in the same Ge25As30S45 thin film. The vector hologram was recorded by (RCP+LCP) polarizations beams while the scalar hologram was recorded by two linearly s-polarized beams. The pump intensity was 4W/cm2. ... 115

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Figure 6.5: Optical microscope images of recorded gratings on the same Ge25As30S45 thin film: (a) scalar gratings written by (s+s) polarization beams; (b) vector gratings written by (RCP+LCP) polarization beams; (c) vector gratings written by (s+p) polarization beams. ... 116 Figure 6.6: Diffraction efficiency of +1 and -1 diffracted orders as a function of the rotation angle of the quarter-wave plate (the elasticity of the incident probe beam polarization). Ge25As30S45 thin film thickness is 7 µm. Lines are guide to the eye. ... 117 Figure 7.1: Wave front of the probe beam exiting the GRIN lens measured by the Shack-Hartmann: exposure time was 30 min and the power was 8W/cm2_{. The thickness of the thin film was 5 μm. The asymmetry} of the wave front profile is due to the inhomogeneity of the excitation laser beam. ... 127 Figure 7.2: (a) Lens optical power dependence on pump intensity for different irradiation durations, observed with parallel and perpendicular probe polarizations (with respect to pump polarization); (b) Lens optical power dependence on irradiation time for different pump intensities, observed with parallel and perpendicular probe polarizations (with respect to pump polarization). ... 128 Figure 7.3: Scanning electron microscope (SEM) images of surfaces of a damaged sample. ... 129 Figure 7.4: Modification of the measured optical power over time for different pump intensities examined with two probe polarizations: parallel to irradiation polarization (a) and perpendicular to irradiation polarization (b). ... 130 Figure 7.5: Raman spectra (a) and EDAX elemental quantitative analyses (b) of the Ge-As-S films of 5 µm thickness (freshly evaporated (dashed curve) and 180 days stored in ambient atmosphere after the irradiation (solid curve)). ... 131 Figure 7.6: 2D profiles of the sample before (black curve) and after (red curve) irradiation showing the absence of surface modification (expansion or contraction). Inset: magnification of the surface profile of the irradiated zone. ... 132 Figure 9.1: (a) Scheme of the experimental set-up for the preparation of chalcogenide synthesis ampoule ; (b) Tubular rocking furnace and (c) thermal profile used to melt, fine, quench and anneal the chalcogenide glass within its silica ampoule. ... 140 Figure 9.2: Thin film deposition techniques, from [219]. ... 143 Figure 9.3: Schematic representation of a thermal evaporation chamber: B – heated boat, S – substrate, H – heating system for the substrate, V – vacuum (from [222]). ... 144 Figure 9.4: Schematic illustration of thin film deposition dependence on angle of evaporation beam direction (from [222]). ... 145 Figure 9.5: Schematic representation of the sputtering evaporation apparatus : T – target electrode ; S – substrate electrode ; P – plasma ; V – vacuum and H – heater (from [222]). ... 146 Figure 9.6: Schematic representation of the e-beam evaporation method. ... 147 Figure 9.7: DSC traces of the Ge25As30S45 thin film and crushed bulk glass pieces (y-axis: unit: 0.5 mW/mg/div.). ... 150 Figure 9.8: Quantification (EDAX analyse) of chemical elements of composition Ge25As30S45. ... 151 Figure 9.9: Scheme describing the main electron-matter interactions. ... 152

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Figure 9.10: Schematic energy diagram describing the Rayleigh and Raman scatterings. The line thickness indicates the signal strength from the various transition state shown by black horizontal lines. ... 154 Figure 9.11: Schematic representation of Raman Depolarization Ratio. ... 155

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Ծնողներիս… À Mon Amour…

բոլոր ջանքերի և սիրո համար որ ամեն պահ ինձ հետ է…

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Acknowledgments

I would like to express my gratitude to all those who have supported me throughout this PhD project since my arrival in Quebec City, 4 years ago. It is always difficult to express with words the emotions of gratitude, especially in some foreign language, so my sentences will be shorter than my thoughts.

First, I am deeply indebted to my supervisors Prof. Tigran Galstian and Prof. Réal Vallée for giving me the opportunity to achieve my doctoral studies at the Center for Optics, Photonics and Laser (COPL) at Laval University. The completion of this PhD thesis would certainly not have been possible without their continued support, encouragement, guidance and dedication to this work. I want to express my sincere gratitude for all useful comments, advices, valuable remarks they gave me through the periodic and frequent meetings. Their motivation, enthusiasm, and immense knowledge that they shared honorably with me, were essential to do this work. I want to mention their contribution both in professional and personal aspects, which help me from the first day till now for my integration to the Quebec society.

I would like to thank Prof. Younès Messaddeq to have revised my thesis manuscript prior to its deposition. His insightful comments and suggestions from the point of view of a chemist helped me noticeably in improving my work. His contribution was significant since the beginning of my doctoral studies and I am also sincerely grateful to his research group, for the support in preparing material and for the fruitful discussions during the entire project, especially to Sandra Helena Messaddeq, Prof. Igor Skripachev, Yannick Ledemi and Mohammed El-Amraoui.

Sincere thanks to all the technical and administrative staff of COPL and Laval University for their help and support, in particular to Patrick Larochelle who was always available and able to solve rapidly the technical issues inherent to experimental investigations.

Thanks to the Natural Sciences and Engineering Research Council of Canada (NSERC) agency for their financial support.

I would like to address my thanks to Renan Cariou to have revised and improved my English in the first part of this thesis.

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I also express my thanks to all staff of ultrafast laboratory in Yerevan State University, especially to Prof. Levon Mouradian, Garik Yesayan and Artur Kirakosyan.

Thanks to the Armenian community here in Quebec city, and especially to my friends Karen, Amalya, Elina, Ani, Vahe , Anush and all who were close to me for cheering me up when necessary. Special thanks to Karen for all the important discussions about physics problems that we had.

Finally, I extend special and most important thanks to my family: my mother, my father, my sister and her family who all have supported me in this adventure so far from my home in Erevan, and to my husband met here in Quebec, Yannick, for his patience and unremitting encouragement, for our long scientific (and not) discussions which have made possible the completion of this PhD project. Իմ բոլոր հաղթանակներն ու հաջողութունները նաև ձերն են: Շնորհակալ եմ Ձեզ....սիրում եմ Ձեզ անսահման:

Merci également à ma belle-famille: votre amour sincère et votre soutien m’ont apporté beaucoup plus que vous ne le pensez. Les mots d'encouragement venus du cœur motivent en profondeur. Je vous aime.

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Foreword

This thesis mainly aims at detailing the examinations of photoinduced anisotropy (under irradiation of band gap light) in thermally stable chalcogenide glass (ChG) thin films belonging to the ternary Ge-As-S system. The investigations reported in this work lead to important information related to the physical properties of the studied ChG material, at both macroscopic and microscopic levels, and to the understanding of the mechanisms involved in the studied photoinduced effects, encompassing thus different aspects from fundamental physics science to applicative science for the optics and photonics high-tech industry.

Chapter one proposes a brief review on the subject of ChG glasses and their main photoinduced properties and modifications.

The major and original part of the thesis is based on three first author articles and two proceedings published in peer-reviewed journals during my PhD studies. Chapter two presents a SPIE proceeding published in 2014 (K. Palanjyan, R. Vallée T. Galstian , ‘Band

gap dependence upon thickness of chalcogenide Ge-As-S thin films’, Proc. SPIE 9288, Photonics North 2014, 92880K (2014)) where the band gap study of the selected ChG

material was presented, and co-authored by my two supervisors, Prs. T. Galstian and R. Vallée. The subject and methods of realization were at first discussed with them. The optical experiments of the project were carried out entirely by myself at the COPL laboratories, at Laval University. After discussing the content and structure of the manuscript, I provided the original version for further editing and completing.

The third chapter refers to the basic research axis of this thesis: the study of the photoinduced anisotropy of these ChG thin films. The present chapter presents an article published in 2013 in Optical Materials Express (K. Palanjyan, S.H. Messaddeq, Y.

Messaddeq, R. Vallée, E. Knystautas, T. Galstian, Photoinduced birefringence in Ge-As-S thin films, Optical Materials Express, Vol. 3. Issue 6, pp. 671-683 (2013)). The goal of this

study was to investigate the photoinduced birefringence (PIB) and understand the mechanisms responsible for the observed anisotropic changes. The preparation of bulk samples, their evaporation to obtain thin films and the experimental set-up for the PIB

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measurement were realized within the frame of this work. The measurements and characterizations were performed by using different experimental techniques, as optical spectroscopic transmission, thermal analysis, elemental microanalysis through energy dispersive X-ray spectroscopy, micro-Raman spectroscopy, etc., for a better understanding of the PIB effect through its relationship with the ChG properties and structure. This work was realized in collaboration with the research groups of Profs. Y. Messaddeq and E. Knystautas from Laval University. The respective roles of the authors are as follows: first, all authors participated in the subject discussions. The glass was prepared in the laboratory of Prof. Y. Messaddeq with the help of Dr. S.H. Messaddeq and Dr. I. Skripachev. Then, the preparation of the thin films was achieved thanks to the e-beam evaporator available in the laboratory of Prof. E. Knystautas with the assistance of Dr. S.H. Messaddeq. The thin film characterizations and main optical experiments were then carried out at COPL laboratories by me. Following initial discussions with Profs. T. Galstian and R. Vallée, experiments were performed by S.H. Messaddeq and myself. All results were then discussed with the authors. The manuscript was essentially written by myself, after discussing the content and structure with Profs. T. Galstian and R. Vallée. My version of the manuscript was communicated to all the co-authors who then added some editing and completing remarks.

In the fourth chapter, additional investigation of the PIB effect is reported thanks to a thorough measurement of its local value. Such study of the local value of the PIB, besides the determination of their average values, may be useful for integrated optic and photonic devices. Some approximations were considered to allow the realization of both the experiments and modeling (achieved with MatLab program). A giant local anisotropy was observed from these ChG Ge-As-S thin films. The article was published in 2015 in Optical

Materials Express (K. Palanjyan, R. Vallée, T. Galstian, Observation of giant local photoinduced birefringence in Ge25As30S45 thin films ,Optical Materials Express, Vol. 5 Issue 5, pp.1122-1128 (2015) ) authored by my two supervisors, Profs. T. Galstian and R.

Vallée, and myself. The subject and methods of realization were first discussed with my supervisors. The experimental part was entirely realized by myself at COPL laboratories. After discussing the content and structure of the manuscript, I provided the original version for further editing and completing.

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xxv The fifth chapter reveals the second major result of the current thesis: the photoinduced absorption changes (photoinduced dichroism) of the Ge-As-S thin films. Besides the values of dichroism measured experimentally, a phenomenological model is proposed to explain the asymmetric and non-monotonic behavior of photoinduced absorption dynamical changes, and also to account for the mechanisms which cause these photoinduced modifications. The chapter presents an article published in 2015 in the

Journal of Non-Crystalline Solids (K. Palanjyan, R. Vallée and T. Galstian, Journal of Non-Crystalline Solids 410 (2015) 65–73)). The authors were my two supervisors and

myself. The subject and methods of realization were at first discussed with them. The optics experimental part was entirely realized by me at COPL laboratories. After discussing the manuscript’s content and structure with my supervisors, I provided them my version of manuscript for further editing and completing.

Finally, the sixth and seventh chapters describe two different applications based on the obtained results. The sixth chapter corresponds to the study of the polarized holograms recorded on these ChG thin films with the comparison between the scalar and vector holograms as well as the thermal stability assessment for the latter. The seventh chapter deals with the formation of gradual variation of the refractive index (GRIN lenses) on these ChG Ge-As-S thin films. The studies of the optical performance and the wave front distortions of the obtained lenses were performed upon different experimental conditions. Results were presented at an international conference and published in 2014 in the peer-reviewed SPIE proceedings (K. Palanjyan, R. Vallée T. Galstian, ‘Photoinduced GRIN

lens formation in chalcogenide Ge-As-S thin flms’, Proc. SPIE 9288, Photonics North 2014, 92880L (2014)). In both cases, the authors were my two supervisors and myself. The

subject and methods of realization were at first discussed with them. The optics experiments were entirely realized by me at COPL laboratories. The preparation of these articles was realized according to the same way as for the previous ones.

Aside from these main results, the eight chapter consists in the general conclusion of the thesis while the Appendix brings on one hand some additional descriptions of the experimental method used for the preparation and characterization of the studied bulk and thin film and on the other hand, a review of the known methods for thin films fabrication.

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xxvi

Co-authors:

Tigran Galstian: Center for Optics, Photonics and Laser, Department of Physics,

Engineering Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue de la Terrasse, Québec, G1V 0A6, Canada.

e-mail: galstian@phy.ulaval.ca

Réal Vallée: Center for Optics, Photonics and Laser, Department of Physics, Engineering

Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue de la Terrasse, Québec, G1V 0A6, Canada.

e-mail: rvallee@copl.ulaval.ca

Younès Messaddeq: Center for Optics, Photonics and Laser, Department of Physics,

e-mail: Younes.Messaddeq@copl.ulaval.ca

Sandra H. Messaddeq: Center for Optics, Photonics and Laser, Department of Physics,

e-mail: Sandra.Messaddeq@copl.ulaval.ca

Emile Knystautas : Center for Optics, Photonics and Laser, Department of Physics,

e-mail: Emile.Knystautas@phy.ulaval.ca

Publications (during the PhD thesis):

Reviewed periodicals

 K. Palanjyan, S.H. Messaddeq, Y. Messaddeq, R. Vallée, E. Knystautas, T. Galstian, Photoinduced birefringence in Ge-As-S thin films, Optical Materials Express, Vol. 3. Issue 6, pp. 671-683 (2013).

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xxvii  K. Palanjyan, R. Vallée, T. Galstian, Experimental Observations of Photoinduced Bond Conversions in Ge-As-S Thin Films, Journal of Non-Crystalline Solids, Vol. 410, pp 65–73, (2015).

 K. Palanjyan, R. Vallée, T. Galstian, Observation of giant local photoinduced birefringence in Ge25As30S45 thin films, Optical Materials Express, Vol. 5 Issue 5, pp.1122-1128 (2015).

Conference Presentations

 K. Palanjyan, R. Vallée T. Galstian, ‘Band gap dependence upon thickness of chalcogenide Ge-As-S thin films’, Proc. SPIE 9288, Photonics North 2014, 92880K (2014).

 K. Palanjyan, R. Vallée T. Galstian, ‘Photoinduced GRIN lens formation in chalcogenide Ge-As-S thin films’, Proc. SPIE 9288, Photonics North 2014, 92880L (2014).

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1

Chapter 1 General Introduction

Chalcogenide glasses (ChG) based materials have been studied for several decades and utilized today in a myriad of applications requiring infrared light transparency or transmission, infrared sensing, phase change memories, optical data storage, etc. Dedicated for a long time to military restricted or scientific utilizations, ChG materials are nowadays finding civilian applications. First, the most developed and used technology to date is the optical data storage with the of CD-RW, DVD-RAM, DVD±RW and BLU-ray discs which are based on ChG thin film alloys exhibiting a very fast and reversible transformation from crystal to vitreous state under a specific laser exposure. Second, their remarkable mid-infrared transparency in the two infrared atmospheric windows (3-5 µm and 8-12 µm) is now utilized for civilian night vision and thermal imaging devices, for instance. Indeed, the simultaneous development of vitreous compositions with properties meeting the standards for practical use and large scale production have led to a very cost effective alternative to the single crystal germanium technology for the fabrication of infrared lenses and windows. Such progress, associated with the development of infrared detectors, has resulted in lower manufacturing costs of infrared detection and imaging systems.

This chapter is dedicated to a state of the art of photoinduced phenomena in ChG and ChG thin films. To present an exhaustive review of all the investigations and works reported in this field to date is not the intent here, however a general overview of the unique photosensitive character specific to ChG, which is by far one of the most light-sensitive materials, will be provided.

After a brief historical perspective, the specific properties which make these materials so unique and their related practical applications will be described in more details. The photoinduced phenomena reported in these glasses will then be reviewed. Emphasis will be given to the effects observed under continuous wave (CW) laser

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2

illumination. Finally, the different models proposed in the literature to describe these phenomena at the atomic and molecular short range structural level, including those related with the photoinduced anisotropy changes, will be discussed.

1.1 Chalcogenide Glasses and Thin Films

1.1.1 Definition and history of Chalcogenide Glasses

Generally, glasses or vitreous materials constitute a unique class of materials. They were defined by Zarzycki as non-crystalline solids featuring the phenomenon of glass transition [1]. Among the known glasses, one can distinguish three main categories: the metallic glasses, the organic/polymeric glasses and the inorganic glasses. The latter category comprises the well-known silicate glasses but also the phosphate glasses, the heavy metal oxide glasses, the halide glasses (particularly the fluoride glasses) and the ChG. All these types of glasses are characterized by specific features owing to their chemical composition and will therefore be used for specific applications. For instance, heavy metal oxide glasses possess heavier elements than silicate ones, leading to glasses with higher refractive indices and extended transmission window in the infrared region. However, among all of them, only the ChG are transparent so far in the infrared region, up to 20-25 μm depending on their compositions, making them the most promising candidates for mid-infrared optical and photonic applications like lenses for infrared cameras [2], planar waveguides for integrated optics [3] or infrared sensors [4].

In the 1930’s, a research group from University of Hannover in Germany proposed the term “chalcogen”, which means “ore former” (from “chalcos” old Greek for “ore”) as the characteristic name of the group VI of elements including oxygen O, sulfur S, selenium Se, tellurium Te and “chalcogenides” for their compounds [5] (see Figure 1.1). Twenty years later, in the 1950’s, after the first report on arsenic sulfide glasses, the term chalcogenide was used to distinguish the oxide glasses (essentially silicate glasses) from those based on S, Se and Te. Those glasses (ChG) indeed exhibit a unique mid-infrared transparency compared to the traditional oxide glasses [6]. Following studies on glasses based on selenium and tellurium showed even further transmission than sulfide ones in

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4

Figure 1.2: Photographs of ChG fabricated in the research group of Prof. Younès Messaddeq, COPL, Laval University (source: http://www.cercp.ca). From left to right: gallium germanium sulfide, arsenic sulfide and arsenic selenide glasses.

Nowadays, ChG belonging to the Ge-As-Se, As-S and As-Se systems for instance are commercialized as passive infrared optics and optical fibers by many companies like Amorphous Materials Inc., Umicore IR glass, Schott, CorActive, IRflex, IRradiance, Diafir and others.

1.1.2 Optical Properties of Chalcogenide Glasses

1.1.2.1 Infrared Transmission

The exceptional transparency of ChG in the mid-infrared region directly depends on the nature of the anion, i.e. S, Se or Te. Indeed, due to their high atomic weight, increasing from sulfur (MS = 32.06 g/mol) to tellurium (MTe = 127.6 g/mol) associated with the high atomic weight of the cations (As, Ge, Ga, Sb, etc.) with whom they form the glass, sulfide, selenide and telluride glasses are characterized by very low maximum phonon energies (respectively, 350-425 cm-1_{, 250-300 cm}-1_{and 150-200 cm}-1_{) as} compared to those of oxide (~1100 cm-1_{for silica glass) or fluoride glasses (~540 cm}-1_). As a consequence, owing to their very low multiphonon frequency, sulfide, selenide and telluride glasses exhibit the most extended transmission in the mid-infrared region, up to 20-25 µm, while sulfide glasses, whose anionic weight is lower, use to transmit mid-infrared light up to 10-12 µm. Between them, selenide glasses are transparent to around 15-16 µm. The transmission spectra of these glasses are represented in Figure 1.3 and compared with those of oxide and fluoride glasses.

10 mm 10 mm

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5 Figure 1.3: Optical transmission of the three families of chalcogenide glass, compared to silica and fluoride glass, from [9]. The glass thickness is 2mm.

The mid-infrared optical domain of the electromagnetic spectrum is of major interest from a technological point of view since it includes the telecommunication wavelengths, the thermal radiation domain, the spectral signature range of the so-called greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide, and ozone), the spectral footprint range of the biological molecules, etc.

The need for materials capable to operate (emit, detect or conduct light) in this range is thus continuously growing for many applications as detection of the weak mid-infrared emission lines of an orbiting plane, remote CO2 detection, monitoring in real time of pollutants in the environment [10], help in medical diagnostic through efficient optical biosensors [11], etc., besides the traditional defense & security applications in the mid-infrared (thermal imaging, laser countermeasures, etc.).

1.1.2.2 Optical Band Gap

The multiphonon absorption defines the long wavelength (infrared) cut-off of transparency windows of a glassy (dielectric) material, while its short wavelength cut-off is characterized by the electronic transitions from the valence to the conduction band. ChG are also semi-conducting materials and present therefore a band gap between their valence and conduction bands in their energy diagram, as schematized in Figure 1.4 [12]. As glass is a disordered material, localized states can exist within the material band gap. These localized states form then the band tails which further reduce the minimum energy required for an electronic transition to occur from the valence band to the conduction band, defining the optical band gap. The optical band gap of inorganic glasses usually

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6

corresponds to wavelengths in the ultra-violet and visible range. In ChG, where the anions (S, Se or Te) possess high energy lone-pair electrons, the optical band gap is narrowed. The absorption edge is thus shifted to longer wavelengths: in the green / red and near mid-infrared regions for the sulfide and selenide / telluride glasses, respectively. The coloring and opacity of ChG observed in Figure 1.2 are directly correlated to this optical band gap shift.

Figure 1.4: Schematic representation of the electronic band structure in amorphous semiconducting materials. Arrows A and B, C show the optical electronic transitions in the Weak Tail Absorption (WTA)/Urbach and Tauc regimes, respectively.

In vitreous materials, the absorption can be divided in three regions: the Tauc region, the Urbach and the weak tail absorption (WTA) regions which can be easily identified in the short wavelength edge in the transmission or absorption spectrum (see Figure 1.5) [13]. In the Urbach region (domain B in Figure 1.5), the absorption coefficient α(E) is an exponential function as:

α(E) exp (E/EU)

Where EU is the Urbach energy (characteristic energy related to the width of the valence (or conduction) band tail states, and may be used to compare the “widths” of such localized tail states of different material) and E is the energy of incident photons. In this region take place the electronic transitions between the tail of a band (corresponding to the localized states of a band) and the defects state (represented by the arrows B in Figure 1.4). In the Urbach regime, the values of absorption coefficient α range from 1 to 104

Defects states

Energy state density of a perfectly ordered system

Localized tails states

Ene

rg

y

Energy density of state

B

A

Forbidden_band Valence band Conduction band

C

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7 cm-1_{. In the domain A which corresponds to the weak absorption tail of an absorption} spectrum in Figure 1.5 occur the electronic transitions from tail to tail (represented by the arrows A in Figure 1.4). The absorption coefficient α in this regime is very weak, below 1 cm-1_{. In this case, α}_{shows a gradual exponential behavior:}

α(E) exp (E/EW)

which is referred to as a ‘weak absorption tail’ or ‘residual absorption’. EW is the energy characteristic to the width of the defect states in the bandgap.

Sometimes, the two latter regimes are joined in a single one, the so-called Urbach tail absorption. This regime of absorption depends on the temperature.

Figure 1.5: Typical spectral dependence of the optical absorption coefficient in amorphous semiconductors. In the A and B regions, the optical absorption is controlled by optical transitions between tail and tail, and tail and extended states, respectively. In the C region, the optical absorption is dominated by transitions from extended to extended states. In the domain B, the optical coefficient follows Urbach rule. In the region C, the optical absorption coefficient follows the Tauc’s relation, from[13].

In the Tauc region (domain C in Figure 1.5), the localized electronic states do not contribute to the absorption phenomenon. The transitions between the valence and conduction bands, shown by the arrows C in Figure 1.4, are similar to those observed in ideal crystals. The absorption coefficient α(E) is then given by the Tauc equation:

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8

where E0 is the optical gap energy and E is the energy of the incident photons. These transitions correspond thus to the absorption of higher energy photons (of shorter wavelengths) in comparison with the Urbach absorption. Typical values of absorption coefficient α in the Tauc region are _{. The optical gap energy E0 and Urbach} energy EU can be employed sometimes to characterize the local disorder in a glass structure [12].The good understanding of the above described notions is important when the material under study is an amorphous semiconductor like chalcogenide glassy thin films. Careful studies of the absorption edge may indeed provide useful information not only about the state of disorder, but also about the glass structure and its defect content. The existence of weak bonding arrangement in ChG, evidenced by their extended Urbach absorption tail, can be directly correlated to their extraordinary photosensitivity [14]. The irradiation of ChG glasses with near band gap or sub band gap laser light may generate various effects on these localized states and will be discussed later in this chapter.

1.1.2.3 Refractive Index

Another important characteristic that makes the chalcogenide glass materials unique is their high refractive indices (from 2.0 to 3.6 and above) when compared to those of oxide, phosphate or fluoride glasses (~1.45-1.6). Such glasses with high refractive indices may be used in various photonic devices as photonic crystals or omnidirectional reflectors for instance [15, 16].

The linear refractive index of a material essentially depends on its density and its polarizability. Their high value in ChG is thus easily explained by the fact that these glasses are based on heavy and polarizable elements which are covalently bonded within the network. Depending on the vitreous system and the glass composition within the latter, the ChG may exhibit wide variations of the refractive index, as shown in Figure 1.6 where the refractive index and optical band gap of different vitreous systems are depicted. It can be seen that refractive index and optical band gap can respectively vary from 2 to 3.6 and from 1.5 to 2.8 eV according to the glass composition/system.

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9 Figure 1.6: Compositional variation of the refractive index and optical band gap of chalcogenide glass thin films [17].

Several techniques exist to characterize the refractive index of a glass. One of them is indirect and consists in calculating first the Fresnel losses at normal incidence from a recorded absorption/transmission spectrum through the following relation:

0 0 2 1 1 1 n 1 n R T T            

where R is the reflection losses (light is reflected at both input and output interfaces) and T0 is the maximum transmission at a given wavelength. However, other direct and more accurate methods of refractive index measurement are usually preferred: the ellipsometry and the prism coupling techniques for instance.

1.1.2.4 Optical Non Linearity

Chalcogenide glasses are recognized for their excellent nonlinear optical properties which overcome those of all the known vitreous materials [18]. When a material is exposed to low intense light, its response is linear with the electromagnetic field, E, the polarization P of the material is thus expressed by:

P = (1) E

When exposed to an intense electromagnetic beam, its response becomes nonlinear and its polarization is then expressed by:

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10

P = (1)E + (2) E.E + (3)E.E.E + …

Where (1)_{is the linear susceptibility (first order) while}_(2)_and_(3)_{are the second} and third order susceptibilities, respectively (susceptibilities (n)_{are tensors of n+1 rank).} The coefficients decrease then rapidly for higher order terms. The importance of the second and third order terms depend on the field strengthE.

The polarization P originates from the creation of dipoles due to the displacement of the negative and positives charges in a system (e.g. molecule), under the action of the electric field E. When the electric field E is weak, this charges displacement is weak, the dipole thus oscillates harmonically, and the polarization of the material is linear. On the other hand, when the electric field E is strong, this charge displacement is strong, the dipole oscillation becomes anharmonic, and the material polarization is nonlinear.

The second order nonlinear properties exist only in non-centro-symmetric materials. When an inversion center exists for the considered material, the components of the tensor related to the second order susceptibility cancel each other by symmetry. Therefore, in centro-symmetric materials like glasses, there is no second order nonlinear effect. However, it is possible to break the glass isotropy by different glass treatments, called poling. For example, the thermal poling consists in heating the glass while applying an electric field in order to generate a permanent polarization of the glass, allowing the observation of second order nonlinear effects such as the second harmonic generation, the Pockels effect, the frequency addition, or the optical rectification.

The third order nonlinear properties are present in centro-symmetric materials and more specifically in glasses. Among the different optical nonlinear phenomena, one can cite the third harmonic generation, the frequency addition, the four-wave mixing or the optical Kerr effect. The latter is particularly utilized in telecommunications for signal processing.

As mentioned in the previous section, ChG exhibit high linear and non-linear refractive indices. Susceptibilities (3) of about two orders of magnitude higher to that of silica were measured in ChG [18]. Non-linear refractive index measured in silica is about _⁄_{at 1.06 µm [19]. In arsenic sulfide As2S3 glass, this value is about 80} times higher and incorporation of Se to As-S composition increases it by about 400 times

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11 [20]. This is explained by the presence of covalent, highly polarizable homopolar Se–Se bonds in the glass structure as identified by Raman spectroscopy. The measurements of non-linear optical properties of Ge-Se-As ChG glasses reveal that the substitution of germanium (Ge) for arsenic (As) reinforces the non-linearity while replacing selenium (Se) by arsenic (As) does not [21].

1.1.3 Thermal Properties of Chalcogenide Glasses

Chalcogenide glasses are defined as soft materials because their characteristic temperatures are lower compared to those of the traditional silicate based glasses. These temperatures, i.e. the glass transition temperature Tg, the crystallization temperature Tx and the melting temperature Tm are defined in the Appendix A3. For instance, the glass transition temperature Tg of fused silica glass is around 1100˚C. In comparison, the Tg of arsenic sulfide As2S3 glass is around 180˚C. The network of vitreous As-S is known to be two-dimensional. Addition of germanium to the As-S matrix results in a network reticulation, giving rise to a three-dimensional network and thus induced an increase in Tg to about 300-350˚C, depending on the glass composition. Germanium sulfide based glasses usually have Tg around 300-400˚C, while GaLaS glasses have the highest characteristic temperatures among the ChG (Tg around 450-550˚C). Selenium-based glasses have lower characteristic temperatures than sulfur-based ones, due to lower bond energies and average bond strength. Tellurium-based glasses have even lower characteristic temperatures than selenium-based ones, for the same reason. Therefore, the thermal behavior of a glass is essentially related to its chemical composition and its network reticulation, through the energy of chemical bonds forming the network. Assessing thermal characteristics of a glass, especially chalcogenide glass, is thus crucial not only for its preparation processing but also to understand how its structure evolves with the chemical composition.

Glasses with low tendency to crystallize, i.e. with a large difference between the onset crystallization and glass transition temperatures (see in Appendix A.3), are in general required to produce large bulk samples and optical fibers. However, glasses with increased crystallization ability may also find application in phase change optical memory for instance. The critical cooling rate is also an important key factor for

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glass-12

forming, particularly for thin film fabrication. If the glass melt is cooled, or quenched, at an insufficient cooling rate, crystallization will occur. The fabrication of large chalcogenide glass bulks or preform for optical fiber fabrication is usually restricted to a certain range of glass compositions because of their critical cooling rate. In thin film fabrication processing, very high cooling rate can be achieved, extending thus the glass-formation domain of a vitreous system, as will be discussed in the Appendix A.1-A.2.

Different thermal analysis techniques can be employed to characterize ChG, such as differential scanning calorimetry (DSC) used in this work (and described in Appendix A.3), thermogravimetric (TG) analysis, thermomechanical (TMA) analysis, etc. These techniques permit to probe one property, such as sample heat capacity, weight loss or expansion as a function of temperature under a controlled heating ramp. As briefly mentioned previously, they provide important information not only for optical fiber drawing or precision molding experiments for example, but also for the understanding of glass structure and its related behavior. In the same way, the viscosity of ChG is of major interest for glass processing as for any other glass and is assessed through different thermal analysis techniques depending on the range of viscosity (which is of more than 14 orders of magnitude) of interest.

1.1.4 The Ge-As-S Glass System

Arsenic sulfide As-S glasses have been widely investigated over the past sixty years and are still the subject of many studies besides their current practical utilization for mid-infrared optical glasses and fibers. As can be seen in Figure 1.7, the vitreous domain of the As-S binary system is large around the stoichiometric composition As2S3 (= As40S60, at.%), allowing the fabrication of many possible compositions with sulfur S content ranging from about 55 to 100 at.% (and arsenic (As) content ranging from 45 to 0%, respectively). Glassy arsenic sulfide is also obtained in a second compositional range, around 40 at.% of S (60 at.% of As), while crystallization and/or phase separation is observed below 35% and between 45 and 55% of S. Likewise, such extended vitreous domain can be an advantage to control the glass optical and/or thermal properties by simple adjustment of its elements concentration.

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13 Addition of Germanium (Ge) to the As-S system gives rise to a ternary system whose domain of glass-forming ability may also be particularly vast with a large range of non-stoichiometric compositions, as shown in Figure 1.7. Numerous Ge-As-S compositions different from the stoichiometric GeS2-As2S3 and Ge2S3-As2S3 pseudo-binary vitreous compounds (red dot lines in Figure 1.7) may form glass. Unlike other glasses like those based on oxides or halides, the preparation of non-stoichiometric composition of Ge-As-S system (with an excess or default of S anions) is possible as the preparation can be realized from each single element separately. This provides a unique flexibility to finely tune glass properties such as optical band gap, linear or nonlinear refractive indices, photo-sensitivity, etc.

However, the vitreous domains reported in the literature for the Ge-As-S ternary system are often inconsistent owing to the use of different conditions for glass synthesis (e.g. cooling rate) or unconsidered influence of impurities [22]. Figure 1.7 shows the vitreous domain recently reported by Musgraves and his colleagues [23]. To facilitate the reading of this diagram, three regions within the vitreous domain can be distinguished: (i) first, the S-rich region which contains compositions with S concentration higher than ~66.7 at.% (corresponding to left corner of the diagram in Figure 1.7); (ii) second, a S-poor region, identified in blue and subdivided in three sub regions (labeled A, B, C) which will be described later and; (iii) third, an intermediate region localized between the last two, with S content comprised between 66.7 to about 40 at.%.

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15 leading to the reticulation of the glass network (3D) [24]. Thus, some related glass properties (like glass characteristic temperatures or chemical stability) are greatly improved, making the Ge-As-S glasses promising materials for optical, nonlinear optical, optoelectronic and photonic applications [25, 26].

Figure 1.8: Structure models of (a) three-dimensional continuous random network like fused silica SiO2 or chalcogenide glass GeS2 and (b) two-dimensional distorted layers in As2S3 chalcogenide glass,

from [27].

The intermediate region, comprised between the GeS2-As2S3 tie line and S = 40 at.% line (left-border of the third region, in blue in Figure 1.7) is then considered as sulfur deficient (except for the GeS2-As2S3 tie line). A reduction up to a complete disappearance of the sulfur rings or small chains from the glass structure is observed in this region [23]. Hereby, with the progressive transition from the GeS2-As2S3 tie line to the opposite arbitrary limit, an increase of the content of Ge-Ge and As-As homopolar bonds can be observed. This is due to the fact that Ge cation (whose expected coordination number is four) and As cation (whose expected coordination number is three) do not find enough S anions (whose expected coordination number is two) to form heteropolar bonds.

Last, the S-poor region can be seen here as a highly sulfur deficient compositional range. This domain includes glass compositions with less than 40 at.% of S, while 66.7 at.% are theoretically required to satisfy the glass stoichiometry. An increase of content of the Ge-Ge and As-As homopolar to the detriment of heteropolar Ge-S/As-S ones is therefore expected and experimentally shown by Raman spectroscopic studies [23]. To