Étude du phénomène de démouillage dans les couches-minces de verres de chalcogénures du système As-S-Ag

Dans les domaines de l'optique et de l'électronique, la fabrication de structures nano et microscopiques organisées nécessitent l'utilisation de méthodes coûteuses d'un point de vue énergétique mais aussi de temps de travail et de quantité de matériel : dans la plupart des cas, les structures sont fabriquées par le biais de photo-lithographie, de gravure, et de FIB (sonde ionique focalisée) sur des matériaux tels que le silicium. Ces méthodes, bien que très utilisées et maîtrisées, produisent des structures dont la surface possède une certaine rugosité causée par le processus de gravure. Une alternative, plus simple et moins coûteuse, se trouve dans l'exploitation du phénomène de démouillage qui peut se définir comme étant une diffusion de la matière gouvernée par des forces interfaciales tentant de réduire la surface entre deux matériaux. En plus des effets décrits ci-après, le démouillage permet d'obtenir une réduction de la rugosité de la surface ; propriété très importante pour une application optique. Le phénomène de démouillage est bien connu dans la littérature et particulièrement dans le domaine des recouvrements polymériques (peintures, traitements de surface, etc.) puisqu'il s'agit d'un e et néfaste causant une réduction de la surface de la couche polymérique appliquée sur un matériau (décollement, formation de gouttelettes, etc.). Mais lorsque ce phénomène est contrôlé et appliqué aux couches-minces, il peut s'avérer très intéressant pour la fabrication de structures à l'échelle microscopique voire nanoscopique. De nos jours il existe plusieurs exemples de couches-minces polymériques ayant subit un démouillage contrôlé pour des applications telles que le développement de surfaces anti-réflectives. De même, le démouillage a été appliqué à des couches-minces métalliques pour former des structures possédant des propriétés plasmoniques (Au, Ag) ou magnétiques (FePt). L'application du démouillage aux systèmes vitreux reste cependant peu développée, mais possède d'attrayantes propriétés, particulièrement pour les verres de chalcogénures. Dans cette thèse, nous discutons des différents types de démouillages appliqués aux couches minces de verres de chalcogénures, et plus particulièrement au système Agx(As20S80)100-x. L'avantage de ce système réside dans sa bonne transmission dans l'infrarouge proche et moyen, permettant ainsi de possibles applications dans cette région sous formes de guides d'ondes, résonateurs, etc. Deux méthodes de démouillages sont étudiées : la première, thermique ; la deuxième, photoII induite. Pour le démouillage induit thermiquement, les couches-minces doivent subir un traitement thermique à des températures supérieures à la température de transition vitreuse du verre a n d'obtenir une viscosité suffisamment faible pour que la tension interfaciale entre la couche-mince et le substrat cause le démouillage de l'échantillon. Cette méthode permet l'obtention de structures circulaires quasiment hémisphériques dont la taille et la forme sont contrôlables par changement de la composition chimique et des paramètres thermiques. Pour le démouillage photo-induit, nous avons utilisé différents lasers à ondes entretenues (CW) dont les longueurs d'onde d'émission sont proches de celles des bandes interdites des différentes couches-minces. Le démouillage ainsi causé par l'irradiation laser est différent du démouillage induit thermiquement. L'exposition au faisceau laser va créer des structures démouillées linéairement et de manière perpendiculaire au champs électrique du faisceau incident. En plus du phénomène de démouillage, lorsque la couche-mince est irradiée par un faisceau laser de longueur d'onde proche de celle de la bande interdite, il apparaît un processus photoanisotropique de biréfringence. Nous avons étudié l'impact des différents paramètres liés au faisceau laser tels que la densité de puissance et le temps d'exposition. Ce processus est indépendant du démouillage et va apparaître avant celui-ci.The fabrication of self-assembled nano and microscopic structures has been of great interest in the past decades, mainly for the production of optical devices. The methods of fabrication are based on photo-lithography, liquid and/or gas etching, and focused ion beam (FIB). Those methods are costly and time consuming and might create poor surface quality. In this thesis, we present an alternative method of fabrication based on the dewetting phenomenon. It is de ned as the di usion of matter driven by interfacial forces and a reduction of the shared area between two materials. One interesting property of this process is a decrease in surface roughness leading to good optical quality. The dewetting phenomenon is well reported for polymeric lms as well as metallic thin- lms. In the case of polymeric materials, dewetting can be used for the formation of nano and micro-structures, for example, the fabrication of anti-re ective surfaces. For metallic materials, plasmonic (Au, Ag) and magnetic (FePt) structures can be produced on the surface of metallic thin- lms. However, for glassy thin- lms, dewetting is still in its early stages. In this thesis, we will explore di erent types of dewetting on chalcogenide glassy thin- lms of the Agx(As20S80)100-x system. An interesting property of the prepared thin- lms is their transparency in the infrared region allowing promising applications such as waveguides and resonators. Two distinct approaches are studied in this thesis: the rst one is a thermally-induced process; the second one is based on photo-induced e ects. For thermal dewetting, glassy thin- lms must undergo thermal treatments above the glass transition temperature (Tg) in order to decrease the viscosity until interfacial tensions between the lm and its substrate are high enough to induce dewetting. With this technique, we can obtain quasi-hemispherical structures whose shape and size can be controlled by varying chemical composition and thermal parameters (temperature, time, atmosphere, etc.). For laser-induced dewetting, we use a continuous wave (CW) laser whose wavelength must be near the bandgap of the material. Laser-induced dewetted structures are very di erent from their thermal counterparts: instead of being worm-like or hemispherical, they auto-organise in parallel lines, separated by a distance proportional to the initial thickness of the lm. IV Furthermore, structures are always perpendicular to the electric eld of the incident laser beam. Additionally, when thin- lms are exposed to a laser irradiation of near-bandgap energy, a photo-anisotropic process of birefringence appears. We have reported the e ect of di erent laser parameters such as power density and exposure time. This process of photo-birefringence is independant and appears before dewetting

Laser-induced dewetting of silver-doped chalcogenide glasses

We report the observation of laser-induced dewetting responsible for the formation of periodic relief structures in silver-based chalcogenide thin-films. By varying the concentration of silver in the Agx(As20S80)100-x system (with x = 0, 4, 9 and 36), different surface relief structures are formed. The evolution of the surface changes as a function of laser parameters (power density, duration of exposure, and polarisation) as well as film thickness and silver concentration has been investigated. The scanning electron microscopy and atomic force microscopy images of irradiated spots show periodic ripples aligned perpendicularly to the electric field of incident light. Our results show that addition of silver into sulphur-rich chalcogenide thin-films improves the dewetting when compared to pure As20S80 thinfilms. The changes in surface morphology were attributable to photo-induced chemical modifications and a laser-driven molecular rearrangement

Templated dewetting for self-assembled ultra low-loss chalcogenide integrated photonics

Integrated photonics is of growing interest but relies on complex fabrication methods that have yet to match optical losses of bulkier platforms like optical fibers or whispering gallery mode resonators. Spontaneous matter reorganization phenomenon (e.g. dewetting) in thin-films provides a way for self-assembled structures with atomic scale surface rugosity, potentially alleviating the problems of roughness scattering loss and fabrication complexity. In this article, we study solid-state dewetting in chalcogenide glass thin-films and demonstrate its applicability to the fabrication of high-quality integrated photonics components. Optimal dewetting parameters are derived from a comprehensive experimental study of thin-film properties under high temperature rapid annealing. Atomic scale surface roughness are obtained using dewetting, with RMS values as low as Rq = 0.189 nm. Several integrated photonics components are fabricated using the method and characterized. We show that the use of pre-patterned templates leads to organized, reproducible patterns with large-scale uniformity and demonstrate the record high quality-factor of 4.7 × 106 in compact (R = 50 µm) microdisks, corresponding to 0.08 dB⋅cm−1 waveguide propagation loss. The integrated devices are directly fabricated on standard silicon-on-insulator dice using the micro-trench filling technique and coupled to silicon waveguides, making them readily deployable with existing silicon devices and systems

Silicon subwavelength grating waveguides with high-index chalcogenide glass cladding

Silicon subwavelength grating waveguides enable flexible design in integrated photonics through nano-scale refractive index engineering. Here, we explore the possibility of combining silicon subwavelength gratings waveguides with a high-index chalcogenide glass as a top cladding, thus modifying the waveguiding behavior and opening a new design axis for these structures. A detailed investigation of the heterogeneous SWG waveguide with high-index cladding is presented based on analytical and numerical simulations. We design, fabricate and characterize silicon subwavelength grating waveguide microring resonators with an As20S80 cladding. Thanks to As20S80 negative thermo-optic coefficient, we achieve near athermal behavior with a measured minimum thermally induced resonance shift of −1.54 pm/K, highlighting the potential of subwavelength grating waveguides for modal confinement engineering and to control light-matter interaction. We also show that the chalcogenide glass can be thermally reflowed to remove air gaps inside the cladding, resulting in a highly conformal structure. These types of waveguides can find application in reconfigurable photonics, nonlinear optics, metamaterials or slow light

Sulfur-rich chalcogenide claddings for athermal and high-Q silicon microring resonators

Heterogeneous integration of materials with a negative thermo-optic coefficient is a simple and efficient way to compensate the strong detrimental thermal dependence of siliconon-insulator devices. Yet, the list of materials that are both amenable for photonics fabrication and exhibit a negative TOC is very short and often requires sacrificing loss performance. In this work, we demonstrate that As₂₀S₈₀ chalcogenide glass thin-films can be used to compensate silicon thermal effects in microring resonators while retaining excellent loss figures. We present experimental characterization of the glass thin-film and of fabricated hybrid microring resonators at telecommunication wavelengths. Nearly athermal operation is demonstrated for the TM polarization with an absolute minimum measured resonance shift of 5.25 pm · K−1, corresponding to a waveguide effective index thermal dependence of 4.28 × 10−⁶ RIU · K −1. We show that the thermal dependence can be controlled by changing the cladding thickness and a negative thermal dependence is obtained for the TM polarization. All configurations exhibit unprecedented low loss figures with a maximum measured intrinsic quality factor exceeding 3.9 × 10⁵, corresponding to waveguide propagation loss of 1.37 dB · cm−1. A value of−4.75 ± 0.75 × 10−⁵ RIU · K−1 is measured for the thermo-optic coefficient of As₂₀S₈₀ thinfilms

Universal micro-trench resonators for monolithic integration with silicon waveguides

We present a systematic study of micro-trench resonators for heterogeneous integration with silicon waveguides. We experimentally and numerically demonstrate that the approach is compatible with a large variety of thin film materials and that it does not require specific etching recipe development, thus making it virtually universal. The microresonators are fabricated through in-foundry silicon-on-insulator processing and in-house backend processing. We also report ultra-compact chalcogenide microresonators with radius as small as 5µ and quality factors up to 1.8 × 105. We finally show a proof-of-concept of a novel multilayer waveguide using the micro-trench technique

Silicon-coupled tantalum pentoxide microresonators with broadband low thermo-optic coefficient

Stable microresonators are important integrated photonics components but are difficult to achieve on silicon-oninsulator due to silicon intrinsic properties. In this work, we demonstrate broadband thermally stable tantalum pentoxide microresonators directly coupled to silicon waveguides using a micro-trench co-integration method. The method combines in-foundry silicon processing with a single step backend thin-film deposition. The passive response of the microresonator and its thermal behavior are investigated. We show that the microresonator can operate in the overcoupled regime as well as near the critical coupling point, boasting an extinction ratio over 25 dB with no higher-order mode excitation. The temperature dependent wavelength shift is measured to be as low as 8.9 pm/K and remains below 10 pm/K over a 120 nm bandwidth

Etchless chalcogenide microresonators monolithically coupled to silicon photonic waveguides

Integration of chalcogenide waveguides in silicon photonics can mitigate the prohibitive nonlinear losses ofsilicon while leveraging the mature CMOS-compatiblenanophotonic fabrication process. In this work, wedemonstrate, for the first time, a method of integratinghigh-Q chalcogenides microring resonators onto the sil-icon photonics platform without post-process etching.The method uses micro-trench filling and a novel ther-mal dewetting technique to form low-loss chalcogenidestrip waveguides. The microrings are integrated di-rectly inside silicon photonic circuits through evanes-cent coupling, providing an uncomplicated hybrid in-tegration scheme without the need to modify the exist-ing photonics foundry process. The microrings showa high quality factor exceeding 6⇥105near 1550 nmand propagation losses below 0.7 dB/cm, indicatinga promising solution for low-cost, compact nonlinearphotonic devices with applications in various fieldssuch as telecommunications and spectroscopy

Er3+-doped Ga-Ge-Sb-S glass thin films by PVD deposition

International audienceIn the frame of the major issues related to global warming and pollution, the microsensor based on mid-infrared (MIR) spectroscopy is a useful tool to allow continuous measurement of different bio-chemicals species that disturb our environment. In the aim of developing a MIR source potentially integrated in a microsensor, we fabricated rare earth doped chalcogenide thin films by different phase vapor deposition (PVD) technics. The RF magnetron sputtering, pulsed laser deposition (PLD) and electron beam evaporation were investigated.The selected glass system is Ge-Ga-(Sb)-S with Er3+ ions doping. Er3+ ions show emissions in NIR and MIR at 1.55 µm (4I13/2→4I15/2)and at 2.8 µm (4I11/2→4I13/2) under excitation at 808 nm. Deposition parameters were optimized for the three PVD techniques based on a comparison to define the higher fluorescence efficiency. Sulphide thin films were characterized by means of transmission, AFM, XPS, SEM, EDS, ellipsometry and Raman spectroscopy to better control the deposition behavior