Relaxations and Light-Induced effects in the Chalcogenide Glass As2Se3

Chalcogenide glasses (CGs) exhibit a distinctive set of properties that render them essential both for fundamental research and for advanced technological applications. This Thesis investigates thermally and optically activated quasi-local structural rearrangements in the CG As2Se3 by employing fast differential scanning calorimetry (FDSC) in combination with a dedicated light-irradiation setup. This methodology reveals a series of pronounced phenomena that critically dictate the behavior of the glass. I show that the structure and response of As2Se3 glass are strongly governed by the cooling rate used during vitrification: higher quench rates (QC) produce a more structurally heterogeneous glass. Upon subsequent reheating in FDSC, the observed endothermic and exothermic events directly reflect the underlying α- and β-relaxation dynamics. Importantly, As2Se3 displays clear rejuvenation at elevated quench rates, highlighting the tunability of its glassy state. To probe enthalpy relaxation, I performed isothermal annealing on fast-quenched (FQ) samples, which reveals the Johari–Goldstein (JG) or β-relaxation that is typically challenging to resolve in strong glasses. To further deconvolve the heat-flow signal, I employed temperature-modulated FDSC (TM-FDSC), an advanced form of FDSC that separates reversible from irreversible contributions to the heat capacity. When examining As2Se3 at base modulation frequencies of 0.1 Hz and 1 Hz, the JG relaxation signature is clearly evident at 0.1 Hz, which is entirely different from 1 Hz, underscoring the dynamic character of this secondary relaxation process. A simple Debye-relaxation framework (including single and double relaxation modes) is then used to interpret the data and is shown to capture the main experimental trends. Extending the study beyond purely thermal effects, I designed an FDSC–laser coupling configuration that allows controlled exploration of photo-induced processes in As2Se3 glasses prepared at different quench rates and exposed to a range of irradiation power densities (Pd’s). These experiments demonstrate that light profoundly reorganizes the glass network: it softens the glass while concurrently inducing both rejuvenation and relaxation. Significantly, optical irradiation drives the material toward a distinct iso-enthalpic stationary state whose enthalpy does not depend on either the initial quench rate QC or the applied power density Pd. These photo-induced transformations also shed light on the role and mechanism of the JG relaxation in As2Se3. Collectively, this work shows that thermally and optically driven rejuvenation and relaxation offer powerful tools for device processing, enabling precise control over structural evolution and the production of glasses with finely tailored, application-specific properties.