Competition between photorelaxation and photoexcitation in chalcogenide glasses and the effect of aging

Measurements of transient photodarkening and permanent Bragg reflector formation were performed on fragile As₁₈S₆₅Se₁₇ glass and indicate that photodarkening and photorefraction can only be induced in previously aged glasses while they are prevented in freshly annealed glasses. These phenomena are associated with the photorelaxation process that is revealed to be prominent in fresh glasses and is shown to effectively anneal out the defects produced during photoexcitation thereby resulting in no net permanent changes in the structure. On the other end, aged glasses do not undergo any photorelaxation and exhibit large transient and permanent changes associated with an increase in structural enthalpy as confirmed by Differential Scanning Calorimetry. These results demonstrate that photorelaxation can be a significant problem in fragile glass compositions as it cancels out the effect of photoexcitation and impedes the photodarkening and photorefractive process.