Photodarkening, structural instabilities, and crystallization of glassyAs2Se3induced by laser irradiation

Cw-laser-induced structural transformations in glassy ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ have been studied as a function of the irradiation time using light scattering techniques. The behavior of the material under the influence of the laser irradiation is divided into five stages with respect to the irradiation level: (1) For low intensities (I<1.0 kW/${\mathrm{cm}}^2$), the Rayleigh and Raman scattering intensities show a decay with the irradiation time which is interpreted as due to photodarkening; (2) For irradiation levels (1.0<I≤1.7 kW/${\mathrm{cm}}^2$) the Raman peak position and width show quasiperiodic changes with the irradiation time; (3) At intermediate laser intensities (1.7<I≤1.95 kW/${\mathrm{cm}}^2$) below the threshold intensity for crystallization additional crystallinelike Raman peaks appear and disappear quasiperiodically; (4) For somewhat higher intensities (1.95≤I≤2.1 kW/${\mathrm{cm}}^2$) close to the threshold for crystallization, the crystallinelike Raman peaks appear and disappear for some time but after that they increase and saturate; (5) For intensities above a threshold (I>2.1 kW/${\mathrm{cm}}^2$) the Rayleigh scattering and the crystalline Raman peaks grow rapidly and saturate after a short time to a height which depends on the intensity. We propose a model based on the existence of different metastable states in the glassy matrix having different degrees of disorder and on an athermal laser pumping mechanism of the irradiated material between these metastable states. We start from the photodarkening phenomenon at very low irradiation levels which correspond to transitions between only two states, and the enhancement of more atomic transitions at higher pumping rates up to the formation of submicrocrystalline clusters and their annealing or sequential internal transformations near the glassy-to-crystalline transition and finally their coalescence to form stable microcrystallites above some threshold intensity.