Anisotropic structural change induced by sub-bandgap light in As–S glasses studied by quasi-elastic light scattering

Abstract Changes in the quasi-elastic light scattering spectra with illumination of linearly polarized sub-bandgap light have been investigated by monitoring simultaneously the formation of Bragg gratings in As–S glasses in order to study the mechanism of photoinduced anisotropic structural rearrangement. We have found that the quasi-elastic light scattering intensity and the configurational relaxation time increase with illumination time only in the earlier stage of the formation of Bragg gratings. On the other hand, the low-frequency Raman spectra associated with intermediate or mesoscopic vibrational excitations exhibit changes in the whole stage of Bragg grating formation. The observations indicate that a local bonding rearrangement occurs mainly in the earlier stage, while co-operative mesoscopic fragmentary rearrangements are involved in the whole stage.