The origin of photoinduced anisotropy in germanium sulfide based glasses.

Metastable photoinduced anisotropy (PA) generated by linearly polarized sub-bandgap light in Pr-doped Ge-S-I glasses is reported. A PA minimum is observed in undoped Ge34.5S55.5I10 composition. PA increases with S content in S-rich glasses. PA increases with Ge content at a constant I content in Ge-rich glasses, while addition of I at a constant S/Ge ratio in Ge-rich glasses decreases the PA. The addition of Pr to about 1000 ppm increases PA in Ge-rich glasses, if no Pr-clustering occurs. The explanation of these compositional trends is based on the coexistence of three different molecular phases (structural units) in these glasses, with their relative contribution varying with varying composition. Microscopic models are proposed for diamagnetic defect centers responsible for PA in S-rich, Ge-rich and stoichiometric glasses.