Unusual observation of fast photodarkening and slow photobleaching in a-GeSe2 thin film

We report an unusual observation of fast photodarkening and slow photobleaching in a-GeSe2 thin film, when subjected to 532 nm band gap light illumination. Photobleaching is observed only after complete saturation of photodarkening. A detailed kinetic analysis of photoinduced transmission change reveals that photodarkening is one order faster in comparison to photobleaching. Interestingly, photobleaching has an induction time period and irreversible in nature whereas photodarkening appears to be an instantaneous process with light illumination showing transient characteristics.