Simultaneous microscopic measurements of photodarkening and photoexpansion in chalcogenide films

A near-field scanning optical microscopic analysis is performed on As2S3 film gratings in order to simultaneously collect index and topography images with sub-micrometre resolution. This technique makes it possible to unambiguously study the correlation between photodarkening and photoexpansion at the local scale. The development of a positive index change concomitantly with expansion or contraction in films of different thermal histories suggests that homopolar bonds are not a major contributor to the darkening effect. Photodarkening is instead associated with structurally stable defects that appear to be largely decoupled from the volume change mechanism. While photoexpansion and photodarkening follow the same growth kinetic during irradiation of annealed films with band-gap light, it is clearly shown that their structural origin is different. These results have relevance for grating fabrication since both the relief and the index patterns contribute to the grating efficiency, yet they appear to have distinct behaviour during processing or long-term stability.