Photodarkening in AsS films and its application in grating fabrication

Abstract

Of the many photoinduced effects which occur in chalcogenide glasses, photo-darkening is of prime importance for applications. In order to determine our film compositions we have used an electron microprobe analysis. This analysis shows that in the AsS system for compositions richer in As than As 30S 70 there is good agreement between the film and source compositions. For sulphur-rich compositions however the films seem to be more As rich than the source. This difference has been attributed to the preferential evaporation of sulphur. The results show that, in the AsS system, the extent of photodarkening increases with increasing As concentration. This increase can be explained if photo-darkening involves the formation of AsAs bonds. The photo-darkening becomes negligible around the composition As 30S 70. By knowing the extent of photo-darkening, it is possible to work out the change in absorption coefficient and the change in the film refractive index due to illumination. Our analysis shows that for As-rich compositions, changes in the refractive index as high as 0.1 are induced by photo-darkening. Such a large change in the refractive index makes AsS thin films promising media for optical recording.