PHOTODARKENING IN AS-S FILMS AND ITS APPLICATION IN GRATING FABRICATION

Of the many photoinduced effects which occur in chalcogenide glasses, photo-darkening is of prime importance for applications. In order to d etermine our film compositions we have used an electron microprobe ana lysis. This analysis shows that in the As-S system for compositions ri cher in As than As30S70 there is good agreement between the film and s ource compositions. For sulphur-rich compositions however the films se em to be more As rich than the source. This difference has been attrib uted to the preferential evaporation of sulphur. The results show that , in the As-S system, the extent of photodarkening increases with incr easing As concentration. This increase can be explained if photo-darke ning involves the formation of As-As bonds. The photo-darkening become s negligible around the composition As30S70. By knowing the extent of photo-darkening, it is possible to work out the change in absorption c oefficient and the change in the film refractive index due to illumina tion. 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.