RELAXATIONAL SELF-ENHANCEMENT OF HOLOGRAPHIC GRATINGS IN AMORPHOUS AS2S3 FILMS

A detailed experimental study of the relaxation of holographic grating s in disordered materials is presented. Relaxation parameters of holog raphic gratings in nonannealed As2S3 amorphous semiconductor films hav e been measured as a function of aging time, initial diffraction effic iency, recording light intensity, and grating period. The influence of the readout light intensity and sample thickness and its temperature has also been investigated. Relaxational self-enhancement of gratings were found up to 18 times with respect to the initial diffraction effi ciency, with a saturation value stable over a period of more than 2 ye ars. The relaxational self-enhancement effect is explained in terms of a phenomenological relaxation model with periodically distributed str ess. Under certain conditions, a spatially periodic mechanical stress field resulting from a holographic grating causes anomalous diffusion of unfilled sites. This leads to a density modulation which increases the initial refractive index modulation. The motion of unfilled sites is enabled by the movement of kinetic particles including S atoms. The correlation length of the structural disorder of amorphous As2S3 film s is estimated from the relaxational self-enhancement effect measureme nts to be about 0.5 mum.