Relaxational self-enhancement of holographic gratings in amorphous As2S3 films

Abstract

A detailed experimental study of the relaxation of holographic gratings in disordered materials is presented. Relaxation parameters of holographic gratings in nonannealed As2S3 amorphous semiconductor films have been measured as a function of aging time, initial diffraction efficiency, 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-enhancements of gratings were found up to 18 times with respect to the initial diffraction efficiency, with a saturation value stable over a period of more than 2 years. The relaxational self-enhancement effect is explained in terms of a phenomenological relaxation model with periodically distributed stress. 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 films is estimated from the relaxational self-enhancement effect measurements to be about 0.5 μm.