A nonlinear dynamic effect of formation of photo-induced periodic structures in waveguiding As2S3-Ag thin films

A nonlinear effect of spatiotemporal transformations of a focused linearly polarized Gaussian beam of a He-Ne laser obliquely incident (S and P polarizations) upon an As₂S₃-Ag film containing a small amount of chlorine and deposited onto a glass substrate is found and interpreted. The effect consists in the appearance, in the process of exposure, of beams diffracted and anisotropically scattered by periodic structures arising due to excitation of the waveguide TE₀ and TM₀ modes in the film. The diffraction measurements of the effective refractive indices for the TE₀ and TM₀ modes make it possible to calculate the refractive index and the thickness of the film from dispersion equations. Along with slow evolution of the diffraction and scattering patterns, we have found a persistent optical turbulence in the fringes of scattering from the structures related to excitation of the TE₀ modes. It is suggested that the turbulence is an indication of existence of reversible processes in the mechanism of photosensitivity.