Photodarkening kinetics dependent of temperature have been investigated by measuring the photoinduced changes in optical transmittance of amorphous $As_2S_3$ thin films. Amorphous $As_2S_3$ thin films are deposited on slide glass by vacuum evaporation of bulk $As_2S_3$ in a vacuum of $10^{-6}$ Toor and the typical thickness of the film is 3μm. Illuminations of $As_2S_3$ thin films to light are made with Ar-ion laser at wavelength 514.5nm (2.41eV) which corresponds to band gap energy (2.34eV) of $As_2S_3$. The reversible shift of the absorption edge by band gap illumination and annealing near the glass transition temperature (~450 K) has been explained by the double well potential model. From the experimental results we have confirmed that the thermal transition rates from the metastable state to the ground state at room temperature can be neglected when compared with the optical transition rates, and thus the transient changes at room temperature does not originate from the thermal transitions between the double well potentials. A spectrophotometric measurement, based on the transmission interference spectra of the films, to determine the complex refractive index and the thickness is presented. A phase conjugate wave is generated by degenerate four wave mixing in amorphous $As_2S_3$ thin film. Using the results of the rate equations obtained from the photodarkening kinetics the time dependent behaviors of phase conjugate wave are analytically treated and the theory are compared with those of experiment. Self-diffraction of a hologram recorded in $As_2S_3$ thin film are also treated by applying the Magnusson-Gaylord's multiwave diffraction theory. Photoinduced nonlinear optical changes of amorphous $As_2S_3$, which does not come from the thermal transition at room temperature, are explained by using the density matrix formalism. The treatment based on the double well potential model for the four level system gives a improved analytic expression for nonlinear optical absorption coefficients, and explicitly show the break of reciprocity law between illumination intensity and exposure. Photo-darkening and partial photobleaching kinetics (transient changes) obtained theoretically are compared with the experimental results. Photoinduced anisotropy (birefringence and dichroism) arsing under the action of linearly polarized Ar-ion laser light is detected by observing the transmitted intensity of the probe beam (He-Ne laser, 633nm) behind the crossed polarizer and the temporal behaviors of the signal are theoretically investigated in terms of 3rd order non-linear optical susceptibilities. We have described the application of photoanisotropic $As_2S_3$ thin film to perform the nonlinear optical image processing such as subtraction, addition, contrast reversal and edge enhancement of the images. The experimental results are well explained by the extension of the theory for nonlinear optical polarization. In Boolean algebra, there are 16 logic functions for two input binary. We have demonstrated a implementation of optical parallel Boolean logic gates using the photoinduced anisotropy of amorphous $As_2S_3$ thin films.
