Photodarkening mechanism is investigated in amorphous $As_2S_3$ thin film, which is prepared on a slide glass by vacuum evaporation of bulk $As_2S_3$. The film is illuminated by Ar ion laser beam of wavelength 5145Å and the transmittance change due to photodarkening is measured.
The experimental results show that the reciprocity law about photodarkening is not satisfied and that the absorption coefficient of the film photodarkened for infinite illumination time is a function of the illumination intensity only and irrespective of the previous illumination intensity.
These properties are explained by the proposed model which allows the thermal transition from the metastable state to the stable state. In this treatment we use the previous assumption made by Tanaka et al. that the barrier height is scattered from site to site.
It is possible to record and erase holograms repeatedly in the photodarkened film by a purely optical means. The diffraction efficiency, the recording time, and the erasure time of the recorded hologram are measured for various illumination conditions. The diffraction efficiency is of the order of $10^{-4}$, which is comparable to those of photorefractive crystals in the absence of any applied field. And the reciprocal of the erasure time is nearly proportional to the reading beam intensity.
Amorphous $As_2S_3$ thin film can be used as hologram recording material as well as nonlinear material in DFWM experiment. One-way image transmission using phase conjugation is investigated by using the film as the nonlinear material. The mask object is installed in the path of a backward pump beam which is incoherent to the forward pump beam and the probe beam. A transmission type grating is produced in the $As_2S_3$ film by the forward pump beam and the probe beam. As the grating diffracts the backward pump beam to generate the phase conjugate wave, slow response time of the film is irrelevent in this experiment. It is shown that the image of the mask object is indeed formed by the retrotraversing phase conjugate wave.
