Degenerate four-wave mixing in amorphous $As_2 S_3$ film (thickness-5um) is performed using the cw Ar-ion laser (5145Å). The probe beam intensity is made sufficiently smaller (1-10%) than the pump beam intensity and the phase conjugate wave is generated in the backward direction of the probe beam. The intensity detected has showed the typical time dependent variation which is of great interest.
The intensity of the signal beam ($-10^{-2}mW$), which is the phase conjugate wave, is nearly independent of the forward pump beam intensity, but when the probe beam intensity or backward pump beam intensity is increased, the signal beam intensity increases. When the intensity of the backward pump beam is increased, the first peak in the time dependent signal beam has appeared earlier in time. In order to investigate the above time dependence of the signal beam, the diffraction equation of the extended Kogelnik"s coupled wave theory of volume grating is applied.
Also the Mach-Zehnder interferometer is used to measure the time dependence of Δ(nL) in $As_2S_3$ film. When the laser power is increased, Δ(nL) changes slowly. The time dependent behaviour of Δ(nL) experimentally obtained is substituted to the diffraction which describes the phase conjugate wave generation.
When Δ n.L 《 n.Δ or Δ n.L 》 n. Δ L is assumed, the diffraction efficiency η(t) showed the behaviour which qualitatively agrees with the experimental data. Also in the investigation of the transmitted power through the $As_2S_3$ film, the change Δ L in the thickness has appeared increasingly dominent as laser beam intensity is increased.
From this experiment, it is reasonably concluded that when the light intensity is high, Δ n.L 《 n. Δ L and in the low light intensity, Δ n.L 》 n. Δ L.
