Phase conjugation in amorphous selenium thin films

We report phase conjugation in amorphous selenium thin films with a 25-mW He-Ne laser. The phase-conjugate signal exhibits a maximum efficiency of 18%. Gratings were recorded with spatial frequencies of 275 lines/mm. We have performed micro Raman spectroscopy measurements between and on the lines that form the grating, and the results reveal the presence of amorphous selenium only. Scanning electron microscopy together with profilometry measurements indicate induced variations of the film thickness. However, this relief does not seem sufficient to explain the observed effects.