RECORDING OF HOLOGRAPHIC GRATINGS AND THEIR COHERENT SELF-ENHANCEMENT IN AN a-As 2 S 3 FILM WITH A MINIMUM LIGHT INTENSITY MODULATION

The holographic grating recording efficiency and the coherent self-enhancement efficiency of gratings are experimentally studied depending on the recording light interference fringe visibility in an a-As2S3 chalcogenide film in order to find the minimum visibility. The minimum fringe visibility Mmin = 3.6×10–4 is found, which is determined by the scattered light background. In this case the maximal diffraction efficiency ηmax = 0.05% and the maximal self-enhancement factor ξmax = 5.0, compared to ηmax = 15% and ξmax = 12.3 in the optimal M = 1.0000 case. In the case of two-beam holographic grating recording the maximal diffraction efficiency increases when M is increased, whereas sensitivity decreases. A simple model, based on linear recording with the spatial light intensity distribution governed by M and including intensity-dependent material photosensitivity and grating relaxation, is proposed to explain these results. In the case of coherent self-enhancement both the maximal diffraction efficiency and sensitivity increase with fringe visibility M. This can be explained by the fact that mechanical stress modulation during the initial grating recording depending on M is followed by relaxational structural changes reinforcing the coherent self-enhancement effect. The developed approach may explain the absence of this effect in some azobenzene oligomer films.