Relaxational self-enhancement of holographic gratings in amorphous As2S3 films

Abstract

A detailed experimental study of the relaxation of holographic gratings in disordered materials is presented. Relaxation parameters of holographic gratings in nonannealed AS& amorphous semiconductor films have been measured as a function of aging time, initial diffraction efficiency, recording light intensity, and grating period. The influence of the readout light intensity and sample thickness and its temperature has also been investigated. Relaxational self-enhancements of gratings were found up to 18 times with respect to the initial diffraction efficiency, with a saturation value stable over a period of more than 2 years. The relaxational self-enhancement effect is explained in terms of a phenomenological relaxation model with periodically distributed stress. Under certain conditions, a spatially periodic mechanical stress field resulting from a holographic grating causes anomalous diffusion of unfilled sites. This leads to a density modulation which increases the initial refractive index modulation. The motion of unfilled sites is enabled by the movement of kinetic particles including S atoms. The correlation length of thestructural disorder of amorphous A& films is estimated from the relaxational self-enhancement effect measurements to be about 0.5 'pm.

Recording of holographic gratings and their coherent self-enhancement in an a-As₂S₃ film with a minimum light intensity modulation

“…Self-enhancement (SE) of a non-stationary hologram is an increase in its diffraction efficiency (DE) over time under the stimulus of a single beam light irradiation or simply in the dark [2,3].The SE effect was reported for the first time in 1973 by Gaylord et al [11] for phase holograms in LiNbO 3 :Fe crystals. Three types of SE can be distinguished: (i) coherent SE due to the holographic recording by diffracted waves; (ii) incoherent SE due to the contrast and/or transmission increase of a hologram by incoherent light; (iii) relaxational (or dark) SE due to the contrast and/or transmission increase of a hologram by thermostimulated relaxation processes [2,3].…”

“…This instability manifests itself through internal mechanical stresses arising in the course of preparing these films (e. g. during thermal deposition in the case of a-As 2 S 3 films) [2,8,9]. The mechanical stresses force the disordered state toward a more ordered one and diminish in the course of relaxation [2,8,9]. Both PSC and RSC in a-As 2 S 3 films are manifested as changes in absorption and refractive indices, thickness, density, microhardness and dissolution rate [4,9].…”

“…Yet such studies at a fixed spatial frequency can be regarded as a special case of holographic grating spectroscopy of materials. The aim of this work was to study the recording light interference fringe visibility dependence of HG recording efficiency in an a-As 2 S 3 film by two beams (as usual) and by one beam in the process of HG coherent self-enhancement [2,3] focusing on extremely low visibilities.…”

“…Amorphous As 2 S 3 semiconductor chalcogenide films are used for real-time holographic information storage, optical computing, and for making holographic optical elements [2][3][4]. Diffraction efficiency exceeding 80% [4], high spatial resolution of spatial frequencies up to 9700 lines/mm [4,5], simplicity of a direct recording process for practically permanent holograms which need neither fixing nor development, reversibility, and availability of large samples are some of the attractive features of these materials.…”

“…due to their thermodynamic instability [8]. This instability manifests itself through internal mechanical stresses arising in the course of preparing these films (e. g. during thermal deposition in the case of a-As 2 S 3 films) [2,8,9]. The mechanical stresses force the disordered state toward a more ordered one and diminish in the course of relaxation [2,8,9].…”