A. Ozols

RELAXATIONAL SELF-ENHANCEMENT OF HOLOGRAPHIC GRATINGS IN AMORPHOUS AS2S3 FILMS

A detailed experimental study of the relaxation of holographic gratings in disordered materials is presented. Relaxation parameters of holographic gratings in nonannealed As2S3 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...

Intensity threshold for holographic recording in amorphous As2S3 films

The dependence of holographic recording on recording light intensity has been experimentally studied in nonannealed amorphous As2S3 films. It has been observed that the threshold of the recording intensity is about 10−5 W/cm2 for 10.5‐μm‐thick film and a 1 μm grating period. A phenomenological model is presented to explain the intensity dependence in the 10−5–10−1 W/cm2 intensity range. According to this model the observed intensity behavior is caused by photoinduced spatially periodic mechanical stress.

Holographic recording in amorphous semiconductor films

The present state of the real time holographic recording in amorphous semiconductor films is reviewed including mechanisms, parameters, properties and applications. Effects of the coherent, incoherent and relaxational self- enhancement as well as the influence of the film structure relaxation are considered. Quasi-permanent sub-band-gap light holographic recording is reported for the first time. 157

Nonlinear exposure dependence of the holographic recording and relaxational structural changes in amorphous As2S3 films

The exposure dependence of holographic recording has been experimentally studied in nonannealed amorphous As2S3 films for small exposures ≤1 J/cm2. The square root of the diffraction efficiency is found to depend nonlinearly on exposure. This nonlinearity increases when the film thickness is increased and the holographic grating period is decreased. No exposure threshold has been found down to exposures of 0.025–0.17 J/cm2, depending on period. Different reasons for the observed nonlinearity are discussed. Nonlinearity is explained in terms of a phenomenological model based on the simultaneous action of the photostructural and relaxational structural changes, and this model has previously been used to explain the existence of relaxational self‐enhancement and the intensity threshold for holographic recording in amporphous As2S3 films. The model itself is developed further.

Two-color holographic-grating formation in amorphous As 2 S 3 films

A detailed experimental study of the holographic gratings recorded in nonannealed amorphous As2S3 films by 514.5-nm light in the presence of 632.8-nm readout light is carried out. A strong influence of a continuous 632.8-nm readout is found. The dependences of the maximal first-order diffraction efficiency and the corresponding specific recording energy on the holographic grating period were studied in a wide range of periods from 0.40 to 70.0 μm for 2-yr-old films. The obtained results are discussed in terms of photoinduced structural changes, relaxational structural changes, photoinduced anisotropy, and photoinduced recharging of the localized states in the bandgap. The photoinduced sulphur related D-- and D+-center orientation mechanism is proposed to explain the stimulating action of 632.8-nm light.

Diffraction-efficiency oscillations in amorphous As 2 S 3 films

An experimental study of the holographic gratings recorded in nonannealed, thermally with time relaxed amorphous As2S3 films by 514.5-nm light in the presence of 632.8-nm readout light is carried out. The dependences of the maximal first-order diffraction efficiency on the holographic grating period was studied in a wide range of periods, from 0.40 to 70.0 µm. A peculiar oscillatory diffraction-efficiency temporal behavior occurring under certain conditions is reported. The obtained results are discussed in terms of photoinduced structural changes, relaxational structural changes, photoinduced anisotropy, and photoinduced recharging of the localized states in the bandgap. The diffraction-efficiency oscillations are explained by the model of annihilating defects and by multiwave mixing in a thin dynamic hologram.

Influence of the age of amorphous nonannealed As2S3 thin films on holographic properties

The dependences of the maximal first order diffraction efficiency and the corresponding specific recording energy on the holographic grating period were studied. Grating period was varied from 0.40 to 70.0 μm. Both fresh and aged films were used. A large holographic recording efficiency decrease in the course of aging is found to take place. These changes are due to the effective film grain size increase caused by the relaxational structural changes and atmospheric oxygen exposure. Results are explained with the aid of stress fields induced by the evaporation and holographic recording. The obtained results can be used to optimize the hologram recording in amorphous chalcogenide films.

Polarization holograms and diffraction anisotropy in amorphous chalcogenides

Polarization hologram recording based on the effect of photoinduced anisotropy (PA) is reviewed, focusing on amorphous chalcogenides (ACh). Possible PA mechanisms in ACh are considered. Polarization holographic grating recording in amorphous As?S?Se (a-As?S?Se) films is experimentally studied and analysed in comparison with scalar recording. It is holographically established that linearly polarized 632.8 nm light produces photoinduced anisotropy and the chalcogen related D+, D? centre reorientation and generation mechanism is proposed. It is used to explain the observed peculiarities of polarization (vector) recording in comparison with scalar recording based on photoinduced structural changes: much lower diffraction efficiency (4 ? 10?3?% versus?4%), much larger specific recording energy (6400?J/(cm2?%) versus 20?J/(cm2?%)), difference in spatial frequency response, instability (vector hologram lifetime of about two days versus practically permanent scalar holograms), the absence of hologram self-enhancement (present in scalar recording), and near perfect reversibility. It is also experimentally found that light diffraction from the polarization holographic gratings in a-As?S?Se films is indeed anisotropic since the readout wave polarization diffracted in the minus first order is changed in such a way that the linear signal wave polarization orthogonal to the reference wave polarization is reconstructed. The results obtained are discussed.

Effect of structure relaxation on the holographic recording in amorphous As2S3 films

Abstract Results of the experimental studies of holographic recording in a-As 2 S 3 films, such as exposure and intensity dependences, spatial frequency dependence, are reported. Several peculiarities are found including recording intensity threshold, nonmonotonical intensity dependences, nonlinear exposure curves, the change of the spatial frequency response due to film aging. These peculiarities are explained in terms of the phenomenological model which is based on the simultaneous action of the photostructural changes and the relaxational structural changes.

Mechanisms of holographic recording in amorphous semiconductor films

The mechanisms of holographic recording in (mainly chalcogenide) amorphous semiconductor films are reviewed including photoinduced structural changes, relaxational structural changes, recharging of localized states in the band gap, and photoinduced anisotropy. The holographic properties in these cases are compared. Different effects of hologram self-enhancement are also considered.