We demonstrate a possibility to write efficient and thermally stable volume holographic gratings in a glassy polymer material based on PMMA and phenanthrenequinone with layers prepared, by casting the liquid solution of ingredients on a substrate and drying to a solid state. A high concentration of phenanthrenequinone (up to 4 mol. %) makes it possible to use photosensitive layers of lower thicknesses (50-180 µm) for the recording of efficient holographic gratings. The exposing is followed by a thermal amplification of the grating due to diffusion of residual phenanthrenequinone molecules and fixation by an incoherent optical illumination. We present experimental temporal curves of the refractive index modulation and diffraction efficiency both under the exposure and the heating process. The behavior of the gratings under temperatures up to 140 • C has been studied.

For phase holographic gratings in layers of polymethylmethacrylate, containing phenanthrenequinone in high concentration (nearly 3 mol%), a discrepancy between experimental (up to 9) and estimated (∼45) magnitudes of the thermal diffusion amplification coefficient has been revealed. Analysis of plausible reasons of the lower experimental efficiency of the diffusion amplification has been carried out. The influence of material deformations on the reflection grating formation process was investigated experimentally. It is shown that thermoactivated amplification of holograms under high phenanthrenequinone concentration and its profound modulation are depressed by the arising density 'grating'.

The schemes of light beam transformations by volume dynamic holograms in resonant media revealing the fifth-or higher-order nonlinearities have been theoretically analyzed. N-wave mixing has been realized by changing of the propagation direction or frequency of the read-out wave in the solution of Rhodamine 6G and polymethine 3274U dyes. It has been demonstrated that the experimental results are in good agreement with the theoretical data obtained for a three-level medium model with due regard to absorption from the excited singlet level.

A photorefractive Bi 12 TiO 20 (BTO) crystal is exposed with high intensity pulsed-laser beams 532 nm (the average intensity of the laser was 110 mW/cm 2 , the average intensity per pulse was 2 MW/cm 2 ), and the photo-induced dynamics of the absorption between 480 and 900 nm are studied and explained by two long-lived energy levels in the forbidden band. The relaxation times of the long-lived energy levels are experimentally found to be 10 4 and 10 5 s. The hysteresis character of the absorption coefficient is discussed.

The spectral dependences of absorption photoinduced in a pure bismuth titanium oxide crystal by 532-nm laser pulses are studied. It is shown that optical absorption in the crystal in the range from 492 to 840 nm increases with increasing exposure. The photoinduced absorption relaxes in the dark for more than 60 hours. A model of photoinduced absorption is proposed which assumes the population of two trap centres with the normal energy distribution law for the concentrations of electrons photoexcited from donors to the conduction band. This model well describes the spectral dependences of photoinduced absorption by using the average ionisation energies of the traps E 1 1X60 eV and E 2 2X57 eV. The model is used to estimate the increase in the photorefractive sensitivity of a bismuth titanium oxide crystal in the near IR region, which was earlier observed after exposing the crystal to visible radiation. It is predicted that the speed of response of dynamic holography devices based on BTO crystals exposed to green light can be increased.

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Theoretical and experimental studies of the interaction of two counterpropagating light waves on dynamic reêection gratings formed in the Denisyuk scheme in cubic photorefractive crystals due to the diffusion mechanism of the charge transfer are considered. The results are presented which demonstrate the possibility of using dynamic photorefractive Denisyuk holograms to design adaptive elements of measuring systems based on optical and ébreoptic interferometers.

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