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.

A photorefractive BTO crystal is exposed with cw-laser beams (430 mW/cm 2 at 514 nm), and the photoinduced absorption between 480 and 900 nm is studied. A method of controlling this absorption by low intensity laser radiation of "green" and "red" wavelengths is demonstrated. The physical mechanism can be explained by the redistribution of electrons on the long-lived energy levels into the forbidden band. Dynamical characteristics of the redistribution are estimated.

We present the results of experimental studies of the dynamics of the photoinduced optical absorption in a bismuth silicate crystal subject to continuous laser irradiation with wavelengths of 532 and 655 nm. The semiconductor-laser light beam with the wavelength λ = 655 nm causes the crystal bleaching at this wavelength, whereas its exposure to a shorter-wavelength irradiation from the optical-spectrum green region with a wavelength of 532 nm increases the optical absorption at both wavelengths, of 532 and 655 nm. The experimental results are interpreted using the theoretical model which assumes that the crystal has deep defect centers of two types so that an electron at each of these centers can be in one of the states characterized by different photoionization cross sections.

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the beam propagation with regard to optical activity as well as without considering one is presented. We investigate numerically the influence of the magnitudes of external applied voltage and background illumination and of the size and polarization of input beam on the selffocusing. The conditions required for soliton-like propagation are obtained. We discuss also an evolution of the beam polarization state during propagation distance, which is connected with the optical activity.

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