SPACE-CHARGE WAVES IN PHOTOREFRACTIVE CRYSTALS AND THEIR PARAMETRIC-EXCITATION

It is shown that weakly damped space-charge waves exist in photorefractive crystals with a sufficiently large lifetime-mobility product. The dispersion law, the damping constant, and the quality factor of these waves and their dependence on the crystal parameters and on the experimental conditions are found. The instability of the fundamental grating against excitation of these waves is investigated; the fundamental grating is excited either by a moving interference pattern or by a standing interference pattern plus an oscillating external electric field. The dependence of the threshold and the characteristic exponent of the instability on the wave vectors of the exciting waves and on the experimental parameters is found for the three-dimensional case. It turns out that the strongest instabilities correspond to the cases of optimal enhancement of photoinduced gratings. The theory is compared with the experimental data for BSO crystals.