GRATING OSCILLATIONS AND NONLINEAR EFFECTS IN PHOTOREFRACTIVE CRYSTALS

The theory of grating oscillations in photorefractive materials is dev eloped in the linear and nonlinear approximations for the drift mechan ism of holographic recording. The resonance dependence of dim-action e fficiency on the phase-modulation frequency is predicted for the linea r regime in which holograms are recorded by two beams, one of which is phase modulated. For long drift lengths (KgL0 much greater than 1) th e resonance frequency is shown to be Omega(r) approximate to (tau(1)K( g)L(0))(-1). A more pronounced resonance peak is expected for the non- Bragg diffraction orders. In the nonlinear regime of recording, additi onal resonance maxims at Omega(r)/p (p is an integer) are found. Grati ng oscillations are experimentally studied in thin holograms of Bi12Ti O20. A sharp resonance for the non-Bragg order in the interval 100-200 0 Hz is detected. The position of the resonance is shown to depend on the experimental conditions. The experiment is in excellent agreement with the theory. At a high contrast ratio and in a high external elect ric field, distortions in the resonance dependence at Omega < Omega(r) and even a chaotic frequency dependence are found, which points to th e nonlinear character of grating oscillations. (C) 1998 Optical Societ y of America.