Dispersion law of photorefractive waves in sillenites

We investigate the dispersion law (relationship between Omega(k) and K) of photorefractive waves using a new experimental method based on the excitati on of photorefractive waves by a spatially oscillating interference pattern and on the detection of the first non-Bragg diffraction order. The existen ce of a sharp resonancelike dependence of the output signal on the frequenc y of excitation allows us to analyze the dispersion law with high precision . Bi12TiO20 and Bi12GeO20 single crystals are used for the investigations, and the dependences of Omega(k) on K, on external field, on index of modula tion, and on light intensity are studied. The experimentally observed relat ion Omega(k)proportional to 1/K is in excellent agreement with theory. A co ntribution of nonlinear effects to the spectrum of photorefractive waves is discussed. [S1050-2947(99)00909-9].