Photorefractive ac-enhanced nonlinear response of sillenites: Low- and high-contrast effects

Analytically and numerically we investigate the dependence of the first Fourier harmonics of the space-charge field, induced in an AC-biased sillenite crystal by a light-interference pattern, on the light contrast m. It is shown that within the whole contrast range, 0 < m < 1, these dependences are controlled by the only scalar parameter - the space-charge wave quality factor Q. In the low-contrast limit, m less than or similar to Q(-2), this factor defines the degree of enhancement of the nonlinear response while for larger contrasts it characterizes strong saturation effects. The data obtained are compared with the results of the previous studies of the AC-response. The possibilities of experimental detection of predicted dependences and their possible implications are discussed.