Theory of four-wave mixing in photorefractive media when the response of a medium is nonlinear in respect of the modulation parameter

A theoretical model is constructed for four-wave mixing in a photorefractive crystal where a transmission grating is formed by the drift–diffusion nonlinearity mechanism in the absence of an external electrostatic field and the response of the medium is nonlinear in respect of the modulation parameter. A comparison is made with a model in which the response of the medium is linear in respect of the modulation parameter. Theoretical models of four-wave and two-wave mixing are also compared with experiments.