Phase Conjugation in Photorefractive Media

Phase conjugation (PC) by degenerate four-wave mixing in photorefractive crystals such as LiNb₃3, BSO, BaTiO₃ is different in kind than PC in other media. The effect is non-local and the nonlinearity, say X(³) cannot be obtained from the solution of atomic responses. Strictly speaking, we cannot characterize the medium response by a simple constitutive third order nonlinear constant X(³) since the strength of the grating produced by the interference of two optical beams depends on the period of the gratings, clearly a nonlocal effect.