Phase Conjugation in Photorefractive Media
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Phase conjugation (PC) by degenerate four-wave mixing in photorefractive crystals such as LiNbO3, BSO, BaTiO3 is different in kind than PC in opther 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.