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It is shown that two-step, two-wave mixing makes a substantial contribution to the total three-wave mixing process in a noncentrosymmetric crystal. The efficiency of generating 2₁-₂ was observed as a function of the difference frequency ₁-₂, the electric polarization vector, and the propagation direction. This method allowed the determination of the true third-order susceptibility ^ (3), in both sign and magnitude, in terms of {^ (2) }^2.
Yablonovitch et al. (Mon,) studied this question.