Consideration of the coupled wave equations governing three wave mixing in photorefractives and nematic liquid crystals, when both are exhibiting a local response, shows that the main difference between the two is the self phase modulation that occurs in the liquid crystals. It is this difference that explains the dissimilar variation of the exponential gain factor, as a function of inter-beam angle. The other difference between the two media is the dependence of gain on the total incident intensity for the case of nematic liquid crystals and on the detuning frequency for the case of photorefractive media.
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