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Local changes of indices of refraction observed in poled single crystals of ferroelectric LiNbO3 and LiTaO3 when illuminated by focused light have been studied. The extraordinary index of refraction was observed to decrease as much as 10−3 with a focused Ar laser light of 20 mW intensity as λ=0.488 μ, while the change of ordinary index was much smaller. The effect is attributed to the drifting of photoexcited electrons out of the illuminated region followed by their retrapping near the beam periphery. The space-charge field between these retrapped electrons and the positive ionized centers in the illuminated region causes the observed change of refractive indices via the electro-optic effect of the samples.
F. S. Chen (1969) studied this question.