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Periodic laminar ferroelectric domain superstructures in chromium-doped LiNbO3 have been grown by modulating the bias current during Czochralski growth. Periodic domain patterns stacked along the y axis with domain thicknesses as low as 8 μm have been prepared. The alternating sign of the effective nonlinear optical (nlo) coefficient enables efficient frequency mixing independent of the constraints of birefringence. This is demonstrated by realizing frequency doubling of 1.06-μm YAG : Nd laser radiation using the largest nlo coefficient, d33. For domain thicknesses equal to an odd multiple of the coherence length, d=(2m+1)lc, the second harmonic intensity varies as the square of the domain number. For the optimum case, d=lc, an improvement of the nlo efficiency, as compared to the 90° phase-matched process, by the factor (2/π)d33/d312=15 is inferred.
Feisst et al. (Sun,) studied this question.