OPTICAL 2ND-HARMONIC GENERATION AROUND LOCK-IN PHASE-TRANSITION POINTS OF SOME A2BX4-TYPE FERROELECTRICS

Intensities of optical second-harmonic wave and coherence lengths were measured in the ferroelectric and incommensurate phases of (NH4)2BeF4, K2ZnCl4 and {N(CH3)4}2ZnCl4 cystals. No appreciable second-harmonics were observed in the incommensurate phase of (NH4)2BeF4 and {N(CH3)4}2ZnCl4. In the case of K2ZnCl4, a virgin crystal exhibited the second-harmonic generation in the incommensurate phase but the intensity decreased with a long relaxation time of about one hour toward negligibly weak value. The previously reported SHG in the incommensurate phase should be originated from the symmetry-lowering due to the impurities or defects. The possibility of the quasi-phase match in the incommensurate phase was also excluded in these crystals.