Optical second-harmonic generation around lock-in phase transition points of some A₂BX₄-type ferroelectrics

Abstract

Intensities of optical second-harmonic wave and coherence lengths were measured in the ferroelectric and incommensurate phases of (NH₄)₂BeF₄, K₂ZnCI₄ and {N(CH₃)₄}₂ZnCI₄ cystals. No appreciable second-harmonics were observed in the incommensurate phase of (NH₄)₂BeF₄ and {N(CH₃)₄}₂ZnCI₄. In the case of K₂ZnCI₄, 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.