Optical Second-Harmonic Generation of Ammonium Fluoroberyllate Crystal in the Incommensurate and Ferroelectric Phases

Temperature dependence of the optical second-harmonic intensity of ammonium fluoroberyllate crystal (NH4)2BeF4 in the incommensurate and ferroelectric phases, and temperature dependence of coherence lengths in the ferroelectric phase were precisely measured. Strong second-harmonic intensity was not observed in the incommensurate phase, contrary to a previous report on the same type of crystal. Measured coherence lengths were compared with the incommensurate modulation period determined by the X-ray diffraction method. It was found that the coherence lengths were larger than the modulation period by a factor of 103. These facts indicate that the possibility of quasi-phase matching, i.e. the coincidence of the coherence length and the modulation period, is excluded in the crystal.