Nonlinear optical properties of Rb2ZnCl4 in the incommensurate and ferroelectric phases

Nonlinear optical properties of Rb2ZnCl4 are measured for the incommensurate and ferroelectric phases by means of second-harmonic generation experiments. In the temperature range of the ferroelectric lock-in phase, the nonlinear optical coefficients d33, d32, and d24 and the Miller coefficients are determined using the wedge technique and are shown to be proportional to the spontaneous polarization. In the incommensurate phase of Rb2ZnCl4, the order of magnitude of the second-harmonic intensity corresponding to nonlinear optical coefficients allowed by the local point-group symmetry is estimated to be at least 107 times smaller than the second-harmonic intensity generated in quartz and thus difficult to detect. Furthermore, it is shown that quasi-phase-matching of the fundamental and second-harmonic waves in the incommensurate phase is limited to a very small temperature range of TC