Nonlinear optical properties of Rb2ZnCl4 in the incommensurate and ferroelectric phases.

Abstract

Nonlinear optical properties of ${\mathrm{Rb}}_{2}$${\mathrm{ZnCl}}_{4}$ are measured for the incommensurate and ferroelectric phases by means of second-harmonic generation experiments. In the temperature range of the ferroelectric lock-in phase, the nonlinear optical coefficients ${d}_{33}$, ${d}_{32}$, and ${d}_{24}$ and the Miller \ensuremath{\delta} coefficients are determined using the wedge technique and are shown to be proportional to the spontaneous polarization. In the incommensurate phase of ${\mathrm{Rb}}_{2}$${\mathrm{ZnCl}}_{4}$, 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 ${10}^{7}$ 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 ${T}_{C}$${T}_{C}$+0.01 mK. (${T}_{C}$ is the incommensurate-ferroelectric phase-transition temperature.) The temperature dependence of the second-harmonic intensity in the range ${T}_{C}$198 K is explained by the occurrence of a defect-induced unipolar domain structure.