Nonlinear optical properties ofRb2ZnCl4in the incommensurate and ferroelectric phases

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 δ 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<$T_C$+0.01 mK. ($T_C$ is the incommensurate-ferroelectric phase-transition temperature.) The temperature dependence of the second-harmonic intensity in the range $T_C$<T<198 K is explained by the occurrence of a defect-induced unipolar domain structure.