Simultaneous measurements of the second harmonic generation and of the birefringence of KH2PO4near its ferroelectric transition point

Abstract

We report some measurements of the variation of the nonlinear optical-susceptibility coefficient ${{\chi }_{31}}^{2\omega }$ with temperature and dc electric fields in the vicinity of the 122°K ferroelectric-transition point of K${\mathrm{H}}_2$P${\mathrm{O}}_4$ (KDP). Simultaneous measurements of the birefringence $\Delta n={n_1}^{\prime }-{n_2}^{\prime }$ were also made and this quantity was found to be proportionnal to ${{\chi }_{31}}^{2\omega }$; they are both closely related to the order parameter of the transition. A comparison of our results with some measurements of the spontaneous polarization $P_3$ and of the spontaneous shear $x_6$ is made. Our data indicate that the transition is first order and may well be interpreted in the framework of the Landau theory if terms of sixth and eighth order are taken into account in the free-energy expansion. An attempt is also made to fit our results to the statistical theory of Silsbee, Uelhing, and Schmidt.