Optical and Ferroelectric Properties of Barium Sodium Niobate

Optical absorption, refractive index, dielectric constant, nonlinear optical coefficients, and linear electro-optic coefficients are reported for single-domain crystals of ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$ ${\mathrm{O}}_{15}$ between room temperature and the Curie temperature. Orthorhombic ($\mathrm{mm}2\mathrm{}$) barium sodium niobate, a filled tungsten-bronze structure, is stable to intense laser radiation, and its phasematchable nonlinear coefficients are 3 times those of LiNb${\mathrm{O}}_3$ and LiI${\mathrm{O}}_3$. For the 1.064-μm laser fundamental, the observed phase-match temperatures and the angular half-widths of the phase-matched second-harmonic intensity due to the coefficients $d_{31}$ and $d_{32}$ are found to be in good agreement with the values calculated from refractive-index data. The dielectric constant and electro-optic half-wave voltage data indicate that ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$ ${\mathrm{O}}_{15}$ is a useful electro-optic modulator material. The temperature variation of the spontaneous polarization $P_S$ is deduced from the birefringence, electro-optic, nonlinear optical, and pyroelectric data. It is concluded that the ferroelectric transition in ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$ ${\mathrm{O}}_{15}$ is of the first order.