Photorefractive properties of tungsten bronze ferroelectric lead barium niobate (Pb_1-xBa_xNb₂O₆) crystals

Lead barium niobate (Pb_1-xBa_xNb₂O₆) is a very promising material for photorefractive applications in that it has a high ferroelectric-to-paraelectric transition temperature (300-500°C depending on composition) and simultaneously can have large electro-optic coefficients, because of the nearly vertical morphotropic phase boundary (MPB) near 1-x = 0.63. Pb_1-xBa_xNb₂O₆ crystals have been grown by the vertical Bridgman method for near-MPB compositions (0.5<1-x<0.6), and their photorefractive properties were investigated by the two wave mixing experiments. Material properties such as composition and absorption coefficient varied along the growth direction because of the incongruent charge composition and the nonuniform distribution of impurities. A maximum diffraction efficiency of 50% at λ = 514.5 nm was observed in a 3.2 mm thick nominally undoped as-grown crystal. The photorefractive sensitivity increased as the wavelength decreased. The gain coefficient also increased with decreasing wavelength, changing from 0.5 cm^-1 at λ = 633 nm to above 8 cm^-1 at λ = 465 nm.