Optical nonlinearity and dielectric properties of noncentrosymmetric crystals of RE borotungstate family Ln3BWO9
The second harmonic generation method was used to prove the noncentrosymmetric structure of Ln3BWO9 borotungstates for elements of the RE series from lanthanum through gadolinium. The crystals exhibit moderate optical nonlinearity in the absence of phase matching between the radiation of the neodymium laser and its second harmonic. An increase in the optical nonlinearity in the middle of the RE series is caused by a more severe distortion of the WO6 polyhedra in crystals. In the range 500-800 K, Tb3BWO9 crystals exhibit dielectric relaxation, while at higher temperature, they are conductors of the Arrhenius type. Both phenomena were analyzed for the a- and c-cuts of Tb3BWO9 single crystals using the model of oxygen ion transfer over vacancies.