The nonlinear optical properties of the XYB2O6 family of compounds

Abstract The mixed niobate–borate crystals KNbB2O6 and RbNbB2O6 are monoclinic with space group Pna21 and point group m. Some of their properties are favourable for applications in non-linear optics including exceptional resistance to laser damage, UV cut-off at wavelengths below 290 nm and large d-coefficients. Although they have refractive indices that are similar to KTiPO5 their birefringences are smaller than anticipated for structures containing planar borate rings and distorted NbO6 octahedra, leading to large acceptance angles for second harmonic generation (SHG). Type 1 phase matching has been observed at 532 nm in RbNbB2O6. The birefringence of KNbB2O6 is smaller than that of RbNbB2O6, and KNbB2O6 is not phase-matchable at this wavelength. Nevertheless, it is possible to tune the birefringence of KNbB2O6 by replacing some K+ ions by the larger Rb+ ions so that non-critical phase matching may be possible at room temperature.