The nonlinear optical properties of the XYB2O6 family of compounds

The mixed niobate-borate crystals KNbB2O6 and RbNbB2O6 are monoclinic with space group Pna2(1) and point group m. Some of their properties are favoura ble for applications in non-linear optics including exceptional resistance to laser damage, UV cut-off at wavelengths below 290 nm and large d-coeffic ients. Although they have refractive indices that are similar to KTiPO5 the ir birefringences are smaller than anticipated for structures containing pl anar borate rings and distorted NbO6 octahedra, leading to large acceptance angles for second harmonic generation (SHG). Type 1 phase matching has bee n observed at 532 nm in RbNbB2O6. The birefringence of KNbB2O6 is smaller t han that of RbNbB2O6, and KNbB2O6 is not phase-matchable at this wavelength . Nevertheless, it is possible to tune the birefringence of KNbB2O6 by repl acing some K+ ions by the larger Rb+ ions so that non-critical phase matchi ng may be possible at room temperature. (C) 2001 Elsevier Science B.V. All rights reserved.