Effect of stresses on the OH libration bands in proton-exchanged LiNbO3 optical waveguides

IR reflection spectroscopy was used to study the oxygen-hydrogen libration bands in proton-exchanged lithium niobate optical waveguides. It is established that in the case of beta(i)-HxLi1-xNbO3 phases generated on x-cut LiNbO3 the new anisotropic libration mode at 890-940 cm(-1) is observed in addition to the known isotropic mode at 960-970 cm(-1), whereas the same phases on z-cut exhibit only latter isotropic libration mode. The observed phenomena are explained as a result of anisotropic stresses generated in the x-cut waveguides, whereas the stresses in the structures on z-cut are isotropic. The OH-bond librations are clearly anisotropic in the alpha, kappa(1), and kappa(2) phases generated on both z- and x-cuts, which indicates a fundamental difference of the local symmetry of the proton's position in the beta(i) phases compared to the alpha, kappa(1), and kappa(2) HxLi1-xNbO3 phases.