Abstract

The H⁺–Li⁺ exchange technique in LiNbO³ is useful for the preparation of optical waveguides because it creates a large increase in the extraordinary refractive index. We have considered the reason for this increase both experimentally and theoretically. The exchange rate x of H⁺ and Li⁺ is about 0.7, as measured by nuclear-reaction analysis with a high-energy ¹⁵N ion beam. Electron diffraction shows that the space group does not change, and x-ray diffraction shows that the lattice constant hardly changes. Measurements of the infrared-absorption spectrum suggest that H⁺ is located in the plane where oxygen atoms are packed most closely. Two optimized positions of H⁺ in the plane have been decided by calculating the total energy of clusters that include hydrogen and oxygen atoms by the coupled Hartree–Fock method. The calculated first-order polarizability of clusters that include hydrogen and oxygen atoms is much larger than that of clusters including lithium and oxygen atoms. As a result of the decomposition contributions of molecule orbitals, this seems to arise from the s orbital, which is the lowest unoccupied orbital in all of the clusters. It suggests that the small atomic radius of H⁺ is the reason for the refractive-index change.

© 1990 Optical Society of America