First-principles Study Of Doped Lithium Niobate Crystals

Lithium niobate (LiNbO3referred LN) have good physical properties of electro-optics, acousto-optics, nonlinear optics, besides that its thermal property and chemical property are stabile. It can be widely used in optical modulator, optical switche, optical parametric oscillator, double frequency device, photorefractive devices, optical waveguide substrate and the optical isolator, etc... Due to the crystal preparation technique, numerous applications and researches were carried out on congruent lithium niobate crystal (CLN). In recent years, thanks to the different techniques for preparing, the researchs of near-stoichiometric lithium niobate crystal become more common.The photorefractive effect of the crystal can be increased or decreased by doping. In this paper, we select photorefractive and anti-photorefractive crystals Fe:LN and Mg:LN as research samples. Hydrogen ions affect the photorefractive effect of LN crystal. Due to the crystal growth process, it cannot be avoided that hydrogen ions enter in LN crystal. Generally, the influence of hydrogen ions upon photorefractive effect would be ignored, that is disadvantage for anti-photorefractive application. For the iron-doped LN crystals, due to the instability of iron ion, it present+2and+3valences inside the crystal. In references, the energy level of Fe ion was always marked as one single level, which is very unfavorable in photorefractive effect research and application.The main works are shown below:1-. We use density functional theory to study lithium niobate (LN) and Mg-doped lithium niobate crystals with accidental introduction hydrogen ions. We think that the H ions occupy lithium site, offset from this position, close to oxygen plane and unite with one of the oxygen atom to form OH". Through modeling crystal structures of LN. Mg:LN and H:Mg:LN, we find that absorption line edge of each crystal displaces each other, we propose the formation of OH2-,OH-structure, and explain mechanism of its influence upon absorption line.2%Based on first-principles density functional theory, we study band structure of stoichiometric lithium niobate crystal SLN and the energy levels of Fe ion with the different valence in Fe:SLN crystals. The results show that the forbidden band of SLN depends on the2p level of O2-and4d level of Nb5+. For Fe:SLN crystal, Fe2+and Fe3*can both occupy Li site; Fe3+can occupy Nb site, but Fe2+can’t occupy Nb site, and the energy levels of Fe ions in different sites have a distinction of0.41eV in maximum.