Research On Growth And Optical Properties Of Scndium Doped LiNbO₃Crystal

Lithium niobate (referred to as LN) crystal is a high-quality synthetic crystalwhich possesses a lot of excellent properties, such as large electro-opticalcoefficient and nonlinear optical coefficient, acousto-optical effect, ferroelectricityand so on. LN crystals have been widely used as holographic storage materials andnoncritical phase matching materials. However, severe light scattering can beinduced by high density light in LN crystals. One way to achieve better opticalproperties in LN crystal is through impurity element doping. In this dissertation, inorder to improve the resistant ability to optical damage, Sc2O3was doped into LNcrystals.Sc:LN crystals were grown by the pulling method. The crystals grown are freeof macroscopic defects and present good optical homogeneity. The suitable growthconditions of doped crystals are explored. Under these conditions, compositionalinhomogeneity due to solute segregation is greatly decreased and it’s easier to growcrystals without cracks.According to X-ray power diffraction results, the lattice structure of Sc:LNcrystals are very similar to that of the congruent sample. This indicates that Sc3+canonly replace Li+, Nb5+or NbLi4+. X-ray rocking curves of LN crystal samples showthat the crystals have good crystallinity.The dependence of the change of the absorption band in the IR spectra andabsorption edge in the UV-Vis spectra and Raman spectra on the change of Sc2O3were investigated. The threshld concentration and location of Sc in Sc:LN crystalsare further discussed. When the Sc doping concentration is below the threstholdconcentration, Sc3+will replace NbLi4+first, and when it’s up to the thresholdconcentration, all of NbLi4+are replaced and then Sc3+will occupy normal Li+andNb5+sites.The birefringence anisotropic method was carried out to prove that crystalsgrown possessed good optical homogeneity.The photorefractive resistance of Sc:LN crystals was studied by transmissionfacula distortion method. Crystal’s photorefractive resistance were enhanced by thedoping of Sc2O3. The1.5mol%doped crystal shows the strongest resistant ability.Two-wave coupling method was used to study the photorefractive properties ofLN crystals. It was found that with the increase of scandium, time of setting gratingwas shortened and the changes of refractive indexes decreased. This may be causedby replacing intrinsic defects in LN crystals, thus leading to increase ofphotoconduction. Frequency doubling experiment shows that with irradiation of1.064μm laser,the efficiency of frequency doubling increased with the increasing dopingconcentration of Sc. It is found that the threshold doping concentration is near1.5mol%. It’s up to28.3%when the doping concentration is1.5mol%. Although this isnot good enough comparing to reported results, this may be caused by not coatinganti-reflection film and some other reasons.