Study On The Electronic Structures And Optical Properties Of Doped LiTaO₃ Crystals

LiTaO₃ crystal is a multifunctional material with the excellent characters on integrates electro-optic,acousto-optic,piezoelectric,non-linear photorefractive and laser activity,and has a wide range of applications.It is used as Q-switched light,optical frequency conversion,and infrared detectors in laser technology.In fiber optic communication,electro-optic modulators can be made,especially in optical volume holographic storage,due to the perfect non-volatility and long dark decay time,it shows superior storage performance.Isomorphic to LiNbO3 crystal,this crystal has similar storage advantages,it has become one of the most popular holographic storage photorefractive materials.The intrinsic defect structure of the LiTaO3 crystal allows the crystal to be doped with various impurity ions,thereby improving the photorefractive and light damage resistance of the LiTaO3 crystal,making the storage performance more ideal and expanding its practical applications in various aspects.Holographic storage research found that different combinations of ions doped in LiTaO₃ crystals would exhibit different storage characteristics.At present,most of the researches on LiTaO₃ at home and abroad are experimental studies,but the theoretical research on the electronic structure and optical properties of doped LiTaO3 crystals has rarely been reported.In this paper,the electronic structures and optical properties of pure LiTaO3 crystal,variety single-doped and double-doped Li TaO3 crystals were investigated by first principle based on the density functional theory.The results show that:The impurity energy levels of photorefractive ion Fe³⁺doped LiTaO₃ appear within the band gap are mainly contributed by the Fe3d orbital,and the band gap width is 3.05 eV.After doping with magnesium,no new impurity energy levels are generated in the forbidden band.When the concentration of Mg is less than or exceeds the threshold?slightly less than 6 mol%?,the forbidden band widths are 2.72 eV and 2.45 eV,respectively.The absorption spectra of the Fe ions in crystals show two absorption peaks at417 nm and 745 nm which are attributed to the transition of the electrons of the splitting orbital of Fe3d.For Fe:Mg:LT crystal,the absorption peak intensity lower than that of Fe doped LiTaO₃,with a slight shift.Specially,with the concentration of Mg ion attaining the threshold value,the short-wave peak moves slightly to 457nm,while the long-wave peak at745 nm disappears.The absorption peaks corresponding to the splitting orbital T_2gg and E_g of Fe ions are related to the occupancy of iron ions.With the Mg ion concentration at the threshold value,the Fe ions could occupy Ta positions,and the absorption peak corresponding to E_g disappear.When using the 457nm waveband for functional absorption,the use of higher Mg doping will not adversely affect the absorption,when using the745nm waveband,Fe occupying Li sites is more advantageous than Fe occupying Ta sites in storage applications and should not be used highly doped with magnesium.Within the forbidden band gap,the photorefractive ions Mn and Cu doped LiTaO₃crystals show the impurity energy levels.The band gap of each doped crystal is narrower than that of pure lithium tantalate crystal,and the light absorption in the visible region is significantly enhanced.The deep and shallow centers of the Cu and Mn co-doped sample are at 314nm and 587nm,respectively.The dynamic range and sensitivity of the storage parameters can be improved by increasing the amount of copper doping.Analysis suggests that the co-doped sample with Mn at Ta site would achieve more competitive than that with Mn at Li site in optical storage application.In photorefractive ion double-doped lithium tantalate crystals,different impurity ion combinations will show different optical properties and affect the dual optical storage parameters,so it is necessary to select codoping ions according to the simulating datum of the different samples.