Study On The Tri-doped Lithium Niobate Crystals By The First Principles

LiNbO₃ crystal（LN）is a multifunctional material with the excellent characters on integrates electro-optic,acousto-optic,piezoelectric and non-linear photorefractive,and has a wide range of applications.In the information society,high-density storage and high-speed processing of information are becoming increasing demanded.LiNbO₃crystal with excellent photorefractive properties performs well in the application of optical volume holographic storage technology,and it is considered to be one of the most the preferred materials.The application of optical volume holographic storage technology has the characteristics of high storage density,high redundancy,parallel addressing and fast access.Two-center holographic storage technology can avoid volatilization during reading.It is a practical technology with good properties as stability,long life,easy to read and rewrite.The realization of two-center holographic storage requires external or inherent defects to form two photorefractive centers in the gap（the deep and shallow energy levels in the forbidden band）.When recording with long-wavelength light,electrons are excited from the shallow energy level to the conduction band,trapped in the deep energy level,forming a grating.Use short-wavelength light for erasing.Long-wavelength light is used for reading,which will not damage the deep-level grating.Experimental researches on optical storage of triple-doped lithium niobate crystals with double photorefractive ions and one anti-photorefractive ions have shown good storage effect,but the theoretical research on electronic structure and optical properties is still lacking.In this paper,the electronic structures and optical properties of pure LiNbO₃ crystal and tri-doped LiNbO₃ crystals are investigated by first principle based on the density functional theory.The results show that:The extrinsic defect levels within the forbidden band of Cu:LiNbO₃ crystal and Fe:LiNbO₃ crystal are mainly contributed by the 3d orbits of Cu ions and of Fe ions respectively.The forbidden band widths are 3.45 eV and 3.42 eV respetively.In Cu:Fe:LiNbO₃ crystal,the impurity levels are contributed together by the 3d orbits of Cu and Fe ions;the forbidden band width is 3.24 eV;the absorption peaks are formed at1.36 eV,1.90 eV,2.47 eV and 3.01 eV.The Cu:Fe:Mg:LiNbO₃ and Cu:Fe:Mg（E）:LiNbO₃ crystals present the forbidden band widths of 2.89 eV and 3.30eV severally;the absorption peaks are formed at 2.45 eV,1.89 eV and 2.89 eV,2.59eV,2.24 eV,respectively.When the Mg²⁺concentration reaches the threshold,the absorption edge will purple-shift,and the Fe³⁺ions will occupy theNbsites,causing the crystal field to change,thereby changing the positions and intensities of the absorption peaks.The peak at 2.9 eV can be chosen as erasing light,and the peak at 2.5 eV as write and read light in the two-center nonvolatile holography.The tri-doped crystal with Mg²⁺concentration over the threshold shows obvious absorption peak at 2.9 eV and stronger absorption at 2.5 eV,that is beneficial for this application.Strong absorption of write light could shorten the time to reach the saturation of diffraction efficiency,then increase the dynamic range（M/#）and the sensitivity（S）.Meanwhile,in this Mg doping condition,write time could be shortened,so optical damage could be weakened,and finally the image quality could be optimized.The band gap of each doped system is narrower than that of pure lithium niobate crystal,and impurity levels appear in the band gap.The extrinsic defect levels within the forbidden band of Cu:LiNbO₃ crystal and Mn:LiNbO₃ crystal are contributed by the3d orbits of Cu ions and of Mn ions respectively,with forbidden band width 3.39 eV and 3.34 eV respetively.In Cu:Mn:LiNbO₃ crystal,the impurity levels are contributed together by the 3d orbits of Cu and Mn ions,with forbidden band width 3.32 eV;the absorption peaks are formed at about 3.17 eV,2.88 eV and 1.90 eV.The Cu:Mn:Mg:LiNbO₃ and Cu:Mn:Mg（E）:LiNbO₃ crystals present the forbidden band widths of 3.17 eV and 3.27 eV severally;the absorption peaks are formed at 3.05eV,2.79 eV,1.85 eV and 3.17 eV,2.82 eV,1.97 eV,respectively.The Mg²⁺concentration over the threshold would make the absorption edge to purple-shift comparing to the below-threshold condition;that would also let part of Mn²⁺ions occupying theNbposition,therefore change the positions and intensities of the absorption peaks.In the two-center storage application,the erasing light and the writing light would be chosen at the absorption peaks of 3.17 eV and 2.82 eV in Cu:Mn:Mg（E）:LiNbO₃ crystal.With the stronger absorption intensity and shorter wave length of the writing light in this crystal comparing to other test crystals,the time could be shorten to reach the saturation of diffraction efficiency,then the important parameters of the dynamic range（M/#）and the sensitivity（S）could be improved.Choosing suitable high concentration of Cu and Mg ion would help to this application.Also in this crystal the absorption peaks of 2.82 eV and 1.97eV could be used as other deep-shallow photorefractive absorption in the two-center storage application.