Study On The Electronic Structures And Optical Properties Of Two Photorefractive Ions Doped Lithium Niobate Crystals

Lithium niobate crystal is a widely used photoelectric material.It has been considered as one of the most preferred material in optical storage technology because of its excellent photorefractive properties.Due to the special defect structure of lithium niobate crystals,it's possible to make a variety of impurity ions doped in the crystal to expand practical applications.The two photorefractive ions doped lithium niobate could overcome the volatilization of stored information during the readout process.Up to now the two-center holographic storage technology of lithium niobate crystal has been deeply researched and acquired a lot of outstanding achievements.While,researchers mainly focus on the experimental study of practical application and pay much less attentions on its microcosmic mechanism through theoretical method.It is necessary to reveal the mechanisms of the charges transporting in these crystals,that would be the precondition to understand the photorefractive effect in crystals,and to optimize finally the parameters of the holographic storage.Under these circumstances,the electronic structures and the optical properties of variety photorefractive ions doped lithium niobate crystals are studied using the first-principles based on density functional theory in this paper.The results show that:The impurity bands of Cu and Mn doped lithium niobate crystals appear within the band gaps,which are contributed by Cu 3d orbital and Mn 3d orbital.The band gap of each doped crystal is narrower than that of pure lithium niobate crystal.There are three absorption peaks at 432nm,554nm and 718nm respectively in Cu and Mn co-doped lithium niobate crystal.The first one is shallower than that of Cu mono doped,the last one is weaker than that of Mn mono doped and the second one is non-photorefractive peak and associated with the concentrations of Mn³⁺,the difference intensity of this peak in different co-doped crystal comes down to the electron transitions between Cu and Mn ions.In the two-center holographic storage,The deep and shallow center of Cu and Mn co-doped sample are 432 nm and 718 nm respectively,which are deeper and shallower than those of the Cu and Fe co-doped lithium niobate crystal and permit to apply suitably higher Cu ion concentration to obtain more superior dynamic range and recording sensitivity.In two photorefractive ions doped lithium niobate crystals,the difference of optical properties due to different doped ion combinations affects the storage parameters.So it's necessary to take beforehand simulating calculations of different candidate ions to choose the matched dopant.The impurity bands of Fe and Ni mono and co-doped lithium niobate crystals are mainly contributed by the Fe 3d,Ni 3d orbitals.For co-doped sample,there are deep and the shallow two center structures provided by Ni ion and Fe ion respectively.The smaller value of C_Fe²⁺/C_Fe³⁺makes the Fe ion more likely to substitutes for Nb site in co-doped sample,these crystals would achieve more competitive shorter sensitized and recorded light wavelength than that of the Fe occupy Li site in the holographic storage application.The former crystal has two absorption peaks at 410 nm and 605 nm which could realize non-volatile holographic storage.The deeper center could prevent the deep center information destroyed by other light wavelength,and could permit higher concentration of Ni ion to raise the dynamic range and sensitivity.