铜铁镁三掺铌酸锂晶体的第一性原理研究

利用基于密度泛函理论的第一性原理,研究了Cu:Fe:Mg:LiNbO3晶体及对比组的电子结构和光学特性.研究显示,单掺铜或铁铌酸锂晶体的杂质能级分别由Cu 3d轨道或Fe 3d轨道贡献,禁带宽度分别为3.45和3.42 eV;铜、铁共掺铌酸锂晶体杂质能级由Cu和Fe的3d轨道共同贡献,禁带宽度为3.24 eV,吸收峰分别在3.01,2.53和1.36 eV处;Cu:Fe:Mg:LiNbO3晶体中Mg^2+浓度低于阈值或高于阈值(阈值约为6.0 mol%)的禁带宽度分别为2.89 eV或3.30 eV,吸收峰分别位于2.45 eV,1.89 eV或2.89 eV,2.59 eV,2.24 eV.Mg^2+浓度高于阈值,会使吸收边较低于阈值情况红移;并使得部分Fe^3+占Nb位,引起晶体场改变,从而改变吸收峰位置和强度.双光存储应用中可选取2.9 eV作为擦除光,2.5 eV作为读取和写入光,选取Mg^2+浓度达到阈值的三掺晶体在增加动态范围和灵敏度等参量以及优化再现图像的质量等方面更具优势.

First-principles study of Cu:Fe:Mg:LiNbO3 crystals

In this paper the electronic structures and optical properties of Cu:Fe:Mg:LiNbO3 crystals and their comparative groups are investigated by first-principles based on the density functional theory to explore the characteristics of charge transfer in crystals and analyse the parameters of the two-colour holographic storage technology based on optical properties of crystals. The basic crystal model is built as a supercell structure 2 ×2 × 1 of near-stoichiometric pure LiNbO3 crystal with 120 atoms, including 24 Li atoms, 24 Nb atoms and 72 O atoms. Above that the five doped crystal models are established as follows: the copper doped LiNbO3 crystal(Cu:LiNbO3), the ferri doped LiNbO3 crystal(Fe:LiNbO3), the copper and ferri co-doped LiNbO3 crystal(Cu:Fe:LiNbO3), the copper, ferri and magnesium tri-doped LiNbO3 crystal(Cu:Fe:Mg:LiNbO3) with doping ions at Li sites, and the copper, ferri and magnesium tri-doped LiNbO3 crystal(Cu:Fe:Mg(E):LiNbO3) with ferri ions at Nb sites and magnesium ions at both Li sites and Nb sites. The last two models represent the concentration of Mg ions below the threshold(~6.0 mol%) and over the threshold respectively. The charge compensation forms are taken successively as CuLi^+-VLi^-, FeLi^2+-2VLi^-, FeLi^2+-CuLi^+-3VLi^-, MgLi+-FeLi2+-CuLi^+-4 VLi^-i MgLi^+-3MgLi^+MgNb^3-FeNb(2-2CuLi^+) CuL+iin doped models. 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 3 d orbits of Cu ions and the 3 d orbits of Fe ions respectively. The forbidden band widths are 3.45 eV and 3.42 eV respetively in these two samples. In Cu:Fe:LiNbO3 crystal, the impurity levels are contributed by the 3 d orbits of Cu and Fe ions;the forbidden band width is 3.24 eV;the absorption peaks are formed at 1.36, 2.53, and 3.01 eV. The Cu:Fe:Mg:LiNbO3 and Cu:Fe:Mg(E):LiNbO3 crystal presentthe forbidden band width of 2.89 eV and 3.30 eV respectively;the absorption peaks are formed at 2.45, 1.89 eV and 2.89, 2.59 eV, 2.24 eV, respectively. In Cu:Fe:Mg:LiNbO3 crystal, the weak absorption peak at 3.01 eV disappears, beacause of the superposition of the red-shifted absorption edge and the next bigger peak. The peak locations move slightly, which can be explained by the crystal field changing under the different doping concentrations and the different occupying positions of doping ions. In Cu:Fe:Mg(E):LiNbO3 crystal, the absorption peak near 2.5 eV is stronger than that of the other tri-doped crystal, which may be caused by the deference in occupancy among Fe ions. 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 at2.9 eV and stronger absorption at 2.5 eV, which is beneficial for this application. The strong absorption of write light can 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 can be shortened, so optical damage can be weakened, and finally the image quality can be optimized.