晶体电光和非线性光学效应的离子基团理论(Ⅲ)——利用畸变氧八面体的离子基团模型计算LiNbO3,LiTaO3,KNbO3,BNN晶体的电光和倍频系数

本文用文献1]中所提出的(MO₆)离子基团模型,计算LiNbO₃,LiTaO₃,KNbO₃,BNN等晶体的电光和倍频系数。假设在O_h对称时,这些晶体中的氧八面体具有相同的能级和对称波函数,则通过群表示理论就可得到在C_3ν,C_2ν对称性时氧八面体的能级和对称波函数,并进而用ABDP理论计算它们的电光和倍频系数。计算结果和实验符合得相当好。本文并从理论上解释了这些晶体的倍频系数大小、符号和氧八面体畸变间的关系,由此可以得到以下两个结论:(1)畸变氧八面体的离子基团模型不但适用于钙钛矿型材料,同时也适用于钨青铜型、LiNbO₃型材料。(2)在钨青铜型、LiNbO₃型的材料中,仍是“离子键”对电光和倍频效应作出主要贡献,同时由于LiTaO₃的共价键成份比LiNbO₃大,因而LiTaO₃的非线性光学效应比LiNbO₃小。

AN IONIC GROUPING THEORY OF THE ELECTRO-OPTICAL AND NON-LINEAR OPTICAL EFFECTS OF CRYSTALS (Ⅲ)——A THEORETICAL CALCULATION OF THE ELECTRO-OPTICAL AND OPTICAL SECOND HARMONIC COEFFICIENTS FOR LiNbO3, LiTaO3,KNbO3, AND BNN CRYSTALS BASED ON A DEFO

Based on (MO₆) ionic grouping model as previously proposed in (I), we have calculated the various electro-optical coefficients as well as the optical second harmonic coefficients for LiNbO₃, LiTaO₃, KNbO₃, and BNN crystals. If we assume that the oxygen-octahedra in the crystal lattice of these crystals of O_h symmetry all possess the same energy level and wavefunctions, then the energy level and the wavefunction for LiNbO₃, LiTaO₃, KNbO₃, and BNN erystals of C_3ν, or C_2ν symmetry can be found upon applying the theory of group representations. Furthermore, by using the ABDP theory, we have calculated their electro-optical and optical second harmonic coefficients. It is interesting to note that without introducing any adjustable parameters the calculated values agree satisfactorily with the experimental data. The dependence of the magnitude and the sign of the optical second harmonic coefficients for these crystals upon the degree of the oxygen-oetahedra distortion has also been interpreted theoretically in this paper. We have thus arrived at the following conclusions:(1) The ionic grouping theory of the deformed oxygen-octahedra model proves to be appropriate not only for the perovskite-type structure but also for the tungsten bronze and LiNbO₃ type structures.(2) In crystals of the tungsten bronze and LiNbO₃ type structure, it is the ionic bonds that make major contribution to the electro-optical and optical second harmonic effects. As the covalent nature of bonds in the LiTaO₃ crystal exceeds that in the LiNbO₃ crystal, the non-linear optical effects in the LiTaO₃ erystal are weaker than those in the LiNbO₃ crystal.(3) The magnitude and the sign of the optical second harmonic coefficients for these crystals depend upon the degree of the oxgen-octahedra distortion.