无机硼酸盐体系的二阶非线性光学晶体材料

Certain Inorganic Borate Crystals as Excellent Second-Order Nonlinear Optical Materials in the UV-Region

总结了无机硼酸盐二阶非线性光学晶体材料的结构特征，即无中心对称性和阴离子基团的π共轭类芳香性结构；讨论了阴离子基团理论处理实际体系时遇到的困难及其假设中的不尽合理之处。以BBO（低温相偏硼酸钡β-BaB2O4）和LBO（三硼酸或LiB3O5）晶体为例，从含组态作用的INDO/S记结合态求和方法计算得到的结果分析后表明，BBO和LBO的价带具有相同的原子轨道组成，导带的底部，在BBO中主要是Ba原子轨道的贡献；在LBO中主要是（B3O5）-基团的贡献。前者对二阶极化率的贡献主要来自环外O原子的2p轨道到Ba原子的电荷转移，而后者的贡献则主要来自[B－O]阴离子基团内的电荷转移．计算得到的BBO和LBO的价带和导带间的电子跃迁能分别是6.28eV和7.26eV，相当好地接近于紫外吸收边的测量值6．43eV和7.78eV，最大的二阶非线性极化系数Xyyy（7．18X10-9esu）和Xzyy（2.38X10－9esu）相当接近实验测量值d22（5．30X10－9esu）和d32（2．79X10－9esu）。

It has been discovered in Fujian Institute in the mid-eighties that certain non-centro-sym-metrical inorganic borate crystals with quasiaromatic π conjugate benzene-ring-like anionic groupsare excellent second-order nonlinear optical materials in the uv-region. In this communication weshall present evidences to show that some of the assumptions of the so-called anionic group theoryare not at all reasonable. There is no doubt that the cations in these crystals do play some role inthe production of such second-order nonlinear optical response. The BBO(β-BaB2O4) and LBO(LiB3O5) crystals are taken as the examples for discussion. The structures of the electronic ener-gy bands are calculated using the INDO/S method with configuration interactions, followed bycombination with the sum-over-states method for the calculations of the frequency-dependent andstatic second polarizabilities of microscopic species, Ba3 (B3O6), and Li2B6O10. The macroscopicsecond susceptibilities of BBO and LBO crystals are estimated after the relationship between themicroscopic and macroscopic species is taken into consideration. It is shown that the valencebands have contributions from the same atomic orbitals in these two crystals, but the low edge ofconduction band in BBO has contributions from the Ba atomic orbitalsi while in LBO, there arecontributions from the antibonding interaction orbitals between the B and O atomic orbitals. It isalso shown that the excited charge-transfer states from the O 2p orbitals to the Ba atomic orbitalsmake significant contributions to the susceptibility in the BBO crystal; but those from the O 2porbitals to B atomic orbitals make major contributions to the susceptibility in the LBO. The cal-culated absorption edges of the Ba3 (B3O6)2 (197 nm 6. 28 eV) and the Li2B6