尿素晶体的线性和非线性光学性质的从头算研究

Ab Initio Study of Linear and Nonlinear Optical Properties of Urea Crystal

采用从头计算平面波赝势方法(PWP)和局域密度近似,计算了尿素晶体的电子能带结构和光学性质,计算表明尿素晶体属直接带隙晶体,计算带隙为5.11eV。在剪切操作下,采用密度矩阵理论计算了介电常数和倍频系数,理论结果与实验符合甚佳,并成功地将二阶非线性光学系数谱的特征峰与介电函数光谱结合起来,按单、双光子共振项进行了分析。不同跃迁过程对二阶非线性光学系数的贡献的研究表明,二阶非线性光学极化率在低频下几乎全部来自于虚电子过程。

A systematic ab initio study of the second-order optical response functions and the dielectric function of urea crystals was performed. Specifically, the dielectric function and the second-harmonic generation response coefficient over a wide frequency range have been calculated. The highly accurate full-potential projected augmented wave （PAW） method was used in the calculation. The results are in good agreement with the experiments. The microscopic mechanism of the linear and nonlinear optical properties of urea crystal has been analyzed by coupling optical spectrum and the calculated electronic structure. The virtual electron process and virtual hole process have also been performed, we find that the virtual hole process is nearly no contribution to nonlinear optical response.