Ab Initio Study Of The Organic Nonlinear Optical Material

Nonlinear optics is a subject to study the interaction of intense laser with matter, and it is a rapidly developed subject after the birth of the laser. As the important embranchment of the field of modern optics, nonlinear optics is attracting people's extensive attention. It has become an important project to find new nonlinear optical materials with strong nonlinear optical properties and quick response due to their attractive applications, for instance, in modern laser technology, optical communication, data storage, optical information processing, and other fields. Much emphasis has been put out for organic nonlinear optical materials because they have many advantages, such as large nonlinear optical coefficient, wide response wave band, good flexibility, high optical damage threshold, low cost, and easy combination and modification.The thesis has studied the linear and nonlinear optical properties of organic materials; we evaluate the dielectric function and the second-harmonic generation response coefficient over a wide frequency range and the relationship between the optical properties and the electronic structure. The main contents and results are represented as follows.Crystalline urea is among the first organic materials found to have an application in nonlinear optics, also it is among the simplest organic crystals which have a second-harmonic response. So it's very significant to study the microscopic mechanism of the nonlinear optical properties of urea crystal. A systematic ab initio study of the electronic structure and the optical properties of urea crystal is presented. Our results are in good agreement with the experiments. The virtual electron and virtual hole process has also been performed. We find that the virtual hole process has nearly no contribution to nonlinear optical response of the urea crystal.NMBA is a promising candidate for nonlinear optical applications, one of its nonlinear optical coefficients is very large. And the molecular structure of NMBA is"D-π-A", to which many researcheres pay a attention recently. We report for the first time the electronic and optical properties of NMBA obtained by the full-potential projected augmented wave method. The study of the electronic structure shows that the band structure of NMBA, which is flat and not seriously dispersive is typical of an insulating system. The contribution of 2p states of N and O in nitro to the bottom of conduction bands is very large, the nitro is the main contribution resource of the linear and nonlinear properties. From the electron deformation density maps of the molecule, we can see that NMBA possesses two primary intermolecular hydrogen bond and the charge transfer appears from the electron-donating group CH3 to the electron-withdraw group NO2, two phenyl act as a good electronic medium through which charge can be transferred, thereby the SHG response is enhanced. The optical properties of NMBA show a considerable anisotropy. The magnitude of the peaks ofεx′′x (ω) is much greater than that of the peaks ofε′y′y (ω) andεz′′z (ω), andχ1 1 is the dominant. The study of the contributions of different transitions to the SHG coefficients shows that the virtual electrons process is dominant and virtual hole process can not be neglected. The contribution to the large componentχ1 (12) from virtual hole process is about 30% of the total static limit of SHG coefficient.