Theoretical Study On Nonlinear Optical Properties Of Molecular Materials

It is well known that when light transmits through matter, it interacts with the matter. If the matter responds nonlinearly with the light, the process belongs to nonlinear optical regime. In this category, some new frequencies are generated, and the independent transmission principle and the linear superposition principle are not reasonable any more. In order to observe the nonlinear optical phenomenon, one must use the light with high intensity. Therefore, since the laser was invented in 1961, one has known more properties about the nonlinear optics. One has usually used materials with high nonlinear optical property to study the nonlinear optical phenomenon, which can be widely used in the laser frequency doubling, laser frequency mixing, parameter amplification and oscillation, integrated optics, optical communication, beam direction, beam distortion removing, image amplification and transformation, optical information disposing, optical signal controlling, optical limiting and threshold monitoring, complete optical connecting, optical computer and so on. Because the organic molecules with large nonlinear optical property have many merits, such as wider response wave band, better flexibility, high optical damage threshold and lower cost, easier synthesizing and modification, one has paid much attention to these materials. The theoretical and experimental results have shown that multi-branch molecules have strong nonlinear optical properties. Compared with the one-dimension molecules, the multi-branch molecules have longer conjugation length, better charge transfer property and better two-photon absorption property.The thesis studies the linear and nonlinear optical properties of a series of multi-branch molecules by the quantum chemical theory on the ab initio level with the density functional theory method. The structure-property relation is presented. The main contents and results are as follows. I. One-photon and two-photon absorption properties of a series of multi-branch molecules.The one-photon and two-photon absorption properties of a series of multi-branch molecules are calculated on the DFT level. First, we use the HF method to optimize structures of the molecules. Then the response theory is applied for calculating one-photon and two-photon absorption properties of the molecules. The computational results indicate that the multi-branch molecules with better planar and symmetric structures show better two-photon absorption property.II. Linear and nonlinear optical properties of a series of molecules with differentπcenter.We have theoretically designed a series of one dimensional organic molecules with asymmetrical structure. These molecules have push-pull structures with differentπcenters. The linear and nonlinear polarizability and the two-photon absorption cross-section are calculated. The results show that molecules with two double bonds asπcenter have larger second order polarizability and the longer conjugation length also contributes to the larger second order polarizability and two-photon absorption cross-section.There are six chapters in this thesis. The first chapter gives a brief introduction to the nonlinear optical phenomenon and the development process of nonlinear optics; The quantum chemical method to optimize the molecular structure and calculate the energy of the excited states is introduced in the second chapter, including the HF method and the DFT theory. In chapter three, nonlinear optics based on the quantum mechanics is introduced, and the methods to calculate the two-photon absorption cross-section, including the sum-over states method, the few states method, the finite field method and the response function theory are also presented. In the forth chapter, the one-photon and two-photon absorption properties of a series of multi-branch molecules are presented. In the fifth chapter, we discuss the linear and nonlinear optical properties of a series of molecules with differentπcenter. The summary and prospect are presented in the last chapter.