The Study Of Molecular Nonlinear Optical Properties

In the last few decades, scientists have shown great interest in the study of new nonlinear optical materials, due to their widely use and attractive application foreground, for instance, in modern laser technology, optical communication, data storage, optical information processing, and other fields. It has become an important project to find new nonlinear optical materials of high efficiency. Much emphasis has been put on organic nonlinear optical materials because they have many advantages, such as wide response wave band, good flexibility, high optical damage threshold, low cost, and easy combination and modification.Much attention has been shown to Para-nitroaniline (pNA) for its strong nonlinear optical response. However, up to now, the theoretical results and the experimental ones do not agree very well. In this thesis, the nonlinear optical properties of pNA molecule are thoroughly studied.The nonlinear optical properties of pNA molecule in gas phase are studied on the base of time dependent density functional theory. First, the geometry of PNA molecule in gas phase is optimized on the base of density functional theory. Then, the nonlinear optical properties of pNA molecule are studied by using time dependent density functional theory and few states approach. The first order nonlinear hyperpolarizabilities in second-harmonic generation process and electro-optical pockels effect are computed. The basis set effects in ab-mitio quantum-chemical computation are mainly studied. The correction to the nonlinear optical properties due to the electronic correlation energy is discussed and the relationship between the first order hyperpolarizabilities and the frequencies of the external electric field is also investigated.The solvent effects on the nonlinear optical properties of PNA molecule are also chiefly studied on the base of time dependent density functional theory. The polarized continuum model is used to simulate the influence of the solvent environment on the solute molecule. In the first place, the geometry optimization of pNA molecule in each solvent is redo by usingthe polarized continuum model on the base of density functional theory. Secondly, the nonlinear optical properties with pNA molecule in different solvents are studied through using the polarized continuum model and few states approach on the base of time dependent density functional theory, and the first order nonlinear hyperpolarizabilities in second harmonic generation process are computed. It is found that the relationship between the nonlinear optical properties and the polarities of the solvents is non-monotonic, which is quite different from the previous conclusions.The hydrogen bonding effects and vibrational effects on the nonlinear optical properties are also investigated in this thesis. And it is found that the hydrogen bonding effects are quite important while the vibrational ones are not that obvious.The content of this thesis is as follows. The first chapter gives a brief introduction of linear and nonlinear optics and also the developing situation of the nonlinear optical properties. In the second chapter, the nonlinear optical properties are described from both macroscopic and microscopic views, at the same time, giving the definitions of the linear and nonlinear polarizabilities. Then, the explicit expressions of the linear and nonlinear polarizabilities are obtained by using quantum mechanical perturbation theory and density matrix formalism, respectively. The different approaches developed recently for calculating molecular nonlinear optical properties are summarized in chapter three. In the fourth chapter, the research process and the results are given in detail. The main conclusions are given in the last chapter.