Nonlinear Optical Properties Of Organic Materials And The Theory Of Two-photon Absorption

Quantum chemistry can investigate not only the interaction of forming chemical bond but also electronic structure of the molecule. Furthermore it can probe into the relation between the various interaction of compound and the character of electronic structure by the experiment. With the development of computer condition and the theory method, it is possible to investigate the character of complex big organic molecule and polymer with rigid theory.The nonlinear origin of organic material is different from the inorganic material's. The nonlinear origin of inorganic material derives from the electronic structure. The nonlinear origin of organic and polymer materials is mainly determined by the molecule structure. The construction of organic crystal and polymer is through van der strength of molecular units. The interaction of intramolecular valence bond is stronger than that of intermolecular Van der Waals strength, so coupling interaction is weak between electronic structure of the single molecule and the other's. We can think of every molecule as independent nonlinear origin, The coupling interaction of adjacent molecule is through local field. We can also call the material of organic and polymer molecular materials.It is the best advantage that we can devise the nonlinear of the organic materials in molecule level, so that we can get the best optical nonlinear response and other optical character. The hyperpolarizability formula is obtained by the perturbed theory, radiation field, dipolar coupling effects and approximation of . In combination with the result of ab initio Gaussian and result of semi-experience quantum chemistry ZINDO, I compile the program by using the Fortran to calculate the hyperpolarizability and two photo absorption cross-sections. Firstly, I calculate a few small molecules and compare the result with other methods. The results are well in accordance with others. This method has a lot of advantages over others. For example, the program of ADF do not calculate the THG and IDRI, the finite-field method only add a certain electronic field, this leading to its result can not be compare to experiment directly. Furthermore, this method uses second hyperpolarizability to calculate the third hyperpolarizability, this leads to a certain errors. Through devising a series of molecules, when the conjugation length increases, the nonlinearities of the system increases. While the conjugation length increases in a certain degree, the nonlinearities of the system does not increase. Hence we can instruct the experimental synthesis in theory.