The Interaction Between Laser And Organic System

Since the invention of the first laser in 1960, the interaction of laser and medium has been an interesting project. In the last few decades, much emphasis has been put on organic 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. And as organic synthesis technology goes ahead, more and more organic materials and macromolecule compounds with good optical properties appeared. It is often need to consider the propagation properties of the laser, when studying the optical properties of these organic materials. Furthermore with the development of laser technology, properties of the interaction between laser and medium are enriched. Especially the development of ultrashort pulse technology is very fast in recent years, and this makes the study of the interaction between laser and medium come into a completely new area, in which many problems have to be considered again. The interaction of ultrashort pulse laser with atoms and molecules is currently being one of the hottest subjects in the field of the light-matter interaction.Much attention has been paid to /rara-nitroaniline (pNA) for its strong nonlinear optical response. In this thesis, we select it as the medium to interact with the laser pulse. First, the geometry of pNA molecule is optimized on the base of density functional theory (DFT). Then the excited state energies and electric dipole moments of pNA are calculated by the time-dependent DFT. All the works are done on GAUSSIAN-98 program pack. The solvent effects on geometrical and electronic properties of pNA molecule are further considered. The results indicate that the molecule only has one charge transfer state in the range from ultraviolet radiation to visible light. Therefore, pNA molecule can be taken as a two-level system in low energy range when interacting with laser pulse.After knowing the parameters of/?NA molecule, we study the propagation properties of the laser pulse in /?NA by use of semiclassical theory, namely, electric-magnetic fields are described by the Maxwell equations, and the medium of />NA is studied by Bloch equations. The Maxwell equations and Bloch equations are related by polarization. Then the coupledMaxwell-Bloch equations are built up. We can obtain the propagation properties of the laser in the medium by solving these equations. At the same time the spatial-temporal evolution of the population can be got. An iterative predictor-corrector finite-difference time-domain method is used to solve the M-B equations numerically. In the thesis, we mainly study resonance propagation properties of long pulses (ps) and ultrashort (fs) pulses in pNA. For the ultrashort pulses, an unresonance case is also considered.For the long pulses, we find the pulses can very nicely satisfy the area theorem during transportation. During the calculation we use rotating wave approximation and slowly varying envelope approximation, which are found very effective. While for the ultrashort pulses, the carrier wave of the electric field is used, and the spatial-temporal evolution of the carrier wave and population is studied. At the same time we work out the spectrum evolution of the electric field along the distance. Some new phenomena are found for the ultrashort pulses. The population exhibits a step character when the electric field is zero, higher and lower frequency appear when the pulses are propagating in the molecules, and the width of the electric spectrum is widened. The In ultrashort pulse still satisfies the area theory, but for larger area pulses the area theory is not applicable. The rotating wave approximation and slowly varying envelope approximation which are applied in the calculation of long pulses are not applicable for ultrashort pulses. The parity approximation was mostly used before, that is to say the dipole moment of the molecule was neglected. In our work, the results are obtained beyond parity approximation. At last we consider the unres...