Nonlinear Interaction Of Laser And Organic Molecules

This thesis represents mainly two aspects research: one is the theoretical calculation of nonlinear optical properties of several types organic molecules emphasized on two-photon absorption cross sections and solvents effects using modern quantum chemical calculation methods; the other is to investigate propagation of ultrashort laser pulses in the nonlinear molecules materials by solving full Maxwell-Bloch equations employing predictor-corrector finite-difference time-domain method. The main contents and results are illustrated as follows:1. The one-photon and two-photon absorption properties of a series of 1,4-dimethoxy-2,5-divinyl-benzene derivatives are investigated by use of the analytic response theory at Hartree-Fock level. The numerical results show that these molecules have strong two-photon absorption cross sections. In the visible light region, the maximal one-photon absorption strengths of the molecules occur in the first excited state. Furthermore, the maximal two-photon absorption cross sections of the D-π-A typed molecules still appear in the first state, but for D-π-D typed molecules the second excited state has the maximal two-photon absorption cross section. The optical properties of molecules are closely related to the electric ability of donor and acceptor. The symmetry substituted molecules can not be determined to have larger two-photon absorption cross section than the asymmetry substituted molecules for such kind ofπ-center. The charge-transfer process for the charge-transfer states is displayed, and then the mechanism of photopolymerization is qualitatively discussed.2. Solvent effects on the two-photon absorption of a symmetrical diamino substituted distyrylbenzene chromophore have been studied theoretically. For this symmetrical molecule, calculations demonstrate that the dielectric medium has a rather small effect both on the bond length alternation and on the one-photon absorption spectrum, but it affects significantly the two-photon absorption cross section. It is found that both one- and two-photon absorptions are extremely sensitive to the planarity of the molecule, and the absorption intensity can be dramatically reduced by the conformation distortion. It has led to the conclusion that the experimentally observed anomalous solvent effect on the two-photon absorption of dialkylamino substituted distyrylbenzene chromophores can not be attributed to the intrinsic properties of a single molecule and its interaction with solvents. For this molecule, the vibronic effects on the one-photon absorption spectrum are also discussed.3. Linear and two-photon absorption properties of interacting polar chromophores including monomer and two types of dimmers have been theoretically investigated by using time-dependent density functional theory as well as analytical response theory. Few-states model is applied to explain the relationship of two-photon absorption properties among the studied molecules. In contrast to exciton model, our calculations get good agreement with experiment not relying on the experimental parameters.4. The resonant propagation of femtosecond laser pulse in 4-trans- [p-(N,N-Di-n-butylamino)-p-stilbenyl vinyl] pyridine medium with permanent dipole moments is simulated by solving full Maxwell-Bloch equations using predictor-corrector finite-difference time-domain method. In optical regime, this molecule has only one charge-transfer state, and the molecule can thus be simplified as a two-level system. The numerical results show that the permanent dipole moments have an obvious effect on the propagation of the ultrashort pulse laser. The ideal self-induced transparency disappears for 2πpulse, and second harmonic spectral components occur significantly due to the two-photon absorption process. For 6πpulse, continuum frequency generation is produced and a shorter duration pulse in time domain with 465 as is obtained.5. The supercontinuum generation of spectra are especially analyzed. It is found that the supercontinuum generation is strongly modulated by both area and width of the pulse, resulting from the interference between the splitting pulses in time-domain and the implication of the time-energy uncertainty relation. The effect of the permanent dipole moment on the supercontinuum generation is discussed in detail.6. The phase-dependent features of ultrashort laser pulse resonant propagation in a two-level dipolar molecule are demonstrated by solving full Maxwell-Bloch equations. The numerical results show that the carrier propagation and the spectrum evolution of the pulse are sensitive to its initial phase and the phase sensitivity is more obvious for larger area pulse. The phase-dependent feature is more evident in dipolar molecule because the permanent dipole momentum makes the nonlinear effects stronger.This paper is organized as follows. The first chapter gives a brief introduction of nonlinear optics and also the developing process of the nonlinear optics and nonlinear optical materials. The development and applications of ultrashort laser pulse are also described. In the second chapter, the different approaches for calculating molecular nonlinear optical properties are summarized, including sum-over-state, few-state model, finite field, and response theory methods. The fundamental ways of representing the solvent environment are discussed at the same time. The linear coupling model accounted for the vibronic effects on linear optical properties are illustrated in the third chapter. In the fourth chapter, the theory and techniques of generation of ultrashort laser pulse and several important nonlinear phenomena are introduced and the famous area theorem and self-induced transparency are given in detail. The derived Maxwell-Bloch equations beyond and within slowly varying envelope approximation and rotation-wave approximation are located in chapter five. The predictor-corrector finite-difference time-domain method is elucidated in the same chapter. From the sixth chapter to eleventh chapter, my own study works are showed and the main results are alluded above. At last, the conclusions and prospect are given in the twelfth chapter.