Dynamical Process Of Strong Two-photon Absorption Of Organic Molecule System

Since the early 1990's, ultrashort laser pulses with extreme intensity make quantic systems such as atoms and molecules endure extremely strong optical radiation without ionization, when interacting with ultrashort lasers. Series of nonlinear optical effects appear, different from that in the nanosecond and picosecond region, and these developments lead the study of the interaction between laser and matter into a completely new area, in which many problems have to be considered again. On the other hand, as organic synthesis technology goes ahead in the last few decades, more and more organic materials and macromolecule compounds appear with good optical properties. In this thesis, with the most promising ultrashort laser technology and organic materials syncretized, the interaction between ultrashort pulse laser and organic molecule medium is investigated theoretically, in order to design and apply laser properties, to forecast optical features of materials. Para-nitroaniline (PNA) receives much attention for its strong nonlinear optical response. Here, we study the propagation of ultrashort laser pulse in PNA molecule medium, especially the effects of permanent dipoles. Firstly, its geometry in gas phase is optimized on the base of density functional theory with a 6-31++G* basis set .The excited state energies, the permanent dipole moment of the ground state, and electric dipole transition moments are calculated by use of time-dependent density functional theory. The total work is done on GAUSSIAN-98 program pack. In lower energy region, the molecule has only one charge-transfer (CT) state,i.e., the one-photon transition probability is very large between the CT state and the ground state. It is well known that the CT state of a molecule determines its optical properties, so PNA molecule can be taken as a two-level system in low energy range when interacting with laser pulses. The dipole moments of the CT state can be calculated using Finite Field approach. After that, we use semi-classical theory. That is to say, the laser field is considered as classical electric-magnetic field with definite amplitude and phase, and described by Maxwell equations, while the PNA molecule medium is considered as a quantum system and described by Bloch equations. The laser field and quantum system are related by a macroscopical polarization, then the coupled Maxwell-Bloch equations are built up. By use of an iterative predictor-corrector finite-difference time-domain method, we solve the Maxwell-Bloch equations numerically and obtain the propagation properties of the ultrashort laser and the temporal -special evolutions of energy-state population. The conclusions are as follows. The standing slowly varying envelope and the rotating wave approximations fail to give a good description for the propagation of ultrashort pulse in the PNA molecule medium. When the pulse with the excitation energy of the charge-transfer state propagates in this medium, the smaller area pulse still satisfies the area theory, but the carrier reshaping leads to the disappearance of self-induced transparency when the area of a pulse is equal to 2Ï€, while for larger area pulses the area theory is not applicable, due to carrier wave Rabi flopping. The permanent dipole moments of the molecule make the propagation deviate from the area theorem further and make the pulse split more quickly. The population exhibits a step character when the electric field is zero. Higher and lower frequencies appear, especially the double of carrier frequency component, and the spectral width broadens as the increase of the propagating distance. When the pulse with the half of the excitation energy of the charge-transfer state propagates in this medium, the pulse won't split, only being modulated severely. The double of carrier frequency component is visible distinctly, which indicates the high two-photon absorption properties of the molecule. Both in the resonant and nonresonant case,the phase dependence of carrier wave reshaping, the time-dependent population of the excited state and the corresponding spectra increases, as the field strength increases for fixed pulse duration, and the phase dependence decreases as pulse duration increases for fixed field strength. Continuum and distinct peaks can be achieved through the control of the relative phase of two-color ultrashort laser pulses. To sum up, the presence of the permanent dipoles has two pronounced effects relative to the absence of the permanent dipoles: (1) for one-photon transitions, the laser-molecule coupling is markedly reduced; (2) the modification of the selection rules allows simultaneous two-(a process forbidden for a two-level system if its permanent dipoles are zero) and one-phone excitation to occur; (3) the phase dependence increases due to the extra nonlinear effects introduced by the permanent dipoles.