Dynamics Study On Optical Limiting Property Of Two-photon Absorption Molecular Materials

The main purpose of this thesis is to study the carrier-envelop phase (CEP) dependence of ultrashort pulse propagation in a polar molecule and get a new approach for CEP determination. In addition, we focus on the interaction between laser pulse and organic materials with two-photon absorption (TPA) property and optical limiting behavior. The main contents and results are summarized as follows:1. CEP dependence of ultrashort pulse propagation in a polar moleculeWe take DBASVP as an example, quantum chemical calculations show DBASVP has only one charge-transfer (CT) state in lower energy region, therefore-the transition scheme can be simplified as a generalized two-level model, including the ground state So, the CT state S1 in the visible light regime. We theoretically investigate the CEP dependence of ultrashort pulse propagate in DBASVP by numerically solving the full-wave Maxwell-Bloch equations with an iterative predictor-corrector finite-difference time-domain (FDTD) technique. Interestingly, it is shown that unipolar half-cycle pulses can be generated during the two-photon resonant propagation of few-cycle pulse in the polar molecule medium. Moreover, the main features of the soliton pulse, such as pulse direction and the number, depend crucially on the CEP of the incident pulse, which could be utilized to determine the CEP of a few-cycle ultrashort laser pulse from a mode-locked oscillator. In addition, it is found that the information of the CEP of the initial incident pulse can be "remembered" by the population difference near the front face of the medium thus provides another useful approach to determine the CEP information of the incident pulse.2. Optical limiting effect induced by two-photon absorptionPorphyrins and grapheme oxide (GO) possess specific nonlinear optical (NLO) properties due to their significant π-electron conjugation system. It is expected that, through the conjugation of GO and porphyrins, we have succeeded in enhancing the NLO properties of several GO-porphyrins composites. The NLO studies of several hybrid materials and individual porphyrins have been studied by numerically solving the rate equations and field intensity equation with an iterative predictor-corrector FDTD technique in picosecond time domain. Our numerical results show that when the incident intensity is relatively weak, the medium show constant transmittance, indicating linear absorption process, while increasing the incident intensity, the transmittance decreasing remarkably due to enhanced TPA. Moreover, hybrid materials show enhanced nonlinear absorption properties compared with individual porphyrin molecules due to the strong electron acceptor capability of the GO molecule. Moreover, Zn hybrid materials show better absorption abilities than other materials due to its heavier central metals effect. Theoretical works help to explain experimental works.Secondly, we take a serious of ladder-type oligo-p-phenylene-cored chromophores (1FL-4FL) as example. Quantum chemical calculations show that the compounds can be simplified as a generalized three-level model. We focus on the TPA property and optical limiting behavior of these organic materials in the picosecond pulse with an iterative predictor-corrector FDTD technique by numerically solving the rate equations and field intensity equation. Our numerical results reveal that an increase in the π-conjugated length of ladder-type oligo-p-phenylene for these chromophores leads to an enhanced optical limiting behavior and an increase in TPA cross section. Increasing the thickness of the absorber will enhance the interaction between the field and the media, thus more and more energies can be stored in the medium, ultimately results in better optical limiting ability of the medium. For long pulse, two-step TPA takes over one-step TPA and becomes the main mechanism to enhance the dynamitic TPA. Theoretical works help to explain experimental works.