Dynamical Determination Of Nonlinear Optical Parameters Of Strong Two-photon Absorption Molecular Materials

At present, ultrashort laser pulses with extreme intensity has developed quickly, and more and more organic materials and macromolecule compounds appear with good optical properties. The interaction between laser pulse and organic materials came into a completely new area. We study the interaction between laser pulse and molecules by taking strict solutions and adopting a series of approximations. We focus on the optical limiting properties of molecular materials, and study effects of optical ionization, solvent, Lorentz local field correction on their nonlinear optical behaviors.1,Effect of optical ionization on optical limiting properties of molecular materials.Taking 4,4'-bis(diphenylamino) stilbene molecule as medium, we study the interaction between the pulse and the medium by numerically solving the Maxwell-Bloch equations using an iterative predictor-corrector and finite-difference time-domain method. The influences of ionization on properties of optical limiting are investigated. The results show that effect of ionization on optical limiting properties is closely related to the propagation distances and pulse intensities. When the propagation distance is large and the input pulse intensity is strong enough, the ionization effect is favorable for the optical limiting.2,The effect of solvent on dynamical TPA and optical limiting of molecular media.The dynamical two-photon absorption (TPA) cross section as well as optical limiting of a 4,4'-bis(dimethylamino) stilbene (BDMAS) molecular medium for the nanosecond and femtosecond laser pulses is studied. This molecular medium can be described by a cascade three-level model in the visible light regime. Our numerical results show that the BDMAS molecular medium exhibits a strong optical limiting behaviour. The saturation TPA in the femtosecond time domain can be observed, and materials with larger nonlinear absorption cross section would be much easier to be saturated. Due to the contribution of the two-step TPA, the dynamical TPA cross section of BDMAS for nanosecond pulses is about three orders of magnitude larger than that for ultrashort femtosecond pulses. Special attention has been paid to the solvent effects on the optimal limiting performance. With an enhancement of the polarity of solvents, the dynamical optical limiting window becomes broader. In the origin of optical limiting, the dynamical TPA cross section of BDMAS decreases when the polarity of solvents increases, which is in good agreement with the experiment.3,Effect of Lorentz local field correction on propagation of ultrashort laser pulse in one-dimensional para-nitroaniline(PNA) molecules.The effect of Lorentz local field correction (LFC) on propagation of ultrashort laser pulses in para-nitroaniline molecular medium under the resonant and nonresonant conditions is investigated by solving numerically the full-wave Maxwell-Bloch equations beyond slowly-varying envelope approximation and rotating-wave approximation. The effect of the LFC is considerably obvious when pulses with large areas propagate in the dense molecular medium. In the case of resonance, the group velocity of the sub-pulses split from the incident pulse along propagation is severely decreased by the LFC, especially for the latest sub-pulse. However, in the case of nonresonance, the influence of the LFC on temporal evolution of the pulse is less obvious and lacks of homogeneity with increase of incident pulse area, propagation distance and molecular density.