Dynamics Of Optical Nonlinearities Induced By Molecular Translational And Rotational Motions

The dynamics of molecular motion in liquids has attracted a lot of attention for many years. Knowledge of the dynamics of molecular is central to an understaning of physical and chemical origin of mechanisms. Compared to the solid and gas states, liquids are rather poorly understood on a molecular level. In this dissertation, we focus on the molecular motion induced optical nonlinear effects in liquids. The recently developed time-resolved pump-probe technique is employed to investigate the molecular motion induced nonlinear refraction dynamics. The advantages of this technique compared to optically heterodyne detected optical Kerr effect (OHD-OKE) is that it allows direct measuring the sign as well as magnitude of the nonlinear refraction responses.Nonsymmetrical transition from reverse-saturable absorption (RSA) to saturable absorption (SA) is observed in the Z-scan experiments of CuPcTs/DMSO solution. The nonsymmetrical transition from RSA to SA is ascribed neither to saturation of excited-state absorption nor to thermal induced mass transport, the so-called Soret effect. In our consideration, strong nonlinear absorption causes the rapid accumulation of the non-uniform kinetic energy of the solute molecules. The non-uniform kinetic field inevitably causes the migration of the solute molecules. A kinetic energy-gradient theory is presented to interpret the experimental results. Moreover, a two-beam pump-probe Z-scan method is proposed to investigate the translational motion of solute molecules induced mass transport. With this method, the'pure'mass transport effect is extracted from influence of electronic excitations induced nonlinear absorptions. The single peak of the experimental curves clearly indicates the solute molecules migrating out from the centre of beam. The slightly shift of the peak represents the cumulative effect of mass transport. The time-resolved pump-probe technique with a phase object (PO) is employed to investigate the nonlinear refraction responses of nitrobenzene with 21-ps laser pulses. The slow decay which is attributed to the rotation of the molecules and the electron distortion indcued instaneous refractive dynamics are observed. A molecular rotational dynamic mode provides an excellent description of the entire temporal profile of the refractive signal. The coefficients andare obtained by theoretical fits. Z-scan experiments with different input pulse energy with pulse width of 21ps are also performed, and the saturation of molecular reorientation is observed. Moreover, results obtained from Z-scan experiments with 150-fs laser pulses indicate that nonlinear refraction mainly attributes to the instanteous electronic hyperpolarizability contribution in sub-picosecond regime. However in the picosecond regime, the molecular orientation dominates the nonlinear refraction.The PO pump-probe experiments with polarization sate of the pump beam perpendicular as well as parallel to that of the probe beam are performed to investigate the molecular rotation induced anisotropy of the nonlinear refraction in nitrobenzene and carbon disulfide. Further research indicates that the molecular orientation induced nonlinear refraction is a third-order nonlinear phenomenon. The derived molecular orientation induced nonlinear refraction coefficients of nitrobenzene meet the relation n2 ? ? 2n2⊥, which is coincident with the studied theoretical model.The nonlinear refraction dynamics of several simple liquids are investigated by PO pump-probe experiments with the polarization state of the pump beam perpendicular to that of the probe beam. The molecular rotation induced energy transfer between the pump and the probe beams is observed. Based on the coupling theory and the PO pump-probe theory, the rotational lifetime of liquids are obtained.