Study On The Manipulation Of Ultrashort-laser-pulse Induced Nonlinear Effects By Molecular Alignment

Intense ultrashort laser pulses propagating in air include several nonlinear optical effects, such as optical Kerr self-focusing, multi-photon ionization, plasma defocusing, self-phase modulation, self-steepening and group velocity dispersion. The dynamical balance between Kerr self-focusing and plasma defocusing leads to a self-guide filament with a propagating range longer than Rayleigh length. Molecules in air can be aligned by a linearly polarized ultrashort pulse and the excited molecules present periodic revivals of alignment after the pump pulse.In this dissertation, based on the manipulation of the ultrashort pulse propagation in air by molecular alignment, we investigate the modulation of the plasma grating and the third-harmonic generation in the pre-aligned molecules and propose an ultrafast optical gating by molecular alignment and explored an application of this gating in ultrafast optical imaging. Mainly includes the following:1. A novel ultrafast optical gating is proposed based on the molecular alignment in air. The optical gating has unique features including birefringence, femtosecond-scale switching time, periodic revivals, spatial quasi-lens effect and spectral modulation.2. A proof-of-principle experiment of ultrafast optical imaging by using impulsive alignment of diatomic molecules in air is carried out. By employing the pump-probe technique, the image information is written into the molecular alignment by the writing pulse and then can be read out at periodic molecular alignment revivals by a reading pulse. The capacities of raised and intagliated monochromatic imaging, as well as colorful and even holographic-like imaging are experimentally demonstrated.3. Experimentally investigate the modulation of the third harmonic generation (THG) in a filament by molecular alignment. Due to the perturbations of the effective linear and third-order susceptibility induced by the molecular alignment, the THG is enhanced or suppressed by following the impulsive molecular alignment. And the spectral modulation of the third harmonic and is also studied by comparing with the case of the fundamental wave from the same filament.4. Experimentally demonstrate the precise manipulation of the multiphoton-ionization-induced plasma grating by impulsive molecular alignment. The plasma grating induced in the pre-aligned molecules is modulated due to the alignment-dependent-ionization, leading to the modulation of the linear diffraction efficiency. And the dependence of the plasma-grating-enhanced nonlinear THG on the impulsive molecular alignment is also investigated.5. Experimentally investigate the role of the pump filament on the filamentation dynamics of succeeding collinearly-propagating probe pulse. The clamping intensity and electron density of the probe filament are modulated by the plasma and impulsive molecular alignment created by the pump filament. Also, the roles of the Kerr effect, plasma effect, and molecular alignment are indentified in the interaction between the two collinearly-propagating filaments around zero time delay.