Ultrafast-laser-induced Molecular Alignment And Nonlinear Optical Effects

The impulsive rotational Raman excitation induced by laser pulse will force gas molecules align parallel with the laser pulse polarization direction. When CW laser pulse or nanosecond laser pulse are used, the induced molecular alignment stars almost at the time that the pulse stars, and become random align as long as the pulse off. While the femtosecond laser induced molecular alignment will repeat itself at a long time, until the rotational wave packet loses coherence. Molecular alignment caused spatiotemporal modulation can greatly change the nonlinear character of the gaseous media, which influences the frequency spectrum and pulse width of subsequent laser pulse. These could be used at molecular-orbital reconstruction, high-order harmonic generation, M-XFROG laser pulse diagnosis, as well as revivable ultrafast optical buffer and imaging with molecular rotational wave-packets. Meanwhile, molecular alignment facilitates the ultra-fast chemical reaction and the nonlinear progress in gaseous media. In fact, there are inevitable Kerr effect and plasma defocusing accompanying with molecular alignment, so when talk about molecular alignment and its applications, it is important to distingue the act time, degree and mode of the three different effects.In this paper, we mainly introduce molecular alignment mechanism and the companied Kerr effect and plasma contribution, our main content includes the following parts:1. Every non-spherical polarizable molecular placed in a laser electric field will generate an induced dipole moment, which interacts with the laser field to form a torque, which forces the molecular axis to rotate toward parallel with the direction of the field polarization. According to the comparability of impulsive pulse width with alignment period, molecular alignment could be divided into two groups: adiabatic alignment and non-adiabatic alignment. The statistic metric《cos2θ》 could be used to measure the molecular alignment in both of the two generalities.2. The pre-aligned molecules exerts orientation-dependent refractive index changes, we proved that a Gaussian-shaped spatial distribution pump pulse induced pre-aligned molecules acted as a newly established gas-phase nonlinear lens, which caused the subsequent pulse experiences spatical (de)focusing. Based on the (de)focusing effect, we developed a direct measurement of molecular alignment, where the parallel and perpendicular alignments were clearly characterized, and the intensity depended Kerr effect and plasma contribution were distinguished from measured signals.3. Based on the (de)focusing effect, we illustrated the electronic Kerr effect in measured alignment signal, explained the cooperated or competed relationship of them. When the probe pulse polarization parallel with the pump, Kerr effect could increase the signal peak strength and push the peak forward to zero time delay. At the perpendicular case, Kerr effect caused a small peak near zero time delay. Meanwhile, the nonlinear refractive index was extracted straightforwardly.4. Referring to the methods used in researching Kerr effect, we studied plasma influence. Direct retrieved plasma effect based on alignment-induced spatiotemporal modulation. Pointed out the diversity of plasma mechanism, and understood the rate of plasma producing grow exponentially depend on pulse intensity. At this experiment, we offered a reliable measurement of plasma density.