The Ultrafast Manipulation And Application Of Molecular Alignment And Orientation

The impulsive rotational Raman interaction between femtosecond laser pulse and molecules induces the molecular alignment or orientation phenomena when the laser field propagates in gaseous media. The ultrafast manipulation of molecular alignment or orientation can be realized via adjusting and controlling the parameters of femtosecond pulse. Comparing with random oriented molecules, aligned molecules show a lot of special optical properties and be able to induce many unusual physical phenomena, which are unobservable in ordinary condition, such as the alignment based birefringence, alignment induced spatial focusing and defocusing effect and so on. All these oriented-molecule based effects have tremenduous application potential, such as the alignment based inoinzation enhancement, the promotion of high harmonic generation efficiency, and the modulation of propagation of ultrashort laser pulse. The manipulation of molecular alignment and orieantion by various approaches is to realize all kinds of oriented-molecule based applications.This dissertation mainly focus on the ultrafast manupation of molecular alignment and orientation, with the aim of realizing some oriented-molecule based applications. We carry out research through experiments as well as throretical calculation, mainly including the following:1. The characterization of fourth harmonic wave is performed via the technique of molecular alignment based cross-correlation frequency resolved optical gating (MXFROG). Meanwhile, The measurement of fundermental wave and third harmonic wave is realized simultaneously by MXFROG. Moreover, we managed to realized the complete characterization of elliptically polarized feotosecond laser pulse by introducing the Stokes parameters in the experiments, and therefore extend the scope of application of MXFROG.2. The molecular alignment singal is closely related to the intensity of incidence field so that the alignment singal created by Gaussian-shaped femtosecond laser field would induce spatial focusing and defocusing effect. By making use of such effect, we measure the alignment singals of oxygen and acetylene under the identical experimental conditions. With the available nonlinear parameters of oxygen, we managed to retrieve the corresponding unknown parameters of acetylene by comparing the molecular alignment singals of oxygen and acetylene.3. Theoretical investigation on the mechanism and properties of the interaction between femtosecond dual-color field and polar molecules has been performed. The influence of the parameters of the dual-color field upon the molecular orientation degree is also discussed. It is also theoretically proved that if the energy of dual-color pulse is given, when the intensity ratio between fundamental wave and second harmonic wave equals2, the dual-color field can induced maximum molecular orientation degree.4. Comparison investigation between dual-color field and terahertz field in creating molecular orientation has been performed. It is theoretically proposed that the combination of dual-color field and terahertz field can make good use of their respective advantages, and the calculation shows that the combination fields can enhance the molecular orientation degree by a factor of2. The proposal is suitable both to the molecules with longer and the shorter revival periods.