Control Of Femtosecond Filament And Terahertz Generation By Molecular Alignment

The terahertz (THz) radioation with frequency between 0.1 to 10 THz shows significant applications in many fields by virtue of its preponderance of rich information containing, sub-picosecond pulse duration, few-cycle, low photon energy and so forth. Intense THz radiation can be remotely generated by femtosecond laser pulse induced filamentation, without detrimental absorption attenuation by the presence of water vapor in air. It is quite interesting to effeciently control the strength and polarization of the generated THz especially in an ultrafast all-optical approach. Field-free revivals of impulsive molecular alignment through nonadiabatic rotational Raman excitation can be used to influence the formation of filamentation and thus the simultaneously generated THz radioation.In this thesis, by using the impulsive alignment of the diatomic molecules in air, we studied the interaction of two spatially parallel lauched femtosecond filaments at different wavelengths and the generation of THz radiation by a two-color pulse. The following studies have been performed:1. The interaction of two spatially parallel launched femtosecond filaments at the fundamental-wave and second-harmonic frequencies was studied in air. Besides the Kerr effect within the pulse duration, the impulsive alignment of the diatomic molecules in the fundamental-wave filament led to controllable and field-free achievable attraction, fusion, or repulsion of the second-harmonic filament. The molecular-alignment-assisted filament interaction was further confirmed by the fluorescence intensity variation and spectral modulation of the second-harmonic filament at various molecular alignment revivals. This provided us an ultrafast-flexible field-free approach to precisely control the intense ultrashort filaments. 2. All-optical ultrafast control of THz generation from a two-color femtosecond pulse (composed of the fundamental and second-harmonic waves) in air was demonstrated by using impulsive molecular alignment. By tuning its time delay to properly match various molecular alignment revivals, the THz generation from the two-color pulse could be promoted or decreased due to the spatial cross-(de)focusing effect and the alignment-dependent ionization probabilities of the prealigned diatomic molecules in air. For the two-color pulse of orthogonally polarized fundamental and second-harmonic components, the polarization of the generated THz radiation could be controlled by the field-free molecular alignment, which functioned as a transient dynamic wave plate for the THz radiation. The plasma effect on the THz generation of the two-color pulse was also observed, leading to additional amplitude and polarization control of the THz radiation.