Study Of The Generation And The Control Of The Femtosecond Laser Pulse Induced Terahertz Radiation

It has attracted more and more research interest in terahertz generation with its low photon energy, abundant physical and chemical information, picosecond pulse-width and short few cycle pulse. The terahertz wave is between microwave and infrared wave with its frequency0.1-10THz, and the wavelength0.03-3mm. As known, the counterbalance between the Kerr self-focusing and the plasma self-defocusing effects produced the femtosecond dual-color pulse filament, which could be used as an efficient terahertz radiation source. During this process, it is quite necessary to find efficient method to control the terahertz intensity and polarization. Previous research has shown that the molecular alignment induced by the femtosecond pulse could modify its nonlinearity, and further adjust the nonlinear process conveniently.In this dissertation, based on the molecular alignment, we investigate the modulation of the terahertz intensity and polarization and the nonlinear process during the plasma channel. Mainly includes the following:1. Experimentally demonstrate the physical mechanism of the terahertz generation derived from the dual-color pulse. This project measures the phase difference of the dual-color pulse accurately, and shows the function relationship between the terahertz electric field and the dual-color pulse phase difference. It proves that the physical mechanism of the terahertz radiation can be explained by the photo current model as the terahertz electric field is proportional to the sin θ in our results, where θ is the phase difference of the dual-color pulse. It has important significance to find higher power terahertz radiation source.2. The terahertz generation from dual-color pulse filament was controlled by molecular alignment induced (de)focusing effect. Molecular alignment effect modulated the refractive index of the air. For the subsequent dual-color pulse, parallel molecular alignment increased the refractive index acting as a positive lens to increase the ionization of the dual-color pulse, and the terahertz generation increased. On the contrary, the perpendicular molecular alginment decreased the refractive index acting as an negitive lens for the subsequent dual-color pulse. The ionization of the dual-color pulse was decreased and the terahertz generation decreased. On the other hand, the molecular alignment leads to the change of the nonlinearity, and the phase velocity also changed during the propagation process, which adjusts the dual-color pulse phase difference and further controls the intensity and polarization of the generated terahertz. Both linearly and elliptically polarized terahertz radiations can be obtained as the field-free molecular alignment in air functioned as a transient dynamic wave plate.3. The pre-existing plasma and a biased DC electric field was experimentally used to control the intensity and the polarization of the THz radiation. The biased DC electric field increased the plasma photo current to produce stronger terahertz radation. The pre-existing plasma controled the intensity and the polarization by the additional refractive index induced by its presence. These provided efficient methods to improve the terahertz generation.