Theoretical Research On Generating Terahertz Radiation From Gas Plasma Induced By Ultrashort Lasers

Recently, research on terahertz wave generation from gas plasma induced by strong laser field has attracted the attention of many researchers. Compared with other methods, this scheme generates terahertz radiation with high intensity and wide spectrum, and there is no need to worry about the loss of medium, phonon resonance and absorption. Besides, using the reverse process of its principle, one can perform detection simultaneously.In this dissertation, based on the popular photo-current model, we have made a series of exploration on how to get intense terahertz output from several aspects.First, we describe the two-color photo-current model, add a third harmonic light to the ?-2? field, and study the effect of relative phase on strength of radiation. Through a detailed analysis on some specific values, we give a simple but physically clear explanation for this influence brings by the 3? light.Second, we study the impact of adding a constant magnetic field in terms of changing the spatial symmetry of electron motion. The discrete expression of the electron's speed under the light field and magnetic field is carefully derived, and calculations for electromagnetic radiation are performed. Simulation results show that addition of the magnetic field not only enhances the radiation strength, but also reduces the influence of the relative phase, resulting a more stable output.Finally, we try to alter the monochromatic light in two ways: reducing pulse width and introducing a linear chirp. As we know, chirped laser leads to broadening of spectrum, and the wave form of few-cycle laser itself is asymmetric, all of which lead to enhanced terahertz emission, and these two effects are cumulative. Besides, we also find that broadening of spectrum on high- frequency direction increases with the chirp parameter, while broadening on the low-frequency direction are showing a mysterious fluctuation.The investigation presented in this thesis provides a few way of enhancing terahertz radiation and gives our perspective, which can be used for future research theoretically and experimentally.