| Terahertz generation via gas plasma induced by a focused femtosecond laser beam has attracted a considerable amount of interest owing to its capability of producing broadband and high-power terahertz pulse. This scheme is a very promising terahertz source for future THz application. The laser is a combination of a fundamental femtosecond with a wavelength of 800 nm and its second harmonic pulse. Based on the photocurrent model, this thesis studies on the characteristics of terahertz wave emission from gas plasma, the main works of the thesis are as follows:(1) In the one dimensional case, characteristics of terahertz emission is studied. The influence of laser intensity, relative phase of fundamental and second harmonic laser pulse, Ionization saturation is discussed.(2) The influence of wavelength of laser on terahertz generation is discussed. In a certain range, the terahertz strength is positive correlated to the wavelength of laser. A scheme of terahertz generation using a fundamental pulse and its half harmonic pulse is carried out, and its feasibility is explained.(3) Combining with the actual scheme of terahertz generation, in the two dimensional case, characteristics of terahertz wave emission are discussed, especially the reason of elliptically terahertz generation is explained. In theory, the elliptically of terahertz is not only attributed to the influence of plasma, but also the polarization change of laser induced by the BBO crystal.(4) The mechanism of plasma filament is studied. Combined with the Characteristic of filament, its influence on laser transmission is discussed, including spectral broadening, dispersion and cross-phase modulation. Based on this, the terahertz emission from plasma filament is also studied with photocurrent model. |
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