In this work, we investigated the generation of THz short pulses by two-color mid-infrared laser pulses induced gas plasma in ambient air by numerical analysis. In the simulation, the central wavelength of the input laser varied from 2.5 μm up to 4.0 μm. Our result indicates that the generated THz pulse intensity increases at longer wavelengths significantly. In our simulations, the fundamental pulse intensity and duration were kept fixed yielding no significant difference between the generated electron densities indicating that the asymmetry of the electric field has a major role in the THz generation efficiency by increasing the velocity of electrons. Our results also show that the fundamental pulse and the THz pulse can spectrally overlap, which makes it difficult to separate them spectrally. The possibility of the relative phase control between the fundamental and the second harmonic pulses with a single plate is also examined. Our calculations show that the best materials are the fluorides for controlling the relative phase.
We report on the numerical investigation of the generation of THz short pulses by two-color laser pulses in ionized gas plasma in ambient air. One of the major aspects of this study is to find the ideal conditions of second harmonic pulse generation in a suitable nonlinear crystal, where the generated THz pulse is the most intense at given input pulse parameters. We found that an optimal thickness can be found depending on the input pulse parameters, which is defined by two opposing phenomena, the frequency conversion and the linear dispersion. On one hand, a thick crystal can generate energetic second harmonic, but group velocity mismatch spoils the temporal overlap, while on the other hand, a thin crystal does not have enough conversion efficiency. The optimal thickness of one of the most common nonlinear material, BBO was investigated between 1 μm and 500 μm in regards two of the major laser pulse parameters, the pulse duration from 25 fs up to 100 fs and the fluence from 1 Jcm-2 up to 5 Jcm-2. Our investigation concluded that the optimal thickness increases with the pulse duration and decreases with the fluence.
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