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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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