Characterization of electron density in laser-induced plasma with terahertz wave

Terahertz (THz) wave is of fundamental importance in fusion plasmas diagnostics . In this work, we demonstrate the experimental results of the laser induced plasma diagnostics at the THz frequency range. Resonance absorption has been observed and identified through THz spectroscopy, which can be understood by a Lorentz-modified Drude model. The absorption is studied by varying the polarization and energy of the plasma-generated pulse. The results suggest no significant polarization dependence on the absorption features. As the laser intensity of the plasma-generated pulse grows, the central frequency of the resonance absorption increases, as well as the electron density. This indicates the intrinsic relationship between the plasma frequency and the resonance absorption. Therefore, the resonance absorption at the THz frequency range provides an alternative to characterize plasma properties, especially for the plasma with typical frequency at the THz range. Furthermore, our results are supplements to the existing theories on THz generation from the filament. When using the filament as a source to generate the THz wave, the absorptions and the phase variations, as determined by the electron density, should be considered, both in calculations and experiments.