Theoretical research on terahertz air-breakdown coherent detection with the transient photocurrent model

The physical mechanism for sensing broadband terahertz (THz) wave via using femtosecond (fs) laser induced gas plasma without any local accessory near the plasma, i.e. THz air breakdown coherent detection, is systemically investigated by utilizing the transient photocurrent model. Previous observed results, such as conversion from incoherent to coherent detection, can be numerically obtained. Further calculations and analysis show that it is through modification of the gas ionization process, and not acceleration of freed electrons or through a four-wave-mixing (FWM) process, that the THz waveforms can be encoded into the detected second harmonic (SH) signals.