A transient photocurrent model is used to explain terahertz emission from gas plasma irradiated by two-color laser pulses, with one the second harmonic of the other. In this optical process, the relative phase between the two fields is very critical to the terahertz generation efficiency. In order to find out the optimal phase, many cases with relative phase from − π/2 to π/2 are studied. Results show that the case at π/4 can obtain the largest intensity.

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.

A transient photocurrent model is used to explain terahertz (THz) emission from gas plasma irradiated by two-color laser pulses, with one the second harmonic of the other. Taking multiple degrees of ionization into account, the gas ionization process at different irradiating laser pulse width is discussed. Then the corresponding terahertz spectrums and waveforms in this optical process are calculated, results showing that keeping the irradiating pulse energy invariant, the ultimate THz emission can be significantly increased by shortening the irradiating pulse width. Besides, by comparison with the corresponding results when only considering single ionization, one can see that the double ionization begins playing an important role under the intensity range of our interest, and thus should not be neglected.