Based on the theoretical model of field ionization current for terahertz (THz) emission in laser-gas interaction, the parameter optimization for strong THz emission via the two-color laser scheme is explored. It is found from numerical calculation that the ionization current is due mainly to the first-order ionization process and the contribution from high-order ionization is ignorable. In order to produce stronger THz emission, the ionization current can be enhanced by adjusting the amplitude ratio between the fundamental and its second harmonic laser pulses as well as their relative phase difference. The ionization current can also be increased by use of laser pulses at mid-infrared and by changing the gas species. Under the same laser pulse and gas density conditions, the ionization current from the Helium gas is almost twice that from the Neon gas.

Based on a theoretical model and numerical simulations, the ionization currents and subsequent terahertz (THz) emission induced by the interaction of a few-cycle laser pulses with He gas targets are studied. It is shown that owing to the large transverse current generated by field ionization with few-cycle laser pulses, strong THz emission can be generated. The change of the carrier phase of the few-cycle laser pulses leads to the variation of the ionization currents. Correspondingly, the THz emission amplitude shows the characteristic as a periodic function of the carrier phase, which is also confirmed by one-dimensional particle-in-cell simulations. For a given carrier phase, the THz emission amplitude is not proportional to the laser amplitude. It shows at least two peaks at certain laser amplitudes. When the gas density profile is not uniform, the emission amplitude has a similar dependence on laser amplitude and carrier envelope phase, but the THz pulse duration and spectrum are quite different.