Abstract
Laser-induced gas-plasma or filament can be used as the emitter of intense, coherent and broadband terahertz (THz) wave. The generation process can be well-described by transient photocurrent model, in which the transient current caused by two-color laser field is the source of broadband THz radiation. The air molecules are ionized by focused two-color laser field, and free electrons driven by the laser field will form the directional transient current radiating the THz wave. The relative phase between two-color laser field is an important factor that will affect the output THz yield. Simulation of THz emission by the transient photocurrent model is carried out in this work. Because of the stronger directional transient current under the influence of asymmetric electric field, the THz yield will be much stronger when the relative phase equals to π⁄2 than 0. The dependences of other parameters on generating THz radiation are also discussed. These include pumping intensity, power conversion efficiency, wavelength, and pulse duration. The dependence of pumping intensity on THz emission has been compared to the experimental results. Furthermore, by multi-color laser field are also investigated.. On the other hand, the plasma density is much higher in solid-state materials than that in the gas. Therefore, we also conducted studies of THz generation from solid-state material, fused silica.