Numerical Study of the Effect of Pre-Pulse on THz Generation by Two-Color Laser-Induced Microplasma

In this article, a photocurrent model is used to investigate terahertz (THz) generation in two-color laser-induced preformed air microplasma. Comprehensive 2-D numerical particle-in-cell (PIC) simulations are done to understand the most important mechanism of THz generation in laser-induced microplasma. The model used in this simulation involves both ionization current (transverse current) and ponderomotive forces (longitudinal current). The laser pulse is divided into two types, a pre-pulse and a delayed main pulse, which propagate in a parallel direction (i.e., collinear configuration). Our simulation shows that the THz amplitude significantly reduces in the presence of a pre-pulse compared to the case with no pre-pulse. Also, in the study of how a time delay between the pre-pulse and the main pulse affects THz radiation, it has been demonstrated that THz radiation decreases as the time delay increases. Finally, through the investigation of the main pulse and pre-pulse intensity ratio, it emerged that the THz amplitude decreased with an increase in that ratio. The results point to the major role of tunneling ionization in THz generation via laser-induced microplasma.