Effects of multi-color femtosecond laser beams and external electric field on transition-Cherenkov THz radiation

Abstract

Terahertz (THz) radiation via transition-Cherenkov mechanism through effects of multi-color femtosecond laser beams and an external electric field is investigated. In this scheme, the electromagnetic radiation driven by the nonlinear longitudinal current density during two- and three-color filamentation has been evaluated. Variations of the electric field of generated THz waves based on the number of harmonics have been considered. The findings indicate that the addition of the second and third harmonic of laser pulse enhances the THz radiation of filament. The superposition of harmonics of the laser pulse increases the dipole-like current density behind the ionization front, and when the velocity of the ionization front exceeds the light speed, the enhanced Cherenkov radiation electric field generates stronger THz radiation in the far-field zone. The angular distribution of radiation pattern in the forward direction was obtained, and the effects of different parameters on generated THz wave patterns were examined. The polarization of laser pulses and the ionization rate as well as the length of filament play a crucial role in the generation of peak frequency and bandwidth of the THz radiation spectrum. The present study shows that an external electric field induces more dipole-like current density leading to an increase in radiation power with no change in directivity.