No 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.

Terahertz (THz) radiation generation based on a nonlinear induced electron current density in the interaction of two-color femtosecond laser beams with collisional and magnetized plasma is investigated. The nonlinear electron current density that is responsible for THz radiation includes a share from nonlinear ponderomotive force and a part from relativistic term. In this work, relativistic effects along with the effects of variation of interacting plasma density, external magnetic field, and electron collision frequency on THz wave generation have been considered. The reduction of electron collision frequency along with rising in the magnitude of the external magnetic field generates more power without any deformation of radiation patterns. The angular distribution showing the radiation pattern of THz radiation in the forward direction is obtained and the effect of laser pulse second harmonic and variation of electric field on the directivity of generated waves is examined. The change in two-color laser beam intensities both in relativistic and nonrelativistic regimes leads to the generation of THz wave electric fields with different orders of magnitude. The numerical analysis indicated that the utilization of laser pulse second harmonic in a two-color scheme enhances the THz electric field leading to more intense radiation compared to single-color mechanism. The model also shows the effect of external magnetic field and electron collision frequency on the radiation efficiency of THz generation for linear and circular polarizations of two-color laser pulses.