Cross-focusing of two chirped Gaussian laser beams and THz wave generation in plasmas of multi-ion species under a weakly relativistic and ponderomotive regime

Abstract

The generation of terahertz (THz) wave by beating of two high intensity chirped Gaussian lasers in a multi ion species plasma is numerically studied by taking into account the weak relativistic and ponderomotive regime of interaction. The coupled differential equations for beamwidth parameters are extracted by introducing dielectric function of such plasma and using WKB and paraxial ray approximations. Amplitude of THz radiation at beat frequency resultant from the nonlinear current density induced by beat ponderomotive force of cross-focusing of beams was obtained. The impacts of chirp frequency parameter, initial laser intensity and initial ionic species density (specifically, presence of singly and doubly charged ions) in the plasma on THz generation were discussed. Our numerical results reveal that THz radiation generation strongly depends on the chirp frequency parameter. A specific range of chirp frequencies exists for self-focusing as well as THz generation with \textquotedblleft turning point\textquotedblright where the THz emission reaches its maximum value. The results show that the strength of self-focusing and consequently the generated THz radiation get reduced by increasing the density of doubly charged ionic species in the plasma due to the suppression of the nonlinear effects.