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

The generation of terahertz (THz) waves via the 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 the dielectric function of such plasma and using WKB and paraxial ray approximations. The amplitude of THz radiation at beat frequency resulting from the nonlinear current density induced by the beat ponderomotive force of the cross-focusing of beams was obtained. The impacts of the chirp frequency parameter, initial laser intensity and initial ionic species density (specifically, the 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 a 'turning point', where the THz emission reaches its maximum value. The results show that the strength of self-focusing and consequently the generated THz radiation are reduced by increasing the density of doubly charged ionic species in the plasma due to the suppression of the nonlinear effects.