On the effect of stimulated raman scattering on THz radiation in the laser plasma interactions

Abstract

In present study, two-dimensional Particle-In-Cell (PIC) simulations are used to investigate the interaction of ultrashort high-power laser pulses with an under-dense plasma medium. The results of the numerical simulations are followed by some mathematical procedures to obtain the terahertz electric field and the growth rates of the Stimulated Raman Forward (RFS) and Backward Scattering (RBS). The influences of incident laser pulse intensity and initial plasma density are examined on RFS and RBS instabilities and their consequent effects on the characteristics of THz radiation generation. It is seen that at the lower initial plasma densities i.e. 0.0001nc, the larger laser intensities will lead to more broadband and intense THz signals. Where nc=1.74 × 1027m−3 is known as critical plasma density corresponding to the laser pulse frequency. On the other hand, at the higher plasma densities, i.e., 0.0026nc, due to the instabilities growth, the effects of higher laser pulse intensities on the THz emission are insignificant. While for all the considered plasma densities and incident laser intensities, RBS inevitably affects the THz generation, RFS starts to grow for the initial plasma densities of n0=0.0026nc at the incident laser intensities higher than 1 × 1015Wcm−2. Hence, a higher bound is established on the pump intensity which is based on the RFS growth initiation. Thus, to achieve a stable THz signal for the initial plasma density of n0=0.0026nc (4.5 × 1018 cm−3), the incident laser intensity needs to be less than 1 × 1016Wcm−2.