Terahertz radiation generation by nonlinear mixing of two lasers in a plasma with density hill

Abstract

An analytical formalism of the terahertz (THz) radiation generation by beating of two lasers in a plasma with the density hill is investigated. The lasers propagate obliquely to the density gradient, and the nonlinearity arises through the ponderomotive force. The density gradient renders the ponderomotive force driven beat frequency nonlinear current density J→NL to possess a nonzero curl (∇×J→NL≠0) when θ is finite, giving rise to the THz radiation generation. The plasma frequency peak ωpmax is below the frequency difference of the lasers (ωpmax<(ω1−ω2) cos θ, where ω1 and ω2 are the frequencies of the laser, and θ is the angle that is arrived by their propagation vectors with the density gradient) to avoid THz reflection. The THz power conversion efficiency decreases with the THz frequency, increases with the plasma frequency and electron temperature, and maximizes at an optimum angle of incidence. For our set of parameters, the radiated THz power is about 0.15 GW.