Resonant third harmonic generation in a laser produced thin foil plasma

Abstract

An analytical model is developed for third harmonic generation (THG) efficiency from a high-density inhomogeneous plasma produced by laser irradiation of a thin metallic film. The laser suffers strong reflection from the critical layer. The superposition of forward and backward waves creates a quasistatic density ripple of wave number 2k and a second harmonic density ripple at 2ω, 4k, where ω and k are the frequency and wave number of the laser. The density ripple couples with the oscillatory electron velocity at ω, k to produce a nonlinear current at 3ω, 5k driving a resonant third harmonic radiation in the region where 5k=k3 and k3 is the wave number at 3ω frequency. As the density scale length of the plasma is increased, the efficiency of THG increases. The same behavior is reproduced in two particle in cell simulation by launching a laser in a rippled density underdense plasma of phase-matched density and a ripple period of half-laser wavelength.