Resonant third harmonic generation in a laser produced thin foil plasma

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 2 omega, 4k, where omega and k are the frequency and wave number of the laser. The density ripple couples with the oscillatory electron velocity at omega, k to produce a nonlinear current at 3 omega, 5k driving a resonant third harmonic radiation in the region where 5k=k(3) and k(3) is the wave number at 3 omega 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. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2998827]