Third harmonic generation of a short pulse laser in a plasma density ripple created by a machining beam

Abstract

An intense machining laser beam, impinged on a gas jet target, causes space periodic ionization of the gas and heats the electrons. The nonuniform plasma pressure leads to atomic density redistribution. When, after a suitable time delay, a second more intense laser pulse is launched along the periodicity wave vector q⃗, a plasma density ripple nq is instantly created, leading to resonant third harmonic generation when q=4ωp2∕(3ωcγ0), where ωp is the plasma frequency, ω is the laser frequency, and γ0 is the electron Lorentz factor. The third harmonic is produced through the beating of ponderomotive force induced second harmonic density oscillations and the quiver velocity of electrons at the fundamental. The relativistic mass nonlinearity plays no role in resonant coupling. The energy conversion efficiency scales as the square of plasma density and square of depth of density ripple, and is ∼0.2% for normalized laser amplitude ao∼1 in a plasma of 1% critical density with 20% density ripple. The theory explains several features of a recent experiment.