Density structure in low-order harmonic emission from laser-plasma interactions

Abstract

Multiple harmonic generation from intense laser light incident on dense plasma targets is well-established. Both the number of harmonics generated and the corresponding power in each is strongly dependent on the intensity of the incident light. In this paper, attention is focused on an aspect of harmonic generation that has hitherto been largely overlooked, namely its dependence on plasma density. In particular we show that the conversion efficiencies of odd harmonics generated for normally incident light are strongly resonant in character, the number of resonances increasing with harmonic order. In addition to this resonant behaviour, the reflected power in low-order harmonics is further complicated by plasma emission at plasma resonant densities. Only at densities well above the resonances does the radiated harmonic power decay monotonically with density. The density dependence predicted by theory for low-order harmonics is confirmed by particle-in-cell simulations.