Density effects in harmonic emission from intense laser interactions with overdense plasmas

Although the harmonics generated when intense laser light interacts with solid targets are broadly understood in terms of their dependence on the incident intensity, less attention has been paid to the ways in which their behaviour depends on plasma, density. In this paper the conversion efficiencies of the odd harmonics generated when light is incident normally on the target surface are shown to be strongly resonant in character. The number of resonances is governed by harmonic order and their behaviour with density is further complicated by Langmuir wave emission at the corresponding plasina resonant densities for successive harmonic orders. Only at densities well above these resonances does the emission decay monotonically with density. The density dependence predicted by theory is confirmed by particle-in-cell simulations.