Femtosecond envelope of the high-harmonic emission from ablation plasmas

We characterize the temporal profile of the high-order harmonic emission from ablation plasma plumes using cross-correlations with the infrared (IR) laser beam provided by 2-photon harmonic+IR ionization of rare gas atoms. We study both non-resonant plasmas (lead, gold and chrome) and resonant plasmas (indium and tin), i.e. plasmas presenting in the singly-charged ions a strong radiative transition coinciding with an harmonic order. The cross-correlation traces are found very similar for all harmonic orders and all plasma targets. The recovered harmonic pulse durations are very similar to the driving laser, with a tendency towards being shorter, demonstrating that the emission is a \stef{directly} laser-driven process even in the case of resonant harmonics. This provides valuable input for theories describing resonant harmonic emission and opens the perspective of a very-high-flux tabletop \textsc{xuv} source for applications.