Resonance-enhanced high-harmonic generation

In this paper we present some results of a high-harmonic generation experiment in argon, performed with a Ti:sapphire laser (787 nm, 140 fs) and a spatially shaped focus (flat top). The range of intensities is 3-100 TW cm^-2. The intensity dependence of the harmonic signal shows an interesting behaviour of the 13th harmonic, namely an initial suppression followed by a strong enhancement at @85 TW cm^-2, which exceeds the magnitude of the 11th harmonic. This behaviour is a consequence of a resonant atomic process. Numerical calculations were also performed. They partly reproduced the experiment.