Resonance-enhanced high-harmonic generation

In this paper we present some results of a high-harmonic generation experim ent in argon, performed with a Ti sapphire laser (787 nm, 140 fs) and a spa tially shaped focus (flat top). The range of intensities is 3-100 TW cm(-2) The intensity dependence of the harmonic signal shows an interesting behav iour of the 13th harmonic, namely an initial suppression followed by a stro ng enhancement at similar or equal to 85 TW cm(-2), which exceeds the magni tude of the 11th harmonic. This behaviour is a consequence of a resonant at omic process. Numerical calculations were also performed. They partly repro duced the experiment.