Resonant high harmonics and the role of phase matching

Resonance high order harmonic is an exciting phenomenon predicted long time ago but discovered only recently Explanations for the resonance enhancement of a single harmonic rely either on the single atom response or on the collective process of phase matching In this work we try to discriminate between the two possibilities by measuring the coherent lengths of all the harmonics and compare them Information about the coherent lengths of the harmonics allow us also to demonstrate quasi phase matching enhancement of the harmonics yield