Coherence and resonance effects in high-order harmonic generation

We present experimental measurements of harmonic generation in xenon using a 1064-nm 40-psec Nd-doped yttrium aluminum garnet laser. The harmonic yield is studied as a function of the position of the laser focus in the atomic beam. It shows regular oscillations whose period decreases with increasing harmonic order and which can be interpreted as phase-matching effects due to the tight focusing geometry. Moreover, the amplitudes of the oscillations vary rapidly with the laser intensity, a signature of resonance effects in the harmonic generation process.