High-order harmonic generation in xenon: intensity and propagation effects

We present experimental results on high-order harmonic generation in xenon, using a Nd:YAG laser (1064 nm). The harmonic generation efficiency is studied as a function of the laser intensity in the 1013 W/cm2 range. The harmonic conversion rates show a significant change of behavior when the medium becomes ionized (∼1013 W/cm2). This effect can be understood as the change of the nonlinear response of the medium from neutral atoms to ions and also as the influence of free electrons. We study the role of propagation effects (phase matching) by performing experiments using three different lenses (75, 200, and 300 mm) to focus the incident laser beam, thus changing the interaction geometry. Although for each harmonic the conversion rate is strongly dependent on the confocal parameter b (as b3), the behavior of the harmonic frequency distribution shows almost no influence of the focusing geometry.