Abstract
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.
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