Theoretical aspects of intense field harmonic generation

The authors present theoretical studies of high-order harmonic generation in a rare-gas medium. The experimental results obtained at Saclay with a 1064 nm Nd-YAG laser in the 1013 W cm-2 intensity range are summarized. The harmonic emission strengths, first decrease rather steeply for the first orders, then form a long plateau up to the 21st harmonic in xenon, or up to the 33rd harmonic in argon, before decreasing again rather abruptly. The theoretical description of these experiments consists first in the calculation of the photoemission spectra emitted by a single atom. The spectra are obtained by numerically integrating a time dependent Schrodinger equation for the laser-excited rare-gas atom. Second, one must account for collective effects in the medium, described by Maxwell's equations. A theoretical framework for describing the generation and propagation of harmonics in strong laser fields is developed. An numerical solution of the propagation equations for the harmonic fields in xenon at 1064 nm provides results which agree well with experimental data.