Calculations of high-order harmonic-generation processes in xenon at 1064 nm

Abstract

We perform detailed calculations of harmonic conversion in a 15-Torr jet of xenon into which a 1064-nm-wavelength 36-ps-pulse-width laser has been tightly focused, so that the peak intensity ranges from 5\ifmmode\times\else\texttimes\fi{}${10}^{12}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The single-atom emission rates are obtained by integrating the time-dependent Schr\"odinger equation. We employ an improved atomic model which includes excitation and ionization through states with both low-lying ionic cores. The propagation equations are solved using a general and efficient finite-difference technique. Excellent agreement with experimental data is obtained. We consider the effect of the defocusing of the pump beam by the free electrons in the saturation regime, and find it to be small for the conditions studied here. The spatial, temporal, and spectral profiles of the harmonic emission are presented. Significant blueshifts of the harmonics' spectral line shapes are observed in the saturation regime. Finally, using model polarizations, we discuss how the harmonic fields build up in the nonlinear medium, through a series of interferences. It allows us to understand why efficient phase matching can be achieved in a strong-field regime for the laser-atom interaction.