High-order Harmonic Generation During Multiphoton Ionization of Gases

Abstract

We present theoretical work on high-order harmonic generation accompanying multiphoton ionization of atoms in intense laser fields. We first review some elements of the theory of harmonic generation, particularly the limitations imposed by perturbation theory and some aspects of phase-matching at high pump intensities. A new non-perturbative approach to the theory based on the direct numerical solution of the time-dependent Schrödinger wave equation is described. Selected results of this theory are presented, including: (a) evidence for a plateau of high-order harmonic peak intensities and a relatively abrupt cut-off of this plateau, closely similar to what has been reported experimentally in noble gas atoms, (b) specific predictions for harmonic production by H, He and Xe atoms, and (c) analytical and numerical evidence for the correlation of harmonic production spectra with above-threshold photoelectron spectra under certain circumstances.