Propagation of High-Order Harmonics through an Ionizing Gas

Abstract

We present numerical simulations of the interaction of intense, ultrashort laser pulses with gases, at intensities of 1014–1016 W/cm2, which are currently of particular interest for extremely high-order harmonic generation. We treat the atomic dynamics and the plasma propagation aspects self-consistently, which is vital if the effect of free-electron phase matching is to be correctly included. Single-atom harmonic spectra show that extremely high-order harmonics can be generated at or near the point of ionization. In an ionized medium, however, these will not be able to propagate efficiently, and the plateau cutoff will be determined largely by plasma dispersion. The harmonic radiation will then also be spatially defocused and spectrally blueshifted as a result of this plasma formation, and accurate prediction of these effects requires an ‘integrated model’. Our approach is based on solutions to a one-dimensional Schrödinger to describe the atomic behavior, coupled with plane-wave propagation.