High-order harmonic generation in rare gases with an intense short-pulse laser

TLDR

The study investigates high‑order harmonic generation in rare gases using a 794‑nm, 10^14–10^15 W cm⁻² Ti:sapphire laser. The experiment measures harmonic yields in neon and argon across all orders while varying laser intensity with a 794‑nm, 10^14–10^15 W cm⁻² Ti:sapphire pulse. Harmonic yields rise steeply in the cutoff, plateau slowly, then saturate upon ionization; the cutoff scales more weakly with intensity than single‑atom theory predicts; high‑order harmonics up to the 65th in argon and 45th in xenon are observed at 10^15 W cm⁻², attributed to ion‑based generation, and both harmonic and fundamental spectra blueshift when the medium ionizes.

Abstract

We present experimental studies of high-order harmonic generation in the rare gases performed with a short-pulse titanium sapphire laser operating at 794 nm in the ${10}^{14--}$${10}^{15}$-W/${\mathrm{cm}}^{2}$ range. The harmonic yields generated in neon and in argon are studied for all orders as a function of the laser intensity. They vary first rather steeply, in the cutoff region, then much more slowly in the plateau region, and, finally, they saturate when the medium gets ionized. The dependence of the high-order harmonic cutoff with the laser intensity in neon and argon is found to be lower than that predicted in single-atom theories. We observe high-order harmonics in argon and xenon (up to the 65th and 45th, respectively) at ${10}^{15}$ W/${\mathrm{cm}}^{2}$, which we attribute to harmonic generation from ions. We also show how the harmonic and fundamental spectra get blueshifted when the medium becomes ionized.