AUTHOR=Kurz Heiko G. , Kretschmar Martin , Binhammer Thomas , Nagy Tamas , Ristau Detlev , Lein Manfred , Morgner Uwe , Kovačev Milutin TITLE=High-order harmonic spectroscopy in an ionized high-density target JOURNAL=Frontiers in Physics VOLUME=10 YEAR=2022 URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.836287 DOI=10.3389/fphy.2022.836287 ISSN=2296-424X ABSTRACT=

We use high-order harmonic spectroscopy to study ionization dynamics in a macroscopic target with tunable density, spanning over six orders of magnitude. In an in situ pump-probe experiment, the target is prepared at different densities with varying degrees of laser-induced ionization. High-order harmonic radiation is generated in the pre-ionized target, and a steepening in the decrease of the harmonic yield is observed for increasing pre-ionization, allowing not only to identify the contributing quantum paths during high-order harmonic generation but also in determining the amount of ionization within the target. The measurements allow probing of ionization dynamics in laser-induced plasma with high spatio-temporal resolution and are specifically of interest for the optimization of the harmonic generation process in high-density targets with number densities of up to 1022 cm−3.