Extreme-ultraviolet-initiated High-harmonic Generation in Ar$^{+}$

TL;DR

This study uses advanced computational methods to explore how combining extreme-ultraviolet and infrared lasers can enhance high-harmonic generation in ionized argon, revealing control mechanisms and electronic structure effects.

Contribution

It demonstrates the control over ionization pathways and harmonic yield in Ar$^{+}$ using combined XUV and IR pulses, highlighting the influence of electronic structure and timing.

Findings

Significant enhancement of harmonic yield with combined XUV and IR pulses.

Alteration of electron emission pathways due to XUV pulse presence.

Observation of a pronounced Cooper minimum in the harmonic spectrum.

Abstract

We employ the R-matrix with time-dependence method to investigate extreme-ultraviolet-initiated high-harmonic generation (XIHHG) in Ar$^{+}$. Using a combination of extreme-ultraviolet (XUV, $92\textrm{ nm}$, $3\times 10^{12}\,\textrm{Wcm}^{-2}$) and time-delayed, infrared (IR, $800\textrm{ nm}$, $3\times 10^{14}\,\textrm{Wcm}^{-2}$) laser pulses, we demonstrate that control over both the mechanism, and timing, of ionization can afford significant enhancements in the yield of plateau, and sub-threshold, harmonics alike. The presence of the XUV pulse is also shown to alter the relative contribution of different electron emission pathways. Manifestation of the Ar$^{+}$ electronic structure is found in the appearance of a pronounced Cooper minimum. Interferences amongst the outer-valence $3p$, and inner-valence $3s$, electrons are found to incur only a minor suppression of the harmonic intensities, at least for the present combination of XUV and IR laser light. Additionally, the dependence of the XIHHG efficiency on time delay is discussed, and rationalized with the aid of classical trajectory simulations.