Highly nonlinear ionization of atoms induced by intense high-harmonic pulses

TL;DR

This paper presents a novel approach to optimize high-harmonic generation (HHG) for studying highly nonlinear ionization processes in atoms using modest laser pulses, enabling new insights into XUV-induced nonlinear phenomena.

Contribution

It introduces a new method to optimize HHG intensity by scaling experimental parameters differently, allowing the study of highly nonlinear ionization with lower-energy driving lasers.

Findings

Successfully ionized Ar up to Ar$^{5+}$ using modest laser pulses.

Demonstrated the ability to induce multi-photon ionization processes in atoms.

Showed that optimized HHG can access highly nonlinear regimes in the XUV range.

Abstract

Intense extreme-ultraviolet (XUV) pulses enable the investigation of XUV-induced nonlinear processes and are a prerequisite for the development of attosecond pump - attosecond probe experiments. While highly nonlinear processes in the XUV range have been studied at free-electron lasers (FELs), high-harmonic generation (HHG) has allowed the investigation of low-order nonlinear processes. Here we suggest a concept to optimize the HHG intensity, which surprisingly requires a scaling of the experimental parameters that differs substantially from optimizing the HHG pulse energy. As a result, we are able to study highly nonlinear processes in the XUV range using a driving laser with a modest ($\approx 10$ mJ) pulse energy. We demonstrate our approach by ionizing Ar atoms up to Ar$^{5+}$, requiring the absorption of at least 10 XUV photons.