Highly non-linear ionization of atoms induced by intense high-harmonic pulses

B Senfftleben,T Nagy,A Hoffmann,M Sauppe,M J J Vrakking,M Kretschmar,J Tümmler,D Rupp,I Will,B Schütte

doi:10.1088/2515-7647/ab87fd

Abstract

Intense extreme-ultraviolet (XUV) pulses enable the investigation of XUV-induced non-linear processes and are a prerequisite for the development of attosecond pump - attosecond probe experiments. While highly non-linear processes in the XUV range have been studied at free-electron lasers (FELs), high-harmonic generation (HHG) has allowed the investigation of low-order non-linear 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 non-linear processes in the XUV range using a driving laser with a modest (≈ 10 mJ) pulse energy. We demonstrate our approach by ionizing Ar atoms up to Ar5 + , requiring the absorption of at least 10 XUV photons.

Highlights

Three key features of intense XUV pulses from free-electron lasers (FELs) and high-harmonic generation (HHG) sources have opened up new possibilities for a range of fields from life science to material science and fundamental physics: (i) Intense XUV pulses provide the possibility to perform pump-probe experiments, in which a first XUV pump pulse initiates dynamics in an atom or a molecule, and these dynamics are probed by a second, time-delayed XUV probe pulse
This capability has been extensively used with femtosecond time resolution at FELs and has been implemented with attosecond time resolution using HHG sources [2, 3]. (ii) Intense, coherent XUV pulses have enabled single-shot coherent diffractive imaging (CDI) of isolated nanotargets with a resolution in the tens of nanometers range
While these experiments are predominantly performed at FELs [4], CDI on He nanodroplets using an intense HHG source was recently reported [5]. (iii) High XUV intensities enable the study of non-linear optics in this spectral range

Summary

Introduction

Three key features of intense XUV pulses from FEL and HHG sources have opened up new possibilities for a range of fields from life science to material science and fundamental physics: (i) Intense XUV pulses provide the possibility to perform pump-probe experiments, in which a first XUV pump pulse initiates dynamics in an atom or a molecule, and these dynamics are probed by a second, time-delayed XUV probe pulse. Advantages of HHG sources compared to FELs include their smaller size and lower costs resulting in easier access to these sources Another important point is that two-color XUV-optical pump-probe experiments, which are one of the preferred applications of ultrashort XUV pulses, are challenging at FELs due to timing jitter between the optical and XUV beams [8]. We demonstrate in this paper that for a given laboratory size the XUV intensity on target can be optimized by using an NIR focusing element with a relatively short focal length This enables the generation of a smaller XUV beam waist radius in the experiment and a higher XUV beamline transmission. We achieve an estimated XUV intensity in our laboratory of 7 × 1014 W cm−2, allowing us to study highly non-linear processes, which we demonstrate in this paper by ionizing neutral Ar atoms up to Ar5 +

HHG intensity scaling

Experimental setup

Estimation of the XUV intensity

Multiphoton ionization of Ar atoms

Findings

Summary