High harmonic generation in mixed XUV and NIR fields at a free-electron laser

Jan Troß,Carlos Trallero,Bastian Manschwetus,Benjamin Erk,Rebecca Boll,Cheng Jin,Dimitrios Rompotis,Christopher Passow,Shashank Pathak,Sadia Bari,Daniel Rolles,Adam Summers,Anh-Thu Le,Vinod Kumarappan,Patrik Grychtol

doi:10.1088/2040-8986/ac4318

Abstract

We present the results of an experiment investigating the generation of high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field provided by a free-electron laser (FEL), a process referred to as XUV-assisted high-order harmonic generation (HHG). Our experimental findings show that the XUV field can lead to a small enhancement in the harmonic yield when the XUV and NIR pulses overlap in time, while a strong decrease of the HHG yield and a red shift of the HHG spectrum is observed when the XUV precedes the NIR pulse. The latter observations are in qualitative agreement with model calculations that consider the effect of a decreased number of neutral emitters but are at odds with the predicted effect of the correspondingly increased ionization fraction on the phase matching. Our study demonstrates the technical feasibility of XUV-assisted HHG experiments at FELs, which may provide new avenues to investigate correlation-driven electron dynamics as well as novel ways to study and control propagation effects and phase matching in HHG.

Highlights

We present the results of an experiment investigating the generation of high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field provided by a free-electron laser (FEL), a process referred to as XUV-assisted high-order harmonic generation (HHG)
Driven by several technological break-through that led to the development of powerful x-ray free-electron lasers (XFELs) [1] and versatile high-order harmonic generation (HHG) sources [2,3,4], the last two decades have witnessed the emergence and rapid maturing of the new field of ultrafast x-ray science [5, 6]
Typical HHG spectra produced by the NIR pulses in the presence of the XUV pulses from the FEL are shown in the bottom right of figure 1 and in figure 2(b)

Summary

Introduction

Driven by several technological break-through that led to the development of powerful x-ray free-electron lasers (XFELs) [1] and versatile high-order harmonic generation (HHG) sources [2,3,4], the last two decades have witnessed the emergence and rapid maturing of the new field of ultrafast x-ray science [5, 6]. A number of years ago, it was shown theoretically that by combining an attosecond XUV pulse train with a nearinfrared (NIR) laser field, it is possible to achieve a dramatic enhancement of the HHG yield in a single atom or molecule [8,9,10] as well as in a macroscopic ensemble of atoms [11, 12] Soon after these calculations were published, some of their predictions were experimentally verified, demonstrating an enhancement of the harmonic yield by a factor of five when the XUV field was present—even without phase control between XUV and NIR pulses [13, 14]. It was suggested that HHG radiation produced by a combination of an XUV pulse and an NIR field could be used to study correlationdriven dynamics such as Auger decay or correlation-driven inner-valence hole dynamics in excited atomic or molecular systems [19]

Methods

Results

Conclusion