High-Gain Harmonic Generation with temporally overlapping seed pulses and application to ultrafast spectroscopy

TL;DR

This paper investigates the effects of temporally overlapping seed pulses in high-gain harmonic generation for free-electron lasers, demonstrating that saturation effects can be characterized and enabling ultrafast spectroscopy at very short delays.

Contribution

It provides a detailed analysis of nonlinear effects in collinear HGHG with overlapping seeds, supported by experiments and simulations, advancing ultrafast spectroscopy techniques.

Findings

Autocorrelation and interferometry data are sensitive to saturation effects.

Time-resolved spectroscopy is feasible at delays shorter than seed pulses.

Overlapping seed pulses can be used effectively in ultrafast spectroscopy.

Abstract

Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave-packet interferometry experiments at photon energies > $23\,$eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration.