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.
Highlights
The extension of coherent nonlinear spectroscopy techniques to the extreme ultraviolet (XUV) and x-ray spectral regimes would allow the study of photoinduced dynamics in real-time with unprecedented temporal resolution and site/chemical selectivity [1,2,3]
We find that the phase modulation method is capable of extracting the system’s response even for temporally overlapping seed pulses, saturation occurring during the High-Gain Harmonic Generation (HGHG) process leads to strong depletion of the wave-packet interferometry (WPI) signal
In order to understand the experimental results we performed numerical simulations based on high-order interferometric ACs including a simplified model of the XUV generation in the HGHG process, which is described
Summary
The extension of coherent nonlinear spectroscopy techniques to the extreme ultraviolet (XUV) and x-ray spectral regimes would allow the study of photoinduced dynamics in real-time with unprecedented temporal resolution and site/chemical selectivity [1,2,3]. For this development, the generation and control of phase-locked XUV/x-ray pulse sequences and selective background-free detection of weak nonlinear signals is essential [4]. Due to the detection of photons, the overall sensitivity of FWM schemes is limited by stray light
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