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.

中文翻译：

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具有暂时重叠的种子脉冲的高增益谐波生成及其在超快光谱学中的应用

在自由电子激光器（FEL）中高增益谐波产生（HGHG）工艺的共线双脉冲晶种是一种有前途的方法，可以促进在极端紫外线（XUV）光谱范围内的各种相干非线性光谱方案。但是，在使用单个非线性介质的共线排列中，时间重叠的种子脉冲可能会引入非线性混合信号，从而在短时延下损害实验。在这里，我们通过扩展最近出版物中描述的分析来详细研究这些影响（Wituschek等，Nat。Commun。，11，883，2020）。在大于23 eV的光子能量下进行了高阶条纹分辨自相关和波包干涉测量实验，并进行了数值模拟。事实证明，自相关和波包干涉测量数据都对饱和效应非常敏感，因此可用于表征HGHG过程中的饱和度。我们的结果进一步暗示，即使对于小于种子脉冲持续时间的时间延迟，时间分辨光谱实验也是可行的。