Free-electron lasers producing ultrashort pulses with high peak power promise to extend ultrafast non-linear spectroscopic techniques into the extreme-ultraviolet–X-ray regime. Key aspects are the synchronization between pump and probe, and the control of the pulse properties (duration, intensity and coherence). Externally seeded free-electron lasers produce coherent pulses that can be synchronized with femtosecond accuracy. An important goal is to shorten the pulse duration, but the simple approach of shortening the seed is not sufficient because of the finite-gain bandwidth of the conversion process. An alternative is the amplification of a soliton in a multistage, superradiant cascade: here, we demonstrate the generation of few-femtosecond extreme-ultraviolet pulses, whose duration we measure by autocorrelation. We achieve pulses four times shorter, and with a higher peak power, than in the standard high-gain harmonic generation mode and we prove that the pulse duration matches the Fourier transform limit of the spectral intensity distribution.

产生具有高峰值功率的超短脉冲的自由电子激光器有望将超快非线性光谱技术扩展到极紫外-X 射线区域。关键方面是泵和探头之间的同步，以及脉冲特性（持续时间、强度和相干性）的控制。外部种子自由电子激光器产生可以与飞秒精度同步的相干脉冲。一个重要目标是缩短脉冲持续时间，但由于转换过程的有限增益带宽，缩短种子的简单方法是不够的。另一种方法是在多级超辐射级联中放大孤子：在这里，我们展示了几飞秒极紫外脉冲的产生，我们通过自相关测量其持续时间。