Najmeh S. Mirian, Michele Di Fraia, Simone Spampinati, Filippo Sottocorona, Enrico Allaria, Laura Badano, Miltcho B. Danailov, Alexander Demidovich, Giovanni De Ninno, Simone Di Mitri, Giuseppe Penco, Primož Rebernik Ribič, Carlo Spezzani, Giulio Gaio, Mauro Trovó, Nicola Mahne, Michele Manfredda, Lorenzo Raimondi, Marco Zangrando, Oksana Plekan, Kevin C. Prince, Tommaso Mazza, Richard J. Squibb, Carlo Callegari, Xi Yang, Luca Giannessi

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.