Characterization and first applications of attosecond soft X-ray pulses at the European XFEL

Based on advanced electron bunch compression techniques [1], intense ultrashort X-ray pulses at high repetition rate are now available for user experiments at the European X-ray Free Electron Laser (EuXFEL) in Germany. Pulse durations down to about 300 attosecond and pulse energies up to 1 mJ can be generated in the soft X-ray photon energy region between 400 and 3000 eV enabling a multitude of novel scientific applications in the field of non-linear physics and time-resolved studies. To fully exploit these high performances of the EuXFEL, a set-up for single pulse characterization has been permanently installed at the Small Quantum Systems (SQS) scientific instrument allowing to determine simultaneously the temporal and spectral distribution of the individual soft X-ray pulses using an assembly of electron time-of-flight spectrometers for electron angular streaking and a high-resolution grating soft X-ray spectrometer, respectively. After the first proof-of-principle experiment [2] showing the enhancement of nonlinear processes, in particular the formation of double-core hole states, via application of sub-femtosecond pulses, further studies have taken advantage of the possibility to generate two independently tunable attosecond pulses for time-resolved experiments on the corresponding ultrafast timescale. Future options for experimental investigation using circularly polarized attosecond soft X-ray pulses, already demonstrated, as well as the additional application of ultrashort UV pulses generated in a hollow core fiber [3], to be installed, will be discussed in the talk.

[1] S. Serkez et al, J. Opt. 20, 024005 (2018)

[2] L. Funke et al., 10.48550/arXiv.2408.03858 (2024)

[3] J. C. Travers et al., Nat. Photon. 13, 547–554 (2019)