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Self-seeded x-ray free-electron laser with high spectral density and photon energy

Lu Cao1, Tianyun Long1, Winfried Decking1, Marc Guetg1, Vitali Kocharyan1, Naresh Kujala2, Christoph Lechner2, Anders Madsen2, Theophilos Maltezopoulos2 et al.

Giovanni Perosa2, Weilun Qin1,*, Evgeni Saldin1, Matthias Scholz1, Svitozar Serkez2, Andrei Trebushinin2, Jiawei Yan1, Shan Liu1,†, and Gianluca Geloni2,‡

  • *Present address: Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China; Present address: Spallation Neutron Source Science Center, Dongguan, China.
  • †Contact author: shan.liu@desy.de
  • ‡Contact author: gianluca.aldo.geloni@xfel.eu
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Phys. Rev. Applied 26, 034071 – Published 29 September, 2026

DOI: https://doi.org/10.1103/gfpd-6bvp

Abstract

Hard x-ray experiments, such as high-resolution imaging and scattering of complex materials, increasingly demand pulses from x-ray free-electron lasers (XFELs) with high photon energy, high spectral density, and narrow bandwidth. Here, we report the demonstration of hard x-ray self-seeding at 18 keV at the European XFEL. This demonstration narrows the bandwidth to 0.8 eV, compared with 20.5 eV in self-amplified spontaneous emission operation, and increases the average spectral density by a factor of 6, reaching 170  μJ/eV within the same experimental campaign. To advance narrow band operation toward even higher photon energies, we explore the harmonic generation mechanism in self-seeding mode, utilizing the second-harmonic generation self-seeding (SHGSS) scheme, where an initial seed is established at half the target photon energy and the harmonic is coherently amplified downstream. Start-to-end simulations at the 30 keV frontier revealed that while direct self-seeding yielded negligible amplification under typical experimental beam conditions, the SHGSS scheme could deliver an average spectral density of approximately 50  μJ/eV with a 0.25 eV bandwidth using the beamline supplemented with superconducting undulators. With these goals in mind, we experimentally investigated SHGSS at 15 and 18 keV, providing the first proof-of-principle for this scheme. Our results are expected to support cutting-edge experiments in materials science, structural biology, and beyond.

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Phys. Rev. Applied 26, 034071– Published 29 September, 2026
  • Received 5 May 2026
  • Revised 28 July 2026
  • Accepted 31 August 2026
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