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Fully coherent soft x-rays from the nitrogen K-edge to beyond 1 keV

G. Penco1,*, E. Allaria1, L. Badano1, A. Brynes1,†, C. Callegari1, G. Campri1, F. Capotondi1, A. Caretta1, P. Cinquegrana1 et al.

M. B. Danailov1, G. De Ninno1,2, A. Demidovich1, M. Di Fraia8, S. Di Mitri1,3, E. Ferrari4, L. Foglia1, G. Gaio1, L. Giannessi1,5, G. Kurdi1, S. Laterza1, M. Malvestuto1, M. Manfredda1, C. Masciovecchio1, I. Nikolov1, E. Pedersoli1, G. Perosa6, O. Plekan1, E. Principi1, P. Rebernik1, E. Roussel1,7, N. Shafqat1, A. Simoncig1, P. Sigalotti1, S. Spampinati1, C. Spezzani1, P. Susnjar1, M. Trovò1, M. Veronese1, and M. Zangrando1,8

  • *Contact author: giuseppe.penco@elettra.eu
  • †Present address: Cockcroft Institute, Sci-Tech Daresbury, Keckwick Lane, Daresbury, Warrington WA4 4AD, United Kingdom.
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Phys. Rev. Research 8, 043019 – Published 7 October, 2026

DOI: https://doi.org/10.1103/xlrl-kq7k

Abstract

Externally seeded free-electron lasers (FELs) provide radiation with full longitudinal and transverse coherence in the extreme ultraviolet and soft x-ray spectral ranges, inaccessible to conventional laser sources. Until now, externally seeded operation has been limited to the extreme ultraviolet and soft x-ray regime, while shorter wavelengths have been accessed primarily through stochastic self-amplified spontaneous emission processes (or derivatives thereof, one based on self-seeding) or through nonlinear harmonic generation at strongly reduced pulse energy. At the FERMI FEL facility, we have demonstrated stable operation in high-gain harmonic generation (HGHG) covering the nitrogen K-edge, most of the water-window spectral range, and approaching the oxygen K-edge by combining short-wavelength ultraviolet seeding (200 nm), increased electron beam energy, and preservation of an ultralow relative slice energy spread at the 10−5 level. The emitted radiation at such short wavelengths still exhibits the typical narrow bandwidth and wavelength stability of the HGHG scheme enabling, for example, time-resolved x-ray absorption spectroscopy measurements. Longitudinally coherent emission at 1 nm is obtained as the third harmonic of the fundamental FEL emission. Despite the lower intensity typical of nonlinear harmonic emission, these results demonstrate that external seeded FELs can be extended to nanometer wavelengths, establishing deterministic, fully coherent soft x-ray generation and surpassing the previous wavelength operation limits. This advance opens new opportunities for coherent control and nonlinear x-ray experiments that require both high photon energy and longitudinal coherence, and demonstrates a promising pathway toward further extension of externally seeded FELs to shorter wavelengths.

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Phys. Rev. Research 8, 043019– Published 7 October, 2026
  • Received 23 June 2026
  • Accepted 11 September 2026
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