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Poincaré beams from a free electron laser

Nature Photonics volume 19, pages 946–951 (2025)Cite this article

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Abstract

Poincaré beams are light beams that have spatially inhomogeneous polarization structure that spans a finite portion of the Poincaré sphere. This feature bestows the beams with intriguing topological properties and has led to a surge in research on their fundamental characteristics, their controlled generation and on emerging applications. Here we present an experimental demonstration of a Poincaré beam generated in the extreme ultraviolet (16.7 nm) at the FERMI free electron laser (FEL). The ‘star’ type Poincaré beam is generated by exploiting the phase and intensity structure intrinsic to FEL radiation without relying on optical elements. We controlled the spatial polarization distribution through a precise overlap and power balance between two FEL pulses, each with different transverse phase distributions and orthogonal circular polarizations. The spatial polarization structure was mapped in detail and shows extensive coverage of the Poincaré sphere, in agreement with analytic predictions. This method of in situ Poincaré beam production in FELs enables straightforward flexibility in the orientation and balance of polarization states, and can readily be extended to other vector beams and to shorter wavelengths enabling novel science applications in modern light sources.

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Fig. 1: Polarization distribution for a superposition of Laguerre–Gaussian modes.
Fig. 2: Illustration of experimental set-up.
Fig. 3: Poincaré beams before and after linear polarizer.
Fig. 4: Mapping the polarization distribution of the FEL Poincaré beam and analysis of polarization topology.

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Data availability

Source data used to generate Figs. 3 and 4 are publicly available via figshare at https://figshare.com/projects/Poincar_Beams_from_a_Free_Electron_Laser/248654 (ref. 56). The intensity dataset from the complete phase scan is also available at the same location. Additional data supporting the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by US Department of Energy Contract 218 number DE-AC02-76SF00515 and award number 2021-SLAC-100732. We thank W. Fawley, E. Allaria and L. Giannessi for useful discussions and suggestions, and the FERMI operations team for their contributions to this project.

Author information

Author notes

  1. Alexander Brynes

    Present address: ASTeC, STFC UKRI, Warrington, UK

  2. Alexander Brynes

    Present address: The Cockcroft Institute of Accelerator Science and Technology, Warrington, UK

Authors and Affiliations

  1. SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    Jenny Morgan & Erik Hemsing

  2. Elettra-Sincrotrone Trieste, Trieste, Italy

    Primož Rebernik Ribič, Flavio Capotondi, Alexander Brynes, Michele Manfredda, Giovanni De Ninno, Luka Novinec, Matteo Pancaldi, Emanuele Pedersoli, Alberto Simoncig, Carlo Spezzani & Marco Zangrando

  3. University of Nova Gorica, Nova Gorica, Slovenia

    Giovanni De Ninno

  4. Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venezia, Italy

    Matteo Pancaldi

  5. CNR - Istituto Officina dei Materiali (IOM), Trieste, Italy

    Marco Zangrando

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Contributions

J.M. and E.H. proposed the experiment. The experiment was designed by J.M., E.H., F.C. and P.R.R., with contributions from G.D.N. and A.B. The end station was prepared by F.C., M.P. and E.P., with M.Z., M.M., A.S. and L.N. responsible for transporting the radiation beam. C.S. and G.D.N. prepared the machine for the experiment. The experiment was conducted by J.M., E.H., P.R.R., F.C., A.B. and M.P. Data analysis and paper preparation were carried out by J.M. and E.H., with input from all authors.

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Correspondence to Jenny Morgan.

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Nature Photonics thanks Giuseppe Sansone and Shigemi Sasaki for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Spiral intensity profile from uniformly polarised undulators.

Intensity profile of the radiation captured after the Zr filter with all undulators emitting the same polarisation.

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Morgan, J., Rebernik Ribič, P., Capotondi, F. et al. Poincaré beams from a free electron laser. Nat. Photon. 19, 946–951 (2025). https://doi.org/10.1038/s41566-025-01737-7

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