X-rays with Orbital Angular Momentum for spectroscopy and imaging

Résumé :

In addition to the spin angular momentum (SAM) associated to the light polarization, Laguerre-Gaussian light beams carry also an orbital angular momentum (OAM) of ℓ /photon [1]ℏ associated to an azimuthal dependence exp(iℓϕ) of the electric field phase. Over the last thirty years, OAM beams at vis-IR wavelengths found applications in fields as different as biology, telecommunication and imaging [2]. The azimuthal phase dependence, with a singularity on the propagation axis, is accompanied by a radial modulation of the intensity (ring-shaped beams), properties that have been used to modify local magnetic ordering, to improve the spatial resolution in microscopy, and to enhance the edge sharpness in phase-contrast imaging.

Over the last decade, several approaches to the generation of OAM beams at shorter wavelengths, from XUV to hard x-rays, were proposed. Potential applications are often based on the extrapolation of previous work carried out in the vis-IR range. For instance, as for the SAM, the handedness imposed by the OAM has been exploited to study magnetic materials [3] and chiral molecules [4]. The interest of extending the use of OAM beams from the vis-IR to the x-ray range has been growing steadily over the last few years. Nonetheless, the offer of user accessible beamlines and endstations remains limited, especially when one aims at independently varying both SAM and OAM in a controlled way. At the SEXTANTS beamline of the SOLEIL synchrotron, we have implemented and commissioned a new setup for soft x-ray spectroscopy (absorption and resonant scattering experiments) with OAM beams [5].

1. L. Allen at al., Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes, Phys. Rev. A 45, 8185 (1992).
2. Y. Shen et al., Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities, Light: Science & Applications 8, 90 (2019).
3. M. Fanciulli et al., Electromagnetic theory of Helicoidal Dichroism in reflection from magnetic structures, Phys. Rev. A 103, 013501 (2021); Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices, Phys. Rev. Lett. 128, 077401 (2022); Magnetic vortex dynamics probed by time-resolved magnetic helicoidal
dichroism, Phys. Rev.Lett. (2025).
4. J. R. Rouxel et al., Hard X-ray helical dichroism of disordered molecular media, Nature Phot. 16, 570 (2022).
5. P. Carrara et al., Soft x-rays with Orbital Angular Momentum for resonant scattering experiments at the SOLEIL synchrotron, J. Synchr. Rad. 33, 858 (2026).

Contact : matteo.dastuto@neel.cnrs.fr