Maurizio Sacchi
In addition to the spin angular momentum (SAM) associated to the light polarization, Laguerre-Gaussian beams carry also an orbital angular momentum (OAM) of ℓℏ/photon [1] 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 biology, telecommunication, imaging and quantum technologies [2]. Their capability to exert a mechanical torque was exploited in the so-called optical spanner process. The azimuthal phase dependence, with a singularity on the propagation axis, is accompanied by a radial modulation of the intensity (ring-shaped), properties that have been used to modify magnetic ordering, to improve the spatial resolution in microscopy, and to enhance the edge sharpness in phase-contrast imaging.
Over the last decade, the generation of OAM beams at shorter wavelengths, from XUV to hard x-rays, has found an increasing number of applications, often based on extrapolations of previous work carried out in the visible range. For instance, as for the SAM, the handedness imposed by the OAM has been exploited to perform x-ray spectroscopic studies of magnetic materials [3] and of chiral molecules [4], and a recent ptychography study [5] showed that the attainable spatial resolution in the reconstructed XUV images increases with ℓ. Applications of OAM beams in the x-ray range and their implementation at pulsed x-ray sources open new perspectives for element-selective and time-resolved spectroscopic and imaging studies of magnetic structures [6].
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); T. Ruchon, M. Fanciulli, M. Sacchi, Magneto-Optics with light beams carrying orbital angular momentum, in The 2022 magneto-optics roadmap (A. Berger, P. Vavassori Eds.), J.Phys.D: Appl.Phys. 55, 463003 (2022).
M. Fanciulli et al., Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices, Phys. Rev. Lett. 128, 077401 (2022).
4. J. R. Rouxel et al., Hard X-ray helical dichroism of disordered molecular media, Nature Phot. 16, 570 (2022).
5. M. Pancaldi et al., High-resolution ptychographic imaging at a seeded free-electron laser source using OAM beams, Optica 11, 403 (2024).
6. M. Fanciulli et al., Magnetic vortex dynamics probed by time-resolved magnetic helicoidal dichroism, submitted.