Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices – Highlight ELETTRA FERMI

Light is a privileged tool to both drive and analyze the time evolution of magnetic materials in pump-probe experiments. Photons carrying a spin angular momentum (SAM) with charge equal toσℏ, with σ = ±1corresponding to left or right circular polarization (Fig. 1a), have been used intensely for decades in circular dichroism experiments to discriminate and study the magnetic properties of materials, as well as the chirality of chemical enantiomers. It is less common in magnetic dichroism studies to exploit the fact that a photon beam can also carry an orbital angular momentum (OAM) with topological charge equal to ℏ (with ℓ ϵ ℤ), which corresponds to a helical wavefront (Fig. 1b). Such OAM beams, also called “light springs”, show a well-defined handedness analogous to circular polarization. However, the associated magnetic helicoidal dichroism (MHD), stemming from the different optical properties for beams with OAM values of opposite charge, remained up to now mainly the object of theoretical studies.

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M. Fanciulli, M. Pancaldi, E. Pedersoli, M. Vimal, D. Bresteau, M. Luttmann, D. De Angelis, P. R. Ribic, B. Rösner, C. David, C. Spezzani, M. Manfredda, R. Sousa, I.-L. Prejbeanu, L. Vila, B. Dieny, G. De Ninno, F. Capotondi, M. Sacchi, and T. Ruchon., Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices,Phys. Rev. Lett. 128, 077401 (2022).


Caption : Sketches of SAM (a) and OAM (b) photon beams. Blue arrows represent the polarization vectors(electric field), while blue maps represent the intensity in a given transverse plane. Phase wavefronts are shown in orange.