Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices

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 ℓℏ (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.
Recently OAM beams with femtosecond and attosecond pulse durations and wavelengths in the extreme ultraviolet (XUV) range became available. In this framework, the primary goal of our experiment was to demonstrate that magnetic structures can yield, in the XUV range, strong MHD signals that depend on the exact topology of their magnetization configuration. This novel effect has the potential to open up new perspectives for fundamental spectroscopic studies, both in the static and in the dynamic regimes, allowing for instance a dynamical mapping of the spin texture.
 The first experimental proof of magnetic helicoidal dichroism was carried out at the DiProI end-station of the FERMI free-electron laser, using p-polarized 52.8 eV photons (matching the Fe-3p resonance) from the FEL-1 source and measuring the resonant scattering close to the Brewster extinction condition. Well-defined OAM values were imposed on the XUV beam by using specially designed spiral zone plates (Fig. 2a). The sample was a ~15 μm wide Fe-Ni disk (Fig. 2b), whose shape was designed to form at its center a clean remanent magnetic vortex with reconfigurable clockwise (CW) or counter-clockwise (CCW) circulation. The MHD for several OAM values (ℓ= ±1 in Fig. 2c,d) was obtained by taking the difference divided by the sum of two images of the scattered intensity collected for CW and CCW magnetic vortices. The experimental results compare extremely well with calculations based on recent theoretical models (Fig. 2e,f), providing a sound basis for future applications of magnetic helicoidal dichroism.
MHD represents a novel approach in magneto-optics, highly promising for studying complex spin textures, e.g. in topologically protected magnetic structures. It will also help designing new devices that can be used to either impose or probe the OAM of light beams. Moreover, many applications based on angular momentum transfer can be envisaged, from data encoding and processing to magnetization control, that exploit the unlimited orbital momentum ℓℏ of an OAM beam (where ℓ can take any positive or negative integer value) compared to the spin value σℏ of circularly polarized photons (where σ can be only ±1). 
From a fundamental point of view, MHD is a new and original testing tool for structured light beams. The extension of the application of OAM beams from the visible range to the XUV range and beyond introduces new fundamental concepts and opens up new opportunities for the development of structured light spectroscopies applied to scattering and imaging experiments to study magnetic, and more generally chiral, systems.
 

Figure 1.  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.
 

MHD represents a novel approach in magneto-optics, highly promising for studying complex spin textures, e.g. in topologically protected magnetic structures. It will also help designing new devices that can be used to either impose or probe the OAM of light beams. Moreover, many applications based on angular momentum transfer can be envisaged, from data encoding and processing to magnetization control, that exploit the unlimited orbital momentum ℓℏof an OAM beam (where ℓcan take any positive or negative integer value) compared to the spin value σℏ of circularly polarized photons (where σ can be only ±1). 
From a fundamental point of view, MHD is a new and original testing tool for structured light beams. The extension of the application of OAM beams from the visible range to the XUV range and beyond introduces new fundamental concepts and opens up new opportunities for the development of structured light spectroscopies applied to scattering and imaging experiments to study magnetic, and more generally chiral, systems.

Figure 2.  SEM images of an ℓ=1 spiral zone plate (a) and of the Fe-Ni sample (b) used for MHD measurements. Arrows in (b) depict the remanent magnetization resulting from micromagnetic simulations.(c,d) difference divided by the sum of two scattered intensity images obtained for CW and CCW vortices, measured for ℓ=-1 and ℓ=+1. (e,f) corresponding MHD calculations. Copyright 2022 by The American Physical Society


 

This research was conducted by the following research team:

Mauro Fanciulli,^1,2Matteo Pancaldi,³Emanuele Pedersoli,³Mekha Vimal,¹David Bresteau,¹Martin Luttmann,¹Dario De Angelis,³PrimožRebernik Ribič,³Benedikt Rösner,⁴Christian David,⁴Carlo Spezzani,³Michele Manfredda,³Ricardo Sousa,⁵Ioan-Lucian Prejbeanu,⁵Laurent Vila,⁵Bernard Dieny,⁵Giovanni De Ninno,^3,6Flavio Capotondi,³Maurizio Sacchi,^7,8and Thierry Ruchon¹

¹ LIDYL, Université Paris-Saclay, CEA, CNRS, Gif-sur-Yvette, France
² Laboratoire de Physique des Matériaux et Surfaces, Cergy Paris Université, Cergy-Pontoise, France
³ Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
⁴ Paul Scherrer Institut, Villigen-PSI, Switzerland
⁵ IRIG-SPINTEC, Université Grenoble Alpes, CNRS, CEA, Grenoble, France
⁶ Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia
⁷ Institut des NanoSciences de Paris, CNRS, Sorbonne Université, Paris, France
⁸ Synchrotron SOLEIL, L’Orme des Merisiers, Gif-sur-Yvette, France

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