Mauro Fanciulli
The spin angular momentum (SAM) of light plays a central role in the study of magnetism, e.g. in techniques such as X-ray magnetic circular dichroism (XMCD), while the orbital angular momentum (OAM) degree of freedom has been much less exploited. I will present the classical electromagnetic theory we developed for the case of scattering of light carrying OAM by a non-uniform magnetic material, an extension of the magneto-optic Kerr effect (MOKE). It is found that a differential signal is found in the reflected beam profile when switching the OAM or magnetization sign, leading to the so-called magnetic helicoidal dichroism (MHD) [1]. In particular, MHD can give information about the overall topology of the magnetic structure under the probing helicoidal beam. I will also present the first experimental observation of MHD measured at the FERMI free electron laser on a permalloy magnetic vortex with an XUV beam at the Fe 3p resonance [2]. The agreement of the experimental results with the theoretical predictions opens up two directions. On one hand, the extension of MHD to the time domain, allowing to track the picosecond demagnetization and remagnetization dynamics and the transient modification of magnetic topology after a femtosecond infrared pulse [3]. On the other hand, the possibility to explore fundamental properties of light-matter interaction such as the transfer or angular momentum and the role of photon spin-orbit interaction [4].
References:
[1] M. Fanciulli et al., Physical Review A 103, 013501 (2021)
[2] M. Fanciulli et al., Physical Review Letters 128, 077401 (2022)
[3] M. Fanciulli et al., Physical Review Letters, in press (2025)
[4] M. Luttmann et al., submitted