Magneto-optical Kerr effect with twisted light beams: the origin of helicoidal dichroism in the reflection off magnetic vortices

Studying magnetization configurations of ever more complex magnetic structures has become a major challenge in the past decade, especially at ultrashort timescales. Most of current approaches are based on the analysis of polarization and magnetization-dependent reflectivity. We introduced a different concept, centered on the coupling of magnetic structures with light beams carrying orbital angular momentum (OAM), which was recently tested it in an experiment with magnetic vortices. Upon reflection by a magnetic vortex, an incoming beam with a well-defined OAM l gets enriched in the neighboring OAM modes l +/- 1. It results in anisotropic far-field profiles, which leads to a Magnetic Helicoidal Dichroism (MHD) signal. In this paper we provide a detailed analysis of MHD for the case of a magnetic vortex, providing an intuitive explaination in terms of transverse MOKE. The analysis of MHD allows to retrieve the complex magneto-optical constants. This method, which does not require any polarimetric measurement, is a new promising tool for the identification and analysis of magnetic configurations such as vortices, with a possible extension to the femtosecond to attosecond time resolution.