Optomagnetism in plasmonic nanostructures

Abstract

Optically-induced magnetism has drawn considerable interest in the past years for its ability to speed up magnetic processes. For example, static magnetic fields have been demonstrated to be generated in non-magnetic plasmonic (gold) nanoparticles and nano-apertures [1]. Such a phenomenon has been analyzed as the result of the inverse Faraday effect. Inverse Faraday effect in plasmonic structures can be predicted with a hydrodynamic description of the free electron gas of a metal [2]. More generally, the hydrodynamic model provides reference equations for describing optical nonlinearities in plasmonic nanostructures [3]. It is usually admitted that the inverse Faraday effect (IFE) originates from the spin angular momentum (SAM) of light. We evidence that part of the IFE in metals is induced by the orbital angular momentum (OAM) of light[4]. Using a simplified hydrodynamic model of the free electron gas of a metal, we theoretically investigate the IFE and resulting optomagnetism in a thin gold film as well as in axis-symmetric plasmonic nanostructures under illumination with various focused light beams carrying spin and/or orbital angular momenta [5, 6]. The resulting static magnetic field exhibits resonant behaviour and found to be maximum and dramatically confined at the corners and edges of the plasmonic structures, which reveals the ability of metallic discontinuities to concentrate and tailor static magnetic fields on the nanoscale. Plasmonics can thus generate and tune static magnetic fields and strong magnetic forces on the nanoscale, potentially impacting small scale magnetic tweezing and sensing as well as the generation of magneto-optical effects and spin-waves.