Magnetic moment generation in small gold nanoparticles via the plasmonic inverse Faraday effect

We theoretically investigate the creation of a magnetic moment in gold nanoparticles by circularly polarized laser light. To this end, we describe the collective electron dynamics in gold nanoparticles using a semiclassical approach based on a quantum hydrodynamic model that incorporates the principal quantum many-body and nonlocal effects, such as the electron spill-out, the Hartree potential, and the exchange and correlation effects. We use a variational approach to investigate the breathing and the dipole dynamics induced by an external electric field. We show that gold nanoparticles can build up a static magnetic moment through the interaction with a circularly polarized laser light at the localized surface plasmon (LSP) resonance. We analyze that the responsible physical mechanism is a plasmonic, orbital inverse Faraday effect, which can be understood from the time-averaged electron current that contains currents rotating on the nanoparticle's surface. The computed laser-induced magnetic moments are sizable, of about $0.35\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$/atom for a laser intensity of $45\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.16em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}$ at LSP resonance.