Enhancing magneto-optical effects is crucial for size reduction of key photonic devices based on non-reciprocal propagation of light and to enable active nanophotonics. We disclose a so far unexplored approach that exploits dark plasmons to produce an unprecedented amplification of magneto-optical activity. We designed and fabricated non-concentric magnetoplasmonic-disk/plasmonic-ring-resonator nanocavities supporting multipolar dark modes. The broken geometrical symmetry of the design enables coupling with free-space light and hybridization of dark modes of the ring nanoresonator with the dipolar localized plasmon resonance of the magnetoplasmonic disk. Such hybridization generates a multipolar resonance that amplifies the magneto-optical response of the nanocavity by ~1-order of magnitude with respect to the maximum enhancement achievable by localized plasmons in bare magnetoplasmonic nanoantennas. This large amplification results from the peculiar and enhanced electrodynamic response of the nanocavity, yielding an intense magnetically-activated radiant magneto-optical dipole driven by the low-radiant multipolar resonance. The concept proposed is general and, therefore, our results open a new path that can revitalize research and applications of magnetoplasmonics to active nanophotonics and flat optics.
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