  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.
