Enhanced magnetic modulation of light polarization exploiting hybridization with multipolar dark plasmons in magnetoplasmonic nanocavities.

Magneto-optics: nanocavity enhancement Magnetic control of light can be greatly enhanced by custom-designed magnetoplasmonic nanocavities, report European scientists. Alberto Lopez-Ortega and coworkers from Spain, Luxembourg and Sweden fabricated arrays of plasmonic gold-ring nanoresonators that feature a ferromagnetic nanodisk located asymmetrically within them. The coupling and hybridization of multi-polar dark modes and the dipolar plasmon resonance within the nanocavity results in a large amplification of the cavity's magneto-optical response. As a result, the polarization of re-emitted light can be controlled via modulation of a weak external magnetic field. The approach may ultimately enable the construction of efficient nanophotonic devices for performing polarization-based tasks such as light isolation, rotation, modulation and sensing. In the future, the implementation of the concept using different ring materials and an optimized geometry could bring further and substantial improvements in the strength and tunability of the magneto-optical effects. Enhancing magneto-optical effects is crucial for reducing the size of key photonic devices based on the non-reciprocal propagation of light and to enable active nanophotonics. Here, we disclose a currently unexplored approach that exploits hybridization with multipolar dark modes in specially designed magnetoplasmonic nanocavities to achieve a large enhancement of the magneto-optically induced modulation of light polarization. The broken geometrical symmetry of the design enables coupling with free-space light and hybridization of the multipolar dark modes of a plasmonic ring nanoresonator with the dipolar localized plasmon resonance of the ferromagnetic disk placed inside the ring. This hybridization results in a low-radiant multipolar Fano resonance that drives a strongly enhanced magneto-optically induced localized plasmon. The large amplification of the magneto-optical response of the nanocavity is the result of the large magneto-optically induced change in light polarization produced by the strongly enhanced radiant magneto-optical dipole, which is achieved by avoiding the simultaneous enhancement of re-emitted light with incident polarization by the multipolar Fano resonance. The partial compensation of the magneto-optically induced polarization change caused by the large re-emission of light with the original polarization is a critical limitation of the magnetoplasmonic designs explored thus far and that is overcome by the approach proposed here.