Fig. 5. Illustration of the resonant mechanism leading to the large magneto-optical enhancement in the NCRD nanocavity at the Fano resonance maximum.
a Separation of the hybrid mode into its dipolar (pO and ph) and multipolar (S6) components (these are the surface charge maps corresponding to phase ϕ0 in the first column of Fig. S3). The panel also shows the time evolution (retardation ϕ − ϕ0) of the surface charge at the points marked by the 4- and 5-pointed stars. b Calculated and measured optical reflectivity spectra for the NCRD, Au-RI and Py-DI structures. Red dashed lines mark the wavelength of 820 nm, showing that the reflectivity of the NCRD is only marginally higher than that of the Au-RI. The reflectivities of both the NCRD and Au-RI are approximately 25% higher than that of the Py-DI. c A sketch of the electrodynamics of the Py disk generating an electric dipole (pO) triggered by the electric field Ei of an incident linearly polarized electromagnetic radiation and a magneto-optically activated electric dipole (pMO) by a magnetic field H. pO and pMO of the Py nanoantenna inside the NCRD nanocavity are enhanced (by a factor of ~5) with respect to a bare Py disk (Py-DI) by hybridization with the mode S6 of the Au-RI, which results in the introduction of a dipolar mode, ph, in the S6 mode. This is qualitatively depicted by the relative size of electric dipoles pO and pMO in the Py-DI and in the NCRD. In the NCRD nanocavity, hybridization generates a bonding dipolar mode pO + ph. In the Py-DI system, both pO and pMO are generated by radiant (bright) LPR modes, and the resulting H-induced polarization change in the reflected radiation, Er, is determined by their ratio (MOA ∝ |pMO|/|pO|, θK ∝ Re[pMO/pO], and εK ∝ Im[pMO/pO]). The large enhancement of the H-induced polarization change in the NCRD system is a consequence of the low-radiant character of the bonding dipolar mode (pO + ph), while pMO maintains its enhancement and radiant character