Magnetic Purcell enhancement by plasmon-induced magnetic resonance of the nanoparticle-on-mirror configuration

We theoretically reveal that plasmonic nanocavities with 1 nm gaps between the nanosphere and gold film support multiple resonances possessing ultrahigh-field confinement and enhancements for both the electric and magnetic fields. Here, a circulated electric line supported by the plasmonic mode implies that a magnetic dipole is introduced in the center of the gap region. A high magnetic enhancement factor of up to 100 is observed, and multipolar expansion demonstrates that symmetry breaking suppresses the radiation of the dipole mode. The surface charges accumulate in the gap area and form the electric-quadrupole (Qe) mode and magnetic-dipole (m) modes in the gap region, and then the interference between the dominant Qe and m modes gives rise to a pronounced magnetic-based Fano resonance in the scattering spectrum, thus boosting the magnetic response in the scattering valley. Next, we introduce a magnetic dipole in the gap region, where a significant radiative magnetic Purcell factor of up to 200 is observed, following the Fano valley of the scattering spectrum. Furthermore, the ultrasmall magnetic mode volume and high magnetic field at the Fano valley frequencies demonstrate this phenomenon. This study unlocks new possibilities for single-photon emission from the magnetic-dipole transition in the nanoparticle-on-mirror configuration.