Plasmon-Induced Optical Magnetism in an Ultrathin Metal Nanosphere-Based Dimer-on-Film Nanocavity

Recently, plasmon-induced optical magnetism has appealed much research interest in nanophotonics and plasmonics due to intriguing applications in optical metamaterials, and ultrasensitive plasmonic nano-metrology, among many others. Here, a strong in-plane magnetic dipolar resonance in an ultrathin plasmonic nanocavity consisting of a silica-coated gold nanosphere dimer coupled to a gold thin film is observed experimentally and explained theoretically. Multipolar expansion and numerical simulation disclose that such magnetic resonance is induced by a displacement current loop circulating around a nanometer thick triangular region in the cavity. The spectral response and radiation polarization of the magnetic mode are visualized by using a polarization-resolved dark-field imaging system at the single-particle level. The resonance responses of this magnetic mode highly depends on cavity gap thickness, nanosphere dimension, and the incident angle, allowing straightforward resonance tuning from the visible to near-infrared region and thus opening up a new avenue for magnetic resonance-enhanced nonlinear optics and chiral optics.