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

Recently, plasmon‐induced optical magnetism has attracted 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. Plasmon‐induced optical magnetism represents a promising approach to generate pronounced magnetic response in nonmagnetic‐metal nanostructures. Here, a strong in‐plane magnetic dipolar mode in a gold nanosphere‐based dimer‐on‐film nanocavity is observed, its physical origin is theoretically disclosed, and the geometry‐ and excitation‐dependent resonance frequency and scattering amplitude are demonstrated.