Breaking the Electric-Dipole Selection Rule via a Plasmonic Nanocavity Excited by a k-Space Filter-Assisted Radial Vector Beam

Breaking the electric-dipole selection rule in molecular spectroscopy is of great significance for manipulating vibrational state transitions and developing unconventional photofunctions of molecules. In this study, a static plasmonic nanocavity composed of a gold (Au) nanosphere on a silver (Ag) substrate was excited using a radial vector beam with a tunable spatial frequency component. The resulting nanocavity-plasmonic mode has a significantly enhanced electric-field gradient to visualize the electrical-quadrupole transition in the molecule. The static plasmonic nanocavity is tunable by regulating the spatial frequency component of the excitation beam. Thus, the interaction between the electric field/electric-field gradient of the nanocavity-plasmonic mode and the molecular polarizabilities has been accurately identified. This innovative nanospectral platform provides unique opportunities for studying weak physical and chemical processes in molecules.