Plasmonic Bi‐Cavity Nanostructure for Efficient Light Collection and Localization

Tip‐enhanced Raman spectroscopy (TERS) typically relies on high‐numerical‐aperture (NA) excitation to generate a strong axial field at the tip apex, limiting working distance and constraining sample geometries. Here, we show that a plasmonic bi‐cavity tip, the plasmon‐tunable tip pyramid (PTTP), supports a hybrid antenna‐cavity response when the nanopyramid length and plateau length are co‐tuned. In this configuration, an in‐plane surface‐plasmon‐polariton cavity supported by the plateau hybridizes with the longitudinal antenna mode of the nanopyramid, efficiently funneling energy to the apex under radially polarized, on‐axis excitation. Finite‐element simulations identify as a design‐critical parameter governing a Fabry‐Pérot‐like SPP resonance, with co‐tuning of producing a sequence of geometrical resonances associated with maximal apex . Experiments on monolayer graphene confirm robust near‐field enhancement using a dry objective with . By relaxing the high‐NA requirement, this approach increases working distance and compatibility with constrained environments, advancing practical implementations of nano‐Raman spectroscopy.