Interplays of dipole and charge-transfer-plasmon modes in capacitively and conductively coupled dimer with high aspect ratio nanogaps

Abstract

Charge transfer plasmons (CTPs) in coupled plasmonic cavities have gained attention for their interesting resonance mechanisms and potential applications in sensing. The authors present the first investigation of the CTP mode in top-down fabricated tall plasmonic nanostructures with deep nanogaps, whose excitation becomes possible by the interaction of transversal and longitudinal plasmonic modes. Hybridization of dipole and CTP modes at different parts of the plasmonic nanostructures, for both capacitively coupled dimer of sub-5 nm gap and conductively coupled dimer with junction bridge is extensively studied. Importantly, the authors demonstrate the conditional emergence of a hybrid dimer-CTP mode based on the cladding refractive index and analyte thickness, which can present an opportunity for highly specific surface sensing applications. The sensing performances of these modes are evaluated, showing a figure-of-merit of ≈18.4 and Q ≈ 44 for the hybrid dimer-CTP mode in the visible spectrum. Surface sensing based on alkanethiol adsorption shows surface sensitivity as high as ≈3.2 nm/carbon-chain for this mode, which is higher than the typical values obtained by LSP structures.