Interplays of Dipole and Charge‐Transfer‐Plasmon Modes in Capacitively and Conductively Coupled Dimer with High Aspect Ratio Nanogaps

Abstract

The capacitive, inductive, and conductive couplings in coupledcavity structures have been explored in the literature. The capacitive coupling refers to the near-field coupling of electric dipoles, with some notable examples in plasmonic oligomers and dolmen structures. [27] The most widely used plasmonic structure for light-matter interaction is the plasmonic dimer, formed by two capacitively coupled plasmonic disks separated by a nanoscalegap, which has been demonstrated in the enhancement of quantum emitters [28,29] and detection of adsorbed molecules. [30,31] Inductive coupling, on the other hand, refers to the coupling of magnetic dipoles, such as between vertically stack split-ring resonators (SRR) in 3D stereo-metamaterial. [32,33] Stronger electromagnetic coupling is expected for decreasing inter-antenna spacing. As the antennas become physically connected, the nature of coupling changed from capacitive to conductive. In this situation, a shared conduction current between the antennas allows for the surface charge redistribution, resulting in a new plasmon mode. [34] The role of conductive coupling can be seen in the normal excitation of higher-order magnetic mode in the U-shape SRR, where the optical excitation of a horizontal electric dipole along in the SRR bottom arm leads to the excitation of the antiparallel vertical electric dipoles in the SRR side arms due to the surface charge distribution along the SRR structures. 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/carbonchain for this mode, which is higher than the typical values obtained by LSP structures.

Optimizing Interparticle Gaps in Large‐Scale Gold Nanoparticle Supercrystals for Flexible Light‐Matter Coupling

“…However, a limit can be expected for gap sizes below 1 nm, when interparticle carrier transport due to tunneling and hopping becomes possible and increasingly dominant leading to the breakdown of collective plasmon modes. [21] Additionally, due to the rather high mobility of Au-surface-atoms, coalescence (necking) becomes more likely at very small separations. [22] Figure 2 shows 2D hexagonal supercrystals which were obtained by selfassembly of different AuNP sizes and PSSH molecular weights.…”