We theoretically reveal that the enhanced electric-field concentration in the conventional plasmonic dimers arise from charge redistribution. The redistribution is governed by the quasistatic Coulomb interactions, where the self-interaction energy of collective charges within a monomer plays an irreplaceable role. This finding is further confirmed by the additional red shifts of hybridized modes in plasmonic nanorod dimers at short distances. Conversely, the situation in a composite cavity consisting of a plasmonic nanorod and a dielectric disk is quite different, where the charges of the nanorod are merely amplified while maintaining the same distribution pattern as an individual nanorod, resulting in near-field concentration remaining at the level of an individual nanorod. Inspired by the charge interaction characteristics in dimers, we further constructed a plasmonic dimer cavity modified with molecules attached to the gap-facing surfaces. By leveraging the Coulomb interactions between small molecules and the collective charges of the metal, one can obtain enhanced charge concentration around the gap-facing surfaces. The corresponding coupling strength between the molecules-modified cavity and a single quantum emitter can be enhanced by several times or more compared to the unmodified case, with the enhancement factor dependent on parameters such as molecular number, transition dipole moment, and molecule-cavity distance.

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• Cavity quantum electrodynamics
• Nanophotonics
• Near-field optics
• Plasmonics