Electronic quantum effects in mesoscopic plasmonic nanostructures significantly influence their optical properties. Here, by exciting plasmon, Bennett, and charge-transfer modes, we demonstrate that they can be strongly coupled in purely metallic devices. Using the Feibelman $d$ parameter framework, we derive an analytical expression for the coupling strength, showing its inverse dependence on the square root of the structure's length scale ($g\propto 1/\sqrt{L}$). Thus, we establish a nanophotonic platform where nonclassical states, such as Bennett and charge-transfer modes, interact strongly with purely classical ones, such as surface plasmons, providing an indirect strategy to probe and exploit quantum electronic phenomena in optical systems.

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• Plasmonics
• Nanostructures
• Density functional theory
• Hydrodynamic models
• Transformation optics