Collective multimode strong coupling in plasmonic nanocavities

Plasmonic nanocavities enable access to the quantum properties of matter, but are often simplified to single mode models despite their complex multimode structure. Here, we show that off-resonant plasmonic modes in fact play a crucial role in strong coupling, and determine the onset of a novel collective interaction. Our analysis reveals that $n$ strongly coupled plasmonic modes, introduce up to $n(n+1)/2$ oscillation frequencies that depend on their coupling strengths and detunings from the quantum emitter. Furthermore, we identify three distinct regions as the coupling strength increases: (1) single mode, (2) multimode, and (3) collective multimode strong coupling. Our findings enhance the understanding of quantum dynamics in realistic plasmonic environments and demonstrate their potential to achieve ultra-fast energy transfer in light-driven quantum technologies.