Collective multimode strong coupling in plasmonic nanocavities

Abstract: 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.