Collective Plasmonic-Molecular Modes in the Strong Coupling Regime

We demonstrate strong coupling between molecular excited states and surface plasmon modes of a slit array in a thin metal film. The coupling manifests itself as an anti-crossing behavior of the two newly formed polaritons. As the coupling strength grows, a new mode emerges, which is attributed to long range molecular interactions mediated by the plasmonic field. The new, molecular-like mode repels the polariton states, and leads to an opening of energy gaps both below and above the asymptotic free molecule energy.