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. 
Adi Salomon, Robert J. Gordon, Yehiam Prior, Tamar Seideman, Maxim   Sukharev
cond-mat.mes-hall cond-mat.mtrl-sci physics.optics