Strongly Interacting Plasmon Nanoparticle Pairs:  From Dipole−Dipole Interaction to Conductively Coupled Regime

We have investigated the optical response of periodic arrays of metallic (gold) nanoparticles composed of a pair of particles on each lattice site. By varying the interparticle separation within the pairs from dielectric proximity to conductive contact on a nanometer scale, we observe an abrupt, large renormalization as well as a splitting of the surface plasmon polariton energy. These spectral anomalies are ascribed to a transition whereupon the interparticle dipole−dipole interaction is shunted and the plasmon polaritons exhibit multipolar behavior, including a very high local concentration of electromagnetic energy in the vicinity of their conductive contact.