Long-range dipole-dipole interactions in a plasmonic lattice.*

Spontaneous emission of quantum emitters can be enhanced by increasing the local density of optical states whereas engineering dipole-dipole interactions require modifying the two-point spectral density function. Here, we experimentally demonstrate long-range dipole-dipole interactions (DDIs) mediated by surface lattice resonances in a plasmonic nanoparticle lattice. Using angle-resolved spectral measurements and fluorescence lifetime studies, we show that unique hybrid plasmonic modes mediate long-range DDI between donor and acceptor molecules. The fluorescence lifetime measurements show density-dependent non-exponential decay dynamics. We observe significant and persistent long-range dipole-dipole

interaction strengths at room temperatures for a range of densities that map to 800 nm mean nearest-neighbor separation distance between donor and acceptor dipoles, a factor of 100 larger than free space. Our results pave the way to engineer and control long-range DDIs between ensembles of emitters at room temperature.