  We develop a theory of cooperative emission of light by an ensemble of
emitters, such as fluorescing molecules or semiconductor quantum dots, located
near a metal nanostructure supporting surface plasmon. The primary mechanism of
cooperative emission in such systems is resonant energy transfer between
emitters and plasmons rather than the Dicke radiative coupling between
emitters. We identify two types of plasmonic coupling between the emitters, (i)
plasmon-enhanced radiative coupling and (ii) plasmon-assisted nonradiative
energy transfer, the competition between them governing the structure of system
eigenstates. Specifically, when emitters are removed by more than several nm
from the metal surface, the emission is dominated by three superradiant states
with the same quantum yield as a single emitter, resulting in a drastic
reduction of ensemble radiated energy, while at smaller distances cooperative
behavior is destroyed by nonradiative transitions. The crossover between two
regimes can be observed in distance dependence of ensemble quantum efficiency.
Our numerical calculations incorporating direct and plasmon-assisted
interactions between the emitters indicate that they do not destroy the
plasmonic Dicke effect.
