We identify a new mechanism for cooperative emission of light by an ensemble of N dipoles near a metal nanostructure supporting a surface plasmon.The cross-talk between emitters due to virtual plasmon exchange leads to a formation of three plasmonic super-radiant modes whose radiative decay rates scales with N , while the total radiated energy is thrice that of a single emitter. Our numerical simulations indicate that the plasmonic Dicke effect survives non-radiative losses in the metal.Radiation of a dipole near a metal nanostructure supporting surface plasmon (SP) is attracting renewed interest due to possible biosensing applications [1]. While early studies mainly focused on fluorescence of molecules near rough metal films [2], recent advances in near-field optics and in chemical control of molecule-nanostructure complexes spurred a number of experiments on single metal nanoparticles (NP) linked to dye molecules [3,4,5,6,7,8] or semiconductor quantum dots [9]. Emission of a photon by a dipole-NP complex involves two competing processes: enhancement due to resonance energy transfer (RET) from an excited dipole to a SP [10], and quenching due to decay into optically-inactive excitations in the metal [11]. These decay channels are characterized by radiative, Γ r , and non-radiative, Γ nr , decay rates, respectively, and their balance is determined by the separation, d, of the emitter from the metal surface [12,13]. The emission is most enhanced at some optimal distance, and is quenched close to the NP surface due to the suppression of quantum efficiency, Q = Γ r / (Γ r + Γ nr ), by prevalent non-radiative processes. Both enhancement and quenching were widely observed in fluorescence experiments on Au and Ag nanoparticles [3,4,5,6,7,8]. In recent single-molecule measurements [6,7,8], the distance dependence was in a good agreement with single-dipole-NP models [12,13], prompting proposals for a NP-based nanoscopic ruler [8].In this Letter, we identify a novel mechanism in the emission of light by an ensemble of dipoles located near a nanostructure supporting a localized SP. A typical setup would involve, e.g., dye molecules [3,4,5] or quantum dots [9] attached to a metal NP via DNA linkers. Namely, we demonstrate that RET between individual dipoles and SP leads to a cross-talk between the emitters. As a result, the emission of a photon becomes a cooperative process involving all dipoles in the ensemble. This plasmonic mechanism of cooperative emission is analogous to the Dicke effect for N radiating dipoles in free space, confined within a volume with characteristic size smaller than the radiation wavelength λ [14,15,16]. In that case, the cooperative emission is due to photon exchange between the emitters that gives rise to super-radiant (SR) states with total angular momentun 1 and enhanced radiative decay rate ∼ N Γ r 0 , where Γ r 0 is the decay rate of an isolated dipole. In contrast, in plasmonic systems, the dominant coupling mechanism between dipoles is SP exchange, i.e., excitation of a virtual SP in a na...

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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.

Metal photoluminescence (MPL) originates from radiative recombination of photoexcited core holes and conduction band electrons. In metal nanostructures, MPL is enhanced due to the surface plasmon local field effect. We identify another essential process in plasmon-assisted MPLexcitation of Auger plasmons by core holes-that hinders MPL from small nanostructures. We develop a microscopic theory of plasmon-enhanced MPL that incorporates both plasmon-assisted enhancement and suppression mechanisms and derive the enhancement factor for MPL quantum efficiency. Our numerical calculations of MPL from Au nanoparticles are in excellent agreement with the experiment.