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Article Dans Une Revue Optics Letters Année : 2010

Fluorescence quenching by a metal nanoparticle in the extreme near-field regime

Résumé

We study the spontaneous decay rate of a dipole emitter close to a metallic nanoparticle in the extreme near-field regime. The metal is modeled using a nonlocal dielectric function that accounts for the microscopic length scales of the free electron gas. We describe quantitatively the crossover between the macroscopic and microscopic regimes and the enhanced nonradiative decay due to microscopic interactions. Our theory is in agreement with results previously established in the asymptotic near-and far-field regimes. The controlled modification of spontaneous emission is a central issue in photonics. Modifications of the spontaneous decay rate of molecules close to metallic surfaces [1] or atoms in cavities [2] have become textbook examples. The development of nano-optics techniques has stimulated the use of metallic nanopar-ticles or tips to act on the excited-state lifetime [3], on the fluorescence intensity [3,4], and on the radiation pattern [5,6] of isolated emitters, leading to the concept of optical nanoantenna. The interplay among the enhancement of the excitation intensity, nonradiative decay, and changes in the radiation pattern [7,8] offers useful degrees of freedom. Fluorescence enhancement can be optimized for imaging applications or single photon sources, while efficient quenchers can be designed for biochemical applications [9,10]. In this Letter we study quantitatively the spontaneous decay rate of a single emitter coupled to a metallic nanoparticle, up to a regime in which the mac-roscopic description of the electrodynamics of the metal surface breaks down. This regime is expected when the distance to the metal surface is on the order of the microscopic length scales driving the electron dynamics. In this regime, the metal surface has to be described using a spatially nonlocal dielectric function. In the context of molecular fluorescence, a general formalism and the main trends have been described by Ford and Weber [11]. More recently, a giant enhancement of the nonradiative decay rate due to microscopic interactions at a plane metal surface has been predicted [12], and a simplified nonlo-cal model has been used to describe the change in the radiative and nonradiative decay rates of molecules adsorbed on small nanoparticles [13]. In the present study, we describe the full crossover between the far-field regime and the extreme near-field regime (up to physical contact) in the case of nanoparticles with size R satisfying ᐉ Ͻ R Ӷ␭, where ᐉ is the electron mean free path and ␭ is the emission wavelength. Note that this condition does not include the case of very small particles ͑R Ͻ 10 nm͒ [13,14] in which other mechanisms, such as quantum confinement, can be involved. Handling the full emitter-nanoparticle distance range requires a more sophisticated model than that used in [12,13] and allows us to determine precisely the breakdown of the macro-scopic approach, thus providing an answer to a recurrent issue in nano-optics [15]. In the weak-coupling regime, the spontaneous decay rate of a dipole emitter located at position r takes the form ⌫ = ͑2/ប͉͒p͉ 2 Im͓u · G͑r , r , ␻͒ · u͔, where p is the transition dipole, u = p / ͉p͉, and ␻ is the emission frequency [16]. The dyadic Green's function G describes the electrodynamic response of the environment. It connects an electric dipole at position r to the radiated electric field at position rЈ through the relation E͑rЈ , ␻͒ = G͑rЈ , r , ␻͒ · p. In free space, the decay rate is obtained from the vacuum Green's function and reads ⌫ 0 = ␻
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Dates et versions

hal-01233702 , version 1 (30-11-2015)

Identifiants

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Etienne Castanié, Matthieu Boffety, Rémi Carminati. Fluorescence quenching by a metal nanoparticle in the extreme near-field regime. Optics Letters, 2010, 35 (03), pp.291. ⟨10.1364/OL.35.000291⟩. ⟨hal-01233702⟩
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