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Physical Review Letters

Spontaneous Emission of Europium Ions Embedded in Dielectric Nanospheres

Hannes C. Schniepp, Vahid Sandoghdar

Published Dec 3, 2002• 133 citations
Publisher

Abstract

We measure fluorescence lifetimes of emitters embedded in isolated single dielectric nanospheres. By varying the diameters of the spheres from 100 nm to 2 microm and by modifying their dielectric surrounding, we demonstrate a systematic change of paradigm in the spontaneous emission rate, as we cross the border from the superwavelength regime of Mie resonances to the nanoscopic realm of Rayleigh scattering. Our data show inhibition of the spontaneous emission up to 3 times and are in excellent agreement with the results of analytical calculations.

DOI
10.1103/physrevlett.89.257403

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We measure fluorescence lifetimes of emitters embedded in isolated single dielectric nanospheres. By varying the diameters of the spheres from 100 nm to 2 microm and by modifying their dielectric surrounding, we demonstrate a systematic change of paradigm in the spontaneous emission rate, as we cross the border from the superwavelength regime of Mie resonances to the nanoscopic realm of Rayleigh scattering. Our data show inhibition of the spontaneous emission up to 3 times and are in excellent agreement with the results of analytical calculations.
Summary form only given. Over more than three decades many experiments have shown that the rate of spontaneous emission from an emitter can be strongly modified due to its electromagnetic surrounding. It is also well-known that the fluorescence lifetime of an emitter changes when embedded in dielectric media where aside from a change in the density of states, it is influenced by the local fields in the dielectric. The goal of our work has been to study the transition between the regime of an emitter in vacuum to that of a bulk dielectric medium, emphasizing the domain where the emitter's environment has a subwavelength extent. In our experiment we have investigated the dependence of the spontaneous emission rate of Eu/sup 3+/ ions in polymer nanospheres of different diameters, ranging from 20nm to 2000nm. In order to perform well-defined control experiments, we have combined atomic force microscopy and fluorescence confocal microscopy to make measurements only on isolated single spheres lying on a glass substrate.
Summary form only given. Over the last three decades of the 20 th century many experiments have demonstrated that the rate of spontaneous emission for an emitter is determined by its electromagnetic surrounding. This phenomenon has been usually studied for atoms or molecules placed in the vicinity of mirrors or in optical resonators. Furthermore, it is also well-known that the fluorescence lifetime of an emitter changes after being embedded in dielectric media where aside from a change in the refractive index, it experiences the local fields in the dielectric. For a spherical medium larger than the order of a wavelength it is known that the coupling to Mie modes of this dielectric resonator has a large influence on the emission rate of an emitter inside. However, when the medium becomes comparable or smaller than a wavelength one can no longer speak of resonator modes. Here we report on an experiment where we have studied the variation of the spontaneous emission rate as the dielectric medium surrounding an emitter is made smaller and smaller down to a few nanometers.
Spherical SiO2 colloids with two different diameters (175 nm, 340 nm) were doped with erbium at different concentrations. The spheres show sharply peaked photoluminescence centered at 1.535 μm, due to intra-4f transitions in Er3+. From measurements of the Er decay rate for different Er concentrations the decay rate of isolated Er ions (i.e., in absence of concentration quenching) was determined for the two colloid diameters. The data were compared to spontaneous emission rates derived from calculations of the local optical density of states in the colloids. The calculation predicts a large difference in the spontaneous emission rate for both colloid sizes (61 vs 40 s−1), in perfect agreement with the measured data.
We observed the change in fluorescence lifetime from a molecular dye embedded in a colloidal suspension of polystyrene spheres when the mean separation between spheres remains less than the wavelength of emission. At high densities of colloidal particles the spontaneous emission is inhibited, while as the density of colloidal particles is lowered the spontaneous emission is enhanced. This phenomenon is interpreted in terms of coherent interference from the electric field radiated by the molecular dipole and the reradiated field from the dielectric spheres.
The spontaneous-emission rates of emitters embedded in a thin layer 0 of a loss-free dielectric depend on their optical environment, i.e., on the optical properties of the two media 1 and 2 adjoining layer 0. For electric- and magnetic-dipole transitions the spontaneous-emission rates, normalized with respect to those in an infinite medium 0, are expressed in terms of the Fresnel reflection coefficients for plane and evanescent waves incident from medium 0 on the interfaces to the planar-stratified or homogeneous media 1 and 2, respectively. From this result which is valid for arbitrary layer thicknesses d0{d}_{0}d0​ we derive an approximation for extremely thin layers of optical thickness ${n}_{0}{d}_{0}\ensuremath{\ll}\frac{\ensuremath{\lambda}}{8}$ (where $\ensuremath{\lambda}$ is the emission wavelength) between two homogeneous loss-free dielectric media 1 and 2. For the normalized spontaneous-emission rates as functions of the refractive indices nj{n}_{j}nj​ of media j=0,1,and2j=0, 1, \mathrm{and} 2j=0,1,and2 analytical expressions are obtained. We have used these expressions previously without proof [Phys. Rev. B 21, 4814 (1980)] to analyze experimentally observed changes in the fluorescence lifetime of Eu3+{\mathrm{Eu}}^{3+}Eu3+ ions in varying optical environments, which yielded the quantum efficiency of the emitting state.