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Nanotechnologies in Russia

Spontaneous and induced emission in dielectric nanoparticles

Tasoltan T. Basiev, Yu.V. Orlovskii, Konstantin K. Pukhov

Published Oct 1, 2008• 13 citations
Publisher

Abstract

An analytical expression is proposed for the rate of radiative decay of excitations of optical centers in an ellipsoidal nanoparticle that is considerably smaller than the light wavelength ( a, b, c ≪ λ ) and is embedded in the surrounding medium. This expression is compared with the corresponding expression for a bulk crystal. The time of radiative decay in nanoparticles spherical in shape and identical in size (2 R ≪ λ ) is derived as a function of the dielectric characteristics of a nanocomposite and the nanoparticle volume fraction C in suspensions and aerosols. The expressions describing the emission and absorption cross sections in spherical nanoparticles are obtained. The experimentally measured and theoretically calculated decay times of metastable levels of rare-earth impurity ions in YAG and Y 2 O 3 crystalline nanoparticles are compared with those for bulk crystals with the same structure.

DOI
10.1134/s1995078008090036

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An analytical expression is proposed for the rate of radiative decay of excitations of optical centers in an ellipsoidal nanoparticle that is considerably smaller than the light wavelength ( a, b, c ≪ λ ) and is embedded in the surrounding medium. This expression is compared with the corresponding expression for a bulk crystal. The time of radiative decay in nanoparticles spherical in shape and identical in size (2 R ≪ λ ) is derived as a function of the dielectric characteristics of a nanocomposite and the nanoparticle volume fraction C in suspensions and aerosols. The expressions describing the emission and absorption cross sections in spherical nanoparticles are obtained. The experimentally measured and theoretically calculated decay times of metastable levels of rare-earth impurity ions in YAG and Y 2 O 3 crystalline nanoparticles are compared with those for bulk crystals with the same structure.
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.
Spontaneous emission rate is not only affected by the properties of the material itself, but also depends on the dielectric properties of the surrounding media. Recently, some work has been carried out to study the dependence of spontaneous emission rates of the luminescent-ion-containing nanometer particles on the refractive index of the media. Based on the free distribution of nanometer particles in media, the pro-bability density of luminescent lifetime is analyzed. Since nanometer particles distribute freely in media, the number of nanometer particles in a given volume surrounding the luminescent center obeys the Poission distribution. The local relative refractive index nr, which is the ratio of media effective refractive index to particle refractive index, is affected by the particle number of nanoparticles within a sphere of radius half of the radiative wavelength. The probability density function of nr can be obtained according to the dependence of nr on the particle number. Based on the relation between nr and luminescence lifetime, the analytical form of probability density function of lifetime is obtained. A calculation has been done based on the parameters given in Meltzer's experiment (Eu3+ doped Y2O3 in methanol). The result shows that the relative fluctuation of luminescence lifetime is too small to be observed. However, with increasing size of nanometer particles or density of nanometer particles, the relative fluctuation of lifetime increases greatly and could be observed.
The change of intensities of optical transitions of atoms, molecules, and their ions in the near field of nanoobjects is of interest for researchers from both basic and applied points of view. The concept of a near field was used as early as the 1960s to study the mechanisms of hypersensitive transitions of rare-earth (RE) ions. In this work, it is once more emphasized that, precisely for these transitions, changes in characteristics under the action of local properties of the medium are especially strong and, correspondingly, these transitions are especially promising for use. The Judd method for the calculation of the intensities of hypersensitive transitions of RE ions taking into account the inhomogeneous ligand field is extended to RE ions in the near field of nanoobjects. A simple analytical expression for the Judd-Ofelt intensity parameter Ω 2 for RE ions in the field of spherical nanoparticles of a subwavelength size is derived.
The photoluminescence of nitrogen-vacancy (NV) centers in diamond nanoparticles exhibits specific properties as compared to NV centers in bulk diamond. For instance large fluctuations of lifetime and brightness from particle to particle have been reported. It has also been observed that for nanocrystals much smaller than the mean luminescence wavelength, the particle size sets a lower threshold for resolution in Stimulated Emission Depletion (STED) microscopy. We show that all these features can be quantitatively understood by realizing that the absorption-emission of light by the NV center is mediated by the diamond nanoparticle which behaves as a dielectric nanoantenna.
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.