The radiative lifetime, R , of the 5 D 0 metastable excited state of Eu 3ϩ ions in nanocrystalline monoclinic Y 2 O 3 samples is about four times longer than that in the micron size powder of the same material. The Eu 3ϩ radiative lifetime was measured in nanocrystals surrounded with air as well as those immersed in different liquids. It is shown that the radiative lifetime changes with the index of refraction of the immersion medium and provides a unique test of the standard formula relating R and the oscillator strength. The magnitude of the effect is determined by the ''filling factor'' ͑the fraction of the sample volume occupied by nanocrystals͒ which can therefore be determined. ͓S0163-1829͑99͒51144-2͔The prospect of modifying the radiative lifetime of centers in solids is a tempting goal with a variety of potential applications to lasers, displays, etc. One means to do this is to alter the local environment of the center by putting it in different hosts or by selecting certain sites. Such a process simultaneously alters the spectrum of the center. An alternative method modifies the radiation modes to which the center couples by placing the material in some sort of cavity. In the present, it is shown that the lifetime can by modified, within certain limits, without altering the spectral properties, by placing nanoparticles, whose size is much less than the wavelength of light, in different media. By this means, the local environment of the center remains unchanged while its coupling to the light is modified both by changing the density of radiative modes and by modifying the surrounding medium polarizability. While this idea has been previously discussed in an earlier work, 1 the particle size was about the same as that of the light wavelength, making a quantitative comparison with theory difficult, and only the modification of the effective index of refraction was considered; the effective polarizability change produced by altering the surrounding medium was not considered.Nanocrystalline insulating materials doped with rare earth and transition metal ions exhibit optical properties which are significantly different from that of bulk materials.2-4 Such differences are expected due to: ͑i͒ the confinement effects on the vibrational spectra of nanocrystals, ͑ii͒ the increased role of the impurity ions at the surface whose optical properties are modified by the disorder at the surface, and ͑iii͒ the alteration of the electronic bands of the matrix. This makes nanocrystals interesting both in terms of their fundamental properties and because of their potential in a variety of applications.We report here the results of a study of the radiative lifetime, R , of the 5 D 0 metastable excited state of Eu 3ϩ ions in nanocrystalline Y 2 O 3 which is drastically different from that in the bulk material.The nanocrystalline Y 2 O 3 :0.1%Eu 3ϩ samples were prepared by condensation after laser evaporation as described previously.5 In the present work nanocrystalline samples with particle size distributions centered at app...

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The low-temperature homogeneous broadening of the electronic transitions of Eu 3ϩ and Pr 3ϩ rare-earth impurity ions in Y 2 O 3 and LaF 3 nanocrystals embedded into amorphous materials ͑polymer and oxyfluoride glass ceramics͒ was studied with hole-burning and fluorescence line narrowing techniques. It is shown that the homogeneous linewidth is determined by the interaction of the impurity ions contained in the nanocrystals with the two-level systems ͑TLS's͒ of the surrounding glass matrix. A comparison of the experiments with a calculation provides direct evidence for the long-range nature of the interaction with the TLS's. DOI: 10.1103/PhysRevB.64.100201 PACS number͑s͒: 78.67.Ϫn, 78.67.Bf It is well known that when rare-earth ͑RE͒ ions are doped into glasses, their dynamics are governed by interactions with the two-level systems ͑TLS's͒ of the glass. Experiments in many systems provide evidence that at low temperatures the homogeneous linewidths, ␥ h , in these systems obey a power law in temperature, ␥ h ϳT ␣ with 1Ͻ␣Ͻ2. Theoretical calculations show that this behavior is predicted for interactions with the TLS's of the glass. [1][2][3][4][5] With the recent availability of nanocrystals containing RE ions, it is of great interest to determine whether RE ions separated from the glassy TLS's by the crystalline nanoparticle in which they are contained also exhibit interactions with TLS's when the nanoparticles are embedded in an amorphous matrix such as a glass. This can provide an independent test of the TLS model and can determine the length scale of the interactions.It has been recently determined that materials consisting of insulating nanocrystals doped with RE ions embedded into amorphous ͑glassy͒ matrices possess nearly identical spectra to those of RE ions in single crystals of the same crystalline composition and structure.6 This is not unexpected, as the optical spectra of RE ions are determined by the short-range local environment of the RE site, which ͑with the exception of the ions at the nanocrystal-glass interface͒ remains unperturbed in crystallites of a few nanometers size. The sharp line spectra allow one to spectrally isolate ions in the nanoparticles from those in the amorphous matrix. Here we apply the technique of spectral hole burning to examine their dynamical properties. This work is motivated, in part, by the interest in these materials for applications such as hole-burning memories 7 and optical processors. 8,9The mechanisms responsible for changes in the dynamical properties of the excited states of RE ions in insulating nanocrystals embedded in amorphous matrix, compared to single crystals, can be grouped into two categories: ͑i͒ those connected with size restriction effects and ͑ii͒ those caused by the interaction of RE ions with the amorphous environment surrounding the crystallites. The effects of the first kind are due mainly to the modification of the phonon spectrum of the nanocrystals at low frequencies due to their sizerestricted nature as reported for ''free-standing'' na...

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