Evidence for long-range interactions between rare-earth impurity ions in nanocrystals embedded in amorphous matrices with the two-level systems of the matrix

Abstract

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...

Optical properties of Eu3+-doped zinc borate glasses

“…Rare earth (RE) -doped glasses are interesting materials for optical devices such as lasers [1], fiber amplifiers [2], ultraviolet (UV) detectors [3] and hole-burning high-density memory [4]. Doping of glasses with terbium (Tb), europium (Eu), samarium (Sm) or dysprosium (Dy) leads to light emission in the visible range [5].…”

Dynamical processes of Ln3+ ions doped in LaF3 nanocrystals embedded in transparent oxyfluoride glass

“…The energy level structure of Ln 3+ ions, determined by their local environment, remains nearly the same for the ions doped in nanocrystals and in the bulk crystals, except for an increased inhomogeneous broadening seen in the nanocrystals. The optical dynamics of Ln 3+ phase is greatly affected by the surrounding glass matrix and the confinement due to their sizerestricted nature [3][4][5][6]. For isolated Eu 2 O 3 nanoparticles and Eu-doped Y 2 O 3 nanoparticles in 5-30 nm diameter range, the spectral hole burning (SHB) line width increases as the particle size decreases.…”

Photon echo studies of LaF3:Pr3+ nanocrystals in glass

“…ðBT 1:7 Þ below 5 K with the appearance of a strongly temperature-dependent process whose temperature behavior is best described by BT 4:5 above 5 K: The data of Meltzer et al [3,4] exhibits a similar nonzero extrapolated value at T ¼ 0 K; a nearly linear temperature dependence for Pr 3þ below 4 K and up to 12 K for Eu 3þ and for Pr 3þ ; a strong temperature dependence ðBT 5 Þ [6] was found above 5 K: They also found a similar result for Y 2 O 3 :Eu 3þ nanocrystals embedded in a polymer where a nearly linear temperature dependence was observed. As usually observed in glasses, the homogeneous line widths for Eu 3þ are much less than those of Pr 3þ : Macfarlane and Dejneka ascribed the quasilinear temperature dependence for Tm 3þ below 5 K to interactions with the TLS which may extend into the disordered interface region and the strong temperature dependence above 5 K to an interaction with the confined modes of the nanocrystals.…”

“…1. Data includes results on Eu 3þ and Pr 3þ [3,4] and Tm 3þ [5]. The glasses are of similar but not identical compositions.…”