Rare Earth Ions In Glasses Research Articles

Insight into the Partitioning and Clustering Mechanism of Rare-Earth Cations in Alkali Aluminoborosilicate Glasses

Rare-earth (RE) containing alkali aluminoborosilicate glasses find increasingly broad technological applications, with their further development only impeded by yet-poor understanding of coordination environment and structural role of RE ions in glasses. In this work we combine free induction decay (FID)-detected electron paramagnetic resonance (EPR), electron spin echo envelope modulation (ESEEM), and MAS NMR spectroscopies, to examine the coordination environment and the clustering tendencies of RE3+ in a series of peralkaline aluminoborosilicate glasses co-doped with Nd2O3 (0.001-0.1 mol%) and 5 mol% La2O3. Quantitative EPR spectral analysis reveals three different Nd3+ forms coexisting in the glasses: isolated Nd3+ centers, dipole-coupled Nd clusters (Nd-O-X-O-Nd, where X = Si/B/Al), and spin-exchange-coupled Nd clusters, (Nd-O-Nd) and (Nd-O-La-O-Nd). Extensive RE clustering is observed at high RE2O3 concentrations, with more than 90% REs converting to dipole- and exchange-coupled Nd clusters already at [RE2O3] = 0.01 mol%. ESEEM analysis of the EPR-detectable Nd centers indicates a Na/Si-rich environment (four Na+ per Nd3+) for the isolated Nd3+ centers and the Na/Si/B-rich environment (2-3 Na+ and 1-2 boron per each Nd3+) for the dipole-coupled Nd clusters, while the EPR-undetectable exchanged-coupled RE clusters are predicted to exist in a Na/B-rich environment. The RE clustering induces nano-scale glass phase separation, while the Na/B-rich environment of the RE clusters implies a depletion of the same elements from the remaining host glass. Based on our results, we develop a mechanistic model that explains the high tendency of RE3+ to form clusters in alkali aluminoborosilicate glasses.

Analysis of the luminescent spectra of Eu3+ in glasses

The spectroscopy properties of lanthanide ions in glasses differ remarkably from crystal and powder samples due to superposition of transitions from many ions with different local environments. Eu3+ ions were doped in lead–borosilicate and boro–tellurite glass samples to probe the structural information and to reveal their effects on the luminescent properties of lanthanide ions. Three emission peaks were observed for 5D0→7F0 transition, with peak energies linearly dependent on the excitation wavelengths, and correlation between the intensity ratio of 5D0→7F0 versus 5D0→7F2 and the width of 7F1 was observed. The former indicates that there are three subsets of Eu3+ sites in the samples, with 7F0 being pushed downwards by 7FJ (J=2, 4, and 6) by crystal-field J-mixing, and the latter can be quantitatively modeled by the adaptation of the theoretical model of Wen et al. [Phys. Chem. Chem. Phys. 12, 9933 (2010)] to account for the corrections due to the inhomogeneous broadening of 7FJ (J=0, and1) crystal-field levels. The methods explored here can be applied to study other glass hosts for luminescent materials.

Visible Luminescence of Tm$^{3+

Tm, Er, and Yb codoped calcium-aluminoborate glasses were prepared. Intense visible luminescence was observed under excitation at a 325-nm wavelength. It was demonstrated that violet and blue luminescences (392, 454 and 478 nm) were mainly contributed by Tm, and green (514, 528 and 559 nm) and red (652 nm) luminescences were mainly contributed by Er. Energy transfer processes among Tm, Er, and Yb were evidenced, in which various colors of the luminescence could be easily tuned by adjusting the concentration and the kind of the rare earth ions in glasses.

Nonoxide Glass-Forming Systems-Glass Formation and Structure, and Optical Properties of Rare-Earth Ions in Glasses

Nonoxide glasses composed of halides and/or chalcogenides (sulfides, selenides, and tellurides) as their main components have been extensively investigated due to their characteristic structures and properties. Among them, the present review focuses on glass formation based on nonfluoride-halide (halides other than fluoride) systems and the structure of the glasses as well as the transition properties of rare-earth ions in the nonoxide glasses. Systematic examination of glass formation in nonfluoride-halide systems based on AXn-MX-M′X2 with a single halide ion revealed that the systems of A=Zn, Cd, Ag, Cu and Li are vitrified by a conventional melting-cooling method, where X=Cl, Br, or I, M=alkali metal (K, Rb, Cs), and M′=alkaline earth metal (Ba). The coordination structure around the cations, An+, was investigated by means of X-ray absorption spectroscopy and other methods. The structural model for these halide glasses is also discussed. The studies about the transition properties of rare-earth ions in the nonoxide glasses showed that the multiphonon relaxation rates are strongly suppressed. Detailed analyses on the fluorescent decay curves for Er3+-doped Ga2S3-based glasses are presented.

Optical properties and local structure of rare-earth-doped amplifier for broadband telecommunication

Optical properties and local structure of rare-earth ions in glasses for broadband optical fiber amplifiers are reviewed. In a series of Er-doped glasses with broad 1.5 μm emission, heavy-metal oxide glasses are attracting great interest. Anomalous compositional dependences of optical properties of Er-doped antimony silicate glasses are shown. Structural implication of small compositional dependence observed for the Judd–Ofelt Ω 6 parameter and local phonon energy are discussed.

Site-selective fluorescence spectroscopy of Eu 3+ and Sm 2+ ions in glass

Fluorescence line–narrowing or site-selective fluorescence experiments on Eu 3+ and Sm 2+ ions in glass are reviewed. It is shown that this technique gives detailed information that is difficult to obtain by other means, such as site-dependent homogeneous widths, inhomogeneous broadening mechanisms, distribution of crystal-field components in glass, and so on. A simple theoretical treatment to consider only the second-order terms of the crystal-field potential is discussed and is shown to be very effective. It will be emphasized that J-mixing, i.e., wave function mixing between 4f N states through the static crystal-field potential, plays a significant role in the optical properties of rare-earth ions in glass.

Room temperature persistent spectral hole burning in x-ray irradiated Eu3+-doped borate glasses

Irradiation of x-rays has induced room-temperature persistent spectral hole burning (PSHB) in Eu3+-doped borate glasses melted under an inert atmosphere. Defects were formed by x-ray irradiation and these defects, especially electron trapping centers near rare-earth ions in glasses, were responsible for the PSHB. Electrons were released from these defects upon irradiation of a burning light. Photoreduction of Eu3+ to Eu2+ by trapping these electrons resulted in the formation of persistent spectral holes.

Stark levels analysis for Er 3+-doped oxide glasses: germanate and silicate

A model of effective crystal field Hamiltonian based on the principle of descending symmetry and using the symmetry group chain scheme is proposed to describe the average of local symmetries occupied by rare earth (RE) ions in glasses. Only three crystal field parameters, one for each rank, have to be fitted in this Hamiltonian. Based on the absorption and emission spectra measured in a conventional way at 13 K, the Stark levels in manifolds from 4I 15/2 to 2H 9/2 and their widths of Er 3+ in two oxide glasses: germanate and silicate, are investigated. In the process, the scalar crystal field strength, homogeneous and inhomogeneous effective widths of levels are analyzed, and the former models and results about optical properties of rare earth ions in glasses are discussed. The cubic crystal field term is found to give the major contribution to the crystal field strength around the rare earth ion when the weak axial distortion completely removes the degeneracy.

Saturation of multiphonon relaxation rates of rare-earth ions in glasses and phonon localization in disordered structures

The effect of temperature on the saturation of the multiphonon relaxation rate is investigated. The relaxation rate is deduced from the decay time of the 3H 4 multiplet of Tm 3+ ions, measured from 10 K up to 550 K for different excitation powers in a tellurite glass. Increasing the temperature, at fixed excitation power, the multiphonon-relaxation rate ( W nr) increases as usual. However, an hysteresis is observed, a discontinuity appears at 350 K due to an irreversible compositional change of the glass as confirmed by thermal analysis. Two domains shall be considered: before (phase I) and after (phase II) the glass modification. In each of these domains, at fixed excitation density, the non-radiative decay rate follows the classical law, the mediating phonon frequency remains unchanged whatever the excited state density. Applying our modified model for the W nr dependence with the excited state density, at fixed temperature, allows to deduce the phonon diffusion length. We find that this critical distance is in good agreement with the glass cohesive length deduced from the boson peak recorded by Raman scattering. The results are discussed in the frame of phonon localization in disordered structures.

Saturation effect on multiphonon relaxation rates of rare earth ions in glasses at high excitation power

An increase of the radiative quantum efficiency with increasing the excitation power is observed for several multiplets of rare earth ions in glasses. The effect of the excited state density on multiphonon relaxation rates is investigated for different rare earth excited states in oxide or fluoride based glasses. The multiphonon relaxation rate follows as usual an exponential gap law with a coefficient depending on the excited state density. A model is proposed to take into account for the saturation of the multiphonon process with increasing the excitation power. This bottleneck effect is related to the specific propagation, in disordered structures, of highly energetic vibrations resulting from the multiphonon process. Application of our model to optical data allows us to derive a phonon diffusion length which is in good agreement with the phonon mean free path estimated from thermal conductivity, heat capacity and sound velocity. Furthermore, the results are compared with the medium range order deduced from the frequency of the boson peak recorded in Raman scattering.

Crystal field analysis of Er 3+-doped glasses: germanate, silicate and ZBLAN

A simple model of average crystal field Hamiltonian based on the principle of descending symmetry and using the group chain scheme is proposed to describe the average of local symmetries occupied by rare earth ions in a disordered medium. Only three crystal field parameters, one for each rank, have to be fitted in this Hamiltonian. Based on the absorption and emission spectra measured in a conventional way at 13 K, the Stark levels in manifolds from 4I 15/2 to 2G(1) 9/2 of Er 3+ in two oxide glasses: germanate and silicate and one fluoride glass: ZBLAN, are investigated. The scalar crystal field strength is analysed, and former models about optical properties of rare earth ions in glasses are discussed. The crystal field strength is decreasing from silicate to germanate and finally to fluoride glass. In the oxide glasses, the rare earth ions can occupy positions with symmetries distorted from D 4, D 4 h and their isomorphic groups. In the fluoride glass, the rare earth ions can occupy positions more freely with symmetries distorted from D 4, D 4 h and also from D 3, D 3 d and their isomorphic groups. In all studied glasses, the cubic crystal field terms give the major contribution to the crystal field strength around the Er 3+ ion when the weak axial distortion removes completely the degeneracy.

Optical transitions of rare earth ions for amplifiers: how the local structure works in glass

Several properties of 4f optical transitions of rare earth ions in glasses utilized as optical amplifiers for telecommunications are discussed. For the Er 3+: 1.55 μm transition, the role of Judd–Ofelt Ω 6 parameters is presented. With compositional change in structure of aluminosilicate glasses, the changes of the radiative cross section, quantum efficiency, and flatness, is observed, which are especially important for the amplifiers in the wavelength-division-multiplexing (WDM) network system. Moreover, the energy level structures and resultant spectral properties of Pr 3+, Nd 3+ and Dy 3+ ions, all of which are 1.3 μm-active ions, are compared. The hypersensitivity of Dy 3+ transitions appears especially in chalcogenide glasses, where the non-radiative loss due to multiphonon emissions is minimized. Compositional variations of the branching ratio and radiative cross sections, and their dependence on the Ω 2 parameters, which are affected by the asymmetry of the rare earth ion sites, are discussed.

An interpretation of the Boson peak in rare-earth ion doped glasses

The Boson peak in the Raman spectra of glasses is an ubiquitous feature. We show that doping a series of fluoride, tellurite and sulfide glass compositions with rare-earth ions, such as Pr 3+, Dy 3+, Nd 3+, Ce 3+, at concentrations between 1000 and 10,000 ppm, increases the amplitude of the Boson peak as compared to the respective undoped glasses. Further addition of rare-earth ions results in saturation and even reduction of the Boson peak ascribed to clustering of dopants and/or devitrification of glass host. Prolonged irradiation of photorefractive sulfide glasses with near-bandgap laser light also results in an increase of the Boson peak. A model is suggested for changes in the intensity and position of the Boson peak with glass composition, doping level, and after prolonged irradiation of photorefractive glasses. We propose that the presence of non-bridging anion atoms, such as F, O or S, (or anion atoms having strained bonds with increased polarizability), determines the Boson peak, as well as the solubility of rare-earth ions in glasses.

Numerical Simulations of the Structure and Spectroscopic Properties of Rare-Earth Doped Glasses

A review of the research that has been devoted to the simulation of the structure and spectroscopic properties of rare-earth doped glasses is presented. Since the seminal papers of Brawer and Weber who have applied Monte-Carlo or molecular dynamics techniques, some other very important results have been reported concerning the local structure of the dopants. As a result, crystal field models have been applied and several pecularities of the optical spectra of rare earth ions in glasses have been understood.

Excitation effects in non-radiative multiphonon decays of rare earth doped laser materials

The non-radiative decay process of rare earth ions in solids, which are known to constitute a whole class of laser materials, has been considered up to now to be independent of excitation intensity. We shall show here that the multiphonon non-radiative decay probabilities of rare earth ions in glasses are reduced at high excitation state densities for the larger energy gaps. The classical exponential energy gap law is shown to ‘rotate’ at higher excitation around the 3.2-phonon point. The observed effect is described in term of a spatial saturation of the accepting mode term with effective diffusion lengths ranging between 54 and 20 Å for excited state density from 10 17 to 10 19 cm −3. This effect which is predicted to be general for multiphonon processes at large gives hints for the obtention of laser effects from otherwise non-radiative transitions provided the excitation density is sufficient to allow the coupling between ions through a common phonon diffusion volume.

Theory of donor fluorescence governed by discrete energy transfers between rare-earth ions in glasses

We have derived the macroscopic response theory of donor fluorescence in rare-earth glasses by calculating a statistical expectation of individual energy transfers from energy donors to energy acceptors. The distribution functions of energy acceptors were classified by their distances of the first-nearest neighbor through the fourth-nearest neighbor to donors. The role of the individual energy transfers was found to be important for the nonlinearity of the donor fluorescence. The experimental results of donor fluorescence of Tb3+ ions were well fitted both to the nonexponential decays caused by pulsed excitation and to the concentration quenching of fluorescence with a coupled parameter set of a critical transfer distance and an energy relaxation rate of the acceptor. The energy transfer depended closely on r-8 for the distance r from Tb3+ to Nd3+ ions or among Tb3+ ions in glasses. In this model the energy transfers from a donor to the first- and the second-nearest-neighbor acceptors was enough to explain the experimental results.

Local structures of rare-earth ions in glasses: the ‘crystal-chemistry’ approach

A ‘crystal-chemistry’ model is proposed to evaluate the rare-earth local structure in glass based on the modified-random-network (MRN) model in glass structure. Three influential factors are summarized, concerning the coordination number, the regularity of the site around the ion, and the spacing between ions. The spectral properties of rare-earth ions in glasses is well described and predicted by the proposed ‘crystal-chemistry’ model. The concentration-quenching effect of rare-earth ions in glasses in terms of ion-ion separation is analyzed by this model in relation to the role as glass-modifier or glass-intermediate of the rare-earth ion in the glass. The model suggests that glasses in which the rare-earth ions act as intermediates are likely to show less concentration-quenching effect.

Theoretical analysis of the fluorescence yield of rare earth ions in glasses containing small metallic particles

We examine the fluorescence yield of rare earth ions in glasses containing a uniform volumetric distribution of metallic particles with small specific volume. The role of energy transfer between the ions and the particles is discussed. Depending on the particle size and the ion emission quantum efficiency, quenching or enhancement of the fluorescence is predicted.

Spectroscopy and nonradiative phenomena of rare earths in glasses: Future applications

The radiative transition probabilities of rare earth ions in glasses can be calculated from the measured intensity parameters and calculated transition matrix elements using the Judd-Ofelt theory. The Judd-Ofelt parameters in oxide, flouride and chalcogenide glasses were obtained for a variety of rare earth ions. These intensity parameters depend on the chemical bonding between the rare earths and the glass matrix and the viscosity of the medium. The non-radiative transition probabilities are calculated using a phenomenological theory, experimentally measured fluorescence lifetimes and quantum efficiencies. The nonradiative transition probabilities show an exponential dependence on the phonon energy of the network-forming material. Experimental results according to this theory show that Er(III) has the highest quantum efficiency in fluoride glasses. Technological implementation of the above theory will be discussed for the lasers, sources for fiber waveguides and luminescent solar concentrators.

Low-Temperature Optical Dephasing of Rare-Earth Ions in Glass

A theoretical description of the temperature-dependent dephasing rate of rare-earth ions in glass is given for temperatures $Tl1$ K. The model invokes elastic dipole-dipole interactions between the rare-earth ions and atomic tunneling systems. The calculation explains features of a recent photon-echo experiment on Si${\mathrm{O}}_{2}$:${\mathrm{Nd}}^{3+}$, namely, the magnitude and ${T}^{1.3}$ dependence of the dephasing rate.