Clustering Of Rare-earth Ions Research Articles

Enhanced 3 μm emission in Ho3+/Yb3+ co-doped bismuth-fluoride glasses by Gd2O3

In this work, (100-b)(45Bi2O3-40GeO2-15BaF2)-bGd2O3-2Yb2O3-0.5Ho2O3 glass was prepared to use a high-temperature melting method(where b= 0, 2.5, 5, 7.5, 10). The absorption and fluorescence spectra indicate that the optimal Gd2O3 content is 7.5 mol% (BGBG7.5HY), with an Ω2 value of 6.20×10−20 cm2 for the main glass, which is superior to that of BGBG0HY glass. This suggests that the incorporation of Gd2O3 enhances the mid-infrared emission capacity. Raman spectroscopy studies demonstrate that the introduction of Gd₂O₃ results in a substantial influx of oxygen atoms into the glass network, accompanied by a transition of [BiO₆] to [BiO₃] within the glass mesh. This is evidenced by a notable redshift of the characteristic peaks at 509 cm−1 and 677 cm−1 in comparison to BGB glass. The introduction of Gd2O3 also expands the distance between Ho3+/Yb3+ ions, inhibits rare earth ion clusters, improves the Ho3+ luminescence center environment, and thus enhances the 3 μm emission performance of bismuth fluoride glass. The maximum emission cross section is 1.82 × 10−20 cm2 and the maximum gain coefficient is 3.71 cm−1. The results show that the bismuth fluoride glass prepared in this paper is an excellent gain material for mid-infrared fiber lasers.

Anti-Stokes cooling in highly ytterbium doped phase separated aluminium-yttrium oxide glass by 4 K

Silica glass is a strong candidate in the development of radiation balanced fiber lasers and amplifiers due to its properties such as high stability, high mechanical robustness and chemical durability, thermal resistance, high purity and so on. However, low rare-earth ion solubility lead to inter-ionic energy transfers processes which cause multi-phonon relaxation through a non-radiative decay process and hinder it from being used for aforementioned applications as well as for optical cooling. In the present work, we report cooling to a record low temperature of −4 K from room temperature at atmospheric pressure in a novel silica based highly Yb doped phase separated oxide glass (named as PS: GAYY glass - aluminium: yttrium: ytterbium) using 20 W of 1037 nm pump wavelength. We further theoretically show a global minimum achievable temperature of ∼165 K in our glass. Here, the basic limitation for laser induced cooling in silica glass associated with the existence of clustering of rare earth ions is significantly mitigated. This was achieved by the addition of suitable co-dopants such as aluminum and yttrium which modified the local rare-earth ion environment to avoid clustering as well as reducing the phonon energy. The fabricated glass using modified chemical vapor deposition process in combination with a solution doping technique achieved a maximum doping concentration of 5.57 × 1026 ions/m3 and exhibits better performance than other silica-based host glasses in terms of both optical properties and laser cooling characteristics. We believe PS-GAYY glasses may open new avenues in the field of solid-state laser cooling, promising exciting and broad applications in radiation balanced lasers and amplifiers.

Early-stage clustering in Tb-doped borosilicate glasses revealed by high-resolution scanning transmission electron microscopy and luminescence properties

Glasses with the base composition 60 SiO2·37 B2O3·3 Na2O (in mol%) were doped with different concentrations of terbium oxide. These glasses show phase separation in borate-rich droplets surrounded by a silicate matrix. By means of scanning transmission electron microscopic analyses including energy dispersive X-ray spectroscopy, structural changes during phase separation are investigated, focusing mainly on the local sites of the luminescent ions. Moreover, phase separation structures of as cast and heat-treated glasses are characterized regarding droplet shape and size, as well as size distribution. The luminescence properties of the phase separated glasses are correlated with their nano- and microstructure. Already in the glass with the lowest Tb3+ concentration, the local environments in which the Tb3+ ions reside in could be identified. This early clustering of the luminescent ions remains undetected in fluorescence lifetime measurements. Only in the time-resolved fluorescence measurement of the glass with the highest Tb3+ concentration, which contains very large Tb-rich agglomerates, an indication of clustering is found. Since clustering of rare earth ions can be detrimental to the performance of an optical material, early detection is beneficial. The studies show that high-resolution transmission electron microscopy can assist with this concern.

Indirect excitation and luminescence activation of Tb doped indium tin oxide and its impact on the host’s optical and electrical properties

The effect of adding terbium to indium tin oxide (ITO) thin films on the electrical, optical and light emission properties was investigated. The films were prepared by radio frequency dual magnetron sputtering, maintaining a high optical transmittance in the ultraviolet and visible spectral regions, and a low electrical resistivity ranging from to . Terbium-related luminescence is achieved after annealing at 470 ∘C in air at atmospheric pressure. Electrical resistivity and optical transmittance were measured after each annealing step to evaluate the compromise between the achieved light emission intensity, electrical and optical properties. Additionally, temperature dependence of Tb-related luminescence quenching was assessed by temperature-dependent photoluminescence measurements, from 83 to 533 K, under non-resonant excitation. Thermal quenching activation energies suggest an effective energy transfer mechanism from the ITO host to the rare-earth (RE) ions. This indirect excitation mechanism was modeled using a spherical potential-well and a tight-binding one-band approximation approaches, describing a short-range charge trapping process and subsequent formation of bound excitons to RE ion clusters.

Relationship between Torsion and Anisotropic Exchange Coupling in a Tb(III)-Radical-Based Single-Molecule Magnet.

The incorporation of paramagnetic ligands within rare-earth ion clusters exhibiting large magnetic anisotropy has provided significant advancement in the design of single-molecule magnets (SMMs) with large blocking temperatures. However, the exchange interaction in such systems is complex and difficult to probe by conventional magnetometry techniques, and little is known about the structural relationships. Inelastic neutron scattering and terahertz electron paramagnetic resonance measurements are used complimentarily to investigate the large exchange interaction between a rare earth-radical pair in a Tb(III)-based SMM complex. The origin of the exchange interaction is investigated for two molecular species in the crystallographic unit cell that exhibit different bonding structures between Tb(III) and a 2pyNO radical. A correlation between the Tb-O-N-C torsion angles and the magnitudes of exchange couplings is found. Interestingly, a large nondegeneracy within the ground-state doublet is present for the larger torsion angle species. It is essential to consider the balance of two channels of exchange coupling, 2p-4f hybridization and 2p-5d charge transfer, to explain this characteristic behavior. The former channel gives the antiferromagnetic interaction, and the latter gives the ferromagnetic one. When an effective Ĵ = (1)/2 Ising-type Hamiltonian is applied, the exchange couplings are evaluated to be antiferromagnetic J(z) = 9.89 meV (79.8 cm(-1)) for the low torsion angle (3.8°) species and J(z) = 7.39 meV (59.6 cm(-1)) for the larger torsion angle (15.8°) species. It is also found that a small percentage of the transverse exchange component must be included for the larger torsion angle to account for the observed nondegenerate ground state. The symmetry of the exchange couplings is discussed by considering the characters of d and f orbitals.

Theoretical calculations of g factors for the tetragonal Ce3+ centers in AWO4 (A=Ca, Sr, Pb) and A′MoO4 (A′=Sr, Pb) crystals

The EPR g factors g// and g⊥ for the tetragonal Ce3+ centers in AWO4 (A=Ca, Sr, Pb) and A′MoO4 (A′=Sr, Pb) crystals with scheelite-type structure are calculated in terms of the complete diagonalization (of energy matrix) method. In the matrix, the crystal field parameters Bkq are estimated from the superposition model where the intrinsic parameters A¯k(R0) are taken as those obtained in very similar (CeO8)13− clusters in MPO4 (M=Y, Lu) crystals. The calculated g-values are in reasonable agreement with the experimental data when both covalence of the rare earth ion clusters, reflected in the presence of an orbit reduction factor k<1, and the local deformation due to the difference between the Ce3+ and host lattice cation radii are considered.

Rare-earth ion clusters in doped crystals with fluorite structure

In BaF2 crystals doped with paramagnetic Yb3+ ions, at concentrations higher than 0.01 at %, an EPR line was observed corresponding to a linear cluster Yb-F-Yb-F oriented along a threefold axis.

Direct spectroscopic observation of the atomic-scale mechanisms of clustering and homogenization of rare-earth dopant ions in vitreous silica

Structural aspects of clustering of rare earth ions in oxide glasses have been studied for the last several years in relation to their applications in photonics. However, the mechanism of homogenization of the spatial distribution of rare earth ions by codoping, typically with Al or P, is still not well understood. In this work we report direct experimental determination of the homogenization mechanism of ${\mathrm{Yb}}^{3+}$ ion clusters in silica glasses doped with $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{wt}.\phantom{\rule{0.2em}{0ex}}%$ ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ and codoped with Al or P, using two-dimensional HYSCORE-EPR spectroscopy. The results lead us to conclude that Yb creates its coordination environment via formation of $\mathrm{Yb}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Si}$ and $\mathrm{Yb}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Yb}$ bonds in a Yb-doped silica glass and even the light codoping with Al starts replacing these bonds with $\mathrm{Yb}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Al}$ linkages. Heavy codoping with P replaces all $\mathrm{Yb}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Si}∕\mathrm{Yb}$ linkages with $\mathrm{Yb}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{P}$ linkages. The formation of a next-nearest neighbor shell of Al or P creates suitable structural pockets, which ultimately leads to homogenization.

Direct observation ofR…Rdistances in rare-earth(R)phosphate glasses by magnetic difference neutron diffraction

A magnetic difference neutron diffraction study of a rare-earth (Tb) phosphate glass has revealed exclusively the Tb...Tb distances. The difference between data taken with and without an applied magnetic field of 4 T shows Tb...Tb pairwise atomic correlations at 3.9 and 6.4 A, respectively, with relative coordination numbers of 1:14. The first distance arises when two Tb3+ ions share a common oxygen neighbor, and indicates a clustering of rare-earth ions. The second distance arises when two Tb3+ ions are coordinated to different oxygens in the same PO4 group, in a near-linear arrangement.

EPR of rare-earth ion clusters in mixed crystals Ba1−x La x F2+x doped with Yb3+ Ion

Electron paramagnetic resonance (EPR) spectra of Yb3+ ions in the mixed crystals (BaF2)1−x X(LaF3) x withx≈0.01 have been investigated by the electron spin echo method at a frequency of 94.9 GHz and compared with the spectra obtained by the continuous-wave EPR at a frequency of 9.5 GHz. Some spectral lines were identified within the framework of the simple cluster model proposed in our previous work (L. K. Aminov, R. Yu. Abdulsabirov, M. R. Gafurov, S. L. Korableva, I. N. Kurkin, S. P. Kurzin, R. M. Rakhmatullin, A. G. Ziganshin: Appl. Magn. Reson. 28, 41–53, 2005). Clusters consist of trigonal dipoles La−F and Yb−F (a rare-earth ion with a charge compensator) and are formed beginning with small concentrationsx≈0.001. The lines identified in the present work were attributed to the clusters with configurations Yb−F−La−F and La−F−Yb−F−La−F.

Structure of dysprosium and holmium phosphate glasses by the method of isomorphicsubstitution in neutron diffraction

The relative distribution of rare-earth ionsR3+ (Dy3+ orHo3+) in thephosphate glass RAl0.30P3.05O9.62 was measured by employing the method of isomorphic substitution in neutron diffractionand, by taking the role of Al into explicit account, a self-consistent model of the glassstructure was developed. The glass network is found to be made from corner sharingPO4 tetrahedra in which there are, on average, 2.32(9) terminal oxygen atoms,OT, at 1.50(1) Å and 1.68(9) bridging oxygen atoms,OB, at 1.60(1) Å. The networkmodifying R3+ ions bindto an average of 6.7(1) OT and are distributed such that 7.9(7) R–R nearest neighbours reside at 5.62(6) Å. TheAl3+ ionalso has a network modifying role in which it helps to strengthen the glass through the formation ofOT–Al–OT linkages. The connectivity of the R-centred coordination polyhedra in(M2O3)x(P2O5)1−x glasses, whereM3+ denotes a networkmodifying cation (R3+ or Al3+), is quantified interms of a parameter fs. Methods for reducing the clustering of rare-earth ions inthese materials are then discussed, based on a reduction offs via thereplacement of R3+ by Al3+ at fixed total modifier content or via a change ofx to increase thenumber of OT availableper network modifying M3+ cation.

X-Ray Absorption and Diffraction Studies of Pr3+, Tb3+ and Er3+-Activated Silica Gels

Rare-earth (Pr3+, Tb3+, Er3+) doped silica xerogels were studied by x-ray absorption spectroscopy and x-ray diffraction. A change of the local environment around rare-earth ions upon xerogel densification at 900–950°C and co-doping with aluminum ions was determined from the rare-earths L3-edge EXAFS signals. The densification process induces a decrease of the coordination number and a compression and deformation of the first coordination shell, composed of oxygen atoms. The second coordination shell, composed of silicon and/or aluminum ions, also experiences some modification, which is attributed mainly to a shortening of the shell radius. No evidence of clustering of rare-earth ions upon densification was observed. X-ray diffraction data on Tb-doped gels confirm the EXAFS results.

Fluorescence line narrowing and decay dynamics in sol–gel glasses containing Eu 3+

Fluorescence line narrowing and emission decay measurements on Eu 3+ doped sol–gel glasses have been used to investigate and compare local Eu 3+ environments in binary SiO 2–TiO 2 glasses, SiO 2 glasses containing Al 3+, and SiO 2 glasses without co-dopant. In particular, clustering of rare-earth ions and the effects of annealing temperature on residual hydroxide concentration are studied. The Al and Ti co-doped samples show pronounced line-narrowing implying little energy migration among Eu 3+ ions. The 5D 0– 7F J fluorescence decays are faster in the Ti glasses.

Towards rare-earth clustering control in doped glasses

In this paper, the basic limitation for optical amplifiers linked with the existence of the so-called clustering of rare-earth (RE) ions is addressed. Clusters of RE dopant are related both to what could be called “interaction clusters” and to “chemical clusters”. Such interaction clusters do not depend on the chemical processes of the glass preparation and cannot be reduced through it. However, the existence of chemical clusters of spatial extension of a few Å can tremendously increase ion–ion interactions of all kinds. The physical process of co-operative luminescence is closely related to chemical clustering. Using this effect with Yb3+ as a probing ion, clear signatures of RE clusters can be obtained for different glass compositions and RE concentrations. Developing this approach, a new doping principle is proposed. It is based on the recognition that introducing the doping RE into the glass composition through given precursor crystals with large RE–RE distances, allows some control of the clustering of RE ions and of their self-quenching. So even after complete melting of the precursor and independently of the final glass composition the glass is shown to keep some memory of the doping procedure.

Rare-Earth Clustering and Aluminum Codoping in Sol-Gel Silica: Investigation Using Europium(III) Fluorescence Spectroscopy

Eu{sup 3+} fluorescence spectroscopy is used to investigate the clustering of rare-earth ions and the effectiveness of aluminum codoping in dispersing and isolating rare-earth ions in sol-gel silica monoliths. Fluorescence line-narrowing (FLN) studies are demonstrated as a useful tool in identifying clustered and isolated Eu{sup 3+}. Clustered Eu{sup 3+} is identified by the lack of a line-narrowing effect, which is attributed to energy transfer between adjacent Eu{sup 3+} ions. Clustering is shown to be significant, even in transparent samples with Eu{sup 3+} concentrations as low as 0.5 wt% Eu{sup 3+}. Addition of Al{sup 3+} as a codopant has a profound impact on the bonding and structure of Eu{sup 3+} in silica. Significant fluorescence line narrowing is observed, which suggests that Al{sup 3+} codoping is effective in dispersing and isolating Eu{sup 3+} ions in the silica matrix. Fluorescence decay studies provide evidence of increased Eu{sup 3+} hydroxylation in the Al{sup 3+}-containing samples. 28 refs., 4 figs., 1 tab.

Fluorescence Line Narrowing Studies of Eu3+-Doped Silica Gel-Glasses

ABSTRACTBroadband fluorescence and fluorescence line narrowing (FLN) spectra are presented for Eu3+ ions in Eu2O3-SiO3 and Eu2O3-Al2O3-SiO3 systems prepared by the sol-gel process. Clustering of rare earth ions in silica glass can be a limiting factor in the design and engineering of optical device materials. The addition of Al2O3 is known to help disperse rare earth ions in sol-gel silica. In order to develop a more detailed understanding of how Eu3+ is incorporated in these materials, time-resolved, laser-induced FLN studies at 77 K have been performed using a pulsed dye laser to selectively excite subsets of ions in the inhomogeneously broadened 7F0⇆ 5D0 manifold. Fluorescence spectra are discussed in terms of rare earth ion clustering, energy transfer, and bonding site symmetry. It is concluded that Eu3+ ions cluster in sol-gel silica and that the addition of alumina assists in dispersing Eu3+ ions.