A pulsed EPR study of clustering of Yb ions incorporated in GeO2 glass

The structural aspects of clustering of Yb³⁺ ions and the paramagnetic behavior of these clusters have been investigated in GeO₂ glasses doped with 140-1100 ppm by weight of Yb₂O₃ using time-domain electron paramagnetic resonance (EPR) spectroscopic techniques. The echo-detected EPR (EDEPR) spectra of Yb³⁺ ions and their unusual dependencies on microwave power and magnetic field have been found to be indicative of the formation of clusters of these rare earth ions in GeO₂ glass that behave as non-Kramers type spin systems. The magnetic field and concentration dependence of phase relaxation rates of Yb³⁺ in these glasses further substantiate such a scenario and indicate the formation of clusters of Yb³⁺ ions. A comparison of the EDEPR spectra with calculated cw-EPR line shapes yields a semi-quantitative measure of the typical cluster size of ?3 Yb atoms and intra-cluster Yb-Yb distances of 3.5-4.0 Å in these glasses at doping levels of ?350 ppm of Yb₂O₃.     

The influence of Yb3+ concentration on Yb:YAl3(BO3)4

The Yb:YAl₃(BO₃) ₄ crystals with different Yb³⁺ doping concentration have been grown by the flux method. The lattice parameters and decomposition of the Yb:YAl₃(BO₃)₄ crystal with different Yb³⁺ doping concentration were measured by X‐ray and DTA method. The transmission and fluorescence spectra of Yb³⁺:YAl₃(BO₃)₄ crystal have been measured. The growth defects of Yb_xY_1‐xAl₃(BO₃)₄ crystals were also detected by using the chemical etching method. The results show that the ytterbium concentration influences these properties of Yb:YAl₃(BO₃)₄. As the Yb³⁺ concentration increased, the crystal lattice parameter was decreased. At high doping level, the absorption peak concerned at about 980 nm shift to short wavelength. It is also found that the perfection of Yb:YAl₃(BO₃) ₄ crystal with low Yb³⁺ doping concentration is better than that with high Yb³⁺ concentration. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced 2 μm emission of Yb–Ho doped fluorophosphates glass

In this paper, 2 μm emission spectra of Yb–Ho doped fluorophosphate glass are investigated and compared with Yb–Tm–Ho doped fluorophosphate glass. The 2 μm emission intensity of Yb–Ho doped fluorophosphate glass is much stronger than that of Yb–Tm–Ho doped fluorophosphate glass, exhibiting that Yb–Ho doping is an appropriate doping system to 2 μm application. As the doping concentration of Yb³⁺ ions in Yb–Ho doped fluorophosphate glass increases, the 2 μm emission intensity increases monotonously and possesses a maximum for 10% Yb ions concentration. Therefore, 10% is the optimization of Yb ions doping concentration for 2 μm emission. Otherwise, the up-conversion emission of Ho³⁺ ions is also studied. Combining with the energy transfer processes, the mechanism is discussed.     

Short-wavelength upconversion luminescence of Yb/Tm co-doped glass ceramic containing SrF2 nanocrystals

The preparation process and upconversion luminescence properties of the Yb³⁺ and Tm³⁺ co-doped glass ceramic containing SrF₂ nanocrystals were investigated. In the glass ceramic, the SrF₂ nanocrystals were embedded uniformly in the glass matrix. The Yb³⁺ and Tm³⁺ ions could be enriched into the precipitated SrF₂ nanocrystalline phase, which provide much lower phonon energy than the glass matrix. The glass ceramic exhibited much stronger upconversion luminescence from ultraviolet to visible than the precursor glass. The upconversion luminescence mechanisms were mainly attributed to Yb³⁺-Yb³⁺ cooperative upconversion, Yb³⁺-Tm³⁺ energy transfer and Tm³⁺-Tm³⁺ cross relaxation.     

Effect of Si doping on near-infrared emission and energy transfer of Bismuth in silicate glasses

We have studied the effects of Si doping on the near infrared (NIR) luminescence observed in low Bi doped ( 0.1 mol% ) glasses and the energy transfer from Yb³⁺ to Bi. The broadband near infrared can only be observed when Si is introduced in the Bi-doped glass. The origin of this fluorescence can be attributed to Bi ions at low valence. Efficient energy transfer from Yb³⁺ to Bi NIR active ions is achieved by co-doping of Si. There is an increment of about ~ 29 times of the emission intensity from Bi-related active center as the Yb³⁺ concentration varies from 0 to 2.0 mol% and the amount of Si is 0.05 mol% under 980 nm excitation. The possible mechanism of energy transfer from Yb³⁺ to Bi is also discussed.     

Materials for high-power fiber lasers

The absorption and emission properties of silica based ytterbium doped preforms, made by Modified Chemical Vapor Deposition, and of the drawn fibers were investigated as a function of the atmosphere during the preform collapsing. Under increasingly reducing conditions for the collapsing atmosphere, a strong UV/VIS absorption is built up, accompanied with an UV excited emission in the visible region, obviously induced by the formation of Yb²⁺ ions. Both the fluorescence lifetime of the Yb³⁺ lasing level and the cooperative fluorescence in the visible are remarkably diminished. Laser power experiments with the different fibers show a strong deterioration of the laser efficiency parallel to the degree of reduction of the doped glasses.     

Rare-earth photoluminescence in sol–gel derived confined glass structures

The characteristics of rare-earth luminescence in selected sol–gel derived confined structures have been examined. Erbium and erbium/ytterbium doped photonic materials and structures have been prepared by sol–gel processing, in the form of silicate optical planar waveguides, modified with titania and hafnia, and 1-D photonic bandgap structures consisting of multilayer stacks of silica and titania. The Er³⁺ ions were found to be sensitive probes of the waveguide glass matrix structure, especially when hafnia-containing nanocrystallites were present, which narrowed and resolved different Stark components of the photoluminescence peaks. In 1-D Fabry–Perot coupled microcavities, efficient energy transfer was observed from Yb³⁺ to Er³⁺ ions when these were present simultaneously in the same defect layer, but not when the two types of ions were isolated in separate defect layers.     

Lasing and refractive indices of nanocrystalline ceramics of cubic Yttrium Oxide Y2O3 doped with Nd3+ and Yb3+ ions

A continuous-wave lasing at 1 μm was excited by the radiation of semiconductor laser diodes at room temperature in nanocrystalline ceramics Y₂O₃ doped with Nd³⁺ and Yb³⁺ ions. The refractive indices of the undoped nanocrystalline Y₂O₃ ceramics were measured in the wavelength range 0.4–9 μm.     

Growth and downconversion luminescence of Ho3+/Yb3+ codoped α‐NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho³⁺/Yb³⁺ codoped α‐NaYF₄ single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF₃ system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho³⁺ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb³⁺ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF₄ single crystals of 0.799 mol% Ho³⁺ and 15.15 mol% Yb³⁺. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho³⁺ to Yb³⁺ increases, which may result from the fluorescence quenching between Ho³⁺ and Yb³⁺ ions, Yb³⁺ and Yb³⁺ ions.     

Spectroscopic properties of Yb3+ ions in halogeno-sulfide glasses

In this work, we report the optical properties of Yb³⁺ ions in halogeno-sulfide glasses of composition (75 − x)GeS₂–25Ga₂S₃–xCsCl (x = 5%, 10%, 15%, 20%, and 25% CsCl). This study includes an analysis of the influence of halide concentration on the absorption and emission cross-sections and lifetimes of Yb³⁺ ions. A blue shift of the absorption and emission bands and a decrease of the absorption and emission cross-sections and transition probability are observed as the halide concentration increases in the glass.     

Investigation of the crystallization process in oxyfluoride glass ceramics codoped with Er/Yb

The oxyfluoride glass ceramics are important up-conversion luminescent materials. Er³⁺/Yb³⁺-codoped transparent oxyfluoride glasses were prepared by melt-quenching and subsequently heat-treated at different times and temperatures, and the crystallization process of fluoride nanocrystals from the glass was investigated. X-ray diffraction (XRD) and fluorescence spectra investigations reveal that fluoride nanocrystals are distributed homogeneously among the glassy matrix for the sample doped with Er³⁺/Yb³⁺. The crystallization process indicates that heat-treatment temperature influences the size of fluoride nanocrystals, while heat-treatment time influences their concentration. Moreover, the red emission intensities increase due to the incorporation of Er³⁺/Yb³⁺ into the fluoride nanocrystals under different heat-treatment methods, which are studied by fluorescence spectra.     

Down conversion luminescence of Tb3+–Yb3+ codoped SrF2 precipitated glass ceramics

In the Tb³⁺–Yb³⁺ codoped glass ceramics with SrF₂ nanocrystals precipitated, the energy transfer mechanism from Tb³⁺ to Yb³⁺ was investigated. The excitation power dependence of emission intensity study showed that the quantum cutting occurs during the energy transfer from Tb³⁺ to Yb³⁺ with the excitation of Tb³⁺ high energy level. However, the one-photon process is the main reason that is responsible for the Yb³⁺ infrared emission. The external quantum yields of Tb³⁺ and Yb³⁺ were evaluated by using an integrating sphere measurement system with the excitation of 377 and 488 nm lasers, which are much lower than the expected quantum efficiencies calculated from Tb³⁺ lifetimes. The external quantum yields in the glass ceramics and as-made glasses were also compared.     

Enhanced 2.0 μm emission and energy transfer in Yb3+/Ho3+/Ce3+ triply doped tellurite glass

Ce³⁺ induced enhancement of Ho³⁺ ~ 2.0 μm emission in Yb³⁺/Ho³⁺ codoped sodium–zinc–tellurite (TNZ) glass was achieved under 980 nm LD laser excitation. The spectroscopic studies show that the upconversion is remarkably reduced by the presence of Ce³⁺. The ~ 2.0 μm fluorescence intensity is nearly triply enhanced, and the energy transfer efficiency from Yb³⁺ to Ho³⁺ is improved from 16.1% to 42.6% by increasing the Ce³⁺ concentration from 0 to 0.8 mol%. The mechanism responsible for the upconversion reduction and ~ 2.0 μm emission enhancement in Yb³⁺/Ho³⁺/Ce³⁺ triply-doped TNZ glass is also discussed. Our results indicate that the Yb³⁺/Ho³⁺/Ce³⁺ triply-doped TNZ glass is a promising candidate material for improving the Ho³⁺ 2.0 μm fiber laser performance.     

Radiation damage of silicate glasses doped with Tb and Eu

In this work, we studied a set of Tb³⁺ (or Eu³⁺) doped silicate glasses in which some amounts of BaO were added to increase glass density. The irradiation-induced damage was investigated by absorbance measurements performed before and after each irradiation with doses ranging from 3 to 237 Gy. Analysed glasses underwent also light yield measurements investigated in terms of light production. The results showed that radiation damage and light yield depend on glass composition and are very low for the Eu³⁺ containing glass and for the Tb³⁺ activated glass which contains also lead. A possible explanation could be that lead and europium favour in the glass matrix the formation of a higher concentration of defects with respect to Tb³⁺ doping ions.     

The spectrum and laser properties of ytterbium doped phosphate glass at low temperature

Low-temperature absorption and fluorescence spectra of the Yb³⁺ ions were measured in phosphate glass with compositions of (60-65)P₂O₅-(4-8)B₂O₃-(5-10)Al₂O₃-(10-15)K₂O-(5-10)BaO-(0-2)La₂O₃-(0-2)Nb₂O₅-(4-8)Yb₂O₃ (mol%). Temperature dependence of lifetime of Yb³⁺:²F_5/2 level was investigated. Laser performance of sample pumped by 940 nm laser diode at low temperature were presented. At 8 K, laser oscillation of diode pumped Yb³⁺: phosphate glass yielded a slope efficiency of 4% and a maximum power of 2 mW, the peak laser wavelength is 1001 nm.     

Visible to near-infrared down-conversion luminescence in Tb3 + and Yb3 + co-doped lithium–lanthanum–aluminosilicate oxyfluoride glass and glass-ceramics

Tb^3 + and Yb^3 + co-doped oxyfluoride glasses were fabricated in a lithium–lanthanum–aluminosilicate matrix (LLAS) by a melt-quench technique. Glass-ceramics were obtained by appropriate heat treatment of the as-prepared glasses. Visible to near-infrared down-conversion luminescence was studied for glass and glass-ceramic samples with different Yb^3 + concentrations. It has been found that the luminescence intensity at 940–1020 nm from Yb^3 + ions increases while the emission lifetime of Tb^3 + ions decreases in the glass-ceramic compared to that in the as-prepared glass, which indicates that the energy transfer efficiency increases in the glass-ceramics compared to that in the as-prepared glass. The down-conversion luminescence also increased for increasing Yb^3 + concentration from 1 mol% to 2 mol%, but decreased for the sample with a high Yb^3 + co-doping concentration of 8 mol%, which is attributed to the concentration quenching.     

Investigation of infrared emission and lifetime in Tm-doped 75TeO2:20ZnO:5Na2O (mol%) glasses: Effect of Ho and Yb co-doping

In this paper we describe fabrication and characterization of rare-earth-doped active tellurite glasses to be used as active laser media for fiber lasers emitting in the 2 μm region. The base composition is (mol%): 75TeO₂-20ZnO-5Na₂O with different concentrations of Tm³⁺, Yb³⁺ and Ho³⁺ as dopants or co-dopants. Optical properties of doped glasses were studied and pumping at 800 nm and at 980 nm were tested in order to compare the efficiency of two pumping mechanisms. Optical characterization carried out on glasses containing only Tm³⁺ ions indicated the optimum concentration of Tm₂O₃ in terms of emission efficiency as 1 wt%. The addition of 5 wt% of Yb₂O₃ to Tm³⁺-doped glasses led to the best results in terms of intensity of fluorescence emission and of lifetime values. Yb and Ho co-doped Tm-tellurite glass was measured in emission.     

Spectroscopic properties of Er-Yb co-doped glass ceramics containing BaF2 nanocrystals

A Er³⁺ and Yb³⁺ co-doped transparent oxyfluoride glass ceramic containing BaF₂ nanocrystals has been prepared. The formation of BaF₂ nanocrystals in the glass ceramic was confirmed by X-ray diffraction. Intense upconversion luminescence in the Er³⁺ and Yb³⁺ co-doped glass ceramic could be observed. Stark splitting of the Er³⁺ upconversion luminescence peaks in the glass ceramic indicated that Er³⁺ and Yb³⁺ had been incorporated into the BaF₂ nanocrystals. Near infrared luminescence decay curves showed that the Er³⁺ and Yb³⁺ co-doped glass ceramic had higher luminescence efficiency than the precursor glass.     

Luminescence and defects of Yb3+-doped sol–gel silica glasses

The optical properties of Yb-doped sol–gel silica glasses were studied by optical absorption and radio-luminescence (RL) measurements, that revealed the typical absorption and emission pattern of Yb³⁺ ions. Moreover, RL bands in the 1.5–3.5 eV interval were also observed, and related to defects of the silica matrix. The RL intensity temperature dependence, investigated in the 10–320 K interval, evidenced the presence of the SiO₂ self-trapped exciton emission at 2.2 eV, whose intensity was rapidly quenched by temperature increasing. At variance, the Yb³⁺ emission intensity increased markedly by temperature increasing. This phenomenon is interpreted by considering a competitive role of point defects in free carrier trapping, evidenced by parallel wavelength resolved thermally stimulated luminescence measurements.     

Spectroscopic studies of lead arsenate glasses doped with nickel oxide

Lead arsenate glasses containing different concentrations of NiO ranging from 0 to 1 mol% (in steps of 0.2 mol%) were prepared. The samples were characterized by X-ray diffraction and differential thermal analysis. A number of studies viz., optical absorption, thermoluminescence, magnetic susceptibilities and IR spectra, have been carried out on these glasses. The bands observed in the optical absorption spectra of the glasses have been analyzed using Tanabe-Sugano diagrams for d⁸ ion; the analysis indicates the presence of Ni²⁺ ions in both tetrahedral and octahedral positions. The increase in the concentration of NiO in the glass matrix shows a gradual transformation of nickel ions from tetrahedral positions to octahedral positions in the glass network. The thermoluminescence light output of the X-ray irradiated glasses has exhibited a glow peak at 353 K (with increasing intensity with an increase in the concentration of NiO) in addition to the conventional peak due to the recombination of electron-hole centers; this glow peak is identified due to ³T₂ → ³A₂ emission transition of octahedral Ni²⁺ ions. The value of the magnetic moment evaluated from the measured magnetic susceptibility show a decreasing trend from 3.84 to 3.10 μ_B with the increase in the concentration of NiO. All these studies indicate an increasing presence of octahedral (lasing) Ni²⁺ ions in these glasses with the increase in the concentration of NiO.