The microstructure of erbium-ytterbium co-doped oxyfluoride glass-ceramic optical fibers

Oxyfluoride transparent glass-ceramics combine some features of glasses (easier shaping or lower than single crystals cost of fabrication) and some advantages of rare-earth doped single crystals (narrow absorption/emission lines and longer lifetimes of luminescent levels). Since the material seems to be promising candidate for efficient fiber amplifiers, the manufacturing as well as structural and optical examination of the oxyfluoride glass-ceramic fibers doped with rare-earth ions seems to be a serious challenge. In the first stage oxyfluoride glasses of the following compositions 48SiO₂-11Al₂O₃-7Na₂CO₃-10CaO-10PbO-11PbF₂-3ErF₃ and 48SiO₂-11Al₂O₃-7Na₂CO₃-10CaO-10PbO-10PbF₂-3YbF₃-1ErF₃ (in molar%) were fabricated from high purity commercial chemicals (Sigma-Aldrich). The fabricated glass preforms were drawn into glass fibers using the mini-tower. Finally, the transparent Er³⁺ doped and Er³⁺/Yb³⁺ co-doped oxyfluoride glass-ceramic fibers were obtained by controlled heat treatment of glass fibers. The preceding differential thermal analysis (DTA) studies allowed estimating both the fiber drawing temperature and the controlled crystallization temperature of glass fibers. X-ray diffraction examination (XRD) at each stage of the glass-ceramic fibers fabrication confirmed the undesirable crystallization of preforms and glass fibers has been avoided. The fibers shown their mixed amorphous-crystalline microstructure with nano-crystals of size even below 10 nm distributed in the glassy host. The crystal structure of the grown nano-crystals has been determined by XRD and confirmed by electron diffraction (SAED). Results obtained by both techniques seem to be compatible: Er₃FO₁₀Si₃ (monoclinic; ICSD 92512), Pb₅Al₃F₁₉ (triclinic; ICSD 91325) and Er₄F₂O₁₁Si₃ (triclinic; ICSD 51510) against to initially expected PbF₂ crystals.     

Effect of heat-treatment on luminescence and structure of Ag nanoclusters doped oxyfluoride glasses and implication for fiber drawing

The effect of heat treatment on the structure and luminescence of Ag nanoclusters doped oxyfluoride glasses was studied and the implication for drawing the corresponding fibers doped with luminescent Ag nanoclusters has been proposed. The heat treatment results, first, in condensation of the Ag nanoclusters into larger Ag nanoparticles and loss of Ag luminescence, and further heat treatment results in precipitation of a luminescent-loss nano- and microcrystalline Ag phases onto the surface of the glass. Thus, the oxyfluoride fiber doped with luminescent Ag nanoclusters was pulled from the viscous glass melt and its attenuation loss was 0.19 dB/cm in the red part of the spectrum; i.e. near to the maximum of Ag nanoclusters luminescence band. The nucleation centers for the Ag nanoclusters in oxyfluoride glasses have been suggested to be the fluorine vacancies and their nanoclusters.     

Structure and luminescence properties of Eu/Tb codoped oxyfluoride glass ceramics containing Sr2GdF7 nanocrystals

Eu/Tb codoped transparent oxyfluoride borosilicate glass ceramics containing Sr₂GdF₇ nanocrystals were fabricated under a reductive atmosphere and the conversion of Eu³⁺ ions to Eu²⁺ ions was observed. The Sr₂GdF₇ nanocrystals with an average size of 32 nm were homogeneously precipitated in the oxyfluoride borosilicate glass matrix, which could be evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. The enhancement of photoluminescence emission intensity, reduction of the relative emission intensities between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁, and long fluorescence lifetimes of Eu²⁺, Eu³⁺, and Tb³⁺ ions revealed that more rare earth ions were partitioned into the low phonon energy environment Sr₂GdF₇ nanocrystals. Under ultraviolet excitation, pure and bright white light emission was obtained in the oxyfluoride borosilicate glass ceramic, which may be a potential blue, green and red-emitting phosphor for white LEDs.     

The influence of erbium doping of Al-N sputtered coatings on their optical properties

Room-temperature photoluminescence (PL) measurements were performed on erbium-doped AlN thin films deposited by r.f. reactive magnetron sputtering on steel and amorphous quartz substrates. The Al/(N+Al) ratio in the as-deposited films varies in the range from 0.48 to 0.51, while erbium content ranges from 0.2 to 3.5 at.%. X-ray diffraction showed that the AlN(Er) films with erbium contents up to 1.4 at.% have the hexagonal AlN crystalline structure and a preferential orientation following the [0 0 1] direction. However, as a consequence of lattice distortion and/or defect generation originated by the incorporation of erbium atoms in the AlN structure, the films became amorphous for higher erbium contents. All the as-deposited films exhibited room-temperature PL at 1.54 μm, indicating that at least part of the erbium species are optically active in the as-grown samples. Subsequent annealing of the films led to an intensification of the PL signal up to a factor of 6 for the films heat-treated at 1075 K. Within the range of chemical compositions studied in this work, the PL signal increased with increasing erbium content. A weak green visible PL was also detected for the samples doped with the highest erbium contents when annealed at high temperatures.     

Influence of erbium doping on phase transition and optical properties of strontium barium niobate

The optical properties of erbium impurities in strontium barium niobate are investigated measuring optical absorption and emission in the visible and near infrared spectral region. For the main fluorescence band at 1.55 μm, an anomalous dependence of the fluorescence decay time on dopant concentration is found which, however, can be consistently explained by reabsorption effects. A Judd–Ofelt analysis of the absorption spectra together with an appropriate analysis of the reabsorption yields a radiative quantum efficiency of approximately 60%. In addition, erbium dopants are shown to efficiently influence the phase transition temperature of strontium barium niobate.     

Thermal and optical properties of tellurite glasses doped erbium

Er³⁺-doped tellurite glasses with molar compositions of 75TeO₂–20ZnO–(5 − x) Na₂O–xEr₂O₃ (x = 0, 0.5, 1, 2, 3, and 4 mol%) have been elaborated from the melt-quenching method. The effects of Er₂O₃ concentration on the thermal stability and optical properties of tellurite glasses have been discussed. From the differential scanning calorimetry (DSC) profile, the glass transition temperature T _g, and crystallization onset temperature T _x are estimated. The thermal stability factor, defined as ∆T = T _x − T _g, was higher than 100 °C. It suggests that tellurite glass exhibits a good thermal stability and consequently is suitable to be a potential candidate for fiber drawing. Furthermore, the stability factor increases with Er₂O₃ concentration up to 2 mol% then presents a continue decrease suggesting of beginning of crystallization of highly doped tellurite glasses. The refractive index and extinction coefficient data were obtained by analyzing the experimental spectra of tanΨ and cos∆ measured by spectroscopic ellipsometry (SE). The complex dielectric functions (ε = ε₁ + iε₂) of the samples were estimated from regression analysis. The fundamental absorption edge has been identified from the optical absorption spectra and was analyzed in terms of the theory proposed by Davis and Mott. The values of optical band gap for direct and indirect allowed transitions have been determined. An important decrease of the optical band gap was found after Er doping. It was assigned to structural changes induced from the formation of non-bridging oxygen. The absorption coefficient just below the absorption edge varies exponentially with photon energy indicating the presence of Urbach’s tail. The origin of the Urbach energy is associated with the phonon-assisted indirect transitions.     

Influence of W5+ ions on the optical properties of doped Bi12TiO20

There have been studied single crystals of undoped and doped Bi₁₂TiO₂₀ with two concentrations of W⁵⁺ (2.62 × 10¹⁷ cm⁻³ and 2.62 × 10¹⁸ cm⁻³). There have been obtained absorption spectra in the energy range of 10,482–15,408 cm⁻¹ by classical measurements. There have been determined the cross-section (σ_a) of the impurity absorption and the oscillator strength of d–d transitions. There have been calculated the refractive index of doped crystals and the concentration of Ti³⁺ ions in an undoped sample through an experiment.     

The influence of CsBr addition on optical and thermal properties of GeGaS glasses doped with erbium

The influence of the addition of CsBr on the optical and thermal properties of GeGaS glass doped with Er has been being investigated. We find that the addition of CsBr into GeGaS glass leads to some improvements in the radiative properties of Er³⁺ ions in this glass system. The GeGaS–CsBr glasses demonstrate much longer radiative lifetimes and stronger photoluminescence for ⁴I_13/2 ?⁴I_15/2^4\hbox{I}_{13/2} \rightarrow{^4\hbox{I}}_{15/2} and ²H_11/2 ?⁴I_15/2 ^2\hbox{H}_{11/2} \rightarrow{^4\hbox{I}}_{15/2} transitions in Er³⁺ ions. Very low Judd-Ofelt parameters and sharp Er³⁺ absorption spectra with multiple peaks suggest the presence of microcrystals. All optical parameters are strongly influenced by the interplay of Ga and CsBr. The glasses studied have been characterized by their basic glass transformation temperatures. The addition of CsBr into GeGaS glass keeping the ratio between CsBr and Ga close to unity is favorable for the incorporation of larger amount of Er. The experimental results are discussed in terms of structural local arrangement induced by CsBr addition.     

Optical and spectroscopic properties of soda lime alumino-silicate glasses doped with erbium and silver

Spectroscopic properties of Ag/Er co-doped soda lime silicate glasses have been studied with the aim of assessing the effective role of silver as a sensitizer for erbium. Changes in spectroscopic properties of Er³⁺ as a function of silver addition to the base composition have been measured. Transmission electron microscopy (TEM), absorption as well as photoluminescence measurements in the visible and infrared spectral region, particularly ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ion were performed; excitation wavelengths in the range from 325 to 808 nm were used. Enhancement of the Er³⁺ luminescence at 1.54 μm was observed when Ag was added.     

Cerium doped soda-lime-silicate glasses: effects of silver ion-exchange on optical properties

Effects of silver ion-exchange on optical absorption (OA) and photoluminescence (PL) spectra of a cerium doped soda-lime-silicate glass at room temperature are investigated. The optical spectra are described in terms of the characteristic transitions 4f↔5d originated in Ce³⁺ ions placed mainly in two different sites of the glass network. As Ag⁺ ions are introduced into the cerium doped glass, they are reduced to elementary silver (Ag⁰) which are favoured by the reaction Ce³⁺+Ag⁺→Ce⁴⁺+Ag⁰. Then, the number of Ce³⁺ ions decrease inversely with depth from the surface contrarily to Ce⁴⁺ ions does, and elementary silver diffuses and aggregates to form nanoparticles. As a consequence of these changes, the OA spectra of exchanged samples increase substantially in the UV range and the luminescence decreases significantly. The high sensitivity of PL together with deconvolution analysis of spectra, however, allows us to detect changes in the excitation and emission spectra from the earlier stages of ion-exchange. This indicates that during the ion-exchange we deal with fast processes (much shorter than 1 min). In fact, transmission electron microscopy observations of samples from the glass exchanged for a short time as 1 min at 325°C show the presence of a scanty number of silver nanoparticles, which confirms this point. Furthermore, with increasing the length of time of ion-exchange, PL spectra exhibit a progressive red shift indicative in part of a covalence increment in the oxygen–cerium coordinated bonding. We observe no luminescence from Ag⁺ ions and other silver molecular species in contrast with other preliminary PL studies on silver ion-exchange in soda-lime-silicate glasses free of cerium. The effect is discussed on the basis of a supplementary increase in the number of Ce⁴⁺ ions mainly due to the reaction Ce³⁺+Ag⁺→Ce⁴⁺+Ag⁰, which prevents efficiently the luminescence of the silver centers.     

Effects of F− on the optical and spectroscopic properties of Yb3+/Al3+-co-doped silica glass

Yb³⁺/Al³⁺-co-doped silica glasses with different F⁻ content were prepared in this work by sol–gel method combined with high temperature sintering. XRF, FTIR and XPS methods were used to confirm the presence of F⁻. The effects of F⁻ on the optical and spectroscopic properties of these glasses have been investigated. It is worth to notice that the F⁻/Si⁴⁺ mass ratio equal to 9% is a significant value showing a real change in the variation trends of numerous following parameters: refractive index, UV absorption edge, absorption and emission cross sections, scalar crystal-field N_J and fluorescent lifetimes. Furthermore, introduction of F⁻ can adjust the refractive index of Yb³⁺/Al³⁺-co-doped silica glass and it is useful for large mode area (LMA) fibers.     

Morphology of CaF2 nanocrystals and elastic properties in transparent oxyfluoride crystallized glasses

An oxyfluoride glass with the composition of 25CaF₂-5CaO-20Al₂O₃-50SiO₂ (mol%) and crystallized glasses containing CaF₂ nanocrystals (10-70 nm) are fabricated. The size and morphology of CaF₂ nanocrystals is examined using transmission electron microscopy and atomic force microscopy (AFM), and elastic properties of crystallized glasses are evaluated using a cube resonance method. The large increase in the glass transition temperature in crystallized glasses suggests that the Al₂O₃-SiO₂ based glass network having a high thermal stability is created due to the formation of CaF₂ nanocrystals. It is suggested from AFM observations that the chemical bonding between CaF₂ nanocrystals and oxide glass matrix is weak. Young’s modulus (E) increases with increasing heat treatment temperature, i.e., E = 88.4 GPa for the glass and E = 93.3 GPa for the sample heat-treated at 700 °C for 1 h. The present study demonstrates that oxyfluoride crystallized glasses containing CaF₂ nanocrystals have good elastic (mechanical) properties, being available in practical device applications even from the mechanical point of view.     

Visible and near infrared luminescence properties of Er-doped LBTAF glasses for optical amplifiers

Trivalent erbium doped lead borate titanate aluminum fluoride (LBTAFEr) glasses of molar composition (50-x)PbO·30H₃BO₃·10TiO₂·10AlF₃·xEr₂O₃ were prepared and characterized applying the Judd-Ofelt theory. The visible and near infrared luminescence spectra obtained by exciting the samples at 486 nm (xenon source) and 514.5 nm (Ar⁺ laser) radiations exhibit (²H_11/2, ⁴S_3/2) → ⁴I_15/2 (547 nm) and ⁴I_13/2 → ⁴I_15/2 (1.53 μm) transitions, respectively. Thermalization of the two emitting ²H_11/2 and ⁴S_3/2 states is taken into account in calculating the significant radiative properties. The intensity parameters are used to calculate the transition probabilities, luminescence branching ratios, radiative decay times and emission cross-sections for the observed emission bands. The McCumber theory has been applied to evaluate the emission cross-section using the absorption cross-section of the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ion. The decay profiles of the (²H_11/2, ⁴S_3/2) emission state were recorded and analyzed.     

Optical and magneto-optical properties of erbium doped InGaN and GaN epilayers

Using combined excitation-emission spectroscopy we have studied the erbium incorporation into GaN and InGaN for in situ doped MOCVD-grown layers and compared them to samples grown by MBE. A smaller up-conversion efficiency for the main site is observed compared to minority sites in the same sample as well as versus all sites from MBE grown samples. Furthermore, we show that the 1.54 μm emission efficiency is reduced with increasing In-content both under excitation above the bandgap in the UV as well as under resonant excitation at around 980 nm. This indicates that non-radiative decay channels for the Er ion are the largest contributing factor for this behavior. Finally, the Zeeman splitting of the excitation and emission lines of Er:GaN under application of magnetic fields up to 6.6 T with B||c-axis is shown.     

Fluorescence properties of Eu3+-doped alumino silicate glasses

Alumino silicate glasses of a very broad range of molar compositions doped with 1 ⋅ 10²⁰ Eu³⁺ cm⁻³ (about 0.2 mol% Eu₂O₃) were prepared. As network modifier oxides Li₂O, Na₂O, K₂O, MgO, CaO, SrO, BaO, ZnO, PbO, Y₂O₃ and La₂O₃ have been used. All glasses show relatively broad fluorescence excitation and emission spectra. For most glasses only a weak effect of the glass composition on the excitation and emission spectra is observed. Although the glasses should be structurally similar, notable differences are found for the fluorescence lifetimes. These increase steadily with decreasing mean atomic weight, decreasing refractive index and decreasing optical basicity of the glasses, which may be explained by local field effects. An exception from this rule are the strontium, barium and potassium containing glasses, which show significantly increased fluorescence lifetimes despite of their high refractive index, optical basicity and molecular weight. The non mono-exponential fluorescence decay curves as well as the fluorescence spectra indicate a massive change in the local surroundings of the doped rare earth ions for these glasses.     

Effects of GeO2 on the thermal stability and optical properties of Er/Yb-codoped oxyfluoride tellurite glasses

The aim of this article was to study the influence of GeO₂ on the thermal stability and optical properties of Er³⁺/Yb³⁺ codoped (70 − x)TeO₂–xGeO₂–PbF₂–BaF₂ (TGEYx) glasses prepared by using a melting method. The properties of Er³⁺/Yb³⁺ codoped glasses were investigated by using differential scanning calorimetry, upconversion luminescence, Raman and optical absorption spectra. The results indicated that TGEY35 glass with the germanate–tellurite mixed network showed the best thermal stability and poor crystallization tendency. With increasing the GeO₂ content, the maximum phonon energy of oxyfluoride tellurite glass network increased, while the phonon density decreased. The upconversion emission intensities enhanced obviously based on the decreasing phonon density of glass hosts, while the increasing red emission (657 nm) with the increase of GeO₂ concentration was attributed to the relative larger maximum phonon energy which matching the energy gap between the pertinent ⁴S_3/2 and ⁴F_9/2 levels.     

Low-energy bead-milling dispersions of rod-type titania nanoparticles and their optical properties

The low-energy dispersion of nanomaterials in the bead-milling process is examined. The effect of milling parameters including bead size, rotation speed, and milling time on the dispersibility of fragile rod-type titanium dioxide nanoparticles is investigated. From experimental data obtained for the morphological, optical, and crystalline properties of dispersed nanoparticles, an unbroken primary particle dispersion in colloidal suspension was obtained only by conducting the bead-milling process using the optimum milling parameters. Deviation from the optimum conditions (i.e., higher rotation speed and larger bead size) causes re-agglomeration phenomena, produces smaller and ellipsoidal particles, and worsens crystallinity and physicochemical properties, especially the refractive index, of the dispersed nanoparticles. We also found that decreases in refractive index induced by the milling process are related to collisions forming broken particles and the amorphous phase on the surface of the particles. In addition, the present low-energy dispersion method is prospective for industrial applications, confirming almost no impurity (from breakage of the beads) was apparent in the final product.     

Synthesis and optical properties of sub-micron sized rare earth-doped zirconia particles

Sub-micron sized crystalline particles of Eu³⁺ and Er³⁺-doped zirconia (ZrO₂) were prepared via a wet chemical sol-gel route and post synthesis annealing. The doping was achieved by introduction of the respective rare earth salts into the zirconia precursor solution, with insitu generation of sodium chloride for stabilization of the particle surface during growth. A series of materials with differing europium content, nominally 0.3, 3 and 6 mol% within the ZrO₂ lattice, were prepared for respective comparison and characterization of their optical properties following annealing at 700 °C. Average emission lifetimes of up to 2.3 ms were observed for the Eu³⁺-doped particles. Particle sizes, approximated from SEM micrographs, were observed in the range 250-400 nm. The synthesis of Er³⁺-doped ZrO₂ particles (0.5 mol%) produced 300 nm sized particles which exhibited emission in the visible and infrared regions after annealing at 1000 °C. X-ray diffraction (XRD) with Rietveld analysis for phase quantification, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectra were used to characterize the samples.     

Spectroscopic properties of the Ce-doped borate glasses

The EPR, optical absorption and photoluminescence (emission and excitation) spectra as well as decay kinetics of a series of the Ce-doped glasses with Li₂B₄O₇, LiKB₄O₇, CaB₄O₇, and LiCaBO₃ compositions have been investigated and analysed. The borate glasses were obtained from the corresponding polycrystalline compounds in the air atmosphere, using standard glass technology. The EPR signals of the isolated Ce³⁺ and pair Ce³⁺–Ce³⁺ centres, coupled by magnetic dipolar and exchange interactions were registered at liquid helium temperatures. The characteristic for glass host broad bands corresponding to the 4f → 5d transitions of the Ce³⁺centres have been observed in the optical absorption and photoluminescence (emission and excitation) spectra. The obtained luminescence decay curves can be satisfactory described by exponential function with lifetimes in the 19.8–26.1 ns range, which depend on the basic glass composition. The local structure of Ce³⁺ centres in the investigated glasses has been considered and discussed.     

Influence of Al2O3 addition on thermal and structural properties of erbium doped glasses

Changes in the structural properties of Er³⁺ doped soda-lime silicate glasses were investigated as a function of Al₂O₃ content. A combined approach of experimental techniques and molecular dynamic simulations (MD) was used to evaluate the structural features directly correlated to the glass properties. The experimental results in term of density, thermal properties as well as microstructural and mineralogical data showed a significant variation when increasing the alumina content from 10 mol% to 15 mol%. These results were compared to the MD information and discussed: changes in erbium and aluminium local configuration, due to the glass structural evolution as a function of the alumina concentration, have been investigated.