Spectroscopic properties of the 1.8 μm emission of Tm3+/Yb3+ codoped TeO2–ZnO–Bi2O3 glasses with efficient energy transfer

Tm³⁺-doped and Tm³⁺/Yb³⁺-codoped TeO₂–ZnO–Bi₂O₃ (TZB) glasses are prepared by melt-quenching method. The Judd-Ofelt intensity parameters (Ω_t t = 2, 4, 6), radiative transition rate, and radiative lifetime of Tm³⁺ are calculated based on the absorption spectra. The 1.8 μm emission of the samples is investigated under 980 nm laser excitation. The absorption, emission cross-sections, and gain coefficient of Tm³⁺:³F₄ → ³H₆ are calculated. The energy transfer processes of Yb³⁺–Yb³⁺ and Yb³⁺–Tm³⁺ are analyzed, the results show that the Yb³⁺ ions can transfer their energy to Tm³⁺ ions with large energy transfer coefficient, and a maximum efficiency of 79%.     

Spectral studies of Sm3+ and Dy3+ doped lithium cesium mixed alkali borate glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Sm³⁺ and Dy³⁺ has been studied in mixed alkali borate glasses of the type 67B₂O₃ · xLi₂O · (32 − x)Cs₂O (x = 8, 12, 16, 20 and 24). The Judd–Ofelt intensity parameters (Ω₂, Ω₄ and Ω₆) are calculated. The radiative transition probabilities (A), radiative lifetimes (τ_R), branching ratios (β) and integrated absorption cross-sections (Σ) are computed for certain excited states of Sm³⁺ and Dy³⁺ ions for different x values in the glass matrix. Stimulated emission cross-sections (σ_p) are obtained for certain emission transitions of two ions in these mixed alkali borate glasses. These parameters are compared for different x values in the glass matrix. Variation of these parameters with x in the glass matrix has been studied.     

1.5 μm Emission and infrared-to-visible frequency upconversion in Er+3/Yb+3-doped phosphoniobate glasses

Sodium phosphoniobate glasses with the composition (mol%) 75NaPO₃–25Nb₂O₅ and containing 2 mol% Yb³⁺ and x mol% Er³⁺ (0.01 ⩽ x ⩽ 2) were prepared using the conventional melting/casting process. Er³⁺ emission at 1.5 μm and infrared-to-visible upconversion emission, upon excitation at 976 nm, are evaluated as a function of the Er³⁺ concentration. For the lowest Er³⁺ content, 1.5 μm emission quantum efficiency was 90%. Increasing the Er³⁺ concentration up to 2 mol%, the emission quantum efficiency was observed to decrease to 37% due to concentration quenching. The green and red upconversion emission intensity ratio was studied as a function of Yb³⁺ co-doping and the Er³⁺–Er³⁺ energy transfer processes.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Systematic study of spectroscopic properties and thermal stability of lead germanate glass doped with rare-earth ions

Glasses with composition 50GeO₂–(50?x)PbO–5PbF₂–xLnF₃ (Ln = Pr³⁺–Yb³⁺) were prepared from commercial raw materials. The content of PbF₂ was constant and amounted to 5 mol% whereas the concentration of luminescent ions was diverse (0.2 and 2 mol%). Thermal stability of the glasses were monitored by differential thermal analysis (DTA). It has been found that increase of rare-earth fluoride content from 0.2 mol% to 2 mol% brings about a shift of the glass crystallization onset from 450 °C to 487 °C for Nd-doped samples and from 466 °C to 482 °C for Tm-doped samples. Optical absorption and emission spectra of Ln active ions in oxyfluoride glass have been investigated at room temperature in the ultraviolet (UV), visible (VIS) and near-infrared (NIR) region. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters Ω_2,4,6, radiative transition probabilities, branching ratios and radiative lifetimes of luminescent levels have been estimated. Analysis of decay curves of luminescence revealed that the increase of rare-earth fluoride content from 0.2 mol% to 2 mol% shortens the lifetime of the ⁴F_3/2 level of Nd³⁺ from 224 μs to 90 μs.     

Upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped fluorogermanate glass and glass ceramic

Energy-transfer excited upconversion luminescence in Ho³⁺/Yb³⁺- and Tb³⁺/Yb³⁺-codoped PbGeO₃–PbF₂–CdF₂ glass and glass–ceramic under infrared excitation is investigated. In Ho³⁺/Yb³⁺-codoped samples, green (545 nm), red (652 nm), and near-infrared (754 nm) upconversion emission corresponding to the ⁵S₂(⁵F₄) → ⁵I₈, ⁵F₅ → ⁵I₈, and ⁵S₂(⁵F₄) → ⁵I₇ transitions, respectively, was observed. Blue (490 nm) emission assigned to the ⁵F_2,3 → ⁵I₈ transition was also detected. In the Tb³⁺/Yb³⁺-codoped system, bright UV–visible emission around 384, 415, 438, 473–490, 545, 587, and 623 nm, identified as due to the ⁵D₃(⁵G₆) → ⁷F_J(J = 6, 5, 4) and ⁵D₄ → ⁷F_J(J = 6, 5, 4, 3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicated that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed.     

EPR and magnetic susceptibility studies of B2O3–Bi2O3–Gd2O3 glasses

Glasses of the xGd₂O₃ · (100 − x)[B₂O₃ · Bi₂O₃] system with 0.5 ⩽ x ⩽ 10 mol% were studied by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Data obtained show that for low gadolinium oxide contents of the samples (x ⩽ 3 mol%) the Gd³⁺ ions are randomly distributed in the host glass matrix and are present as isolated and dipole–dipole coupled species. For higher gadolinium oxide contents of the samples (x > 3 mol%) the Gd³⁺ ions appear as both isolated and antiferromagnetically coupled species. The EPR spectra of the glasses reveal resonance sites with an unexpected high crystalline field in addition to the ‘U’ spectrum, typical for Gd³⁺ ions in disordered systems. This absorption line is due to Gd³⁺ ions that replace Bi³⁺ ions from the host glass matrix and could play the network unconventional former role in the studied glasses.     

Visible up-conversion photoluminescence from IR diode-pumped SiO2–TiO2 nano-composite films heavily doped with Er3+–Yb3+ and Nd3+–Yb3+

Efficient infrared-to-visible conversion is reported in thin film nano-composites, with composition 90% SiO₂–10% TiO₂, fabricated by a spin-coating sol–gel route and co-doped with Er³⁺ Yb³⁺ and with Nd³⁺:Yb³⁺ ions. The conversion process is observed under 808 nm laser diode excitation and results in the generation of green (526 and 550 nm) and red (650 nm) emissions: from the former, and blue (478 nm) and green (513 and 580 nm) emissions from the latter. The main mechanism that allows for up-conversion is ascribed to energy transfer among Er³⁺ and Yb³⁺ ions in their excited states. Up-conversion efficiency for red emission predominates in samples doped with Er³⁺:Yb³. The results illustrate the large potential of this class of materials for photonic applications in optoelectronics devices.     

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.     

Alkaline earth zinc borate glasses doped with Cu2+ ions studied by EPR,optical and IR techniques

Copper ions incorporated into alkaline earth zinc borate glasses 10RO + 30ZnO + 60B₂O₃ (R = Mg, Ca and Sr) and 10SrO + (30 ? x)ZnO + 60B₂O₃ + xCuO (x = 0, 0.1, 0.3, 0.5, and 0.7 wt.%) were characterized by electron paramagnetic resonance (EPR), optical absorption and FTIR techniques. The EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in alkaline earth zinc borate glasses were present in octahedral sites with tetragonal distortion. The spin concentration (N) participating in resonance was calculated as a function of temperature for strontium zinc borate (SrZB) glass sample containing 0.7 wt.% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures and the Curie constant was evaluated from the 1/χ-T graph. The optical absorption spectra of these samples show only one absorption band. The optical band gap energies (E_g) and Urbach energy (ΔE) are calculated from their ultraviolet edges. The FTIR studies show different stretching and bending vibrations of alkaline earth zinc borate glasses.     

Bond character,optical properties and ionic conductivity of Li2O/B2O3/SiO2/Al2O3 glass: Effect of structural substitution of Li2O for LiCl

A glass of composition (20 ? x)Li₂O–xLiCl–65B₂O₃–10SiO₂–5Al₂O₃ where 0 ? x ? 12.5 wt% is prepared using the normal melt-quenching technique. The optical constants and electrical conductivity and their correlation are investigated, furnished and discussed with the substitution of Li₂O for LiCl. The mechanism of the optical absorption and the calculated Urbach energy follow the rule of phonon-assisted transitions. The ionic conduction mechanism is determined by activation energy process. Substitution up to 10 wt% LiCl provides high ionic conductivity (1.9 × 10^?2 Ω^?1 m^?1) due to the high average electronegativity of LiCl which increases the polarizability of lithium ions. The small cation–anion distance approach confirmed the enhancement in ionic conductivity of LiCl containing glass compared to that of Li₂O. Due to the large size of Cl^? ions, there is an expansion of the lattice which in turn broadens the available path windows. For 12.5 wt% LiCl, anomalous density behavior is observed and a reduction in conductivity is occurred, σ = 5.4 × 10^?3 Ω^?1 m^?1. Owing to the model of bond fluctuation, the reduction is attributed to the increase in the alkali halide concentration which creates bottlenecks that hinder the motion of Li⁺ ions. The ionic conductivity character is strongly supported by the behavior of the glass ionicity factor, density, molar volume, refractive index, average boron–boron separation, molar refraction, metallization criterion and non-bridging oxygen concentration of the studied glass.     

Spectroscopic properties and energy transfer in Yb3+–Ho3+ co-doped germanate glass emitting at 2.0 μm

A new kind of germanate glass co-doped with Yb³⁺–Ho3⁺ was prepared. The J-O parameters were calculated to be Ω₂ = (6.59 ± 0.21) × 10^? 20 cm², Ω₄ = (2.77 ± 0.36) × 10^? 20 cm², and Ω₆ = (1.90 ± 0.25) × 10^? 20 cm². The little overlap between the absorption cross section and stimulated emission cross section indicates a non-resonant energy transfer process. The calculation demonstrates that the energy transfer between Yb³⁺ and Ho³⁺ is one-phonon assisted in a great measure. The gain coefficient of Ho³⁺ at 2.0 μm was also calculated. The fluorescence measurement shows the Yb³⁺ co-doping enhances the 2.0 μm emission remarkably.     

EPR and optical studies of Mn2+ ions in Li2O–Na2O–B2O3 glasses – An evidence of mixed alkali effect

EPR and optical absorption spectra of 0.5 mol% MnO₂ doped xLi₂O–(30 − x)Na₂O–69.5B₂O₃ (5 ⩽ x ⩽ 25) glasses have been studied. The EPR spectra exhibit resonance signals characteristic of Mn²⁺ ions. The resonance signal at g ≅ 2.0 is due to Mn²⁺ ions in an environment close to octahedral symmetry, whereas the resonances at g ≅ 4.3 and g ≅ 3.3 are attributed to the rhombic surroundings of the Mn²⁺ ions. The ionic character (A), the number of spins participating in resonance (N), optical band gap energies (E_opt) and Urbach energies (ΔE) show the mixed alkali effect (MAE) with composition. The present study gives an indication that the size of alkalis we choose, is also an important contributing factor in showing the MAE. The variation of N with temperature obeys the Boltzmann law. The optical absorption spectra show a single broad band at ∼21 000 cm⁻¹ corresponding to the transition ⁶A_1g(S) → ⁴T_1g(G) which exhibits a blue shift with x. The theoretical values of optical basicity (Λ_th) have also been evaluated.     

Structural and spectroscopic characterization of poly(styrene sulfonate) films doped with neodymium ions

This work reports the structural and spectroscopy characterization of poly(styrene sulfonate) (PSS) films doped with neodymium (Nd) ions. Nd–PSS films were processed using the acid of poly(styrene sulfonate) – H–PSS and neodymium nitrate – Nd(NO₃)₃; the maximum incorporation of Nd ions in the polymeric matrix was equal 19.3%. The absorption in the UV–Vis–NIR spectral region presents typical electronic transitions of Nd³⁺ ions, with well resolved peaks. The infrared spectra present the transition bands of PSS with characteristic line shape broadening, and the presence of vibrational modes of N–O groups in the range of 1400–720 cm^?1, prove the permanence of Nd(NO₃)_x, with x = 1, 2 and/or 3, in the H–PSS matrix. UV–Vis site selective photoluminescence data indicate that the incorporation of Nd³⁺ introduces a blue shift in PSS emission (325–800 nm), decreasing the interaction between adjacent PSS lateral groups (aromatic rings). Nd³⁺ reabsorption and energy transfer effects between the PSS matrix and Nd³⁺ were also observed. The IR emission of Nd–PSS films at 1076 nm (⁴F_3/2 → ⁴I_11/2) present constant efficiency, independent on Nd³⁺ concentration. The Judd–Ofelt theory was employed to analyze radiative properties. The excitation spectra prove the energy transfer between the polymeric matrix and Nd³⁺. Complex impedance data was used to probe relaxation processes during the charge transport within the polymeric matrix.     

Effect on the fluorescence branching ratio of different synergistic ligands in neodymium complex doped PMMA

A series of neodymium complexes Nd(TTA)₃L_x (where TTA = α-thenoyltrifluoroacetonato, L_x (x = 1–5) = H₂O, triophenylphosphine oxide (Tppo), 2,2-bipyridine (Bipy), 1,10-phenanthroline (Phen) and 2- (N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (Dpbt) were synthesized and incorporated in poly(methyl methacrylate). Their absorption spectra were measured and analyzed using Judd–Ofelt theory. Near-infrared luminescent spectra were studied and the radiative properties have been stimulated. Laser parameters such as effective bandwidths (Δλ_eff), stimulated emission cross-sections (σ_e), and gain bandwidth (σ_g) had also been calculated and compared with other systems. The effect on the fluorescence branching ratio (β) in the ⁴F_3/2→⁴I_11/2 transition of different synergistic ligands had been investigated and the relation between β and Judd–Ofelt parameter Ω₂, Ω₄, Ω₆ was discussed. In conclusion, among the five neodymium complexes, Nd(TTA)₃Dpbt has the largest Ω₂ parameter (33.72 × 10^? 20 cm²), stimulated emission cross-sections, which is found promising to be a candidate for laser materials in further application.     

Optical properties of erbium-doped fluorochlorozirconate glasses

We report the optical properties of a fluorochlorozirconate (FCZ) glass with the composition 53% ZrF₄, 20% NaF, 3.5% AlF₃, 3% LaF₃, 0.5% InF₃, (20 ? x)% BaCl₂, x% BaF₂ with x varying from 0% to 2%, and doped with various amounts of trivalent erbium by the addition of ErCl₃. Annealing of the as-prepared glass in inert (N₂) or reducing (5%H₂ + 95%Ar) atmospheres at temperatures that ensure the conversion of the glass into a glass-ceramic by the nucleation of BaCl₂ nanocrystals, does not significantly change any of Er³⁺ related absorption and photoluminescence (PL) characteristics. We have carried out a Judd–Ofelt analysis of the absorption spectra and obtained Ω₂ = (1.92 ± 0.3) × 10^? 20 cm², Ω₄ = (0.88 ± 0.16) × 10^? 20 cm² and Ω₆ = (0.59 ± 0.08) × 10^? 20 cm², and also the radiative lifetimes of the ⁴I_13/2→⁴I_15/2, ⁴I_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 bands. The radiative lifetime from the Judd–Ofelt analysis for the ⁴I_13/2→⁴I_15/2 band is in good agreement with the experimentally measured PL decay time. The examination of the optical properties of powdered samples with different average particle size does not show any photon trapping effects. We have determined the spectral absorption and emission cross-sections and then estimated the possible spectral optical gain for varying degrees of relative populations of the ⁴I_13/2 and ⁴I_15/2 manifolds.     

Crystal growth and spectral properties of Nd3+:Ca9Gd(VO4)7 crystal

An Nd³⁺:Ca₉Gd(VO₄)₇ crystal with dimensions of ?25×30 mm³ was grown by the Czochralski method. The hardness, thermal expansion coefficient and thermal conductivity coefficient of the crystal were measured. The spectroscopic characteristics of Nd³⁺:Ca₉Gd(VO₄)₇ crystals were investigated. The absorption band at 810 nm has an FWHM of 10 nm, and absorption cross-sections are 5.81×10^?20 cm² for π-polarization and 7.47×10^?20 cm² for σ-polarization at 810 nm. The emission cross-sections at 1067 nm are 4.2×10^?20 and 6.5×10^?20 cm² for π- and σ-polarizations, respectively. The quantum efficiency η_c is equal to 94.3%. To sum up the above results, Nd³⁺:Ca₉Gd(VO₄)₇ crystal can be regarded as a highly efficient solid state laser material.     

EPR study of molybdenum-lead-phosphate glasses

The local symmetry and interaction between paramagnetic ions in xMoO₃(1 ? x)[2 P₂O₅PbO] glasses with 0.5 ? x ? 50 mol% are investigated by EPR spectroscopy. For x ? 10 mol% the isolated Mo⁵⁺ ions surrounded by five oxygen ligands in a square-pyramidal form (C_4v symmetry) prevail. The short range disorder in the environment of Mo⁵⁺ ions is not significantly (ΔR/R ≈ 2%). At high molybdenum content (x > 20 mol%) the dipole–dipole and superexchange coupled Mo⁵⁺ ions appear and their number increases with the MoO₃ content. These two aspects are also correlated with the network modifier and former role of molybdenum oxide in function of its concentration. Thus a strong depolymerization of the phosphate structure and the formation of P–O–Mo or Mo–O–Mo bonds in studied glasses appear.     

Structural impact of iron ions on BaBiBO4 glasses: Spectroscopic and dielectric investigations

Transparent glasses of composition 10BaO.20Bi₂O₃.(70 ? x)B₂O₃.xFe₂O₃ (wt.%) where 0 ≤ x ≤ 2.0, were characterized by XRD and SEM. Physical, spectroscopic and dielectric properties were investigated. At higher dopant of Fe₂O₃, EPR results revealed that, the number of Fe³⁺ ions participate in the resonance is decreased by forming a new signal at g ≈ 3.015 due to increase of antiferromagnetic interaction of Fe³⁺ ions and/or formation of low spin Fe³⁺ ions in the glass matrix. With initial 0.5 wt.% doping of Fe₂O₃, less dense glass is formed with colloids of metallic Bi⁰ atoms. The absorption bands at 604 and 712 nm in F5 glass are ascribed to Bi⁰ and Bi⁺ radicals respectively. No characteristic Fe³⁺ absorption bands (spin-forbidden) are found. Fe²⁺ ions are increased at higher concentration of Fe₂O₃. Higher concentration of Fe₂O₃ is favorable for BO₂O^?, BO₃, BiO₆ and FeO₆ symmetry unit leads to low band gap and high Urbach energy. By doping of Fe₂O₃ the dielectric parameters like dielectric constant (ε′), loss (tanδ and ac electrical conductivity (σ_ac) are found to increase.     

Temperature dependence of Nd3+ → Yb3+ energy transfer processes in co-doped oxyfluoride glass ceramics

A study of the Nd³⁺ → Yb³ energy transfer processes in transparent oxyfluoride glass ceramics has been carried out as a function of temperature in the 100–700 K range. This host is a two-phase optical material that consists of a low-phonon energy fluoride nanocrystalline phase embedded in a predominantly aluminosilicate glassy medium and has shown to be an interesting matrix for rare earth ions. Luminescence decay curves of single Nd³⁺ and Yb³⁺ doped and co-doped samples at different temperatures have been analyzed in order to calculate the energy transfer and backtransfer rates between these ions. Finally, the results have been also investigated to known the phonons involved in the energy transfer processes, concluding at the end that the Nd³⁺ → Yb³⁺ energy transfer rate takes place by the emission of three phonons with energy around 325 cm⁻¹ and in the other hand, Nd³⁺ ← Yb³⁺ energy transfer rate has been found to be non-negligible for temperatures over 370 K with the requirement of absorption of phonons.