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.     

Influence of heat treatment on spectroscopic properties of Er in multicomponent ZrF4-ZnF2-AlF3-YF3-MF2 (M = Ca, Sr, Ba) based glass

Er³⁺ doped multicomponent fluoride based glass was prepared. These precursor fluoride glass samples were then heated using different schedules. Crystalline phase particles were successfully precipitated in the multicomponent fluoride glass samples after heat treatment. The influence of heat treatment on the spectroscopic properties of Er³⁺ in multicomponent fluoride based glass samples were discussed. Small changes of the Judd-Ofelt parameters Ω_i (i = 2, 4, 6) were found in multicomponent fluoride glass samples before and after heat treatment compared to oxyfluoride telluride glass. Preparation conditions used to produce transparent multicomponent fluoride glass ceramics doped with rare-earth ions are discussed.     

Luminescence behavior of Er ions in glass-ceramics containing CaF2 nanocrystals

The upconversion luminescence and near infrared luminescence of the Er³⁺ ions in transparent oxyfluoride glass-ceramics containing CaF₂ nanocrystals have been investigated. The formation of CaF₂ nanocrystals in the glass-ceramics was confirmed by XRD. The oscillator strengths for several transitions of the Er³⁺ ions in the glass and glass-ceramics have been obtained and then the Judd-Ofelt parameters were calculated. The split near infrared emission peaks of the Er³⁺ ions in the glass-ceramics can be observed because the Er³⁺ ions have been incorporated into crystalline environment of the CaF₂ nanocrystals. The upconversion luminescence intensity of Er³⁺ ions in the glass-ceramics increased significantly with increasing heat treated time. The transition mechanism of the upconversion luminescence has been ascribed to a two-photon absorption process.     

Optical properties of Er3+-singly doped and Er3+/Yb3+-codoped novel oxyfluoride glasses

Novel oxyfluoride glasses SiO₂–Al₂O₃–Na₂O–ZnF₂ doped with Er³⁺ and Er³⁺/Yb³⁺ were fabricated. The optical properties of the synthesized glasses were experimentally and theoretically investigated in detail. The experimental and calculated oscillator strengths of Er³⁺ were determined by measurement of the absorption spectrum of Er³⁺-singly doped glass. According to the Judd–Ofelt theory, the experimental intensity parameters were calculated, from which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. The fluorescence lifetime and quantum efficiency for the near-infrared emission of Er³⁺-singly doped glass were determined to be 3.0 ms, and 42%, respectively. Visible upconversion luminescence was observed under 980 nm diode laser excitation. The dependence of the upconversion emission intensity upon the excitation power was examined, and the upconversion mechanisms are discussed.     

Surface plasmon excited infrared-to-visible upconversion in Er-doped transparent glass ceramics

The surface plasmon (SP) excited infrared-to-visible upconversion luminescence has been studied in the oxyfluoride glass ceramics containing Er³⁺-doped fluoride nano-crystals. Upconversion luminescence was observed at glass ceramics. Quantum yield of upconversion luminescence increased with increasing heat-treatment temperature. The transparent glass ceramics heat-treated at 700 °C was covered with a 50 nm gold film, and then attached to an SF10 prism with index-matching oil to make an attenuated total reflection (ATR) illumination. The intense upconversion luminescence bands at 540 and 660 nm were observed at the SP resonance angle by using p-polarized laser beam. The apparent dependences of the upconversion efficiency on laser polarization and on incident angle demonstrated the SP excited upconversion of Er³⁺.     

UV assisted local crystallization in Er3+ doped oxy-fluoride glass

We report on fabrication of Er³⁺-activated LaF₃ nanocrystals in transparent glasses using an original technique, which combines both heat treatment, below glass crystallization temperature, and ultraviolet laser irradiation at 244 nm. The main advantage of this method is to control the spatial localization of the nanoparticles in the glass sample, whereas annealing solely at the crystallization temperature leads to a fully crystallized glass sample. Thermal differential analysis was used to determine the crystallization temperature of the sample. The photoluminescence spectra behaviour of Er³⁺ ions, collected from the UV-irradiated and unirradiated regions, allowed us to follow and to distinguish the structural changes in the glass network under heat treatment and ultraviolet exposure.     

Enhanced luminescent properties of Tb3+ ions in transparent glass ceramics containing BaGdF5 nanocrystals

Tb³⁺ doped transparent oxyfluoride glass ceramics containing BaGdF₅ nanocrystals were prepared. The transmission spectra, photoluminescence spectra, decay time and X-ray excited luminescence spectra of the Tb³⁺-doped glass and glass ceramics were investigated. Energy transfer from Gd³⁺ to Tb³⁺ has been observed in the glass and glass ceramics. The emission intensity of green band (⁵D₄ → ⁷F_J) of the Tb³⁺-doped glass ceramics is enhanced compared with that of the glass under ultraviolet and X-rays, which could be attributed to that the generation of BaGdF₅ nanocrystals with low phonon energy reduces the non-radiative transitions.     

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.     

Crystallization and spectroscopic properties of Eu-doped CaF2 nanocrystals in transparent oxyfluoride glass-ceramics

Transparent oxyfluoride glass-ceramic in the system SiO₂–Al₂O₃–CaF₂–EuF₂ containing Eu-doped CaF₂ nanocrystals were produced by using the controlled crystallization of melt-quenched glass. X-ray diffraction and transmission electron microscopy data have revealed the formation of the CaF₂ nanocrystals of about 65 nm size. Photoluminescence spectra have shown an increase of the splitting of the luminescences associated to the Eu³⁺ ion along with annealing time which is consistent with the Eu³⁺ environment evolving from a glassy to a crystalline state.     

Influence of cationic field strength of modifiers on the 1.53 μm spectroscopic properties of Er3+-doped tellurite glasses

Two series of Er³⁺-doped tellurite glasses with different glass modifiers were prepared by the conventional melt-quenching method. In order to estimate the effect of cationic field strength z/a² of modifiers on the fluorescence properties, the fluorescence spectra were measured. The strength parameters Ω_t (t = 2, 4, 6) for all the samples were calculated based on the absorption spectra and also compared between them. The values of Ω₆ decrease with decreasing of the cationic field strength z/a² of modifiers. As the cationic field strength decrease, the polarization effect of the ligand fields around Er³⁺ increase in the glasses, and which leads to the decreasing of fluorescence peak intensity and bandwidth. The strength of interactions between Er³⁺ ions and OH^? groups, k_OH–Er, depend on the glass composition, are changed with glass modifiers.     

Synthesis and Judd-Ofelt analysis of a sol-gel material doped with a Ln2(Benzoate)6(Phen)2 (Ln = Er/Yb) complex

In this work, we have prepared a sol-gel derived hybrid material directly doped with Er_1.4Yb_0.6(Benzoate)₆(Phen)₂ (Phen = 1,10-phenanthroline) complex, which was reported with intramolecular Yb-Er energy-transfer process in our previous work. The infrared (IR) spectra of the pure complex and hybrid gel material were investigated. The NIR photoluminescence (PL) spectrum of hybrid gel material shows strong characteristic emission of Er³⁺ with broad full width at half-maximum (FWHM) of 70 nm. Judd-Ofelt theory was used in order to analyze the optical properties of Er³⁺ ions in the hybrid gel material.     

Gain cross-section of 1.06 μm emission in Nd-doped SiO2-LaF3 glass-ceramics prepared by sol-gel method

The room temperature near-infrared emission at 1.06 μm corresponding to the ⁴F_3/2 → ⁴I_11/2 laser transition in Nd³⁺-doped SiO₂-LaF₃ transparent glass-ceramics prepared by sol-gel method has been characterized in terms of the product of emission cross-section and fluorescence lifetime. Stimulated emission cross-sections have been calculated from experimental emission spectra using the Füchtbauer-Ladenburg equation. The precipitation of LaF₃ nanocrystals after heat-treatment, confirmed by X-ray diffraction, induces a significant increment of the 1.06 μm near-infrared emission compared with the as made sample. An enhancement of emission cross-section of about 10% has been achieved with increasing temperature of heat-treatment of the nano-ceramming process from 800 to 1000 °C, which is related to the growth of LaF₃ nanocrystals.     

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.     

Fabrication of Er doped oxyfluoride-silicate glass film by pulsed laser deposition for planar amplifier

Pulsed laser deposition (PLD) technique has been employed to fabricate glassy films based on the Er³⁺ doped oxyfluoride lead-silicate glasses. Glassy films have been produced at temperatures between 300 and 400 °C with O₂ pressures ranging from 25 to 175 mTorr. The film microstructure appears to be determined mainly by substrate temperature. The film refractive index increases with higher temperatures and lower O₂ pressures. The photoluminescence intensity of Er³⁺ ions and lifetime are very much dependent on the film microstructure and defects. Fewer fabrication defects and less porous films produce stronger fluorescence intensity and longer lifetime of Er³⁺ ions. At an optimized temperature and O₂ pressure, the Er photoluminescence properties of the film can be reproduced and are very close to those of target glass. Consequently the work reported here suggested a good candidate for further investigation as a thin film material for EDWA.     

The thermal study of oxyfluoride glass with strontium fluoride

A thulium (Tm 3+) doped oxyfluoride glass ceramic containing SrF2 nanocrystals has been presented. Transparent glass ceramic was obtained by heat treating the glass from the SiO2-Al2O3-ZnO-SrF2 system at the first crystallization temperature. Cerammization of glass was studied by DTA/DSC, XRD and SEM methods. It has been found that nanocrystallization of SrF2 strongly depends on the ratio between the components and amount of SrF2. Moreover the rare earth dopant Tm3+ influenced on the thermal properties of glass. The formation of SrF2 nanocrystals in glass ceramic was confirmed by X-ray diffraction. X-ray diffraction analysis of the transparent glass ceramic revealed that the SrF2 nanocrystals are precipitated in the glass matrix. Analysis of the local atomic interactions in the structure of oxyfluoride glass has been used to explain the course of the crystallization.     

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.     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Er3+/Yb3+-codoped silica–germania sputtered films: structural and spectroscopic characterization

We report on the fabrication and spectroscopic characterization of highly photo-refractive Er³⁺/Yb³⁺ coactivated silica–germania slab waveguides, single mode at 1550 nm, deposited by radio-frequency-magnetron-sputtering technique. Details of the sputtering procedure are reported. The structural properties of the films were investigated by Raman spectroscopy. Propagation losses of guided modes were measured at 633 nm and 1320 nm. The emission of the ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ion was analyzed upon excitation of the TE₀ mode at 514 and 981 nm. Back energy transfer from Er³⁺ to Yb³⁺ was observed by measurement of Yb³⁺ emission upon Er³⁺ excitation at 514.5 nm. Photoluminescence excitation spectroscopy was used to obtain information about the Yb³⁺ to Er³⁺ energy transfer process.     

Preparation and upconversion luminescence characterization of 12CaO·7Al2O3:Ho3+/Yb3+ polycrystals

The effect of Yb³⁺ concentration on the fluorescence of 12CaO·7 Al₂O₃:Ho³⁺/Yb³⁺ polycrystals is investigated. The Raman spectra of pure C12A7 under 633‐nm excitation show that the highest photon energy is 787.267 cm⁻¹, which is not much bigger than general fluorides, so it can realize high efficiency upconversion. The upconversion emission spectra suggest that the green upconversion emission centered at 548 nm and the red upconversion emission at 662 nm correspond to the ⁵F₄/⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transition of Ho³⁺ ions, respectively. The intensity of the upconversion luminescence and the ratio of red to green are changed with Yb³⁺ ion concentration. The pump dependence and luminescence decay dynamics spectra show the green and red upconversion emissions are populated by a two‐photon process, and the upconversion mechanisms are analyzed. The relative luminous efficiencies of green and red emissions are 2.035% and 0.7%, respectively. The normalized efficiency obtained for green emission of Ho³⁺ at RT when the sample is excited by 980‐nm light with an absorbed intensity of 7.5 W/cm² is 0.27 cm²/W. This result is comparable to the values obtained in YF₃ for the Yb³⁺, Er³⁺ green emission. The C12A7 with upconversion red and green light will be a promising luminous material.     

Energy transfer between Tm and Er ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in Tm³⁺/Er³⁺ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er³⁺ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm³⁺ ions. This is evidence of the relevant contribution of the route Er₁(⁴I_11/2) + Er₂(⁴I_13/2) → Er₁(⁴I_15/2) + Er₂(⁴F_9/2) to the process. Energy migration among pumped ⁴I_9/2 level reducing the efficiency of the upconversion emission rate (³H_11/2, ⁴S_3/2, and ⁴F_9/2) is observed for Er³⁺ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.