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.     

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.     

Ultraviolet to near-infrared spectral modification in Ce3+ and Yb3+ codoped phosphate glasses

The ultraviolet to near-infrared spectral modification in Ce³⁺ and Yb³⁺ codoped phosphate glasses was realized through the energy transfer from Ce³⁺ to Yb³⁺. The absorption spectra, fluorescence excitation and emission spectra, luminescence decay curves, and time-resolved emission spectra were measured and analyzed. The energy transfer efficiency and concentration quenching efficiency were calculated based on the decay curves of Ce³⁺ 340 nm emission and Yb³⁺ 976 nm emission. The calculated and experimental NIR emission intensities on the Yb³⁺ concentrations were compared and discussed.     

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.     

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.     

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.     

Growth and spectral properties of Er3+/Tm3+ co‐doped LiYF4 single crystal

High quality Er³⁺/Tm³⁺:LiYF₄ single crystals were grown by a Bridgman method. The absorption spectra and luminescent properties of the crystals were studied to characterize the effect of Tm³⁺ on the spectroscopic properties upon excitation of an 800 nm laser diode. The broaden 1.5 μm and the enhanced 2.7 μm emission were observed in the Er³⁺/Tm³⁺ co‐doped LiYF₄ single crystals. Meanwhile, the up‐conversion and 1.5 μm emission intensities from Er³⁺ decrease with increasing the ratio of Tm³⁺ to Er³⁺. The energy transfer processes between Tm³⁺ and Er³⁺ in the Er³⁺/Tm³⁺ co‐doped samples were analyzed. The energy transfer efficiency η_ETE from Er³⁺ to Tm³⁺ is calculated. The highest η_ETE of 65.30% for the sample with 0.296 mol% of Er³⁺, 0.496 mol% of Tm³⁺ concentration was obtained. The present work indicates that Er³⁺/Tm³⁺ co‐doped LiYF₄ single crystal can be a promising material for the potential application in infrared devices.     

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.     

Electroluminescence from Er and Yb co-doped silicon dioxide layers: The excitation mechanism

The excitation mechanism of photo- (PL) and electroluminescence (EL) of erbium ions co-implanted with ytterbium into the SiO₂ layer of light emitting MOS devices (MOSLED) was investigated. Ytterbium implanted and annealed samples exhibit the blue and near infrared electroluminescence. The blue electroluminescence at 470 nm appears due to cooperative up-conversion emission in the Yb³⁺-Yb³⁺ system, and the near infrared EL at 975 and 1025 nm corresponds to transitions from the multiple state ²F_5/2 to the ²F_7/2 ground state in the Yb³⁺ ions. The Er implanted SiO₂ exhibits the luminescence in the blue-green and infrared region. The green and blue peaks correspond to radiative transitions from the ²H_11/2 or ⁴S_3/2 energy levels and from the ²H_9/2 or ⁴F_5/2 energy levels to the ⁴I_15/2 ground state, respectively. We have found that the energy transfer from Yb³⁺ to Er³⁺ ions exists only during photoluminescence excitation. The electroluminescence investigation shows the cooperative up-conversion in the Er³⁺-Yb³⁺ system.     

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.     

Up‐conversion luminescence of ytterbium and thulium codoped potassium yttrium double tungstate crystal

Thulium and ytterbium co‐doped double tungstate Yb³⁺,Tm³⁺: NaY(WO₄)₂ single crystals were prepared by using RF‐heating Czochralski (CZ) pulling method. Its polarized transmittance spectra have been recorded in the region of 290‐2000 nm at room temperature. The energy levels transitions were assigned to the corresponding absorption line. The up‐conversion luminescences at 793 nm and 475 nm were measured when the sample were pumped by 972 nm LD and the energy transfer mechanism between Yb³⁺ and Tm³⁺ ions was analyzed.     

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.     

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.     

Thermal, chemical, optical properties and structure of Er-doped and Er/Yb-codoped P2O5-Al2O3-ZnO glasses

This study was explored in series of the optical, thermal, and structure properties based on 60P₂O₅-10Al₂O₃-30ZnO (PAZ) glasses system that doped with varied rare-earth (RE) elements Yb₂O₃/Er₂O₃. The glass transition temperature, softening temperature and chemical durability were increased with RE-doping concentrations increasing, whereas thermal expansion coefficient was decreased. In the optical properties, the absorption and emission intensities also increase with RE-doping concentrations increasing, When Er₂O₃ and Yb₂O₃ concentrations are over than 3 mol% in the Er³⁺-doped PAZ system and Yb³⁺-doped concentration is over than 3 mol% for Er³⁺/Yb³⁺-codoped PAZ system, the emission intensity significantly decreases presumably due to concentration quenching, formation of the ions clustering, and OH⁻ groups in the glasses network. It is suggested that the maximum emission cross-section (σ_e) is 7.64 × 10^− 21 cm² at 1535 nm is observed for 3 mol% Er³⁺-doped PAZ glasses. Moreover, the maximum σ_e × full-width-at-half-maximum is 327.8 for 5 mol% Er³⁺-doped PAZ glasses.     

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.     

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.     

Nano-crystals precipitation and upconversion luminescence properties in Er3+/Yb3+ co-doped fluoride phosphate glass-ceramics

Er³⁺/Yb³⁺ co-doped transparent glass-ceramics containing Ca₅(PO₄)₃F nano-crystals were prepared and their upconversion properties were investigated. Transparent glass-ceramics were obtained after heat treatment at the first crystallization temperature, TEM images showed that Ca₅(PO₄)₃F nano-crystals of 10–20 nm in diameter precipitated uniformly in the glass matrix, which is similar to the result of those calculated by Scherrer equation. Comparing with the samples before heat treatment, high efficiency upconversion luminescence of Er³⁺ at 547 nm and 667 nm was observed in the glass-ceramics under 980 nm excitation, and the intensity of red emission showed different tendency to that of green emission after nano-crystals precipitation. The reasons for the highly efficient upconversion luminescence in glass-ceramics were discussed.     

Luminescence characteristics of Yb, La codoped yttrium oxide nanopowders

Luminescence characteristics of Yb³⁺, La³⁺ codoped yttrium oxide nanopowders were investigated. The grain size and the crystallinity of (Yb_0.05Y_0.90La_0.05)₂O₃ nanopowders increase with the increase of calcination temperature. The average grain size of the nanopowders calcined at 1100 °C is 66 nm and its cooperative up-conversion luminescence centered at 498 nm was detected due to nanometer size effect and perfect crystallinity. However, the cooperative up-conversion luminescence of (Yb_0.05Y_0.90La_0.05)₂O₃ transparent ceramics was not detected.     

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%.