Mechanisms of Yb sensitization to Tm for blue upconversion luminescence in fluorophosphate glass

The sensitization mechanisms of Yb³⁺ to Tm³⁺ for the blue upconversion luminescence in fluorophosphate glass were studied. Two different mechanisms exist in the sensitization. One is the sequential sensitization that Tm³⁺ is excited from ³H₆ to ¹G₄ through absorbing three photons transferred from Yb³⁺ one by one. Another is the cooperative sensitization that two Yb³⁺ ions form a couple cluster firstly, and then the couple cluster Yb³⁺ ions transfer their energy to Tm³⁺ and excite it to ¹G₄. With the increment of the concentration of Yb³⁺ ions, the sequential sensitization becomes weak and the cooperative sensitization becomes intense, and the transformation trend of sensitization mechanism with the increment of Yb³⁺ concentration can be clarified by the introduction of Tb³⁺ ions in the glass.     

Local thermal effect at luminescent spot on upconversion luminescence in Y2O3:Er,Yb nanoparticles

Y₂O₃:Er³⁺,Yb³⁺ nanoparticles were synthesized using Pechini type sol-gel method and then characterized by XRD, TEM, SEM, Raman spectroscopy, and fluorescence spectrophotometer. Local temperature effect on upconversion luminescence intensities was theoretically analyzed and experimentally tested. These results indicate that a competition process between local temperature at luminescent spot and laser pump power density decides the development trend of upconversion luminescence intensity. Therefore, it can be concluded that the most intensive upconversion luminescence in Y₂O₃:Er³⁺,Yb³⁺ nanoparticles can be achieved at a certain pump power density, which should be slightly below a given constant value (the corresponding threshold of temperature).     

Preparation and upconversion property of TeO2:Er/Yb nanoparticles

Different crystal structure of TeO₂ nanoparticles were used as the host materials to prepare the Er³⁺/Yb³⁺ ions co-doped upconversion luminescent materials. The TeO₂ nanoparticles mainly kept the original morphology and phase after having been co-doped the Er³⁺/Yb³⁺ ions. All the as-prepared TeO₂:Er³⁺/Yb³⁺ nanoparticles showed the green emissions (525 nm, 545 nm) and red emission (667 nm) under 980 nm excitation. The green emissions at 525 nm, 545 nm and red emission at 667 nm were attributed to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of the Er³⁺ ions, respectively. For the α-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles, three-photon process involved in the green (²H_11/2 → ⁴I_15/2) emission, while two-photon process involved in the green (⁴S_3/2→⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emissions. For the β-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles, two-photon process involved in the green (²H_11/2 → ⁴I_15/2), green (⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) emissions. It suggested that the crystal structure of TeO₂ nanoparticles had an effect on transition processes of the Er³⁺/Yb³⁺ ions. The emission intensities of the α-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles and β-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles were much stronger than those of the (α + β)-TeO₂:Er³⁺/Yb³⁺ (3/10 mol%) nanoparticles.     

Significant reduction of upconversion emission in CaTiO3: Yb, Er inverse opals

Inverse opal photonic crystals of Yb³⁺, Er³⁺ co-doped CaTiO₃ (CaTiO₃: Yb, Er) were prepared using self-assembled polystyrene templates combined with the infiltration of sol-gel precursor. The influence of the photonic band gap on upconversion emission of Er³⁺ has been investigated in the CaTiO₃: Yb, Er inverse opals. Significant reduction of the upconversion emission was detected if the photonic band-gap overlaps with the Er³⁺ ions emission band.     

Emissions properties of Ho: I7 → I8 transition sensitized by Er and Yb in fluorophosphate glasses

Emission properties of Ho³⁺ at 2.0 μm and the energy transfer mechanism between Yb³⁺, Er³⁺ and Ho³⁺ ions in fluorophosphate glasses are investigated. The measured emission spectra show that the ⁵I₇ → ⁵I₈ transition of Ho³⁺ upon 980 nm laser diode excitation is strong. Judd–Ofelt intensity parameters (Ω_λ, λ = 2, 4, 6), spontaneous transition probability (A_rad), radiative lifetime (τ_r), absorption cross section (σ_a), stimulated emission cross section (σ_e) and FWHM ×  for the transition of Ho³⁺: ⁵I₇ → ⁵I₈ are calculated and discussed. The obtained results show that the present Yb³⁺/Er³⁺/Ho³⁺ triply-doped fluorophosphate glass can be identified to be a promising material at 2.0 μm emission.     

Frequency upconversion of Tm and Yb codoped YLiF4 synthesized by hydrothermal method

In this paper, YLiF₄ codoped with Tm³⁺ and Yb³⁺ ions was synthesized by hydrothermal method. Yb³⁺ concentration is fixed at 1.5%, and Tm³⁺ concentration is changed from 0.1 to 0.4%. Intense upconversion luminescence is observed when the samples are excited by 980 nm. The dependence of upconversion luminescence on Tm³⁺ concentrations is presented. The results show that upconversion luminescence increases with the Tm⁺ concentration and gets its peak at 0.3 mol%. Under the excitation of 980 nm, the blue emission of 479 nm and the red emission of 647 nm are both duo to two photons process, and the UV emission of 361 nm is attributed to the three photons process. We also analyse the upconversion mechanism and process.     

Enhanced G4 emission in NaLaF4: Pr, Yb and charge transfer in NaLaF4: Ce, Yb studied by fourier transform luminescence spectroscopy

A high resolution luminescence study of NaLaF₄: 1%Pr³⁺, 5%Yb³⁺ and NaLaF₄: 1%Ce³⁺, 5%Yb³⁺ in the UV to NIR spectral range using a InGaAs detector and a fourier transform interferometer is reported. Although the Pr³⁺(³P₀ → ¹G₄), Yb³⁺(²F_7/2 → ²F_5/2) energy transfer step takes place, significant Pr³⁺¹G₄ emission around 993, 1330 and 1850 nm is observed. No experimental proof for the second energy transfer step in the down-conversion process between Pr³⁺ and Yb³⁺ can be given. In the case of NaLaF₄: Ce³⁺, Yb³⁺ it is concluded that the observed Yb³⁺ emission upon Ce³⁺ 5d excitation is the result of a charge transfer process instead of down-conversion.     

Controllable formation of Er–Yb codoped Al2O3 films by the non-aqueous sol–gel method

1 mol%Er³⁺–10 mol%Yb³⁺ codoped Al₂O₃ thin films have been prepared on thermally oxidized SiO₂/Si(110) substrates by a dip-coating process in the non-aqueous sol–gel method from the hydrolysis of aluminum isopropoxide [Al(OC₃H₇)₃] under isopropanol environment. Addition of N,N-dimethylformamide (DMF) as a drying control chemical additive (DCCA) into the sol suppresses formation of the cracks in the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films when the rare-earth ion is doped with a high doping concentration. Homogeneous, smooth and crack-free Er³⁺–Yb³⁺ codoped Al₂O₃ thin films form at the conditions by a molar ratio of 1:1 for DMF:Al(OC₃H₇)₃. A strong photoluminescence spectrum with a broadband extending from 1.400 to 1.700 µm centered at 1.533 µm is obtained for the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films, which is unrelated to the addition of DMF. Controllable formation of the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films may be explained by the fact that the DMF assisted the deprotonation process of Al–OH at the surfaces of gel particles, resulting in enhancement of the degree of polymerization of sols and improvement of the mechanical properties of gel thin films.     

EPR and vibrational studies of YVO4:Tm, Yb single crystal

A well oriented YVO₄ single crystal, with 5% Yb³⁺ and 2% Tm³⁺ nominal doping, was investigated using the Raman and EPR techniques.The EPR measurements suggest that Yb³⁺ ions occupy eight-coordinated Y³⁺ sites forming bisdisphenoids of the D_2d symmetry. An inhomogeneous distribution of rare-earth ions leads to a significant distortion of the local point symmetry (C₁). It seems that strong dipole–dipole interactions between Yb³⁺ ions are responsible for the distortion. As a result, two types of ytterbium magnetic centers appear. They correspond to paired magnetic centers and distorted isolated paramagnetic centers that are strongly sensitive to the magnetic field directions and some imperfections of the crystal. Pair centers can be recorded through the rotation around the c-crystal axis, whereas isolated centers can be measured when the crystal is rotated around the a-crystal axis. With the increasing temperature, the ytterbium signal disappeared at about 23 K and a group of narrow lines became visible. These lines, observed in the range of 240–550 mT, correspond to the Gd³⁺ (S = 7/2) ions, doped to the structure unintentionally from the basic materials.     

Sol-gel synthesis and green upconversion luminescence in BaGd2(MoO4)4:Yb,Er phosphors

Yb³⁺/Er³⁺ codoped BaGd₂(MoO₄)₄ phosphor powders were prepared by the Sol-gel method and the upconversion luminescence properties were investigated in detail. Under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits green upconversion luminescence with peaks at 530 and 550 nm, which are due to the transitions of Er³⁺ (²H_11/2) → Er³⁺ (⁴I_15/2) and Er³⁺ (⁴S_3/2) → Er³⁺ (⁴I_15/2), respectively. Both of the two green emission lines are produced by populating Er³⁺ ions to the excited state through a two-photon process. By monitoring the intensities of the green upconversion luminescence, the optimum conditions for the Sol-gel synthesis were determined when the molar ratio of citric acid to total chelate metal cations was 2:1 and the sintering temperature was at 1073 K. The concentration quenching effect for Er³⁺ was found at the optimum doping concentration of 6 mol%, and the critical distance for the neighboring Er³⁺ was determined to be about 21.5 Å.     

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.     

Optical investigation of Eu:PbF2 ceramics and transparent glass-ceramics

This paper reports an optical investigation of glass-ceramics formed by annealing glasses with compositions 50 GeO₂-40 PbO-10 PbF₂-x EuF₃, x = [0.5; 1; 1.5; 2] and polycrystalline ceramics with composition 100 PbF₂, y EuF₃, with y = 5, 10, 15 and 20. For each material, the photoluminescence spectrum and the photoluminescence lifetimes of the ⁵D₀, ⁵D₁ and ⁵D₂ Eu³⁺ levels are measured. Occurrence of Eu³⁺:β-PbF₂ nanocrystallites in the glass-ceramics is confirmed and total ceramisation requires more than 10% of EuF₃ with respect to PbF₂ in the starting glass.In the Eu³⁺:β-PbF₂ ceramics and glass-ceramics, Eu³⁺ ions replace Pb²⁺ in their regular cubic site, but they interact together to form dimers and higher nuclearity clusters. These two species are easily distinguished according to their photoluminescence decay rate. For the EuF₃ rates investigated here, there are no isolated Eu³⁺ ions in the PbF₂ lattice.A preliminary investigation of the optical properties of co-doped Gd³⁺:Eu³⁺:β-PbF₂ ceramics was also performed. It shows that mixed Gd³⁺-Eu³⁺ dimers and clusters are formed, and that efficient Gd³⁺ → Eu³⁺ energy transfer occurs in these ceramics. The Pb²⁺ ions of the lattice may also be involved in the energy transfer process.     

Fluorescence property investigations on Er-doped oxyfluoride glass ceramics containing LaF3 nanocrystals

The fluorescence properties of oxyfluoride glass ceramics containing nanosized LaF₃ crystals with different ErF₃ doping level were investigated. The spectroscopy analysis indicated that a dominant fraction of ErF₃ had been incorporated into the crystal phase. Broad 1.5 μm emission spectra with full width at half maximum (FWHM) value ranging from about 40–100 nm were obtained, which increased accordingly with ErF₃ doping level. Noteworthily, intense green and red upconversion emissions from samples with high ErF₃ doping level were observed when excited even with a 30 mW diode laser at 976 nm. An overall increment of the upconversion emissions intensity, and, a relative increase in intensity of the red emission with respect to that of the green one were also identified with increasing ErF₃ concentration. The possible upconversion mechanisms were proposed.     

Structure and up-conversion luminescence in sol–gel derived Er–Yb co-doped SiO2:PbF2 nano-glass–ceramics

Transparent oxyfluoride nano-glass–ceramics 90(SiO₂)10(PbF₂) co-doped with 0.3 Yb³⁺ and 0.1 Er³⁺ (mol%) have been prepared by thermal treatment of precursor sol–gel glasses. X-ray diffraction and high resolution transmission electron microscopy analysis pointed out a precipitation of cubic β-PbF₂ nanocrystals of certain diameter in nano-glass–ceramics varying from 10 to 20 nm depending on heat treatment conditions. The incorporation of Yb³⁺ and Er³⁺ dopants in these nanocrystals has been confirmed by signatures of luminescence spectroscopy. Up-conversion luminescence pumped at 980 nm has been detected. Colour tuneability of up-conversion luminescence varying pump power has been analyzed in terms of standard chromaticity diagram. This tuneability opens applications for up-conversion phosphors and three-dimensional optical recording.     

Infrared to visible upconversion luminescence in Er/Yb co-doped CeO2 inverse opal

Infrared to visible upconversion luminescence has been investigated in Er³⁺/Yb³⁺ co-doped CeO₂ inverse opal. Under the excitation of 980 nm diode lasers, visible emissions centered at 525, 547, 561, 660 and 680 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (547, 561 nm), ⁴F_9/2 → ⁴I_15/2 (660 and 680 nm), respectively. The effect of photonic band gap on the upconversion luminescence intensity was also obtained. Additionally, the upconversion luminescence mechanism was studied. The dependence of Er³⁺ upconversion emission intensity on pump power reveals that it is a two-photon excitation process.     

Dependence of optical properties on the composition in Er-doped xNaPO3–(80 − x)TeO2–10ZnO–10Na2O glasses

Transparent and uniform tellurite–phosphate glasses were prepared and the reason why the substitution of NaPO₃ for P₂O₅ can eliminate the coloration of tellurite–phosphate was discussed. The result of TDA indicated that introducing NaPO₃ into tellurite glasses can improve thermal stability of glass hosts. The compositional dependence of absorption cross-sections of ⁴I_13/2, ⁴I_11/2 and ²H_11/2 level, emission cross-section of ⁴I_13/2 level, host phonon energy, up-conversion and 1.5 μm optical emission intensity as well as and quantum yield for ⁴I_13/2 level in PTEr glasses were investigated too. By analyzing obtained data, authors believe that tellurite–phosphate glasses can be used as potential host material for developing optical amplifiers.     

Efficient two-color luminescence of Er/Yb/Li:ZrO2 nanocrystals

We demonstrate the upconversion-photoluminescence spectra of Er³⁺, Yb³⁺ and Li⁺ ions doped ZrO₂ nanocrystals. By introducing Li⁺, emission intensities of single green and single red band increase by a factor of 1.93 and 1.65, respectively. Powder X-ray diffraction data and decreased slopes of the excitation power dependences on upconverted emission intensities give evidences that Li⁺ ions can tailor the local structure of host lattice and improve energy transfer processes from Yb³⁺ to Er³⁺, respectively.     

Synthesis and photoluminescence properties of Yb doped Ba5(PO4)3Cl phosphor for white light-emitting diodes

Yb²⁺ ion doped Ba₅(PO₄)₃Cl phosphor was synthesized by solid state reaction. Four distinct absorption bands were observed in the Ultraviolet (UV) light region due to the electronic transitions of Yb²⁺ ion from ¹S₀ ground state to ²F_5/2(t_2g), ²F_5/2(e_g), ²F_7/2(t_2g), and ²F_7/2(e_g) excited states. The main emission wavelength of the phosphor was around 630 nm. The optimized Yb²⁺ ion concentration was 0.2 mol% (λ_exc. = 400 nm). The calculated critical distance was about 8.729 Å and the concentration quenching was observed above 0.2 mol% due to the electric dipole–dipole interaction.     

Laser-induced time-resolved luminescence of orange kyanite Al2SiO5

Manganese is a very important microelement performing a large number of biological functions in human body. We have detected by spectroscopic measurements manganese in mineral kyanite. In this paper we present laser-induced time-resolved luminescence and optical absorbance spectra of orange, Mn containing kyanite. It was proven the orange color is caused by Mn³⁺. Several luminescence lines and bands were found and ascribed to Mn⁴⁺ and Mn³⁺, emission centers. The spectroscopic technique can be utilized for detection of small amounts of manganese in minerals.     

Investigations of optical and scintillation properties of Tm-doped YAlO3

Pure, 0.1, 0.5 and 1 mol% Tm-doped YAP single crystalline scintillators were grown by the μ-PD method. The XRD analysis confirmed the lattice constants decrease with the Tm concentration. In the transmittance measurement, the absorption bands due to the Tm³⁺ 4f-4f transitions were observed at 265, 360, 485, 690 and 800 nm and they were ascribed to the transition from the ³H₆ ground state to its ¹I₆, ¹D₂, ¹G₄, ³F₃ and ³H₄ excited states, respectively. Strong emission peak due to the ¹I₆-³F₄ transition of Tm³⁺ appeared at 350 nm under X-ray irradiation. The photoluminescence decay time constants related to this transition were evaluated to be from 15.3 to 17.3 μs and the scintillation decay time constants under gamma-ray excitation were estimated to be from 17.5 to 18.8 μs. The Tm 1% doped crystal exhibited the highest light yield of 15, 100 ± 1500 photons/MeV when excited by ¹³⁷Cs gamma-ray radiation.