Synthesis and characterization of Yb3+,Tm3+:Ba2YF7 nanocrystalline with efficient upconversion fluorescence

A novel upconversion luminescence nanocrystals Yb³⁺,Tm³⁺:Ba₂YF₇ were synthesized via the hydrothermal method. They have uniform morphology with a mean size of 30 nm even if annealed at 600 °C. Pumped by 980 nm laser diode the as-synthesized powers emit ultraviolet/blue light, which is in the range of the specific upconversion luminescent spectra of Tm³⁺ ions. After post-annealing at 600 °C in an argon atmosphere for 2 h, their upconversion luminescence intensity is 5 multiple improved and the ultraviolet/blue light can even be seen by the naked eyes under a low excitation power of 20 mW. This indicates that Ba₂YF₇ is a very effective luminescent host material. Excitation power dependences of individual upconversion emission intensity are plotted, which partly uncover the upconversion luminescence mechanism of Tm³⁺ ions.     

Two- and three-photon upconversion of LaOBr:Er3+

In this work, the LaOBr:Er³⁺ (0.1%) powders were prepared by solid state reaction. The structural properties of LaOBr:Er³⁺ were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. The results show that LaOBr:Er³⁺ has low phonon energy, which indicate that LaOBr:Er³⁺ may have high luminescent efficiency. Under excitation into ⁴I_11/2 level of Er³⁺ ions by 980 nm laser, the two- and three-photon upconverted luminescence of LaOBr:Er³⁺ were recorded. The most intense emissions were come from the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions. The upconversion mechanisms were studied in detail through laser power dependence, and results show that excited state absorption is responsible for the upconversion. The upconversion properties indicate that LaOBr:Er³⁺ may be used in upconversion phosphors.     

Frequency upconversion properties of Er3+/Yb3+-codoped lead–germanium–bismuth oxide glasses

The frequency upconversion properties of Er³⁺/Yb³⁺-codoped heavy metal oxide lead–germanium–bismuth oxide glasses under 975 nm excitation are investigated. Intense green and red emission bands centered at 536, 556 and 672 nm, corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively, were simultaneously observed at room temperature. The influences of PbO on upconversion intensity for the green (536 and 556 nm) and red (672 nm) emissions were compared and discussed. The optimized rare earth doping ratio of Er³⁺ and Yb³⁺ is 1:5 for these glasses, which results in the stronger upconversion fluorescence intensities. The dependence of intensities of upconversion emission on excitation power and possible upconversion mechanisms were evaluated and analyzed. The structure of glass has been investigated by means of infrared (IR) spectral analysis. The results indicate that the Er³⁺/Yb³⁺-codoped heavy metal oxide lead–germanium–bismuth oxide glasses may be a potential materials for developing upconversion fiber optic devices.     

Yellow to violet upconversion processes of Nd3+ in neat Cs2NaNdCl6 elpasolite

Yellow to violet upconversion of Nd³⁺ in neat Cs₂NaNdCl₆ elpasolite is demonstrated in this paper. Three intense bands have been observed at 367, 390 and 422 nm originating from ⁴D_3/2 multiplet upon the excitation of 595.5 nm, which is in resonance with the energy of ⁴G_5/2 level of Nd³⁺. The mechanism of the upconversion has been investigated through upconversion emission intensity dependence on the power of the pumping light and luminescence decay. It is found out that the upconversion is a two-photon process. The ground state absorption (GSA)/energy transfer upconversion (ETU) is responsible for the upconversion with comparison of the excitation spectrum and the one-photon absorption spectrum.     

Yellow lighting upconversion from Yb3+/Ho3+ co-doped CaMoO4

Yellow upconversion (UC) luminescence is observed in Ho³⁺/Yb³⁺ co-doped CaMoO₄ synthesized by complex citrate-gel method. Under 980 nm excitation, Ho³⁺/Yb³⁺ co-doped CaMoO₄ exhibited yellow emission based on green emission near 543 nm generated by ⁴F₄, ⁵S₂ → ⁵I8 transition and strong red emission around 656 nm generated by ⁵F₅ → ⁵I₈ transition, which are assigned to the intra 4f transitions of Ho³⁺ ions. The optimum doping concentration of Ho³⁺ and Yb³⁺ was investigated for highest upconversion luminescence. Based on pump power dependence, upconversion mechanism of Ho³⁺/Yb³⁺ co-doped CaMoO₄ was studied in detail.     

Efficient upconversion-pumped continuous wave Er3+:LiLuF4 lasers

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er³⁺:LiLuF₄. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974 nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552 nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552 nm, allowing for efficient cw laser operation.We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774 mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%.To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er³⁺:LiLuF₄ crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974 nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10 mW of output power at 553 nm.     

Comparative study of Er3+ and Tm3+ co-doped YOF and Y2O3 powders as red spectrally pure upconverters

We prepared Er³⁺ and Tm³⁺ co-doped yttrium oxyfluoride (YOF) powder by combustion synthesis and we observed that under near-infrared (λ = 980 nm) laser excitation the characteristic green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) emission of Er³⁺ was suppressed by energy transfer (ET) mechanisms between Tm³⁺ and Er³⁺. The ET process observed in YOF was much more efficient than that observed in standard Y₂O₃ powder prepared under similar conditions. YOF combines the superior mechanical and thermal properties of oxides with low phonon energy of fluorides. Our results show that this material is a serious candidate for use as a red upconversion phosphor.     

Fluorescence spectroscopy of Er3+:LaOBr prepared by NH4Br solid state reaction

The steady luminescent materials La_1−xEr_xOBr (x = 0, 0.0003, 0.01) were synthesized by a new NH₄Br solid state reaction, and the structures were studied using XRD and Raman methods. Under 514.5 nm Ar⁺ laser excitation, the upconversion fluorescence spectra in LaOBr:Er³⁺ were recorded and investigated. It was found that four upconverted emission bands with peaks at 388 nm, 399 nm, 405 nm (violet) and 477 nm (blue) were observed. All these upconverted emissions were assigned, and the upconversion mechanism was deduced to be excited state absorption (ESA), by analyzing the energy level structures of Er³⁺ ions and measuring the power dependence of upconverted emission intensities.     

Energy transfer and energy level decay processes of Er3+ in water-free tellurite glass

This report details the fundamental spectroscopic properties of a new class of water-free tellurite glasses studied for future applications in mid-infrared light generation. The fundamental excited state decay processes relating to the ⁴I_11/2 → ⁴I_13/2 transition in singly Er³⁺-doped Tellurium Zinc Lanthanum glass have been investigated using time-resolved fluorescence spectroscopy. The excited state dynamics was analyzed for Er₂O₃ concentrations between 0.5 mol% and 4 mol%. Selective laser excitation of the ⁴I_11/2 energy level at 972 nm and selective laser excitation of the ⁴I_13/2 energy level at 1485 nm has established that in a similar way to other Er³⁺-doped glasses, a strong energy-transfer upconversion by way of a dipole–dipole interaction between two excited erbium ions in the ⁴I_13/2 level populates the ⁴I_11/2 upper laser level of the 3 μm transition. The ⁴I_13/2 and ⁴I_11/2 energy levels emitted luminescence with peaks located at 1532 nm and 2734 nm respectively with luminescence efficiencies of 100% and 8% for the higher (4 mol.%) concentration sample. Results from numerical simulations showed that a population inversion is reached at a threshold pumping intensity of ∼57 kW cm⁻² for a CW laser pump at 976 nm for [Er₂O₃] = 2 mol.%.     

Upconversion luminescence of Ba(MoO4)h(WO4)1−h:Yb3+/Er3+ nanocrystals synthesized through hydrothermal method

A series of Yb³⁺/Er³⁺ co-doped Ba(MoO₄)_h(WO₄)_1−h upconversion nanocrystals (UCNCs) were prepared via hydrothermal method. The effects of different concentration ratios of Yb³⁺/Er³⁺ and Mo₄O²⁻/WO₄²⁻ on the upconversion luminescence were investigated, and the optimum doping concentrations of Yb³⁺ and Er³⁺ in the Ba(MoO₄)_0.5(WO₄)_0.5 host were found to be 3 mol% and 1 mol%, respectively. Structure of Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺ was identified as the tetragonal in the X-ray diffraction (XRD) results and the particle size observed in the scanning electron microscope (SEM) was about 40 nm. Under excitation of 980 nm semiconductor laser, three emission bands centered at 528, 550 and 660 nm, originating from ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively, were observed for Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺. The pump power dependence research suggested that these bands arise due to two-photon absorption. The variation of CIE coordinate at different excitation powers was observed.     

Ultraviolet upconversion luminescence of Gd3+ and Eu3+ in nano-structured glass ceramics

Ultraviolet multiphoton upconversion emissions of Eu³⁺ (⁵H_3–7, ⁵G_2–6, ⁵L₆ → ⁷F₀) and Gd³⁺ (⁶I_J, ⁶P_J → ⁸S_7/2) are studied in the Eu³⁺ (or Gd³⁺) doped SiO₂–Al₂O₃–NaF–YF₃ precursor glasses and glass ceramics containing β-YF₃ nanocrystals, under continuous-wavelength 976 nm laser pumping. It is experimentally demonstrated that energy transfer from Yb³⁺ to Tm³⁺, then further to Eu³⁺ or Gd³⁺ is responsible for the upconversion process. Compared to those in the precursor glasses, the upconversion emission intensities in the glass ceramics are greatly enhanced, owing to the participation of rare earth ions into the low-phonon-energy environment of β-YF₃ nanocrystals. Hopefully, the studied glass ceramics may find potential applications in the field of ultraviolet solid-state lasers.     

Multicolor upconversion of Er3+/Tm3+/Yb3+ doped oxyfluoride glass ceramics

Er³⁺/Tm³⁺/Yb³⁺ tridoped oxyfluoride glass ceramics was synthesized in a general way. Under 980 nm LD pumping, intense red, green and blue upconversion was obtained. And with those primary colors, multicolor luminescence was observed in oxyfluoride glass ceramics with various dopant concentrations. The red and green upconversion is consistent with ⁴F_9/2 → ⁴I_15/2 and ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ respectively. While the blue upconversion originates from ¹G₄ → ³H₆ transition of Tm³⁺. This is similar to that in Er³⁺/Yb³⁺ and/or Tm³⁺/Yb³⁺ codoped glass ceramics. However the upconversion of Tm³⁺ is enhanced by the energy transfer between Er³⁺ and Tm³⁺.     

Luminescence properties of Yb:Er:KY3F10 nanophosphor and thermal treatment effects

In this work, we present the spectroscopic properties of KY₃F₁₀ nanocrystals activated with erbium and codoped with ytterbium ions. The most important processes that lead to the erbium upconversion of green and red emissions of Er³⁺ were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays of ⁴S_3/2 and ⁴F_9/2 excited levels of Er³⁺ and to determine the upconversion processes and the luminescence efficiencies of erbium in the visible. Analysis of the luminescence kinetics in Yb:Er:KY₃F₁₀ shows a rapid upconversion (Up₁) for the green emission with a time constant of 0.31 μs after pulsed laser excitation at 972 nm for as synthesized nanocrystals, which is faster than the time constant measured for the bulk crystal (23 μs). In addition, it is observed a second upconversion process (non-resonant) (Up₂) responsible for the red emission (Er³⁺), which competes with Up₁ process. However, the luminescence efficiency of the green emission (⁴S_3/2) is observed to be very low (1.6%) for the as synthesized nanocrystal (25 °C). Nevertheless, it increases with the nanopowder heat treatment reaching an efficiency of 99% (T = 550 °C) relative to the bulk crystal. Similar luminescence behavior was observed for the ⁴F_9/2 level (Er³⁺) that emits red emission. X-ray diffraction analysis of nanopowder by Rietveld method reveled that the mean crystallite size remains unchanged (8.3–12.3 nm) after thermal treatments with T ∼ 400 °C, while the ⁴S_3/2 luminescence efficiency strongly increases to 20%. The luminescence dynamics indicates that Er³⁺ ions distribution plays a determinant role in the luminescence efficiency of green and red emissions of Er³⁺ besides also the strong influence on the upconversions processes. The observed luminescence effect is caused by the non-uniform Er³⁺ (and Yb³⁺) ions distribution due to the nanocrystal grown, which introduces a concentration gradient that increases towards the nanoparticle surface. This concentration effect produces strong (Er × Er) cross-relaxations depleting the excited states populations of ⁴S_3/2 and ⁴F_9/2 levels and their luminescence efficiencies in KY₃F₁₀ nanocrystals. The concentration gradient is very accentuated in the as synthesized nanocrystal and gradually decreases with the thermal treatments where the dopant ions can migrate through the lattice towards the nanocrystal’s interior to get a more uniform and random distribution, which is reached after heat treatment to T = 550 °C.     

Synthesis and spectroscopy of transparent colloidal solution of Gd2O3: Er3+, Yb3+ spherical nanocrystals by pulsed laser ablation

The synthesis and the spectroscopy of upconverting nanocolloidal solutions have recently generated considerable interest due to their potential application as biolevels and in biological assays. This paper reports the synthesis of lanthanides doped transparent colloidal solution via pulsed laser ablation (PLA) which is highly fluorescing. Er³⁺, Yb³⁺ co-doped Gd₂O₃ phosphor has been laser ablated to synthesize the colloidal solution in triply distilled water. Spherical shaped nanoparticles of the average diameter in the range of 8–26 nm have been synthesized and characterized. Efficient multicolor upconversion (UC) emission has been observed and possible UC mechanism has been suggested. This approach will provide a method to synthesize highly UC efficient, non-agglomerated, pure transparent nanocolloidal solution for biological applications from already reported efficient phosphors.     

Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca,Sr; B = W,Mo) phosphors

ABO₄ (A = Ca, Sr; B = W, Mo):Er³⁺/Yb³⁺/Li⁺ phosphors tri-doped with different concentrations of Li⁺ ion ranging from 0 to 22.5 mol% were prepared by using a solid-state reaction method. And their upconversion (UC) luminescence properties were in estimated under a 975 nm laser-diode excitation. The four kinds of phosphors (CaWO₄, CaMoO₄, SrWO₄, and SrMoO₄) tri-doped with Er³⁺, Yb³⁺ and Li⁺ ions showed strong green UC emission peaks at 530 nm and 550 nm and weak red UC emission. The intensity of green UC emission of Li⁺ doped samples was several higher than that of Li⁺ un-doped samples due to the reduction of lattice constant and the local crystal field distortion around rare-earth ions. The optimum doping concentration of Li⁺ ions was investigated and the effects of Li⁺ concentration for UC emission intensity were studied in detail.     

Effect of processing parameters on structural,morphological and optical Y2O3:Yb3+/Ho3+ powders characteristics

Rod-like and flake-like up-converting Y₂O₃:Yb³⁺/Ho³⁺ particles which are composed of nanoparticles with size less than 100 nm, are prepared by a simple hydrothermal processing at 473 K (3 h) followed by additional thermal treatment at 1373 K (3 and 12 h). The effect of precursor pH value on the formation of Y₂O₃:Yb³⁺/Ho³⁺ is followed through X-ray powder diffractometry (XRPD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Structural refinement confirms formation of the cubic bixbyte structure (S.G. Ia-3) with the non-uniform accommodation of dopants at C₂ and S₆ cationic sites. Under 978 nm laser excitation, strong green (530–570 nm) up-conversion is observed in all samples. The emission shows a decrease in intensity with an increase in external temperature, indicating FIR (fluorescence intensity ratio) based temperature sensing behavior of 0.52% for the ⁵F₄ → ⁵I₈/⁵S₂ → ⁵I₈ transitions.     

Comparison of laser generation in thermally bonded and unbonded Er3+,Yb3+:glass/Co2+:MgAl2O4 microchip lasers

Pulse laser generation in several Er³⁺,Yb³⁺:glasses thermally bonded with Co²⁺:MgAl₂O₄ was achieved. Peak power in the range of 1.83–7.68 kW with pulse duration between 2.9 and 4.2 ns and energy up to 24 μJ was obtained. The output characteristics for different transmissions of the output couplers were investigated. To show the improvements gained by the thermal bonding procedure, a comparison of thermally bonded and unbonded samples was done in terms of generation efficiency, peak power, beam quality, generated spectra and pulse to pulse jitter.     

Variation of the photoluminescence and vacuum ultraviolet excitation characteristics of BaZr(BO3)2:Eu3+ by the incorporation of Al3+, La3+, or Y3+ into the lattice

The photoluminescence (PL) and vacuum ultraviolet (VUV) excitation properties are studied for the BaZr(BO₃)₂:Eu³⁺ phosphor with incorporating the Al³⁺, La³⁺, or Y³⁺ ion into the lattice. The excitation spectrum shows an absorption band in the VUV region with the band-edge at 200 nm and a very weak charge transfer band of Eu³⁺ at about 226 nm. The luminescence spectrum shows a strong emission at 615 nm (⁵D₀ → ⁷F₂ transition) and weak emission at 594 nm (⁵D₀ → ⁷F₁ transition) in BaZr(BO₃)₂:Eu³⁺, with a good red color purity. The PL intensity is increased by incorporating Al³⁺ into the BaZr(BO₃)₂ lattice. The PL intensity has also increased by incorporating La³⁺ into the lattice, however, the red color purity has deteriorated because of the increased centrosymmetric nature of the site. With the incorporation of Y³⁺ into the BaZr(BO₃)₂ lattice, the PL characteristics of the Eu³⁺ activator resembles that in the YBO₃ lattices. The intensity of the red PL for the Eu³⁺ activator is the highest with good color purity for BaZr(BO₃)₂:Eu³⁺ incorporated with both Al³⁺ (10%) and La³⁺ (0.5%).     

Effects of excitation wavelength and Y3+ content on luminescent properties of YMO4:Dy3+ (M = V,P) phosphors induced by ultraviolet excitation

Y_0.99VO₄:0.01Dy³⁺, Y_0.99PO₄:0.01Dy³⁺ and Y_xVO₄:0.01Dy³⁺ phosphors were synthesized by chemical co-precipitation method. All the samples were characterized by X-ray powder diffraction (XRD) and photoluminescence spectroscopy. XRD results show that the samples only have single tetragonal structure and the crystallinity of Y_0.99VO₄:0.01Dy³⁺ phosphor is higher than that of Y_0.99PO₄:0.01Dy³⁺ phosphor when the heat treatment process is same. Photoluminescence excitation spectra results show that the Y_0.99VO₄:0.01Dy³⁺ and Y_0.99PO₄:0.01Dy³⁺ phosphors can be efficiently excited by ultraviolet light from 250 nm to 380 nm, the former have a wide Dy³⁺–O^2? charge transfer band ranging from 260 nm to 350 nm including a peak at 310 nm, the latter have four peaks at 294 nm, 326 nm, 352 nm and 365 nm. Emission spectra of all the samples exhibit a strong blue emission (483 nm) and another strong yellow emission (574 nm). Moreover, the yellow-to-blue emission intensity ratio and color temperature of emission of Dy³⁺ are strongly related to excitation wavelength in Y_0.99PO₄:0.01Dy³⁺ phosphor, but it is almost not in Y_0.99VO₄:0.01Dy³⁺ phosphor. For Y_xVO₄:0.01Dy³⁺ (x = 0.94, 0.97, 0.99, 1.01, 1.03) phosphors, with increasing value of x, the body color of phosphor changes from yellow to white and the strongest peak in excitation spectra shifts a little to shorter wavelength. It is detrimental to luminous intensity when Y³⁺ content deviate stoichiometric ratio, but the influence of Y³⁺ on the color temperature of emission of YVO₄:Dy³⁺ phosphor is slight.     

Influences of Er3+ content on structure and upconversion emission of oxyfluoride glass ceramics containing CaF2 nanocrystals

Transparent 45SiO₂–25Al₂O₃–5CaO–10NaF–15CaF₂ glass ceramics doped with different content of erbium ion (Er³⁺) were prepared. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses evidenced the spherical CaF₂ nanocrystals homogeneously embedded among the glassy matrix. With increasing of Er³⁺ content, the size of CaF₂ nanocrystals decreased while the number density increased. The crystallization kinetics studies revealed that CaF₂ crystallization was a diffusion-controlled growth process from small dimensions with decreasing nucleation rate. Er³⁺ could act as nucleating agent to lower down crystallization temperature, while some of them may stay at the crystal surfaces to retard the growth of crystal. Intense red and weak green upconversion emissions were recorded for glass ceramics and their intensities increased with the increasing of Er³⁺ content under 980 nm excitation. However, the concentration quenching effect appeared when Er³⁺ doping reached 2 mol%. These results could be attributed to the change of ligand field of Er³⁺ ions due to the incorporation of Er³⁺ ions into precipitated fluoride nanocrystals.