Upconversion emission in antimony–germanate double-clad optical fiber co-doped with Yb3+/Tm3+ ions

In the paper upconversion luminescence properties in Yb³⁺/Tm³⁺ co-doped antimony–germanate glass and double-clad optical fiber were studied. The concentration of lanthanides, which has shown the highest upconversion emission intensity at 478 nm (¹G₄ → ³H₆) and 650 nm (¹G₄ → ³F₄), is 1Yb₂O₃/0.1Tm₂O₃ (mol%) as a result of exciting with a laser diode (976 nm). The lifetime of ²F_5/2 (Yb³⁺) level decreases from 781 μs to 71 μs in the presence of Tm³⁺ 0.1–0.75 mol% respectively. Luminescence decay curve of glass co-doped with 1Yb₂O₃/0.75Tm₂O₃ suggests donor–donor fast migration followed by Tm³⁺ → Yb³⁺ energy transfer. Glass characterized by highest intensity of upconversion luminescence (1Yb₂O₃/0.1Tm₂O₃ mol%) was used as core of double-clad optical fiber made by modified rod-in-tube method. Mechanisms influencing differences in upconversion amplified spontaneous emission of the fabricated optical fiber and bulk glass were discussed. Reabsorption of the amplified spontaneous emission signal along the fibre resulting from Tm³⁺:³H₆ → ¹G₄, transition was observed.     

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.     

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⁺.     

Blue up-conversion emission of Yb3+-doped langbeinite salts

Yb³⁺-doped langbeinite salts were prepared by the solid solution method. X-ray diffraction patterns and vibrational spectroscopy confirmed that all obtained phases are highly pure, iso-structural and they crystallize in the cubic system with the space group P2₁3. The emission luminescence comes from the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions. Moreover, intense blue cooperative emission was observed at 476 nm under excitation in the near infrared at 975 nm.     

Synthesis and two-color emission properties of BaGd2(MoO4)4:Eu3+,Er3+,Yb3+ phosphors

Eu³⁺, Er³⁺ and Yb³⁺ co-doped BaGd₂(MoO₄)₄ two-color emission phosphor was synthesized by the high temperature solid-state method. The structure of the sample was characterized by XRD, and its luminescence properties were investigated in detail. Under the excitation of 395 nm ultraviolet light, the BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ phosphor emitted an intense red light at 595 and 614 nm, which can be attributed to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺, respectively. The phosphor will also show bright green light under 980 nm infrared light excitation. The green emission peaks centred at 529 and 552 nm, were attributed to ⁴H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺, respectively. It indicated that the two-color emission can be achieved from the same BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ host system based on the different pumping source, 395 nm UV light and 980 nm infrared light, respectively. The obtained results showed that this kind of phosphor may be potential in the field of multi-color fluorescence imaging and anti-counterfeiting.     

Upconversion luminescence properties of nanocrystallite MgAl2O4 spinel doped with Ho3+ and Yb3+ ions

The upconversion luminescence spectra of nanocrystallite MgAl₂O₄ doped with 1% of Ho³⁺ and 5% of Yb³⁺ ions after excitation at 980 nm were measured. Influence of excitation regime either continuous or pulse on upconversion mechanisms was shown. For continuous wave (CW) laser excitation upconversion process is due to phonon assisted Excited State Absorption (ESA). For pulse laser excitation upconversion emission is due to Energy Transfer Upconversion (ETU).     

Microstructure and upconversion luminescence of Yb<Superscript>3+</Superscript> and Ho<Superscript>3+</Superscript> co-doped BST thick films

Ba_0.8Sr_0.2TiO₃ (BST) thick films co-doped with Yb³⁺ and Ho³⁺ were fabricated by the screen printing techniques on alumina substrates. The structure and morphology of the BST thick films were studied by XRD and SEM, respectively. After sintered at 1240 °C for 100 min the BST thick films are polycrystalline with a perovskite structure. The upconversion luminescence properties of the RE-doped BST thick films under 800 nm excitation at room temperature were investigated. The upconversion emission bands centered at 470 and 534 nm corresponding to ⁵F₁ → ⁵I₈ and ⁵F₄ → ⁵I₈ transition, respectively were observed, and the upconversion mechanisms were discussed. The dependence of the upconversion emission intensity upon the Ho³⁺ ions concentration was also examined; the emission intensity reaches a maximum value in the sample with 2 mol% Yb³⁺ and 0.250 mol% Ho³⁺ ions. All the results show that the BST thick films co-doped with Yb³⁺ and Ho³⁺ may have potential use for photoelectric devices.     

Near-infrared quantum cutting in Bi3+/Yb3+ co-doped oxyfluoride glasses via cooperative energy transfer for solar cells

The Bi³⁺/Yb³⁺ ion co-doped 55SiO₂–20Al₂O₃–5Na₂CO₃–20CaF₂ glasses are synthesized successfully by a conventional melting-quenching method. High efficient quantum cutting involving the emission of two near-infrared photons for one ultraviolet photon absorbed is realized in the oxyfluoride glasses co-doped with Bi³⁺ and Yb³⁺. An intense characteristic near-infrared emission around 977 nm of Yb³⁺:²F_5/2 → ²F_7/2 transition is obtained when the 303 nm is as excitation wavelength to induce the ¹S₀ → ³P₁ transition of Bi³⁺. The maximum quantum efficiency of our glasses is estimated to be 164.3%. The energy transfer mechanism is proposed to be a cooperative energy transfer via second-order down-conversion process. The glasses could be a potential quantum cutting converter to improve the photovoltaic energy conversion efficiency of crystalline Si solar cells via spectrum modification.     

Visible to infrared energy conversion in Pr3+–Yb3+ co-doped fluoroindate glasses

Processes involving visible to infrared energy conversion are presented for Pr³⁺–Yb³⁺ co-doped fluoroindate glasses. The emission in the visible and infrared regions, the luminescence decay time of the Pr³⁺:³P₀ → ³H₄ (482 nm), Pr³⁺:¹D₂ → ³H₆ (800 nm), Yb³⁺:²F_5/2 → ²F_7/2 (1044 nm) transitions and the photoluminescence excitation spectra were measured in Pr³⁺ samples and in Pr³⁺–Yb³⁺ samples as a function of the Yb³⁺ concentration. In addition, energy transfer efficiencies were estimated from Pr³⁺:³P₀ and Pr³⁺:¹D₂ levels to Yb³⁺:²F_7/2 level. Down-Conversion (DC) emission is observed due to a combination of two different processes: 1-a one-step cross relaxation (Pr³⁺:³P₀ → ¹G₄; Yb³⁺:²F_7/2 → ²F_5/2) resulting in one photon emitted by Pr³⁺ (¹G₄ → ³H₅) and one photon emitted by Yb³⁺ (²F_7/2 → ²F_5/2); 2-a resonant two-step first order energy transfer, where the first part of energy is transferred to Yb³⁺ neighbor through cross relaxation (Pr³⁺:³P₀ → ¹G₄; Yb³⁺:²F_7/2 → ²F_5/2) followed by a second energy transfer step (Pr³⁺:¹G₄ → ³H₄; Yb³⁺:²F_7/2 → ²F_5/2). A third process leading to one IR photon emission to each visible photon absorbed involves cross relaxation energy transfer (Pr³⁺:¹D₂ → ³F₄; Yb³⁺:²F_7/2 → ²F_5/2).     

Investigation on broadband near-infrared emission in Yb3+/Ho3+ co-doped antimony–silicate glass and optical fiber

In the paper antimony–silicate glass and double-clad optical fiber co-doped with ytterbium and holmium ions were investigated. Absorption spectra in infrared (FT-IR) showed characteristic bands: 445, 605, 1037, 1168 cm⁻¹ coming from the vibration of chemical bonds of SbO₃ and SiO₄, respectively. The combination of relatively low phonon energy with a capability for greater separation (avoiding clustering) of optically active centers in the fabricated glasses should allow an effective expansion of spontaneous emission band. The highest intensity of emission at the wavelength of λ_e = 1950 nm resulting from energy transfer between Yb³⁺ → Ho³⁺ ions was observed in the glass co-doped with 1 mol% Yb₂O₃:0.5 mol% Ho₂O₃. As a result of the optical pumping at the wavelength of 976 nm in the produced optical fiber, strong and narrow band of amplified spontaneous emission (ASE) around 2.1 μm, corresponds to the ⁵I₇ → ⁵I₈ transition, were obtained.     

Bifunction in Er3+/Yb3+ co-doped BaTi2O5–Gd2O3 glasses prepared by aerodynamic levitation method

Novel Er³⁺/Yb³⁺ co-doped BaTi₂O₅–Gd₂O₃ spherical glasses have been fabricated by aerodynamic levitation method. The thermal stability, upconversion luminescence, and magnetic properties of the present glass have been studied. The glasses show high thermal stability with 763.3 °C of the onset temperature of the glass transition. Red and green emissions centered at 671 nm, 548 nm and 535 nm are obtained at 980 nm excitation. The upconversion is based on a two-photon process by energy transfer, excited-state absorption, and energy back transfer. Yb³⁺ ions are more than Er³⁺ ions in the glass, resulting in efficient energy back transfer from Er³⁺ to Yb³⁺. So the red emission is stronger than the green emissions. Magnetization curves indicate that magnetic rare earth ions are paramagnetic and the distribution is homogeneous and random in the glass matrix. Aerodynamic levitation method is an efficient way to prepare glasses with homogeneous rare earth ions.     

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.     

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.     

Strong upconversion luminescence in LiYMo2O8:Er,Yb towards efficiency enhancement of dye-sensitized solar cells

Strong green upconversion (UC) emissions induced by a 940 nm laser have been observed in a series of Er³⁺, Yb³⁺ co-doped LiYMo₂O₈ samples. The diffuse reflectance absorption spectra show that the co-doped samples can absorb the photons in the range of 900–1050 nm efficiently, demonstrating the excellent sensitization effect of Yb³⁺ ions. Based on power switched UC luminescence, UC mechanisms are discussed, and the emission spectra excited by 490 nm are also measured to give additional evidence. The results reveal that application of these phosphors may improve the conversion efficiency of DSSCs.     

Influence of Yb3+ content on microstructure and fluorescence of oxyfluoride glass ceramics containing LaF3 nano-crystals

The influence of Yb³⁺ content on structural evolution and fluorescence properties of oxyfluoride glass ceramics containing LaF₃ nano-crystals were systematically investigated. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) experiments indicated that Yb³⁺ ions acted as nucleating agent to facilitate LaF₃ crystallization. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) results verified the incorporation of Yb³⁺ into LaF₃ nano-crystal lattice. The absorption, emission spectra and fluorescence decays were measured. The infrared emission intensity of ⁴F_5/2 → ⁴F_7/2 transition under 980 nm excitation enhanced, while the measured lifetime reduced due to the increase of non-radiative transition probability, with the increase of Yb³⁺ content in glass ceramic. However, when Yb³⁺ doping reached 4.0 mol% the concentration quenching effect appeared.     

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.     

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.     

Study of the line intensity in the optical and magnetooptical spectra in holmium-containing paramagnetic garnets

Studies of line intensity in the optical and magneto-optical spectra in the holmium-containing paramagnetic garnet Ho³⁺:YAG were carried out within the visible spectrum at T = 85 K. Detailed investigation of the magnetic circularly polarized luminescence spectra at 85 and 300 K on ⁵S₂ → ⁵I₈ emission transition in Ho³⁺:YAG was carried out. A quasi-doublet state in the energy spectrum of the Ho³⁺ ions was observed, characterized by a significant magneto-optical activity, which is caused by a large Zeeman splitting of the quasi-doublet. The measurement of the magnetic circular polarized luminescence spectrum carried out within one of the emission lines of the luminescence band ⁵S₂ → ⁵I₈ in Ho³⁺:YAG at 85 K shows significant magneto-optical effects of the intensity change of the emitted light, compared to that measured for the other emission lines in the same luminescent band.     

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 luminescence properties of Yb3+, Tm3+ and Ho3+ co-doped SrGd2(WO4)2(MoO4)2 nano-crystal

Novel up-conversion luminescent SrGd₂(WO₄)₂(MoO₄)₂: Yb³⁺/Tm³⁺/Ho³⁺ nano-crystals were synthesized by hydrothermal method. The composition ratio of rare earth had been investigated. It indicated that when C_Yb³⁺ = 10 mol% and C_Yb³⁺/C_Tm³⁺/C_Ho³⁺ = 10:1.5:2, the emission intensities were the highest. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and up-conversion luminescence spectra were used to characterize SrGd₂(WO₄)₂(MoO₄)₂: Yb³⁺/Tm³⁺/Ho³⁺ nano-crystals and they showed that the sample had high degree of crystallinity, the sample was tetragonal system, and the grain size of the sample was about 56 nm. Three emission peaks, including blue emission peak, green emission peak and red emission peak were observed at 477, 543 and 651 nm corresponding to ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions of Tm³⁺, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺ respectively. All the emission peaks were observed by excitation of 980 nm semiconductor laser. The relationship between up-conversion intensity and excitation power revealed that blue emission at 477 nm was a three-photon absorption process, green emission at 543 nm and red emission at 651 nm was a two-photon absorption process. The quantum yields of the sample were near 3.2%.