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.     

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).     

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.     

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.     

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.     

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.     

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.     

Spectroscopic properties of Eu3+/Nd3+ co-doped phosphate glasses and opaque glass–ceramics

This paper reports the fabrication and characterization of Eu³⁺/Nd³⁺ co-doped phosphate (PNE) glasses and glass–ceramics as a function of Eu³⁺ concentration. The precursor glasses were prepared by the conventional melt quenching technique and the opaque glass–ceramics were obtained by heating the precursor glasses at 450 °C for 30 h. The structural and optical properties of the glass and glass–ceramics were analyzed by means of X-ray diffraction, Raman spectroscopy, UV–VIS–IR absorption spectroscopy, photoluminescence spectra and lifetimes. The amorphous and crystalline structures of the precursor glass and opaque glass–ceramic were confirmed by X-ray diffraction respectively. The Raman spectra showed that the maximum phonon energy decreased from 1317 cm⁻¹ to 1277 cm⁻¹ with the thermal treatment. The luminescence spectra of the glass and glass–ceramic samples were studied under 396 nm and 806 nm excitation. The emission intensity of the bands observed in opaque glass–ceramic is stronger than that of the precursor glass. The luminescence spectra show strong dependence on the Eu³⁺ ion concentration in the Nd³⁺ ion photoluminescence (PL) intensity, which suggest the presence of energy transfer (ET) and cross-relaxation (CR) processes. The lifetimes of the ⁴F_3/2 state of Nd³⁺ ion in Eu³⁺/Nd³⁺ co-doped phosphate glasses and glass–ceramics under 806 nm excitation were measured. It was observed that the lifetimes of the ⁴F_3/2 level of Nd³⁺ of both glasses and glass–ceramics decrease with the increasing Eu³⁺ concentration. However in the case of opaque glass–ceramics the lifetimes decrease only 16%.     

Effect of Ag nanoparticles on the radiative properties of tellurite glasses doped with Er3+, Yb3+ and Tm3+ ions

The present work is devoted to the characterization of the thermal and spectroscopic properties of tellurite glasses, codoped with Er³⁺, Yb³⁺ and Tm³⁺ rare-earth ions and silver nanoparticles (NPs). The techniques used for this investigation were UV–visible and infrared absorption, time-resolved luminescence and thermal lens. Time-resolved luminescence studies indicate efficient Yb³⁺ → Er³⁺ and Yb³⁺ → Tm³⁺ energy transfers and intense Er³⁺ and Tm³⁺ mid-infrared emissions around 1550 nm and 1860 nm, respectively. The presence NPs is found to increase the thermal diffusivity of the materials and to shorten the mid-infrared emission lifetime of both the Er³⁺ and Tm³⁺ ions.     

Energy transfer based photoluminescence spectra of co-doped (Dy3+ + Sm3+): Li2O-LiF-B2O3-ZnO glasses for orange emission

The present paper brings out the results concerning the preparation and optical properties of Sm³⁺ and Dy³⁺ each ion separately in different concentrations (0.3, 0.5, 1.0 and 1.5 mol.%) and also together doped (x mol.% Dy³⁺ + 1.5 mol.% Sm³⁺): Li₂O-LiF-B₂O₃-ZnO (where x = 0.5, 1.0 and 1.5 mol.%) glasses by a melt quenching method. Structural and thermal properties have been extensively studied for those glasses by XRD and TG/DTA. The compositional analysis has been carried out from FTIR spectral profile. Optical absorption spectral studies were also carried out. Sm³⁺: LBZ glasses have displayed an intense orange emission at 603 nm (⁴G_5/2 → ⁶H_7/2) with an excitation wavelength at 403 nm and Dy³⁺: LBZ glasses have shown two emissions located at 485 nm (⁴F_9/2 → ⁶H_15/2; blue) and 574 nm (⁴F_9/2 → ⁶H_13/2; yellow) with an excitation wavelength at 385 nm. Remarkably, it has been identified that the significant increase in the reddish orange emission of Sm³⁺ ions and diminished yellow emission pertaining to Dy³⁺ ions in the co-doped LBZ glass system under the excitation of 385 nm which relates to Dy³⁺ ions. This could be due energy transfer from Dy³⁺ to Sm³⁺. The non-radiative energy transfer from Dy³⁺ to Sm³⁺ is explained in terms of their emission spectra, donor lifetime, energy level diagram and energy transfer characteristic factors. These significantly enhanced orange emission exhibited glasses could be suggested as potential optical glasses for orange luminescence photonic devices.     

Spectroscopic properties and energy transfer in Er–Tm co-doped bismuth silicate glass

In this paper, we investigate the spectroscopic properties of and energy transfer processes in Er–Tm co-doped bismuth silicate glass. The Judd–Ofelt parameters of Er³⁺ and Tm³⁺ are calculated, and the similar values indicate that the local environments of these two kinds of rare earth ions are almost the same. When the samples are pumped at 980 nm, the emission intensity ratio of Tm:³F₄ → ³H₆ to Er:⁴I_13/2 → ⁴I_15/2 increases with increased Er³⁺ and Tm³⁺ contents, indicating energy transfer from Er:⁴I_13/2 to Tm:³F₄. When the samples are pumped at 800 nm, the emission intensity ratio of Er:⁴I_13/2 → ⁴I_15/2 to Tm:³H₄ → ³F₄ increases with increased Tm₂O₃ concentration, indicating energy transfer from Tm:³H₄ to Er:⁴I_13/2. The rate equations are given to explain the variations. The microscopic and macroscopic energy transfer parameters are calculated, and the values of energy transfer from Er:⁴I_13/2 to Tm:³F₄ are found to be higher than those of the other processes. For the Tm singly-doped glass pumped at 800 nm and Er–Tm co-doped glass pumped at 980 nm, the pumping rate needed to realize population reversion is calculated. The result shows that when the Er₂O₃ doping level is high, pumping the co-doped glass by a 980 nm laser is an effective way of obtaining a low-threshold ∼2 μm gain.     

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.     

Spectroscopic investigations on Eu3+ ions in Li–K–Zn fluorotellurite glasses

Eu³⁺ ions incorporated Li–K–Zn fluorotellurite glasses, (70 − x)TeO₂ + 10Li₂O + 10K₂O + 10ZnF₂ + xEu₂O₃, (0 ⩽ x ⩽ 2 mol%) were prepared via melt quenching technique. Optical absorption from ⁷F₀ and ⁷F₁ levels of the Eu³⁺-doped glass has been studied to examine the covalent bonding characteristics, energy band gap and Judd–Ofelt intensity parameters. The emission spectra (⁵D₀ → ⁷F_0,1,2,3,4) of the glasses were used to estimate the luminescence enhancement, asymmetric environment in the vicinity of Eu³⁺ ions, stimulated emission cross section and branching ratios. The phonon side band mechanism of ⁵D₂ level of the Eu³⁺ ions in the prepared glass was examined by considering the excitation and Raman spectra. The radiative lifetime calculated using Judd–Ofelt parameters was compared with the experimental lifetime to estimate the quantum efficiency of ⁵D₀ level of Eu³⁺ ions in Li–K–Zn fluorotellurite glass.     

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⁺.     

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.     

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.     

The 1.53 μm spectroscopic properties of Er3+/Ce3+/Yb3+ tri-doped tellurite glasses containing silver nanoparticles

The metallic silver nanoparticles (NPs) was introduced into the Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses with composition TeO₂–ZnO–La₂O₃ to improve the 1.53 μm band fluorescence. The UV/Vis/NIR absorption spectra, 1.53 μm band fluorescence spectra, fluorescence lifetimes, X-ray diffraction (XRD) curves, differential scanning calorimeter (DSC) curves and transmission electron microscopy (TEM) image of tri-doped tellurite glasses were measured, together with the Judd–Ofelt intensity parameters, emission cross-sections, absorption cross-sections and radiative quantum efficiencies were calculated to investigate the effects of silver NPs on the 1.53 μm band spectroscopic properties of Er³⁺ ions, structural nature and thermal stability of glass hosts. It is shown that Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses can emit intense 1.53 μm band fluorescence through the combined energy transfer (ET) processes from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions under the 980 nm excitation. At the same time, the introduction of an appropriate amount of silver NPs can further improve the 1.53 μm band fluorescence owing to the enhanced local electric field effect induced by localized surface Plasmon resonance (LSPR) of silver NPs and the possible energy transfer from silver NPs to Er³⁺ ions, and an improvement by about 120% of fluorescence intensity is found in the studied Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass containing 0.5 mol% amount of silver NPs with average diameter of ∼15 nm. The energy transfer mechanisms from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions were also quantitatively investigated by calculating energy transfer microparameters and phonon contribution ratios. Furthermore, the thermal stability of glass host increases slightly with the introduction of silver NPs while the glass structure maintains the amorphous nature. The results indicate that the prepared Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass with an appropriate amount of silver NPs is an excellent gain medium applied for 1.53 μm band EDFA pumped with a 980 nm laser diode (LD).