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.     

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.     

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.     

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.     

Up-conversion luminescence properties of Y2O2S:Yb3+,Er3+ nanophosphors

Up-converting yttrium oxysulfide nanomaterials doped with ytterbium and erbium (Y₂O₂S:Yb³⁺,Er³⁺) were prepared with the flux method. The precursor oxide materials were prepared using the combustion synthesis. The morphology of the oxysulfides was characterized with transmission electron microscopy (TEM). The particle size distribution was 10–110 nm, depending on the heating temperature. According to the X-ray powder diffraction (XPD), the crystal structure was found hexagonal and the particle sizes estimated with the Scherrer equation agreeded with the TEM images. Upon the 970 nm infrared (IR) laser excitation, the materials yield moderate green ((²H_11/2, ⁴S_3/2) → ⁴I_15/2 transition) and strong red (⁴F_9/2 → ⁴I_15/2) luminescence. The green luminescence was enhanced with respect to the red one by an increase in both the crystallite size and erbium concentration due to the cross-relaxation (CR) processes. The most intense up-conversion luminescence was achieved with x_Yb and x_Er equal to 0.10 and 0.005, respectively. Above these concentrations, concentration quenching occurred.     

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 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.     

2–3 μm emission and fluorescent decaying behavior in Ho3+-doped tellurium germanate glass

In this work, we report the 2.05 μm emission and ∼3 μm broadband spectra of Ho₂O₃-doped 33GeO₂–30TeO₂–27PbO–10CaO (in mol%) glass under 640 nm laser excitation. Clear emission spectra due to the ⁵I₇–⁵I₈ transition and the ⁵I₆–⁵I₇ transition in Ho³⁺ are observed. The 2.05 μm emission intensity and the full width at half maximum (FWHM) of the ∼3 μm broadband depend on the Ho concentration. The peak stimulated emission cross-section of Ho³⁺ is 6.57 × 10⁻²¹ cm² at 2.05 μm, as calculated by the McCumber theory. The emission spectra are recorded and the maximum emission intensity at 2.05 μm is obtained at a doping level of 0.5 mol% Ho₂O₃ in the glass. A broad and flat emission band from 2700 nm to 3050 nm is observed in 2 mol% Ho₂O₃-doped tellurium germanate glass. The lifetime of the ⁵I₇ state decreases with the increase in Ho³⁺ concentration due to non-radiative relaxation processes. An energy transfer coefficient of 271.88 mol⁻¹ s⁻¹ is obtained.     

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⁺.     

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.     

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.     

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.     

Effects of aging time on phase,morphology, and luminescence by two-photon processes of BiPO4:Er3+, Yb3+ in the solvothermal synthesis

BiPO₄:Er³⁺, Yb³⁺ phosphors were synthesized by the solvothermal process. The phase transformation, morphology, and UC luminescent property were characterized by different analytical techniques. The aging time has obvious influence on the phase, morphology, and luminescence of the samples. With the extension of aging time, the phase of BiPO₄:Er³⁺, Yb³⁺ phosphors changes from hexagon to monocline. The morphology changes from nanorods through nanorugbies to microoctahedra. Under the excitation at 980 nm, BiPO₄:Er³⁺, Yb³⁺ phosphors show green and red UC emissions, which originate from the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions. The green and red UC emission intensities increase gradually with the increase of pumping power. On the basis of the luminescent properties, one can conclude that the two-photon process is involved in green and red UC emissions.     

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).     

Up-conversion luminescence of the NaYF4:Yb3+,Er3+ nanomaterials prepared with the solvothermal synthesis

Up-converting NaYF₄:Yb³⁺,Er³⁺ (x_Yb: 0.20, x_Er: 0.02) nanomaterials were prepared with a microwave assisted solvothermal synthesis to study how the synthesis parameters affect the structure and up-conversion luminescence of the materials and thus their usability as labels in biomedical applications. The purity of the materials was studied with Fourier transform infra-red (FT-IR) spectroscopy and the particle size and morphology with transmission electron microscopy (TEM). The crystal structure was characterized with X-ray powder diffraction (XPD) and the crystallite sizes were calculated with the Scherrer formula. Up-conversion luminescence and luminescence decays were studied with near infra-red (NIR) laser excitation at 970 nm.The presence of the oleic acid was observed in the FT-IR spectra. The TEM images showed small quasi-spherical nanoparticles as well as long nanorods. The XPD measurements revealed that both cubic and hexagonal forms of NaYF₄ were present in the materials. The crystallite sizes ranged from ca. 20 to over 150 nm for the cubic and hexagonal phases, respectively. The characteristic up-conversion luminescence of Er³⁺ in red (640–685 nm; ⁴F_9/2 → ⁴I_15/2) and green (515–560 nm; ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions) wavelengths was observed. The most intense luminescence and the longest luminescence emission lifetime were obtained with the material annealed for 12 h at 177 °C with 1.8 MPa pressure due to the predominance of the well-crystallized hexagonal form of NaRF₄ (R: Y, Yb, Er).     

A facile preparation and the luminescent properties of Eu3+-doped Y2O2SO4 nanopieces

The europium(III)-doped yttrium oxysulfate (Y₂O₂SO₄:Eu³⁺) nanopieces have been prepared via electrospinning followed by calcination at 1000 °C in mixed gas of sulfur dioxide and air. Based on the experimental results, a possible formation mechanism for the nanopieces is that the nanopieces are determined by the directing template of electrospun nanoribbons and the multilayer crystal structure of Y₂O₂SO₄. Besides, the nanopieces show excellent luminescent properties with emissions at 581, 589, 597, 653, 619, and 697 nm resulting from the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transition of Eu³⁺. The peaks of charge transfer and ⁵D₀ → ⁷F₂ transition of Eu³⁺ obviously have red shifts comparing to those of both Y₂O₃:Eu³⁺ nanoribbons and commercial Y₂O₃:Eu³⁺. Moreover, the nanopieces exhibit stronger intensities than the Y₂O₃:Eu³⁺ in excitation and emission spectra. Concentration quenching in the nanopieces occurs when Eu³⁺ concentration is 11 mol%, indicating that the nanopieces have an optimum luminescent intensity under this doping concentration.     

The upconversion luminescence properties of the Yb3+–Ho3+ system in nanocrystalline Y2O2S

Near-spherical Y₂O₂S:Yb³⁺,Ho³⁺ nanocrystals (NCs) with an average particle size of 40 nm were synthesized by the coprecipitation method followed by a solid–gas sulfuration technique. The effects of the Ho³⁺ ion doping concentration on the upconversion luminescence (UCL) property of the NCs was studied through the UCL spectra. Results show that the UCL intensity of Y₂O₂S:Yb³⁺,Ho³⁺ NCs markedly changes with Ho³⁺ ion concentration, and that the Ho³⁺ ion concentration quench is observed at 0.25 mol%. This value is only half as much as that in micron Y₂O₂S prepared by a solid state reaction, which can be attributed to the distinct diffusion mechanism of activator ions in the coprecipitation process. In addition, strong red emissions can be observed in Y₂O₂S:Yb³⁺,Ho³⁺ NCs throughout all Ho³⁺ doping concentrations used. However, the Ho³⁺ in micron Y₂O₂S usually exhibits weak red UCL. Infrared spectra confirm that this result is related to the large vibrational quanta produced by OH⁻ and CO₃ ²⁻ groups adsorbed onto the surface of NCs. These large vibrational quanta can remarkably increase the probability of ⁵S₂–⁵F₅ and ⁵I₆–⁵I₇ multiphonon relaxation, leading to the enhancement of red emissions arising from ⁵F₅ → ⁵I₈ transitions. The UCL mechanism of the Yb³⁺–Ho³⁺ system in nano- and microsized Y₂O₂S is also discussed.     

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%).     

Spectroscopic analysis of a novel Nd3+-activated barium borate glass for broadband laser amplification

Spectroscopic parameters of a novel Nd³⁺-activated barium borate (BBONd) glass have been analyzed for broadband laser amplification. The Judd–Ofelt (JO) intensity parameters were determined through a systematic analysis of the absorption spectrum of Nd³⁺ ions in the BBONd glass. High values of the JO intensity parameters reveal a great centro-symmetrical loss of the Nd³⁺ sites and high covalency degree of the ligand field. The very high Ω₆ intensity parameter value makes evident both a great structural distortion of the Nd³⁺ sites and a strong electron–phonon coupling between Nd³⁺ and free OH⁻ ions, which is consistent with the phonon energy maximum (3442.1 cm⁻¹) recorded by Raman spectroscopy. This strong electron–phonon coupling favors high effective bandwidth and gain bandwidth values of the laser emission (⁴F_3/2 → ⁴I_11/2) of Nd³⁺ ions. The electric-dipole oscillator strengths of all the Nd³⁺ absorption transitions, and in particular that of the hypersensitive transition (⁴I_9/2 → ⁴G_5/2), are enhanced by this great structural distortion of the host. Broadband laser amplification of the ⁴F_3/2 → ⁴I_11/2 emission (1062 nm) of Nd³⁺ ions in the BBONd glass pumped at 805 nm (⁴I_9/2 → ⁴F_5/2 + ²H_9/2) is evaluated through the main fluorescent parameters in competition with non-radiative processes. In general, the BBONd glass exhibits spectroscopic parameters comparable with those reported in the literature for broadband laser amplification into the IR region.     

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).