Upconversion luminescence of Tm/Yb-codoped oxyhalide tellurite glasses

Thermal stability, Raman spectra and blue upconversion luminescence properties of Tm³⁺/Yb³⁺-codoped halide modified tellurite glasses have been studied. The results showed that the mixed halide modified tellurite glass (TFCB) has the best thermal stability, the lowest phonon energies and the strongest upconversion emissions. The effect of halide on upconversion intensity is observed and discussed and possible upconversion mechanisms are evaluated. The intense blue upconversion luminescence of Tm³⁺ in TFCB glass may be a potentially useful material for developing upconversion optical devices.     

Energy transfer and infrared-to-visible upconversion luminescence of Er/Yb-codoped halide modified tellurite glasses

We report on the energy transfer and frequency upconversion spectroscopic properties of Er³⁺-doped and Er³⁺/Yb³⁺-codoped TeO₂-ZnO-Na₂O-PbCl₂ halide modified tellurite glasses upon excitation with 808 and 978 nm laser diode. Three intense emissions centered at around 529, 546 and 657 nm, alongwith a very weak blue emission at 410 nm have clearly been observed for the Er³⁺/Yb³⁺-codoped halide modified tellurite glasses upon excitation at 978 nm and the involved mechanisms are explained. The quadratic dependence of fluorescence on excitation laser power confirms the fact that the two-photon contribute to the infrared to green-red upconversion emissions. And the blue upconversion at 410 nm involved a sequential three-photon absorption process.     

Comparative investigation on spectroscopic properties of Er between Ce-doped and B2O3-added bismuth glasses

The comparative investigation on the spectroscopic properties of Er³⁺ in low phonon energy Bi₂O₃-GeO₂-Ga₂O₃-Na₂O glasses codoped with Ce³⁺ ion and added with B₂O₃ component, respectively, is presented. With increasing Ce₂O₃ content from 0 to 0.8 mol% or B₂O₃ content from 0 to 15 mol%, the lifetime of Er³⁺:⁴I_11/2 level decreases dramatically from 607 to 283 μs or to 197 μs, and the upconversion fluorescence is quenched in both glass samples. The nonradiative energy transfer from Er³⁺:⁴I_11/2→Ce³⁺:²F_5/2 or the enhanced multiphonon relaxation process together with the energy transfer between Er³⁺ and OH⁻ groups are, respectively, responsible for the results. Meanwhile, the lifetime of ⁴I_13/2 level remains almost unchanged in Er³⁺/Ce³⁺-codoped glasses whereas it decreases rapidly in B₂O₃-added cases. As a result, Er³⁺/Ce³⁺ codoping improves the 1.5 μm fluorescence emission intensity, however, B₂O₃ addition has a negative effect on it. The research results indicate that the Er³⁺/Ce³⁺-codoped bismuth glasses will be preferable for obtaining efficient 980 nm pumped EDFA.     

Effect of hydroxyl groups on Er doped Bi2O3-B2O3-SiO2 glasses

A series of Er³⁺-doped Bi₂O₃-B₂O₃-SiO₂-Na₂O glasses with different hydroxyl groups were prepared and the interaction between the Er³⁺ ions and OH groups was investigated. Infrared spectra were measured in order to calculate the exact content of OH groups in samples. The observed increase of the fluorescence lifetime with the oxygen bubbling time has been related to the reduction in the OH content concentration evidenced by infrared (IR) absorption spectra, which confirmed that the OH groups were dominant quenching centers of excited Er³⁺ and a cause of concentration quenching of 1.5 μm band emission. Various nonradiative decay rates from ⁴I_13/2 of Er³⁺ with the change of OH content were determined from the fluorescence lifetimes and radiative decay rates, which were calculated on the basis of Judd-Ofelt theory.     

Upconversion fluorescence property of Er/Yb-codoped novel bismuth-germanium glass

Infrared-to-visible upconversion fluorescence property of Er³⁺/Yb³⁺-codoped novel bismuth-germanium glass under 975 nm LD excitation has been studied. Intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs. The structure of the bismuth-germanium glass has been investigated by peak-deconvolution of FT-Raman spectrum, and the structural information was obtained from the peak wavenumbers. This novel bismuth-germanium glass with low maximum phonon energy (∼750 cm⁻¹) can be used as potential host material for upconversion lasers.     

Hydrothermal synthesis of BaYF5:Yb/Er upconversion luminescence submicrospheres by a surfactant-free aqueous solution route

BaYF₅:Yb³⁺/Er³⁺ upconversion (UC) luminescence submicrospheres have been synthesized by the hydrothermal synthesis method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), scanning probe microscope (SPM), transmission electron microscope (TEM), laser diffraction particle analyzer (LDPA) and UC emission spectra. The as-prepared highly crystalline BaYF₅:Yb³⁺/Er³⁺ submicrospheres are of uniform size depending on different reaction temperatures and reaction times. It is found that the usage of fluoride source NaBF₄ plays the crucial key in the formation of submicrosphere. Under the 980 nm excitation, the UC emission transitions for ⁴F_9/2→⁴I_15/2 (red), ²H_11/2, ⁴S_3/2→⁴I_15/2 (green) in the BaYF₅:Yb³⁺/Er³⁺ submicrospheres came from two-, two-, and two-photon UC processes, respectively. Further, the effects of Yb³⁺ ion concentration, size and surface of as-prepared submicrospheres, and pumping power on the UC luminescence properties of BaYF₅:Yb³⁺/Er³⁺ have also been discussed.     

Investigation of energy transfer and frequency upconversion in Ho/Yb co-doped tellurite glasses

A serials of Ho³⁺/Yb³⁺ co-doped tellurite glasses by pumping 970 nm laser diode (LD) were demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Two intense emission bands were observed in Ho³⁺/Yb³⁺ co-doped tellurite glasses centered at 549 and 664 nm corresponding to Ho³⁺: ⁵S₂(⁵F₄)→⁵I₈ and ⁵F₅→⁵I₈ transitions, respectively. The upconversion intensities of red and green emissions in Ho³⁺/Yb³⁺ co-doped glasses were enhanced largely when increasing Yb₂O₃ content. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model. The energy transfer coefficients were estimated by fitting the simulated curves to the measured ones. The obtained three energy transfer coefficients C_D2, C_D3 and C_D4 were C_D2=5.0×10⁻¹⁸ cm³/s, C_D3=1.5×10⁻¹⁷ cm³/s, C_D4=9.0×10⁻¹⁷ cm³/s.     

Optical absorption spectrum of Cu2O-CaO-P2O5 glasses

Homogeneous CaO-P₂O₅ and Cu₂O-CaO-P₂O₅ glasses were prepared using a melt-quenched method under controlled conditions. The binary glasses were found to be colourless and transparent while the ternary glasses changed from light green to dark green as the Cu₂O content increased. From the absorption edge studies, the values of the optical band gap, E_opt and Urbach energy, ΔE were evaluated. The position of the absorption edge and hence the optical band gap were found to depend on the glass composition. Analysis of the optical band gap shows that for the binary glasses, the value increases as the content of CaO decreases, while for the ternary glasses, the value of the optical band gap increases as the content of the Cu₂O decreases. The density of the glasses was also measured and was found to increase with the increase in CaO and Cu₂O contents.     

Temperature dependence of impurity quenched luminescence in Tm:LiLuF4

The study of the optical properties of a LiLuF₄ crystal doped with Tm³⁺ yielded the discovery of a strong temperature dependence of the Tm-Tm diffusion coefficient. Spectroscopic characteristics have been investigated as a function of the sample temperature, with particular regard to the luminescence decay following pulsed excitation. An appreciable excitation of the lifetime of the ³F₄ manifold is observed over the temperature range 8.9-298 K. The Judd-Ofelt calculations point out a radiative lifetime considerably longer than the experimental one. These facts suggest a theoretical interpretation based on the presence of impurities that quench the manifold and on a temperature-dependent energy migration between Tm³⁺ ions. A one-parameter best fit of the experimental measurements strongly confirms this hypothesis. Weak OH⁻ ion concentration is detected by means of IR and UV spectra, thus supporting the theoretical interpretation.     

Europium doped strontium borate glasses and their optical properties

Optical absorption and luminescence spectra of europium doped strontium borate glasses prepared in different conditions are studied. It is found that the percentage of Eu³⁺ ions varies from 100 to 30% being controlled by the conditions of preparation. The mechanism, favoring reduction of europium to Eu²⁺ state in polycrystalline strontium tetraborate, is much weaker in glasses of the same composition. In samples containing mixed valence europium at densities of 8×10²⁰ cm⁻³, the efficient transfer of optical excitation from Eu³⁺ to Eu²⁺, suppressing the Eu³⁺ luminescence, has been found. The most reliable way of monitoring the percentage of europium ions in different valences for strontium borate glasses is the measuring of absorption at f-f transition ⁷F₀→⁵D₂ of Eu³⁺.     

Optical properties of Er:SrMoO4 single crystal

Er³⁺:SrMoO₄ crystal of high optical quality was grown by the Czochralski method. The room temperature polarized absorption and emission spectra together with the lifetime decay curve were measured. Based on the Judd-Ofelt theory, three intensity parameters, radiative transition rates, radiative lifetimes and fluorescent branching ratios, were obtained. Emission cross-section and gain cross-section around 1.54 μm were also obtained.     

Infrared luminescence from spark-processed silicon

The infrared (IR) photoluminescence (PL) emission of spark-processed silicon (sp-Si) was investigated. A broad and strong room temperature PL peak in the 945 nm (1.31 eV) spectral range was observed when sp-Si was excited with an argon laser. This peak is different from the PL commonly reported for anodically etched porous silicon and other silicon-based materials. The PL intensity increases substantially after annealing sp-Si between 350 and 500 °C in air after which it decreases again. The PL wavelength is observed to peak at 1010 nm by annealing sp-Si near 450 °C. It was further found that the most efficient PL occurs for a Si/O ratio of 0.3, for a small spark gap of about 1 mm, and for spark-processing times in the 15-60 min range.A model for the IR PL is proposed which mirrors that for visible PL. Specifically, it is proposed that the electrons which have been pumped by the laser from the ground state into a broad quasi-absorption band (or closely spaced absorption lines between 1.7 and 2.3 eV) revert back to lower IR levels at 1.31 eV by a non-radiative transition from where they revert radiatively to the ground state by emitting the observed 945 nm light.     

Synthesis and luminescence properties of YVO4:Eu,Bi phosphor with enhanced photoluminescence by Bi doping

YVO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the high-temperature solid-state (HT) method and the Pechini-type sol-gel (SG) method. Spherical SiO₂ particles have been further coated with YVO₄:Eu³⁺,Bi³⁺ phosphor layers by the Pechini-type SG process, and it leads to the formation of core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ phosphors. Therefore, the phase formations, structures, morphologies, and photoluminescence properties of the three types of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors were studied in detail. The average diameters for the phosphor particles are 2-4 μm for HT method, 0.1-0.4 μm for SG method, and 0.5 μm for core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ particles, respectively. Photoluminescence spectra show that effective energy transfer takes place between Bi³⁺ and Eu³⁺ ions in each type of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors. Introduction of Bi³⁺ into YVO₄:Eu³⁺ leads to the shift of excitation band to the long-wavelength region, thus the emission intensities of ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ at 615 nm upon 365 nm excitation increases sharply, which makes this phosphor a suitable red-emitting materials that can be pumped with near-UV light emitting diodes (LEDs).     

Optical and electronic polarizability investigation of Nd-doped soda-lime silicate glasses

This paper reports on different physical and optical properties of Nd³⁺-doped soda-lime silicate glass. The glasses containing Nd³⁺ in (65−x)SiO₂:25Na₂O:10CaO:xNd₂O₃ (where x=0.0-5.0 mol%) have been prepared by the melt-quenching method. In order to understand the role of Nd₂O₃ in these glasses the density, molar volume, refractive index and optical absorption were investigated. The results show that the density and molar volume of the glasses increase with an increase in Nd₂O₃ concentration and consequently generate more non-bridging oxygen (NBOs) into glass matrix. The optical absorption spectra were measured in the wavelength range from 300 to 700 nm and the optical band gaps were determined. It was found that the optical band gap decreases with an increase in Nd₂O₃ concentration. On the basis of the measured values of density and refractive index, the Nd³⁺ ion concentration in glasses, the polarizability of oxide ions and optical basicity were theoretically determined.     

Second-harmonic generation in the thermal/electrical poling (100−x)GeS2·x(0.5Ga2S3·0.5CdS) chalcogenide glasses

Utilizing Maker fringe (MF) method, second-harmonic generation (SHG) has been observed within the GeS₂-Ga₂S₃-CdS pseudo-ternary glasses through thermal/electrical poling technique. The SHG phenomenon was considered to be the result of breakage of the glassy macroscopic isotropy originated from the reorientations of dipoles during the thermal/electrical poling process. Under the same poling condition conducted with 5 kV and 280 °C for 30 min, the maximum value of second-order nonlinear susceptibility χ⁽²⁾ of the poled (100−x)GeS₂·x(0.5Ga₂S₃·0.5CdS) glasses was obtained to be ≈4.36 pm/V when the value of x is equal to 30. Nonlinear dependence of χ⁽²⁾ on compositions of these glasses can be well explained according to the theory related to the reorientation of dipoles.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

Determination of kinetic parameters of thermally stimulated luminescence of Cu-doped BaSO4

Studies of thermally stimulated luminescence (TSL) of doped BaSO₄ with Cu activator have been carried out. The polycrystalline sample of Cu-doped BaSO₄ is prepared by recrystallisation method. The characterization and elemental analysis of BaSO₄:Cu compound are carried out by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). The compound has orthorhombic structure at room temperature. The TSL studies of Cu-doped BaSO₄ sample show two glow peaks at 160 and 227 °C. Comparison of TL intensity of the most intensive glow peak of Cu- and Mn-doped BaSO₄ compounds with that of undoped BaSO₄ shows that addition of Cu and Mn impurity in BaSO₄ compound enhances the TL intensity by about 9 and 3 times, respectively. Among the studied samples, namely undoped, Cu- and Mn-doped BaSO₄, Cu-doped BaSO₄ is found to be the most sensitive. The trap parameters namely order of kinetics (b), activation energy (E), frequency factor (s) and Balarin parameter (γ) associated with the most intensive glow peak, i.e. the 227 °C glow peak of BaSO₄:Cu phosphor were determined by using isothermal decay method and glow curve shape (Chen's) method and these parameters are in good agreement.     

Enhanced blue-green-red up-conversion and 1.3-μm emission of Pr/Yb-codoped oxyhalide tellurite glasses with PbCl2 doping

We report on the blue-green-red up-conversion spectroscopic properties of Pr³⁺/Yb³⁺-codoped oxyhalide tellurite glasses upon excitation of a conventional 980 nm laser diode (LD). Significant enhancement of the blue-green-red up-conversion emission intensity has been observed with increasing PbCl₂ doping. The up-conversion intensity has a quadratic dependence on incident pump laser power, indicating a two-photon process. The population of the Pr³⁺ upper ³P₀ emitting level was accomplished through a combination of a ground state absorption, energy transfer and excitated state absorption. 1.3-μm emission in the second telecom window originated from Pr³⁺:¹G₄→³H₅ transition has also been investigated upon excitation at 980 nm LD. The measured peak wavelength and full width at half-maximum of the fluorescent are 1335 nm and ∼100 nm, respectively. An enhanced 1.3μm emission with increasing PbCl₂ doping has also been observed. Codoping of Yb³⁺ significantly enhance both the blue-green-red up-conversion emission and 1.3-μm emission intensity by way of a nonradiative Yb³⁺:²F₅→Pr³⁺:¹G₄ energy transfer.     

Er3+离子掺杂钡镓锗玻璃上转换发光机理研究

研究了Er3+离子掺杂钡镓锗玻璃的吸收光谱、拉曼光谱和上转换光谱.分析了Er3+离子在钡镓锗玻璃中的上转换发光机理.结果表明:玻璃的最大声子能量为828cm-1,紫外截止波长为275nm.采用800nm和980nmLD激发玻璃样品,在室温下观察到强烈的上转换绿光和红光发射.随着Er3+离子浓度的增加,绿光发光强度先增加后减小,而红光发光强度呈单调递增趋势.能量分析表明:800nmLD激发产生的绿光主要源于Er3+离子4I13/2能级的激发态吸收过程;红光发射主要源于Er3+离子4I13/2能级与4I11/2能级之间的能量转移过程.980nmLD激发产生的绿光主要源于Er3+离子4I11/2能级之间的能量转移过程;而红光发射主要源于Er3+离子4I13/2能级与4I11/2能级之间的能量转移过程和4I13/2能级的激发态吸收过程.通过量子效率分析,发现采用800nmLD激发Er3+离子掺杂浓度为1mol% 的样品时,上转换绿光发光效率最高.