Upconversion luminescence of colloidal solution of Y2O3 nano-particles doped with trivalent rare-earth ions

We have studied upconversion luminescence of colloidal solution of Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺. Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺ show sintering and agglomeration, because they are synthesized by firing a hydroxy carbonate precursor. Colloidal solution of Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺ is prepared through two-step dispersion process and the average diameter of the primary nano-particles is about 50 nm. Under excitation with 980-nm laser diode, upconversion luminescence of colloidal solution of the primary Y₂O₃ nano-particles codoped with 1 mol% Er³⁺ and 5 mol% Yb³⁺ in methyl isobuthyl ketone strongly appeared near 660 nm and weakly near 550 nm.     

Infrared-to-visible upconversion in Er and Er/Yb activated Sb2O4 nanopowders prepared by sol-gel route

We prepared Er³⁺ doped and Er³⁺/Yb³⁺ codoped Sb₂O₄ nanocrystals by the sol-gel method. The Raman, X-ray diffraction (XRD), transmission electron microscope (TEM), and photoluminescence spectra of the samples were studied. The phonon energy of the Sb₂O₄ nanocrystals is very low (the maximum value being 461 cm⁻¹). The upconversion (UC) red emission of the Er³⁺/Yb³⁺ codoped sample is very strong at 975 nm laser diode excitation. The Sb₂O₄ nanocrystals will be a promising luminous material.     

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.     

Spectroscopic investigations of Nd-, Er-, Er/Yb-, and Tm-ions doped SiO2-Al2O3-CaF2-GdF3 glasses

This paper reports on the absorption, visible and near-infrared luminescence properties of Nd³⁺, Er³⁺, Er³⁺/2Yb³⁺, and Tm³⁺ doped oxyfluoride aluminosilicate glasses. From the measured absorption spectra, Judd-Ofelt (J-O) intensity parameters (Ω₂, Ω₄ and Ω₆) have been calculated for all the studied ions. Decay lifetime curves were measured for the visible emissions of Er³⁺ (558 nm, green), and Tm³⁺ (650 and 795 nm), respectively. The near infrared emission spectrum of Nd³⁺ doped glass has shown full width at half maximum (FWHM) around 45 nm (for the ⁴F_3/2→⁴I_9/2 transition), 45 nm (for the ⁴F_3/2→⁴I_11/2 transition), and 60 nm (for the ⁴F_3/2→⁴I_13/2 transition), respectively, with 800 nm laser diode (LD) excitation. For Er³⁺, and Er³⁺/2Yb³⁺ co-doped glasses, the characteristic near infrared emission bands were spectrally centered at 1532 and 1544 nm, respectively, with 980 nm laser diode excitation, exhibiting full width at half maximum around 50 and 90 nm for the erbium ⁴I_13/2→⁴I_15/2 transition. The measured maximum decay times of ⁴I_13/2→⁴I_15/2 transition (at wavelength 1532 and 1544 nm) are about 5.280 and 5.719 ms for 1Er³⁺ and 1Er³⁺/2Yb³⁺ (mol%) co-doped glasses, respectively. The maximum stimulated emission cross sections for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ and Er³⁺/Yb³⁺ are 10.81×10⁻²¹ and 5.723×10^-21 cm². These glasses with better thermal stability, bright visible emissions and broad near-infrared emissions should have potential applications in broadly tunable laser sources, interesting optical luminescent materials and broadband optical amplification at low-loss telecommunication windows.     

Up-conversion luminescence of Er/Yb/Nd-codoped tellurite glasses

Up-conversion luminescence and energy transfer (ET) processes in Nd³⁺-Yb³⁺-Er³⁺ triply doped TeO₂-ZnO-Na₂O glasses have been studied under 800 nm excitation. Intense green up-conversion emissions around 549 nm, which can be attributed to the Er³⁺: ⁴S_3/2→⁴I_15/2 transition, are observed in triply doped samples. In contrast, the green emissions are hardly observed in Er³⁺ singly doped and Er³⁺-Yb³⁺ codoped samples under the same condition. Up-conversion luminescence intensity exhibits dependence of Yb₂O₃-concentration and Nd₂O₃-concentration. Up-conversion mechanism in the triply doped glasses under 800 nm pump is discussed by analyzing the ET among Nd³⁺, Yb³⁺ and Er³⁺. And a possible up-conversion mechanism based on sequential ET from Nd³⁺ to Er³⁺ through Yb³⁺ is proposed for green and red up-conversion emission processes.     

Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.     

Surfactant effect on the upconversion emission and decay time of ZrO2:Yb-Er nanocrystals

Luminescent properties of doped ZrO₂:Er³⁺ and codoped ZrO₂:Yb³⁺-Er³⁺ nanocrystals with average size ∼54 nm were analyzed as a function of non-ionic surfactant (Pluronic F-127) concentration. Surfactant and non-surfactant samples were prepared by the sol-gel micelle process with hydrothermal aging and annealed at 1000 °C for 5 h. The introduction of the surfactant reduces the presence of impurities such as OH and CO₂ on both samples, and increments the tetragonal phase for codoped nanocrystals. It induces an increment larger than 90% and 70% for doped and codoped, respectively, for an optimum molar ratio of 0.0082. The observed enlargement of fluorescence decay time is partly the result of the nanosize effect but is dominated by the reduction of impurities attached on the nanocrystalline surface.     

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.     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

Influence of Yb doping concentration on upconversion luminescence of Er in Lu2O3 phosphor

Cubic phase Lu₂O₃:Er³⁺/Yb³⁺ nanocrystal phosphors were prepared by sol–gel method. Fourier transform infrared (FT-IR) spectra were measured to evaluate the vibrational feature of the samples. Green and red radiations were observed upon 980 nm diode laser excitation. Laser power and Er³⁺ or Yb³⁺ doping concentration dependence of upconversion luminescence were studied to understand upconversion mechanisms. Excited state absorption, cross relaxation and energy transfer processes are the possible mechanisms for the visible emissions.     

Visible upconversion emission and non-radiative direct Yb to Er energy transfer processes in nanocrystalline ZrO2:Yb,Er

Wide band gap Yb³⁺ and Er³⁺ codoped ZrO₂ nanocrystals have been synthesized by a modified sol-gel method. Under 967 nm excitation strong green and red upconversion emission is observed for several Er³⁺ to Yb³⁺ ions concentration ratios. A simple microscopic rate equation model is used to study the effects of non-radiative direct Yb³⁺ to Er³⁺ energy transfer processes on the visible and near infrared fluorescence decay trends of both Er³⁺ and Yb³⁺ ions. The microscopic rate equation model takes into account the crystalline phase as well as the size of nanocrystals. Nanocrystals phase and size were estimated from XRD patterns. The rate equation model succeeds to fit simultaneously all visible and near infrared fluorescence decay profiles. The dipole-dipole interaction parameters that drive the non-radiative energy transfer processes depend on doping concentration due to crystallite phase changes. In addition the non-radiative relaxation rate (⁴I_11/2→⁴I_13/2) is found to be greater than that estimated by the Judd-Ofelt parameters due to the action of surface impurities. Results suggest that non-radiative direct Yb³⁺ to Er³⁺ energy transfer processes in ZrO₂:Yb,Er are extremely efficient.     

NIR to visible frequency upconversion in Er3+ and Yb3+ codoped ZrO2 phosphor

The ZrO₂:Er³⁺ codoped with Yb³⁺ phosphor powders have been prepared by the urea combustion route. Formation of the compounds ZrO₂:Er³⁺ and ZrO₂:Er³⁺, Yb³⁺ was confirmed by XRD. The frequency upconversion emissions in the green and red regions upon excitation with a CW diode laser at ～978 nm are reported. Codoping with Yb³⁺ enhances the emission intensities of the triply ionized erbium in the green and red spectral regions by about ～130 and ～820 times respectively. The emission properties of the ZrO₂:Er³⁺ phosphor powders are discussed on the basis of excited state absorption, energy transfer, and cross-relaxation energy transfer mechanisms.     

Crystalline-structure-dependent green and red upconversion emissions of Er-Yb-Li codoped TiO2

Crystalline structures and infrared-to-visible upconversion luminescence spectra have been investigated in 1 mol% Er³⁺, 10 mol% Yb³⁺ and 0-20 mol% Li⁺ codoped TiO₂ [1Er10Yb(0-20)Li:TiO₂] nanocrystals. The crystalline structures of 1Er10Yb(0-20)Li:TiO₂ were divided into three parts by the addition of Yb³⁺ and Li⁺. Both green and red upconversion emissions were observed from the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ in Er³⁺-Yb³⁺-Li⁺ codoped TiO₂, respectively. The green and red upconversion emissions of 1Er:TiO₂ were enhanced significantly by Yb³⁺ and Li⁺ codoping, in which the intensities of green and red emissions and the intensity ratio of green to red emissions (I_green/I_red) were highly dependent on the crystalline structures. The significant enhanced upconversion emissions resulted from the energy migration between Er³⁺ and Yb³⁺ as well as the distortion of crystal field symmetry of Er³⁺ caused by the dissolving of Li⁺ at lower Li⁺ codoping concentration and the phase transformation at higher Li⁺ concentration. It is concluded that codoping with ions of smaller ionic radius like Li⁺ can efficiently improve the upconversion emissions of Er³⁺ or other rare-earth ions doped luminsecence materials.     

Enhancement of upconversion intensity in Er3+ doped tellurite glass in presence of Yb3+

TeO₂–PbO glasses doped/codoped with Er³⁺/Er³⁺-Yb³⁺ ions have been fabricated by melting and quenching method. Efficient frequency upconversion emissions spanning from blue to red regions corresponding to the ²H_11/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively, upon excitation with 976 nm diode laser radiation have been observed. The variations observed in the intensity of whole upconversion emission spectra due to the presence of the Yb³⁺ ions are reported and discussed in detail.     

Upconverted 5d-4f luminescence from Er and Nd ions doped into fluoride hosts excited by ArF and KrF excimer lasers

We report on observation of upconverted VUV luminescence due to 5d-4f radiative transitions in Er³⁺ and Nd³⁺ ions doped into some fluoride crystals, under excitation by ArF and KrF excimer lasers, respectively. Only spin-forbidden 5d-4f luminescence of Er³⁺ (at 165 nm) was detected from the LiYF₄:Er³⁺ crystal whereas both spin-forbidden (at 169 nm) and spin-allowed (at 160.5 nm) components are observed from the BaY₂F₈:Er³⁺ crystal, the latter being much weaker than in the case of one-photon excitation. Nd³⁺ 5d-4f luminescence at 180 and 173 nm has been detected from the LiYF₄:Nd³⁺ and LaF₃:Nd³⁺ crystals, respectively. The shift of short-wavelength edge of 5d-4f emission spectra towards longer wavelengths is observed under temperature increase from 15 to 293 K. The observed effects in the spectra of Er³⁺ and Nd³⁺ doped crystals were interpreted as a result of reabsorption of 5d-4f luminescence escaping from the bulk of the crystals.     

Photoluminescence of LiNbO3 crystals doped with Er3+ and Yb3+ ions

Results of cooperative phenomena investigations in the impurity subsystem of lithium niobate crystals doped with Er³⁺ and co-doped with Yb³⁺ impurity ions under continuous wave and pulsed excitation at 975 nm and 1064 nm wavelengths are presented. Dependences of some spectroscopic characteristics on the intensity of laser pumping are studied. Based on the pair centers model the analysis of the cooperative luminescence behavior in LiNbO₃:Yb³⁺+Er³⁺ crystals is performed.     

Energy transfer in Er:Eu:Yb co-doped tellurite glasses: Yb as enhancer and quencher

Energy transfer has been studied from Er³⁺ to Eu³⁺ ions on excitation with NIR photons (796 and 980 nm) with and without Yb³⁺ ions. It is found that in one case the presence of Yb³⁺ enhances the fluorescence yield (980 nm excitation) whereas in the other case it quenches (796 nm excitation). Energy transfer from Er³⁺ ion's levels ⁴S_3/2 and ²H_11/2 is verified by decay curve analysis in both the cases. The nature of interaction between the donor (Er) and the acceptor (Eu) ions is found to be dipole-dipole. The energy transfer parameters viz. transfer probability, critical distance etc. have been calculated.     

Upconversion emission enhancement in Er/Yb-codoped BaTiO3 nanocrystals by tridoping with Li ions

Er³⁺/Yb³⁺/Li⁺-tridoped BaTiO₃ nanocrystals were prepared by a sol-gel method to improve the upconversion (UC) luminescence of rare-earth doped BaTiO₃ nanoparticles. Effects of Li⁺ ion on the UC emission properties of the Er³⁺/Yb³⁺/Li⁺-tridoped BaTiO₃ nanocrystals were investigated. The results indicated that tridoping with Li⁺ ion enhanced the visible green and red UC emissions of Er³⁺/Yb³⁺-codoped BaTiO₃ nanocrystals under the excitation of a 976 nm laser diode. X-ray diffraction and decay time of the UC luminescence were studied to explain the reasons of the enhancement of UC emission intensity. X-ray diffraction results gave evidence that tridoping with Li⁺ ion decreased the local symmetry of crystal field around Er³⁺, which increased the intra-4f transitions of Er³⁺ ion. Moreover, lifetimes in the intermediate ⁴ S_3/2 and ⁴I_11/2 (Er) states were enhanced by Li⁺ ion incorporation in the lattice. Therefore, it can be concluded that Li⁺ ion in rare-earth doped nanocrystals is effective in enhancing the UC emission intensity.     

Spectral-kinetic characteristics of Pr luminescence in LiLuF4 host upon excitation in the UV-VUV range

Spectral-kinetic study of Pr³⁺ luminescence has been performed for LiLuF₄:Pr(0.1 mol%) single crystal upon the excitation within 5-12 eV range at T=8 K. The fine-structure of Pr³⁺ 4f ²→4f 5d excitation spectra is shown for LiLuF₄:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr³⁺ ions into 4f 5d involving 4f ¹ core in the ground state. Favourable conditions have been revealed in LiLuF₄:Pr(0.1 mol%) for the transformation of UV-VUV excitation quanta into the visible range. Lightly doped LiLuF₄:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr³⁺³P₀ visible emission upon UV-VUV excitation. The mechanism of energy transfer between Lu³⁺ host ion and Pr³⁺ impurity is discussed.     

Optical thermometry through green and red upconversion emissions in Er/Yb/Li:ZrO2 nanocrystals

Variations of fluorescence intensity ratio of green (generated from Er^{3+ 2}H_11/2 and ⁴S_3/2 levels) and red (generated from the sublevels of Er^{3+ 4}F_9/2 level) upconversion emissions in Er³⁺/Yb³⁺/Li⁺:ZrO₂ nanocrystals have been studied as a function of temperature under 976 nm laser diode excitation. In the temperature range of 323-673 K, the maximum sensitivities of about 0.0134 K^− 1 and 0.0104 K^− 1 are obtained by using green and red emission, respectively. Er³⁺/Yb³⁺/Li⁺:ZrO₂ nanocrystals show potential application value in nanoscale thermal sensor.