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.     

Enhanced near-infrared luminescence in Y2O3:Yb nanocrystals by codoping with Li ion

Y₂O₃:Yb³⁺ nanocrystals codoping with Li⁺ ions were synthesized by glycine combustion method. Codoping with Li⁺ ions leads to about 12 times enhancement of the photoluminescence (PL) intensity around 1 μm, in terms of the increased lifetimes at 1026 nm from 0.384 ms to 1.42 ms at room temperature. The enhancement in the PL intensity could be mostly attributed to the modification of the local symmetry around Yb³⁺ ions by codoping with Li⁺ ions.     

Ultraviolet upconversion luminescence enhancement in Yb/Er-codoped Y2O3 nanocrystals induced by tridoping with Li ions

Ultraviolet (UV) upconversion (UC) luminescence in Yb³⁺/Er³⁺-codoped yttrium oxide (Y₂O₃) nanocrystals can be enhanced by orders of magnitude via tridoping further with Li⁺ ions under diode laser excitation of 970 nm. Sensitized three-photon UC radiations at 390 and 409 nm, corresponding to the ⁴G_11/2→⁴I_15/2 and ⁴H_9/2→⁴I_15/2 of Er³⁺ ions, respectively, present an enhancement time of about 33 times, which is larger than the 24 times enhancement for the UC green radiation. The UV UC radiation at 320 nm that corresponds to the ²P_3/2→⁴I_15/2 of Er³⁺ ions has also been greatly enhanced. Theoretical calculations interpret that all the observed enhancement times of UV UC radiations arise from the prolonged lifetimes of their intermediate states.     

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.     

Continuous-wave dual-wavelength operation at 1062 and 1338 nm in Nd:YAl3(BO3)4 and observation of yellow laser light generation at 592 nm by their self-sum-frequency-mixing

We report simultaneous oscillation in continuous wave at 1062 and 1337 nm in a Nd³⁺:YAl₃(BO₃)₄ nonlinear crystal associated to the infrared laser channels ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 of Nd³⁺. Generation of yellow laser light at 592 nm produced by Type I self-sum-frequency-mixing of both fundamental infrared laser waves is observed under non-optimal phase matching conditions.     

Enhanced white light emission in Er/Tm/Yb/Li codoped Y2O3 nanocrystals

Er/Tm/Yb codoped Y₂O₃ nanocrystals and Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals have been synthesized by sol-gel method, bright white light emission has been observed at 976 nm excitation. The blue, green, and red emissions, respectively, arise from the transitions ¹G₄ → ³H₆ of Tm³⁺, ²H_11/2/⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ion. Moreover, after doping Li⁺ ions into Er/Tm/Yb codoped Y₂O₃ nanocrystals, the white light emission increase greatly. CIE coordinate of Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals is X = 0.32 and Y = 0.36 at 10 W/cm² excitation, which is very close to the standard equal energy white light illuminate (X = 0.33, Y = 0.33).     

Optical and fluorescence spectroscopy of Eu2O3-doped P2O5-K2O-KF-MO-Al2O3 (M = Mg, Sr and Ba) glasses

Fluorophosphate glasses of composition, P₂O₅ + K₂O + KF + MO + Al₂O₃ + xEu₂O₃ (M = Mg, Sr and Ba; x = 0.01, 0.05, 0.1, 1.0, 2.0, 4.0 and 6.0 mol%) were prepared and characterized their optical properties. Crystal-field (CF) analysis revealed a relatively weak CF strength around Eu³⁺ ions in the Ba based fluorophosphate glasses. The Judd-Ofelt parameters have been estimated from the oscillator strengths of ⁷F₀ → ⁵D₂, ⁷F₀ → ⁵D₄ and ⁷F₀ → ⁵L₆ absorption transitions of Eu³⁺ ions and were used to evaluate the radiative properties of the ⁵D₀ → ⁷F_J (J = 0-4) transitions. Considerable variation has been observed in the relative intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions of Eu³⁺ ions due to change in the alkaline earth metal ions. The decay of the ⁵D₀ level shows single exponential and less sensitive to Eu³⁺ ions concentration as well as MgO/SrO/BaO modifiers.     

Inhomogeneous broadening of the dynamically split Kramers spectral line and up-conversion in the pair and quartet centers in CaF2:Nd

The site-selective and time-resolved fluorescence laser spectroscopy and kinetic measurements with high spectral and nanosecond temporal resolution was applied to analyze the high-energy wing of the M and N absorption bands of the ⁴I_9/2(1)→⁴G_5/2(1) crystal-field (CF) transition in a CaF₂:Nd³⁺ (0.6 wt%) crystal at 4.2 K. It was found that at helium temperatures the dynamically split spectral line assigned as the ⁴I_9/2(1)→⁴G_5/2(1) (CF) transition of coherently coupled Nd³⁺ ions in the pair M- and quartet N-centers of CaF₂:Nd³⁺ (0.6 wt%) is inhomogeneously broadened. It consists of the pair M- and quartet N-centers with at least 0.1 A variation of the positions of the fluorescence-excitation spectral lines registered at the ⁴F_3/2(1)→⁴I_9/2(1) CF transition. Small fluorescence-lifetimes variation of the ⁴F_3/2 and ⁴D_3/2 levels from the small variation of the distances R between Nd³⁺ ions in the pair is found. At least 2.7% variation of the value of the Nd-Nd distance R in the pair M-center was determined from the lifetime variation of the ⁴F_3/2 manifold with the assumption of a dipole-dipole interaction between the ions in the pair.The energy transfer up-conversion process responsible for the UV fluorescence observed when pumping the ⁴I_9/2(1)→⁴G_5/2(1) transition has been determined.     

Efficient blue upconversion emission due to confined radiative energy transfer in Tm-Nd co-doped Ta2O5 waveguides under infrared-laser excitation

Intense blue upconversion emission at 480 nm has been obtained at room temperature in Tm³⁺-Nd³⁺ co-doped Ta₂O₅ channel waveguides fabricated on a Si substrate, when the sample is excited with an infrared laser at 793 nm. The upconversion mechanism is based on the radiative relaxation of the Nd³⁺ ions (⁴F_3/2 → ⁴I_11/2) at about 1064 nm followed by the absorption of the emitted photons by Tm³⁺ ions in the ³H₄ excited state. A coefficient of energy transfer rate as high as 3 × 10⁻¹⁶ cm³/s has been deduced using a rate equation analysis, which is the highest reported for Tm-Nd co-doped systems. The confinement of the 1064 nm emitted radiation in the waveguide structure is the main reason of the high energy transfer probability between Nd³⁺ and Tm³⁺ ions.     

Spectral performance and intensive green upconversion luminescence in Er/Yb-codoped NaY(WO4)2 crystal

Using Czochralski (CZ) pulling method, an Er³⁺/Yb³⁺-codoped NaY(WO₄)₂ crystal was prepared. Absorption spectra, emission spectra and excitation spectra of this crystal were measured at room temperature. Some optical parameters, such as intensity parameters, spontaneous emission probabilities and lifetimes, were calculated from absorption spectra with Judd-Ofelt (J-O) theory. Upconversion luminescence excited by a 970 nm diode laser was studied. In this crystal, green upconversion luminescence is particularly intensive. Energy transfer mechanisms that play an important role in upconversion processes were analyzed. Two cross-relaxation processes: ⁴G_11/2 + ⁴I_9/2 → ²H_11/2 (or ⁴S_3/2) + ²H_11/2 (or ⁴S_3/2), and ⁴G_11/2 + ⁴I_15/2 → ²H_11/2 (or ⁴S_3/2) + ²I_13/2, which contribute to the intensive green luminescence under 378 nm excitation, were put forward. Background energy transfer ⁴G_11/2(Er³⁺) + ²F_7/2(Yb³⁺) → ⁴F_9/2(Er³⁺) + ²F_5/2(Yb³⁺) was also demonstrated.     

Synthesis and optical properties of KZnLa0.99Nd0.01(VO4)2 triple vanadate(V)—New promising laser materials

In an attempt to find a neodymium-vanadate system with long lifetime of ⁴F_3/2 level and relatively strong ⁴F_3/2→⁴I_11/2 emission for laser applications, the optical properties of Nd³⁺ in a new KZnLa(VO₄)₂ host is reported. The crystalline samples were obtained at 900 °C in air. The samples were crystallized in monoclinic system and were isostructural with KZnLa(PO₄)₂. KZnLa_0.99Nd_0.01(VO₄)₂ strongly emits in the near infrared range with the maxima at 871.6 and 1057 nm upon excitation through the ⁴F_5/2 level (808 nm) or by the charge transfer bands of VO₄³⁻. The lifetime of ⁴F_3/2 level of Nd³⁺ ion is larger than that observed in other neodymium-vanadates systems.     

Enhanced luminescence of Ca3B2O6:Dy phosphors by Na-codoping for LED applications

The Ca_2.95−yDy_0.05B₂O₆:yNa⁺ (0≤y≤0.20) phosphors were synthesized at 1100 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE), photoluminescence (PL) spectra and thermoluminescence (TL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions on 350 nm excitation was observed at 480 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions and 660 nm (red) due to weak ⁴F_9/2→⁶H_11/2 emissions. The PL results from the investigated Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors show that Dy³⁺ emissions increase with the increase of the Na⁺ codoping ions. The integral intensity of yellow to blue (Y/B) can be tuned by controlling Na⁺ content. By the simulation of white light, the optimal CIE value (0.328, 0.334) can be achieved when the content of Na⁺-codoping ions is y=0.2. The results imply that the Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors could be potentially used as white LEDs.     

The enhancement of luminescence in Co-doped cubic Eu2O3 using Li and Al ions

The co-doping of Li⁺ and Al³⁺ ions drastically enhances the luminescence of cubic Eu₂O₃. The integrated emission intensity of ⁵D₀→⁷F_J bands (J=1-4) at 580-710 nm increases by a factor of about 6.7 in the co-doped Eu₂O₃ compared to the un-doped Eu₂O₃. In order to confirm that the co-doped ions were actually incorporated into the host lattice, the structural characteristics were studied using Raman spectroscopy, XPS, XRD, photoluminescence lifetime, and an SEM. These analyses consistently indicate a certain structural evolution in their results with an increase in the co-doping concentration. Variations in the crystal structure, the crystal morphology, and the intensity variation of the Raman modes at 465 and 483 cm⁻¹ are presented as the evidences showing the incorporation of the co-doped ions into the host. The luminescence enhancement is discussed in terms of concentration quenching, reduction of defect sites, and the modification of the local symmetry of the Eu³⁺ ions, especially in the inversion symmetry sites.     

Rapid synthesis and luminescence of the Eu, Er codoped ZnO quantum-dot chain via chemical precipitation method

ZnO quantum-dot chains codoped with Eu³⁺ and Er³⁺ were synthesized by the chemical precipitation method and the codoping effects on the structures, morphologies and optical properties of the powders were briefly investigated. The X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) results indicated the Eu³⁺ and Er³⁺ were incorporated into the crystal lattice of ZnO host. Transmission electron microscope (TEM) measurements showed the sizes of the ZnO quantum dots decreased with the increase of Eu³⁺ and Er³⁺ doping concentration, and the quantum-dot chains were formed by codoping with Eu³⁺ and Er³⁺. The green emissions in the photoluminescence spectra were attributed to 4f-4f of Er³⁺ inner shell ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, and the characteristic red emissions of Eu³⁺ ions were attributed to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions, respectively. Moreover, the red emission of the Eu³⁺ ions gradually decreased with the Er³⁺ ions doping concentration increased, which may be due to the different energy storage centers in the powders.     

Improved luminescent properties of red-emitting Ca0.54Sr0.16Eu0.08Gd0.12(MoO4)0.2(WO4)0.8 phosphor for LED application by charge compensation

The red-emitting Ca_0.54Sr_0.16Eu_0.08Gd_0.12(MoO₄)_0.2(WO₄)_0.8 phosphor is improved in the emission charateristics by charge compensation, of which chromaticity coordinates (CIE) are x=0.66 and y=0.33. Three approaches to charge compensation are investigated, namely (a) 3Ca²⁺/Sr²⁺→2Eu³⁺/Gd³⁺+vacancy, (b) 2Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+M⁺(M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ employed as a charge compensator) and (c) Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+N⁻ (N⁻ is a monovalent anion like F⁻, Cl⁻, Br⁻ and I⁻ employed as charge compensation ions). Through photoluminescent spectra analyzing the radiative and non-radiative relaxation mechanisms of luminescent system are obtained. Under 20 mA forward-bias current, one red-emitting LED is made by combining 390-405 nm-emitting LED chip and the phosphor. The red-emitting phosphor has broad prospects in LED application field.     

Strong infrared-to-visible frequency upconversion in Er-doped Sr2CeO4 powders

Efficient upconversion (UC) luminescence is demonstrated in Er³⁺:Sr₂CeO₄ powders prepared by combustion synthesis and exposed to near-infrared (∼975 nm) radiation. The UC emission lines observed at ∼530, ∼550 and ∼665 nm correspond, respectively, to ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 4f-4f transitions of Er³⁺. X-ray powder diffraction data showed that the SrCO₃ phase (impurity) is dramatically reduced when Sr²⁺ is partially substituted by Mg²⁺ ions. The UC phenomenon was investigated by use of continuous wave and pulsed laser excitation and the UC mechanism was attributed to energy transfer between excited Er³⁺ ions.     

Infrared and Zeeman effect measurements of ferromagnetically coupled Nd ions in weakly doped LiYF4 crystals

Crystal-field infrared active excitations and photoluminescence of Nd³⁺ ions in weakly doped LiYF₄ have confirmed that the concentration dependent satellite lines accompanying the Nd³⁺ crystal-field optical transitions are due to four ferromagnetically coupled pairs of Nd³⁺ ions in undistorted Y³⁺ sites with the exchange energies J₁=0.9, J₂=1.6, J₃=3.1 and J₄=4.5 cm⁻¹, respectively. A linear Zeeman splitting of the Nd³⁺ ion ⁴F_3/2→⁴I_9/2 transition is observed and the g-factors (g_∥=0.2±0.1; g_⊥=0.97±0.01) associated with the ⁴F_3/2 lowest level are determined.     

Fluorescence and Judd-Ofelt analysis of Nd ions in oxyfluoride glass ceramics containing CaF2 nanocrystals

Judd-Ofelt analyses of Nd³⁺ ions in the oxyfluoride glasses and glass ceramics containing CaF₂ nanocrystals are performed to evaluate the intensity parameters Ω_2,4,6, spontaneous emission probability, radiative lifetime, quantum efficiency, as well as stimulated emission cross-section. The influences of Nd³⁺-doping level and heating temperature on these parameters for the ⁴F_3/2→⁴I_J (J=9/2, 11/2, and 13/2) transitions are systematically discussed. The decrease of intensity parameter Ω₂ evidences the incorporation of Nd³⁺ ions into CaF₂ nanocrystals after crystallization. With increasing of Nd³⁺-doping level, the measured lifetime and quantum efficiency gradually decrease, while the stimulated emission cross-section keeps almost unchanged. For 1.0 mol% Nd³⁺-doped sample, both the emission intensity and the measured lifetime enhance with increasing of heating temperature up to 650 °C. The results indicate that the investigated glass ceramics are potentially applicable as the 1.06 um laser host.     

Luminescent properties of R doped Sr2SiO4:Eu(R=Li, Na and K) red-emitting phosphors for white LEDs

Sr₂SiO₄:Eu³⁺ and Sr₂SiO₄:Eu³⁺ doped with R⁺(R⁺=Li⁺, Na⁺ and K⁺) phosphors were prepared by conventional solid-state reaction and investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. XRD patterns and SEM reveal that the optimal firing condition for Sr₂SiO₄:Eu³⁺ was 1300 °C for 4 h. The excitation and emission spectra indicate that the phosphor can be effectively excited by ultraviolet (395 nm) and blue (466 nm) light and emits intense red light peaked at around 614 nm corresponding to the ⁵D₀→⁷F₂ transitions of Eu³⁺. In the research work, the effect of R⁺ contents on luminescence property and the Eu³⁺ concentration quenching process have also been investigated. The Eu³⁺ concentration quenching mechanism was verified to be a multipole-multipole interaction and the critical energy-transfer distance was calculated to be around 14.6 Å. The dopant R⁺(R⁺=Li⁺, Na⁺ and K⁺) as charge compensator in Sr₂SiO₄:Eu³⁺ can further enhance luminescence intensity, and the emission intensity of Sr₂SiO₄:Eu³⁺ doping Li⁺ is higher than that of Na⁺ or K⁺.     

Photoluminescence characterization of Ce and Dy doped Li2CaGeO4 phosphors

Ce³⁺ and Dy³⁺ activated Li₂CaGeO₄ phosphors were prepared by a solid-state reaction method, and characterized by XRD (X-ray diffraction) and photoluminescence techniques. The characteristic emission bands of Dy³⁺ due to ⁴F_9/2→⁶H_15/2 (blue) and ⁴F_9/2→⁶H_13/2 (yellow) transitions were detected in the emission spectra of Li₂CaGeO₄:Dy³⁺. Ce³⁺ broad band emission was observed in Li₂CaGeO₄:Ce³⁺ phosphors at 372 and 400 nm due to 5d→4f transition when excited at 353 nm. Co-doping of Ce³⁺ enhanced the luminescence of Dy³⁺ significantly and concentration quenching occurs when Dy³⁺ is beyond 0.04 mol%. White-light with different hues can be realized by tuning Dy³⁺ concentration in the phosphors.