Dominant Upconversion Mechanism Research Articles

Electric field induced upconversion fluorescence enhancement and its mechanism in Er3+ doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 transparent ceramic

We have conducted an in-situ upconversion (UC) mechanism study through fluorescence enhancement induced by electric field in the Er3+ doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 transparent ceramic. Under the excitation of a 980 nm diode laser, the sample shows a strong green UC fluorescence, which increases with electric field. A maximum enhancement of 150% was achieved in fluorescence intensity by the applied electric field due the lowering of crystal symmetry by the electric field induced strains. Combined with an in-situ analysis of time-resolved spectra, we proved that the dominant UC mechanism in this system should be the energy transfer (ET) process.

Synthesis and ultraviolet upconversion fluorescence of Tm3+ doped YbF3 microparticles

YbF3 microparticles doped with Tm3+ were synthesised by coprecipitation method, from which intense ultraviolet and blue emission bands are observed under 980 nm excitation. It is found that the upconversion fluorescence intensity of YbF3:Tm3+ synthesised by coprecipitation method is efficiently enhanced comparing with that prepared by solid state reaction method. The dependence of the emission intensity on pumping power was investigated in the Tm3+ doped YbF3. It is believed that two- and three-photon processes perform populations of 1G4 and 1D2 levels of Tm3+ respectively. The dominant upconversion mechanism may be attributed to a cooperative sensitisation process, which makes two Yb3+ ions in excited states cooperatively transfer their energy to a Tm3+ ion. The dependence of the emission intensity on the concentration of Tm3+ was also discussed.

Up-conversion luminescence of Er 3+-doped and Yb 3+/Er 3+ co-doped 12CaO·7Al 2O 3 poly-crystals

The optical properties of Er 3+-doped and Yb 3+/Er 3+ co-doped 12CaO·7Al 2O 3 (C12A7) poly-crystals, synthesized by high temperature solid state method, were investigated in detail. For Er 3+-doped and Yb 3+/Er 3+ co-doped C12A7 poly-crystals, two main emission bands centered around 530/550 nm (green) and 660 nm (red) were observed under 980 nm diode laser excitation via an up-conversion process. The intensity of green up-conversion emission had a strong increase in Er 3+ (1.0 mol.%, 1.5 mol.%, 3.0 mol.%), and the intensity ratio of red to green up-conversion emission had an increase in Yb 3+ (1.0 mol.%, 2.0 mol.%, 10. 0 mol.%)/Er 3+ (fixed at 1.0 mol.%). This detailed study of the up-conversion processes allowed us to identify the dominant up-conversion mechanisms in Er 3+-doped and Yb 3+/Er 3+ co-doped C12A7 poly-crystals.

Efficient oxide phosphors for light upconversion; green emission from Yb3+and Ho3+co-doped Ln2BaZnO5(Ln = Y, Gd)

The optical properties of Yb3+ and Ho3+ co-doped Y2BaZnO5, synthesized by solid-state reactions, are investigated in detail. Under 977 nm excitation (∼25 × 10−3 W mm−2), bright green upconversion emission is observed. Concentration dependence studies at room temperature show that relatively high infrared to visible upconversion efficiencies are obtained with values up to ∼2.6%. The results of power dependence studies and temperature-dependent lifetime measurements allow us to determine the dominant upconversion mechanisms in Yb3+:Ho3+ co-doped Y2BaZnO5oxides. The materials presented in this article constitute new and efficient upconversion phosphors which may find utility in a variety of applications.

Quantitative analysis of different upconversion-mechanism contributions for population of certain level in Er^3+-doped YAG

In Er3+:YAG crystal, dependence of the competition between different upconversion (UC) mechanisms for the population of P3∕22 on the exciting wavelength has been observed. An approximate method has been proposed for quantitative analysis of different UC-mechanism contributions by utilizing the excitation spectra of P3∕22 and G9∕22. The sensitive dependence of the dominant UC mechanism and the fluorescence dynamic behaviors on exciting wavelength observed in the experiment could be well reproduced by this simple and effective method.

Oxide phosphors for efficient light upconversion: Yb3+ and Er3+ co-doped Ln2BaZnO5 (Ln = Y, Gd)

The optical properties of Yb3+ and Er3+ co-doped Ln2BaZnO5 (Ln = Y or Gd), synthesized by solid-state reaction, are investigated in detail. Two main emission bands centered around 548 nm (green) and 673 nm (red) are observed under 977 nm laser excitation via an upconversion process. Studies of the behavior as a function of dopant concentration are described and relatively high infrared to visible upconversion efficiencies of ∼5% are obtained at room temperature. Under modulated 977 nm excitation and for fixed dopant concentrations, the upconverted emission chromaticity can be varied by changing the excitation duration. The results of power dependence studies and lifetime measurements are presented. This detailed study of the upconversion processes allows us to identify the dominant upconversion mechanisms in Yb3+,Er3+ co-doped Ln2BaZnO5 oxides.

Upconversion in Ho3+-doped YbF3 particle prepared by coprecipitation method

YbF3 particles doped with Ho3+ were synthesized by coprecipitation method, from which the ultraviolet and visible emission bands of the Ho3+ and the 480 nm cooperative upconversion emission of Yb3+–Yb3+ are observed under 980 nm excitation. Under the same excitation power, the emission intensity of Ho3+ in coprecipitation method is enhanced by about two times comparing to that in solid-state reaction method. The novel ultraviolet and violet emissions of the Ho3+ are firstly obtained which are centered at 360 (5G2→5I8),391 (3K7→5I8),412 (5G4→5I8), and 446 nm (5G5→5I8). The luminescence decay profiles of 545 and 652 nm visible emissions were obtained with a 980 nm pulsed laser. The excitation power dependence of the emission intensity was also measured and intensity saturation was observed. Based on the level structures of Ho3+, two- and three-photon processes are suggested to perform populations of 5S2 and 5G3 (Ho3+) levels, respectively. The dominant upconversion mechanism may be attributed to a cooperative sensitization process of two excited states of Yb3+ and energy transfers from Yb3+ to Ho3+.

Comparative studies of spectroscopic properties in Er3+–Yb3+ codoped phosphate glasses

Abstract A spectroscopic investigation of an extensive series of commercially available Er 3+ –Yb 3+ codoped phosphate glasses (IOG-1) with different Er–Yb concentrations have been presented based upon spectroscopic measurements and Judd–Ofelt theory. The composition and the atomic concentration of the respective elements of the glass samples were analyzed. Various spectroscopic parameters were obtained to evaluate the performances of these glasses as a laser material in the eye-safe laser wavelength of 1.53 μm. Using the measured fluorescence lifetimes the radiative quantum efficiency of the 4 I 13/2 → 4 I 15/2 transition is estimated to be ∼100% for the glass substrates with an Er 3+ concentration of lower than 2.0 × 10 20 ions/cm 3 . The stimulated emission and absorption cross-sections were also determined and compared. The infrared and upconversion fluorescence were studied, and under 975 nm excitation the dominant upconversion mechanisms are excited state absorption (ESA) and energy transfer (ET) for the green emission, and ET for the red emission. The data obtained here provide useful guidelines on the choice of IOG-1 glasses with the fixed Er–Yb concentrations.

Up-conversion luminescence of Er3+ doped and Er3+/Yb3+ co-doped YTaO4

The synthesis and up-conversion luminescent properties of YTaO4:Er3+ and YTaO4:Er3+/Yb3+ are reported for the first time. According to the measurement results of up-conversion spectra, Yb3+ co-doping can remarkably enhance the green (2H11/2/4S3/2→4I15/2) and red (4F9/2→4I15/2) emissions, but depress the infrared emission (4I9/2→4I15/2). With the increase of the Yb3+ concentration, the intensity of green emission increases, after that, when the Yb3+ concentration increases continuously, the intensity of green emission decreases, while those of the red and infrared emissions increase and decrease alternately. In addition, the up-conversion mechanisms of Er3+ doped and Er3+/Yb3+ co-doped YTaO4 are also discussed. It is found that the transform of up-conversion mechanism from two-step energy transfer to cooperating sensitization takes place when Yb3+ concentration is increased up to 12 mol%. With the further increase of Yb3+ concentration, the energy-back-transfer gradually becomes the dominant up-conversion mechanism, which results in the quenching of the green emission and slight increasing of the red and infrared emissions.

Fabrication of upconversion emissive LaOCl phosphors doped with rare-earth ions for bioimaging probes

Lanthanum oxychloride nanoparticles containing trivalent erbium ions were prepared through a self-hydrolysis process of hydrated chlorides of lanthanum and erbium. The phosphors were characterized by X-ray diffraction to be found in a single oxychloride phase. Irradiation on the powder with a 980-nm laser diode resulted in frequency upconversion into intense green emissions around 525--550 nm and red emissions around 650--670 nm. Stokes emission and excitation power dependence of the upconversion emission were also measured, and the dominant upconversion mechanisms were deduced to be the excited state absorption (ESA) and phonon-assisted energy transfer (PET) for green and red emissions, respectively.

Analysis of up-converted UV fluorescence dynamics in Nd3+ doped ZBLAN glasses

In this work we investigate dynamics of the UV and violet fluorescence from the high lying 4 D 3 / 2 and 2 P 3 / 2 multiplets in Nd 3 + doped fluorozirconate (ZBLAN) glass. Emission properties of several samples with different neodymium ion concentrations (0.3-5 mol.%) were carefully examined under direct and multiphoton pulsed excitation and the dominant up-conversion mechanisms were proposed. The fluorescence lifetimes of the excited 4 D 3 / 2 and 2 P 3 / 2 levels were, according to our knowledge, for the first time determined. Additionally, the 4 D 3 / 2 and 2 P 3 / 2 luminescence quenching was analyzed by investigating the concentration dependencies of decay profiles.

Upconversion spectroscopy and properties of NaYF 4 doped with [formula omitted], [formula omitted] and/or [formula omitted

A spectroscopic investigation of NaYF 4 powders doped with several different concentrations of Er 3 + , Tm 3 + and/or Yb 3 + is described. Rare earth-doped NaYF 4 is known to be a very efficient near-infrared to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calculated. Raman spectroscopy of undoped NaYF 4 is presented here for the first time, demonstrating that the dominant phonon modes in NaYF 4 lie between 300 and 400 cm - 1 . The fact that these phonon modes are also the optically active ones is further demonstrated by temperature-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF 4 samples. Excitation spectroscopy up to ∼ 70000 cm - 1 in an NaErF 4 sample reveals a multitude of Er 3 + 4f excitations, including the illustrious 2F(2) 5/2 one that has not been observed in excitation spectroscopy before. From the low-temperature power-dependence of the emission intensities for an Er 3 + , Yb 3 + codoped NaYF 4 sample, it is concluded that the dominant upconversion mechanism at low temperature is a different one than at room temperature. From direct excitation, the lifetimes of the Yb 3 + F 5 / 2 2 → F 7 / 2 2 , Er 3 + F 9 / 2 4 → I 15 / 2 4 and Er 3 + S 3 / 2 4 → I 15 / 2 4 emissions are determined as a function of temperature for all samples. At elevated temperatures, a significant decrease in the green lifetime is observed, which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er 3 + ions.

CooperativeYb3+−Tb3+dimer excitations and upconversion inCs3Tb2Br9:Yb3+

Green ${\mathrm{Tb}}^{3+}{}^{5}{\stackrel{\ensuremath{\rightarrow}}{{\mathrm{D}}_{4}}}^{7}{\mathrm{F}}_{\mathrm{J}\mathrm{}}$ luminescence visible by eye is observed under near-infrared laser excitation. Optical spectroscopic techniques including absorption, luminescence, and excitation spectroscopy are used to characterize this upconversion (UC) luminescence. The ${\mathrm{Tb}}^{3+}$ UC luminescence is present for all temperatures within a range from 10 to 300 K, and gains intensity by three orders of magnitude between 10 and 300 K. For $T>~100\mathrm{K}$ the dominant upconversion mechanism is the cooperative sensitization of ${\mathrm{Tb}}^{3+}$ by two ${\mathrm{Yb}}^{3+}$ ions. In this temperature regime the ${\mathrm{Tb}}^{3+}$ UC luminescence dominates the visible (VIS) spectrum for all near-infrared (NIR) excitations, resulting in the characteristic green luminescence. At 10 K, the color of the luminescence changes from green to blue, depending on the excitation wavelength corresponding to the dominance of ${\mathrm{Tb}}^{3+}$ UC luminescence or the ${\mathrm{Yb}}^{3+}\ensuremath{-}{\mathrm{Yb}}^{3+}$ cooperative pair luminescence. Two color excitation spectroscopy is performed to directly observe an excited state absorption (ESA) step in the ${\mathrm{Tb}}^{3+}$ UC luminescence excitation spectrum at 10 K. This allows the unambiguous assignment of a type of ground state absorption/excited state absorption (GSA/ESA) mechanism responsible for the upconversion in this system at 10 K. We explain this cooperative interaction in the framework of an ${\mathrm{Yb}}^{3+}\ensuremath{-}{\mathrm{Tb}}^{3+}$ exchange-coupled dimer. An energy level diagram for this dimer is presented. Excitation into dimer levels around 12000--14500 ${\mathrm{cm}}^{\ensuremath{-}1},$ where neither ${\mathrm{Yb}}^{3+}$ nor ${\mathrm{Tb}}^{3+}$ single ions have levels, leads to ${\mathrm{Yb}}^{3+}$ luminescence at 10 K. For laser excitation, 53 $\mathrm{W}/{\mathrm{mm}}^{2},$ resonant with an ESA transition a VIS/NIR photon ratio of ${2.7(10)}^{\ensuremath{-}5}$ is found at 10 K.

Upconversion luminescence in Yb3+ doped CsMnCl3: Spectroscopy, dynamics, and mechanisms

Single crystals of CsMnCl3 doped with 0.9% Yb3+ were grown from the melt by the Bridgman technique and studied by means of variable temperature optical spectroscopy. At cryogenic temperatures, near-infrared Yb3+-excitation around 1 μm leads to intense Mn2+ upconversion luminescence in the red spectral region. This very efficient upconversion process is possible because of magnetic Yb3+–Mn2+ exchange interactions, and a new type of upconversion mechanism is found to be active in this system. The upconversion properties of Yb3+:CsMnCl3 are compared to those of Yb3+:RbMnCl3 and Yb3+:CsMnBr3. The upconversion efficiencies at cryogenic temperatures differ by many orders of magnitude. The bridging geometry between Yb3+ and Mn2+ is found to be a key factor for the efficiency of the process. The highest efficiency is observed for the title compound, and this is correlated with the most likely linear Yb3+–Cl−–Mn2+ arrangement in this crystal. At 15 K the dominant upconversion mechanism in the title compound involves an energy transfer step. By increasing the temperature to 100 K a new and very efficient mechanism involving a sequence of ground state and excited state absorption steps becomes dominant.

Red-to-green up-conversion in Er-doped SiO 2 and SiO 2–TiO 2 sol–gel silicate glasses

Monolithic Er-doped SiO 2–TiO 2 binary glasses of high optical quality were used in an investigation of the effects of different annealing conditions and titanium content on fluorescence yields and decay times of the 4S 3/2 level of Er 3+. In addition, the characteristics of green upconverted fluorescence from the 4S 3/2 level via excitation with red laser light (655 nm) were studied. Energy transfer up-conversion involving two ions in the 4I 11/2 state was determined to be the dominant up-conversion mechanism. The glasses with TiO 2 showed enhanced up-conversion and required lower annealing temperatures for the up-conversion to be observed when compared to SiO 2 glasses doped with Al.