Energy Transfer Upconversion Research Articles

The dominant role of excitation diffusion in energy transfer upconversion of Lu2O3: Tm3+, Yb3+

Emission spectra of Tm3+ and Yb3+ codoped Lu2O3 are studied as a function of the dopant concentration upon 980 nm excitation. The upconverted near infrared emission around 811 nm from Tm3+:3H4→3H6 transition is observed due to two step energy transfers from Yb3+ through Tm3+:3F4 as the intermediate level. The optimal dopants concentrations for the strongest upconversion emission are 0.1 mol% Tm3+ and 4 mol% Yb3+. The dependence of energy transfer coefficients for the two step energy transfers on Tm3+ and/or Yb3+ concentration is studied. The obtained results indicate that the excitation diffusions among Yb3+ ions and among Tm3+ ions mainly control the energy transfer rates for the optimal dopants concentrations and thus dominate the energy transfer upconversion process.

Quadrature frequency resolved spectroscopy of upconversion photoluminescence in GeGaS:Er3+: I. Determination of energy transfer upconversion parameter

The article presents the formulas of quadrature frequency resolved spectroscopy (QFRS) on the upconversion photoluminescence (UCPL) of rare-earth (RE) doped materials on the general M-level model. The formulas are derived in matrix-equation form with the first-order perturbation on the rate equations at the M energy-levels of RE ion. The QFRS spectra for three different UCPL processes, i.e., the excited state absorption (ESA), energy transfer upconversion (ETU) and photon avalanche (PA) via cross relaxation process (CRP) in a particular case of M = 3 are demonstrated with the respective salient features of the excitation power dependence. We have measured the QFRS on the UCPL (4S3/2 → 4I15/2) by systematically varying Er-doping in Ge28.1Ga6.3S65.6 chalcogenide glass from 0.01 to 0.5 at.% as well as 975 nm excitation power. Thereby the relaxation rates k1 at the intermediate level 4I11/2, k2 at the top level 4S3/2 and ETU parameter w are determined as a function of Er concentration. The UCPL dynamics on the basis of the formulas for the 3-level model is interpreted in terms of the determined parameters.

A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of Fe(2,2'-Bipyridine)32.

A meta-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand (CNtBuAr3NC) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr(CNtBuAr3NC)3 complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 °C) is nearly 2 orders of magnitude longer than the current record lifetime for isoelectronic Fe(II) complexes, which are of significant interest as earth-abundant sensitizers in dye-sensitized solar cells. Due to its chelating ligands, Cr(CNtBuAr3NC)3 is more robust than Cr(0) complexes with carbonyl or monodentate isocyanides, manifesting in comparatively slow photodegradation. In the presence of excess anthracene in solution, efficient energy transfer and subsequent triplet-triplet annihilation upconversion is observed. With an excited-state oxidation potential of -2.43 V vs Fc+/Fc, the Cr(0) complex is a very strong photoreductant. The findings presented herein are relevant for replacement of precious metals in dye-sensitized solar cells and in luminescent devices by earth-abundant elements.

Cooperative and non-cooperative sensitization upconversion in lanthanide-doped LiYbF4 nanoparticles.

Lanthanide (Ln3+)-doped upconversion nanoparticles (UCNPs) have attracted tremendous interest owing to their potential bioapplications. However, the intrinsic photophysics responsible for upconversion (UC) especially the cooperative sensitization UC (CSU) in colloidal Ln3+-doped UCNPs has remained untouched so far. Herein, we report a unique strategy for the synthesis of high-quality LiYbF4:Ln3+ core-only and core/shell UCNPs with tunable particle sizes and shell thicknesses. Energy transfer UC from Er3+, Ho3+ and Tm3+ and CSU from Tb3+ were comprehensively surveyed under 980 nm excitation. Through surface passivation, we achieved efficient non-cooperative sensitization UC with absolute UC quantum yields (QYs) of 3.36%, 0.69% and 0.81% for Er3+, Ho3+ and Tm3+, respectively. Particularly, we for the first time quantitatively determined the CSU efficiency for Tb3+ with an absolute QY of 0.0085% under excitation at a power density of 70 W cm-2. By means of temperature-dependent steady-state and transient UC spectroscopy, we unraveled the dominant mechanisms of phonon-assisted cooperative energy transfer (T > 100 K) and sequential dimer ground-state absorption/excited-state absorption (T < 100 K) for the CSU process in LiYbF4:Tb3+ UCNPs.

Infrared emission spectroscopy and upconversion of ZnO-Li2O-Na2O-P2O5 glasses doped with Nd3 + ions

Glasses with composition 20ZnO-10Li2O-10Na2O-(60-x)P2O5-xNd2O3 containing Nd3+ ions were analyzed using density calculations and optical absorption spectroscopy together with excitation and emission luminescence measurements. The Judd-Ofelt intensity parameters (Ω2, Ω4, Ω6) were calculated and their variation correlated with the physical properties. Infrared emission spectra were recorded at an excitation wavelength of 581nm. The emission spectra showed emissions at 872nm, 1053nm and 1325nm which corresponded to the 4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions. A decrease in the emission intensity occurred beyond 1.5mol% Nd2O3 doping which was explained on the basis of concentration quenching of the ions. The infrared to visible upconversion process was also investigated for the prepared glasses. Three main emission bands at 530nm, 590nm and 687nm were observed for excitation at 870nm. These emission bands originated form the 4G7/2 level. Excited state absorption and energy transfer upconversion mechanisms which may be responsible for the upconversion were also discussed in the present work.

Resonance in Er3+ upconversion excitation

Upconversion (UC) processes are discussed on the examples of excited state absorption (ESA) in LaCl3: 1% Er3+ for 980nm excitation and energy transfer upconversion (ETU) in β-NaYF4: 20% Er3+ for 1523nm excitation. The crucial importance of resonance is demonstrated for one- and two-color excitations in LaCl3:Er. For ESA the excitation spectra are convolutions of the individual excitation steps and the UC luminescence intensity shows a linear dependence on the excitation power for each step. For one-color excited ESA and ETU in the low power limit, the exponent of the power dependence is equal to the number of excitation steps. The exponent is reduced due to saturation effects for high power excitation. The ETU excitation spectra resemble the ground state absorption as shown for β-NaYF4:Er. Upon broad-band excitation by sub-Si-band-gap infrared light β-NaYF4:Er shows UC luminescence mainly from the 4I11/2 state around 980nm which efficiently excites a Si solar cell. This Er UC luminescence is two orders of magnitude more intense than that of other rare earth ions under the same experimental conditions.

Stochastic Model of Energy-Transfer Processes Among Rare-Earth Ions. Example of Al2O3:Tm3+

Energy-transfer processes strongly affect the performance of lanthanide-doped photonic devices. In this work, we introduce a simple stochastic model of energy-transfer processes and successfully apply it to the example of cross-relaxation (CR) and energy-transfer upconversion (ETU) in amorphous Al2O3:Tm3+ waveguides on silicon intended for lasers operating at ∼2 μm. The stochastic model is based on the rate-equation formalism and considers two spectroscopically distinct ion classes, namely single ions and ions with neighbors (pairs and clusters), with the corresponding ion fractions being dependent on the doping concentration. We prove that a more accurate description of the luminescence properties of amorphous Al2O3:Tm3+ is obtained when accounting for the presence of these distinct ion classes. Based on the developed model, we derive microscopic CR and ETU parameters of CCR = 5.83 × 10–38 cm6 s–1, CETU1 = 0.93 × 10–40 cm6 s–1, and CETU2 = 7.81 × 10–40 cm6 s–1, and determine the laser quantum efficiency ...

Up-conversion luminescence properties and thermal effects of LaVO4:Er3+ under 1550 nm excitation

Efficiently up-conversion luminescence can be achieved by replacing the traditional sensitizer Yb3+ ions with Er3+ ions in LaVO4 host under 1550nm excitation. In contrast to other up-conversion luminescence materials, such as Y2O2S:Er3+, Y2O3:Er3+, YOCl:Er3+, which exhibit red emission, LaVO4:Er3+ samples present an abnormal green emission under 1550nm excitation. Excited state absorption is the dominating up-conversion luminescence mechanism at low doping concentrations of Er3+, whereas energy transfer up-conversion predominates at high concentrations of Er3+. Infrared thermal imaging is adopted to investigate the thermal effects of LaVO4:Er3+ sample under 1550nm irradiation. It shows a rapid heating process within 1∼5s and LaVO4:0.21Er3+ sample yields a temperature rising of ΔT=66.46°C under 1550nm irradiation (207mW) within 60s. Therefore, the temperature sensor based on up-conversion luminescence of Er3+ ions 2H11/2/4S3/2 thermally coupled levels produced significant measurement errors at low temperatures because of the thermal effects and non-uniform temperature of the incident light spot area.

The basic spectroscopic parameters of Ho3+-doped fluoroindate glass for emission at 3.9 μm

This report details the first study of the fundamental spectroscopic properties of a new optical material for prospective application as a gain medium for fiber laser emission at 3.9 μm. We have investigated the decay processes that are relevant to the excited states of the 5I5 → 5I6 transition in singly Ho3+-doped InF3 (fluoroindate) glass using time-resolved fluorescence spectroscopy. The HoF3 concentration in the glass was 10 mol.%. We excited the 5I7 and 5I6 energy levels using selective excitation at 1982 nm and 1150 nm, respectively. We have established that a strong energy-transfer upconversion process by way of the well-known dipole-dipole interaction between two holmium ions excited to the 5I6 level populate the 5I5 (upper laser) level of the 3.9 μm transition. The 5I6 and 5I5 energy levels emit luminescence with measured peaks located at 2.85 μm and 3.92 μm, respectively and the luminescence efficiencies of these emissions are 78% and 0.2%, respectively. Results from numerical simulations show that for this high Ho3+ concentration, a population inversion for the 3.9 μm transition is reached only for a short time (t < 100 μs) after direct upper laser level pumping at a wavelength of 889 nm.

Red–green–blue upconversion luminescence and energy transfer in Yb3+/Er3+/Tm3+ doped YP5O14 ultraphosphates

Red, green, and blue emissions through up conversion and energy transfer processes were investigated in triply doped YP5O14: Yb3+, Er3+, Tm3+ ultraphosphates upon excitation at 980nm as a function of the Yb3+, Er3+ and Tm3+ concentrations. Highly crystalline tri-doped samples showing a monoclinic form with C2/c space group symmetry were synthesized by a flux method leading to the elaboration of single crystals which were crushed for physical-chemical mainly by X-ray powder diffraction and optical up conversion characterization.The blue, green and red emissions intensity can be adjusted by the Yb3+, Er3+ and Tm3+ amounts. The Er3+ red and green up conversion emissions were examined in the case of the Yb3+/Er3+ co-doped YP5O14 materials. Two- and three-photons processes are observed for the red and green emissions of Er3+ respectively. Three visible red, green and blue up conversion emissions corresponding to Er3+4F9/2→4I15/2 (red), Tm3+1G4→3F4; 3F2,3→3H6 (red), Er3+2H11/2, 4S3/2 →4I15/2 (green) and Tm3+1G4 → 3H6 (blue) emission bands were simultaneously observed in triply doped YP5O14: Yb3+, Er3+, Tm3+ ultraphosphates due to Yb–Er and Yb–Tm up conversion mechanisms. Moreover, a three-photon process is observed for the red emission in the tri-doped materials instead of a 2-photons mechanism observed for samples without Tm, due to a Er3+→ Tm3+ energy transfer involving the Er3+4F9/2 and the Tm3+3F2,3 states.Despite the presence of large phonons energy due to the phosphate material, around 1200cm−1, Er3+ and Tm3+ visible emissions due to up conversion phenomena can be subsequently combined in order to obtain a white light and find new applications in display devices.

Spatial dynamic thermal iteration model for 888 nm end-pumped Nd:YVO4 solid-state laser oscillators and amplifiers

A new theoretical model, spatial dynamic thermal iteration (SDTI) model, for diode-end-pumped solid-state laser systems is developed, which is both applicable to laser oscillators and amplifiers. The influences of pump beam quality, ground state absorption and depletion (GSA/GSD) and energy transfer upconversion (ETU) are included in our model. According to the basic principles of nonradiative transitions and population dynamics, we can obtain the spatial distribution of heat generation and temperature within the laser crystal through numerically solving heat conduction equation by finite element method (FEM). Furthermore, a spatial mesh iteration algorithm is designed to analyze the temperature dependence of absorption cross section, emission cross section and thermal conductivity. Finally, the simulated results of our SDTI model was proved to precisely coincide with the reported experimental results in classical 888nm end-pumped Nd:YVO4 laser oscillator and amplifier systems.

Plasmon enhanced energy-transfer up-conversion in Yb3+-Er3+ co-doped LiNbO3 crystal

We have analyzed the effect of linear chains of metallic Ag nanoparticles on the optical properties of a periodically poled Yb3+-Er3+ co-doped LiNbO3 crystal. By exploiting the broad plasmonic response supported by linear chains of strongly coupled Ag nanoparticles, we demonstrate a 50% of enhancement of the up-converted Er3+ emission under excitation in the f-f transition of Yb3+ ions. The observed intensification is explained in terms of the broad plasmonic spectral response supported by the Ag chains, which overlaps both the Er3+ visible emissions and the Yb3+ absorption band, and the two-photon character of the Yb3+ → Er3+ energy-transfer up-conversion process. The results are of interest for applications involving luminescence up-conversion such as sensing, solar energy conversion, biological imaging or solid-state nanolasers.

Ce3+/Yb3+/Er3+ triply doped bismuth borosilicate glass: a potential fiber material for broadband near-infrared fiber amplifiers

Erbium doped bismuth borosilicate (BBS) glasses, possessing the broadest 1.55 μm near infrared (NIR) emission band among oxide glasses, stand out as excellent fiber material for optical fiber amplifiers. In this work, we demonstrate that both broadened and enhanced NIR emission of Er3+ can be obtained by sensibly combining the effects such as mixed glass former effect, phonon-assisted energy transfer (PAET) and de-excitation effect induced by codopant. Specially, by codoping CeO2 in a controlled manner, it leads to not only much improved optical quality of the glasses, enhanced NIR emission, but also significantly suppressed energy transfer up-conversion (ETU) luminescence which is detrimental to the NIR emission. Cerium incorporated in the glasses exists overwhelmingly as the trivalent oxidation state Ce3+ and its effects on the luminescence properties of Er3+ are discussed. Judd-Ofelt analysis is used to evaluate gain amplification of the glasses. The result indicates that Ce3+/Yb3+/Er3+ triply doped BBS glasses are promising candidate for erbium doped fiber amplifiers. The strategy described here can be readily extended to other rare-earth ions (REs) to improve the performance of REs doped fiber lasers and amplifiers.

NaGd(MoO4)2 nanocrystals with diverse morphologies: controlled synthesis, growth mechanism, photoluminescence and thermometric properties.

Pure tetragonal phase, uniform and well-crystallized sodium gadolinium molybdate (NaGd(MoO4)2) nanocrystals with diverse morphologies, e.g. nanocylinders, nanocubes and square nanoplates have been selectively synthesized via oleic acid-mediated hydrothermal method. The phase, structure, morphology and composition of the as-synthesized products are studied. Contents of both sodium molybdate and oleic acid of the precursor solutions are found to affect the morphologies of the products significantly, and oleic acid plays a key role in the morphology-controlled synthesis of NaGd(MoO4)2 nanocrystals with diverse morphologies. Growth mechanism of NaGd(MoO4)2 nanocrystals is proposed based on time-dependent morphology evolution and X-ray diffraction analysis. Morphology-dependent down-shifting photoluminescence properties of NaGd(MoO4)2: Eu3+ nanocrystals, and upconversion photoluminescence properties of NaGd(MoO4)2: Yb3+/Er3+ and Yb3+/Tm3+ nanoplates are investigated in detail. Charge transfer band in the down-shifting excitation spectra shows a slight blue-shift, and the luminescence intensities and lifetimes of Eu3+ are decreased gradually with the morphology of the nanocrystals varying from nanocubes to thin square nanoplates. Upconversion energy transfer mechanisms of NaGd(MoO4)2: Yb3+/Er3+, Yb3+/Tm3+ nanoplates are proposed based on the energy level scheme and power dependence of upconversion emissions. Thermometric properties of NaGd(MoO4)2: Yb3+/Er3+ nanoplates are investigated, and the maximum sensitivity is determined to be 0.01333 K−1 at 285 K.

27 μm emission in Er^3+ doped glass ceramics containing lutetium oxyfluoride nanocrystals

Intense 2.7 μm emission of Er3+ was achieved in transparent glass ceramics containing orthorhombic Vernier phase lutetium oxyfluoride nanocrystals, which was prepared through conventional melt-quenching and a subsequent thermal treatment method. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of orthorhombic Vernier phase lutetium oxyfluoride nanocrystals in glass ceramics. The preferential incorporation of Er3+ ions into nanocrystals was evidenced by TEM element mapping, stark-split absorption band, and prolongation of luminescence lifetimes. Upon 980 nm laser excitation, enhanced 2.7 μm emission originating from Er3+:4I11/2 → 4I13∕2 transition was obtained in glass ceramics via increasing the heat treatment temperature, which resulted from the inhibited non-radiative relaxation from 4I11/2 level and favored energy transfer upconversion process between the localized Er3+ ions. Our results indicate that lutetium oxyfluoride nanocrystal embedded glass ceramic is a promising candidate for 2.7 μm laser.

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb3+/Yb3+ co-doped Y2O3 transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750°C for 5h. Zr4+ and La3+ ions were incorporated in Tb3+/Yb3+ co-doped Y2O3 nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000nm range for 1mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543nm (green) was observed in these transparent ceramics under 976 and 929nm excitations due to Yb–Tb energy transfer upconversion.

Femtosecond Laser-Induced Upconversion Luminescence in Rare-Earth Ions by Nonresonant Multiphoton Absorption.

The upconversion luminescence of rare-earth ions has attracted considerable interest because of its important applications in photoelectric conversion, color display, laser device, multiplexed biolabeling, and security printing. Previous studies mainly explored the upconversion luminescence generation through excited state absorption, energy transfer upconversion, and photon avalanche under the continuous wave laser excitation. Here, we focus on the upconversion luminescence generation through a nonresonant multiphoton absorption by using the intense femtosecond pulsed laser excitation and study the upconversion luminescence intensity control by varying the femtosecond laser phase and polarization. We show that the upconversion luminescence of rare-earth ions under the intense femtosecond laser field excitation is easy to be obtained due to the nonresonant multiphoton absorption through the nonlinear interaction between light and matter, which is not available by the continuous wave laser excitation in previous works. We also show that the upconversion luminescence intensity can be effectively controlled by varying the femtosecond pulsed laser phase and polarization, which can open a new technological opportunity to generate and control the upconversion luminescence of rare-earth ions and also can be further extended to the relevant application areas.

Temperature and impurity concentration effects on upconversion luminescence in LaInO3 doped with Er3+

In this paper a novel method for synthesis of LaInO3:Er3+ is reported and upconversion luminescence properties of the synthesized material at different temperatures (9–300 K) are studied. The samples were prepared by co-precipitation and subsequent heat treatment of lanthanum, indium and erbium hydroxides. It is shown that the excitation at 980 nm leads to a strong green upconversion luminescence in the material. At the concentrations above 0.1 mol. % of Er3+ the energy transfer upconversion mechanism of the luminescence becomes evident. Further increase of Er3+ content in the material leads to higher red-to-green upconversion luminescence intensity ratio. The mechanisms responsible for the observed variation are discussed.

Designing Upconversion Nanocrystals Capable of 745 nm Sensitization and 803 nm Emission for Deep-Tissue Imaging.

A crystal design strategy is described that generates hexagonal-phased NaYF4 :Nd/Yb@NaYF4 :Yb/Tm luminescent nanocrystals with the ability to emit light at 803 nm when illuminated at 745 nm. This is accomplished by taking advantage of the large absorption cross-section of Nd(3+) between 720 and 760 nm plus efficient spatial energy transfer and migration through Nd(3+) →Yb(3+) →Yb(3+) →Tm(3+) . Mechanistic investigations suggest that a cascaded two-photon energy transfer upconversion process underlies the emission mechanism. This protocol enables deep-tissue imaging to be achieved while mitigating the attenuation effect associated with the visible emission and the overheating constraint imposed by conventional 980 nm excitation.

NIR upconversion emission of Tm3+ doped glassceramics for solar cells applications

The Tm3+ 800nm upconversion emission corresponding to the 3H4→3H6 transition has been obtained upon infrared sub-Si bandgap excitation at 1210nm in Tm3+ doped transparent glasses and glass ceramics with composition SiO2–Al2O3–CdF2–PbF2–YF3. Possible energy transfer mechanisms have been carefully studied through different experimental measurements such as the excitation spectrum, decay rate of the emission and laser pump power versus integrated emission. The results suggest that energy transfer upconversion (ETU) mechanism is responsible for the emission. It is based on the following process: Tm3+(3F4)+Tm3+(3F4)→Tm3+(3H6)+Tm3+(3H4). The upconverted emission is about three times more intense in the glass ceramic samples than in the precursor glasses. This emission can be used to enhance the performances in crystalline silicon (c-Si) solar cells.