Cross-relaxation Efficiency Research Articles

Nanoparticle doping and molten-core methods towards highly thulium-doped silica fibers for 0.79 μm-pumped 2 μm fiber lasers – A fluorescence lifetime study

We study the fluorescence lifetime of Thulium-doped silica fibers for efficient laser operation in the 2 μm “eye-safe” wavelength range pumped by 0.79 μm radiation. A large set of optical fibers prepared by Modified Chemical Vapor Deposition combined with solution doping or nanoparticle doping, and Molten-core method is investigated in terms of fluorescence lifetimes of the 3F4 and 3H4 excited levels, which contain information about the Tm3+ ion environment, the rate of unwanted processes, such as concentration quenching or multiphonon relaxation, as well as the inter-ionic energy-transfer processes, including the cross-relaxation. Maximum evaluated lifetime values are 778 μs for the 3F4 level and 56 μs for the 3H4 level. From the lifetime study, we conclude that for Tm3+ concentrations above 10,000 ppm and enhanced aluminum concentrations above 5 mol. % Al2O3, the cross-relaxation efficiency is enhanced and multiphonon relaxation is remarkably reduced due to the lower phonon energy of the resulting alumino-silicate glass.

Enhancing NIR-II Upconversion Monochromatic Emission for Temperature Sensing.

The upconversion luminescence (UCL) in the second near-infrared window (NIR-II) is highly attractive due to its excellent performance in high-resolution bioimaging, anticounterfeiting, and temperature sensing. However, upconvertion nanoparticles (UCNPs) are normally emitted in visible light, potentially impacting the imaging quality. Here, a monochromatic Er3+-rich (NaErF4:x%Yb@NaYF4) nanoparticles with excitation at 1532nm and emission at 978nm is proposed, both situated in the NIR-II region. The proper proportion of Yb3+ ions doping has a positive effect on the NIR-II emission, by enhancing the cross relaxation efficiency and accelerating the energy transfer rate. Owing to the interaction between the Er3+ and Yb3+ is inhibited at low temperatures, the UCL emission intensities at visible and NIR-II regions show opposite trend with temperature changing, which establishes a fitting formula to derive temperature from the luminous intensity ratio, promoting the potential application of UCL in NIR-II regions for the temperature sensing.

Nd3+, Yb3+:YF3 Optical Temperature Nanosensors Operating in the Biological Windows

This work is devoted to the study of thermometric performances of Nd3+ (0.1 or 0.5 mol.%), Yb3+ (X%):YF3 nanoparticles. Temperature sensitivity of spectral shape is related to the phonon-assisted nature of energy transfer (PAET) between Nd3+ and Yb3+). However, in the case of single-doped Nd3+ (0.1 or 0.5 mol.%):YF3 nanoparticles, luminescence decay time (LDT) of 4F3/2 level of Nd3+ in Nd3+ (0.5 mol.%):YF3 decreases with the temperature decrease. In turn, luminescence decay time in Nd3+ (0.1 mol.%):YF3 sample remains constant. It was proposed, that at 0.5 mol.% the cross-relaxation (CR) between Nd3+ ions takes place in contradistinction from 0.1 mol.% Nd3+ concentration. The decrease of LDT with temperature is explained by the decrease of distances between Nd3+ with temperature that leads to the increase of cross-relaxation efficiency. It was suggested, that the presence of both CR and PAET processes in the studied system (Nd3+ (0.5 mol.%), Yb3+ (X%):YF3) nanoparticles provides higher temperature sensitivity compared to the systems having one process (Nd3+ (0.1 mol.%), Yb3+ (X%):YF3). The experimental results confirmed this suggestion. The maximum relative temperature sensitivity was 0.9%·K-1 at 80 K.

Determination of cross-relaxation efficiency based on spectroscopy in thulium-doped rare-earth sesquioxides

Tm3+ doped cubic rare-earth sesquioxides have been recognized as excellent laser materials operating at ∼ 2 μm. Upon 3H4 excitation by commercially available laser diode around 800 nm, a cross-relaxation (CR) process enables efficient Tm3+ laser originated from 3F4→3H6 transition. The CR process, (Tm1:3H4, Tm2:3H6) → (Tm1:3F4, Tm2: 3F4), cuts one excitation photon into two photons of 2 μm with quantum efficiency of 200%. As a result, efficient CR is needed. The CR efficiency is usually determined based on the emission lifetime of 3H4. However, the CR between the nearest Tm3+ is too fast to emit, overrating the emission lifetime and underrating the CR efficiency. Here, we propose a spectroscopic method for determination of CR efficiency based on emission intensity ratio. Tm3+ doped cubic Y2O3, Lu2O3, Sc2O3, YScO3, LuScO3 and YLuO3 sesquioxides are prepared by solid-state reaction. Near infrared emission spectra and decay curves are studied as a function of Tm3+ concentration. The relationship between CR efficiency and emission intensity ratio is established. Large Stark splittings of 1002 cm−1 and 511 cm−1 are observed for the 3H6 ground state and the 3F4 excited state of Tm3+, respectively in Sc2O3:Tm3+, leading to a long-wave emission at 2.15 μm. The comparison of these six samples reveals that Sc2O3: Tm3+ has the largest CR parameter and Lu2O3:Tm3+ has the highest quantum efficiency for populating the 3F4 from the 3H4 at the same Tm3+ concentration.

Nearly-Unity Quantum Yield and 12-Hour Afterglow from a Transparent Perovskite of Cs2 NaScCl6 :Tb.

Unlike afterglow phosphors, transparent crystals free of scattering issues hold great potential in applications such as volumetric displays and three-dimensional encryption. Here, a double-perovskite host of ca. 85 % transparency, a Cs2 NaScCl6 single crystal, was grown in aqueous solution. Through heavy doping of Tb ions, the crystal exhibited a high photoluminescence quantum yield (PL QY) up to 98.2 %. Both steady-state and transient PL spectroscopic investigation revealed a cross-relaxation efficiency (ηCR ≈99 %) between Tb ions, greatly mitigated the electron saturation on the 5 D3 level and encouraged the radiative route against quenching. The doped crystal displayed an intense afterglow of up to 12 h after ceasing the X-ray excitation, allowing it to be used in a radiation-storage battery which has a linear scaling capacity with loading weight. Our work opens possibilities in photo-stimulated volumetric display and three-dimensional information storage.

Temperature dependent absorption and emission spectra of Tm:CaF2

We present Emission and absorption cross sections of thulium doped calcium fluoride (Tm:CaF2) in the visible to short wave infrared (SWIR) wavelength range for temperatures between 80 K and 300 K. For spectral regions of high and low absorption the McCumber relation and the Fuchtbauer–Ladenburg equation have been used to give reliable results. Furthermore, an estimation for the cross relaxation efficiency is derived from the emission spectra as a function of doping concentration and temperature. In addition, nearly re-absorption-free fluorescence lifetimes for various doping concentrations were studied. It was found that a double exponential fit model is better suited than a migration model to represent the fluorescence decay curves. The measurement results are interpreted in the light of the application of Tm:CaF2 as an efficient active medium in high-energy class diode-pumped solid state lasers.

Quantum tripling in Tm3+ doped La2BaZnO5 phosphors for efficiency enhancement of small band gap solar cells

The emission of three infrared photons at 1700nm by Tm3+ for each absorbed photon at 465nm is reported as a function of the Tm3+ concentration in La2−2xTm2xBaZnO5 phosphors. It was observed that the intensity of the Tm3+3F4→3H6 emission with respect to emissions from the 1G4 state after 1G4 excitation at 465nm increases when the Tm3+ concentration is raised from x=0.001 to x=0.032. This increase is the accumulated effect of four different possibilities for cross-relaxation that can take place between neighboring Tm3+ ions, which explains the efficiency of the quantum tripling process. An optimum in the cross-relaxation efficiency was found for the La1.936Tm0.064BaZnO5 phosphor. At higher Tm3+ concentrations the 3F4→3H6 emission decreases as a result of concentration quenching. Quantum tripling can be used in high power IR phosphor converted LEDs or to increase the efficiency of small band gap solar cells like germanium.

Investigation on visible quantum cutting of Tb3+ in oxide hosts

Visible quantum cutting (QC) through downconversion was observed upon 4f8-4f75d1 excitation of Tb3+ in both BaGdB9O16 and Ca8MgGd(PO4)7 compounds. The QC involves a cross relaxation process between two neighboring Tb3+. Modified and new QC models are proposed based on the QC mechanisms in Gd3+-Tb3+ system. Two calculation equations for the cross relaxation efficiency are suggested according to the energy transfer theory and spectral results. By studying the spectral characteristics of Tb3+ in hosts from fluoride to oxide, it indicates that the visible QC in Gd3+-Tb3+ system may occur mainly depending on the structural features of phosphors, rather than the phonon energies of matrixes. All of the above conclusions have meaning of guidance for investigating other phosphors with QC phenomenon.

Influence of Gd3+ on the visible quantum cutting in green-emitting silicate Na3Gd0.9−xYxSi3O9:0.1Tb3+ phosphors

The visible quantum cutting via cross-relaxation between Tb3+ ions in Na3Gd0.9−xYxSi3O9:0.1Tb3+ phosphors are identified for the first time. It has also been found that with the increase of the ratio of Gd3+/Y3+, the quantum cutting efficiency increases, which indicates the Gd3+ plays an important intermediate role in energy transfer to convert vacuum ultraviolet light to visible light and reinforces the cross relaxation efficiency during the quantum cutting process. In addition, the energy transfer process from Gd3+ to Tb3+ is also investigated and discussed in terms of luminescence spectra.

Structure and spectral-luminescence properties of yttrium-stabilized zirconia crystals activated with Tm3+ ions

Yttrium-stabilized zirconia crystals (ZrO2-13.8 mol % Y2O3-0.2 mol % Tm2O3 and ZrO2-12 mol % Y2O3-2 mol % Tm2O3) have been grown by directed crystallization from a melt in a cold container using direct high-frequency melting. The crystals have been analyzed by X-ray diffraction. The spectral-luminescence properties have been studied, and the cross-relaxation efficiency for Tm3+ ions (3 H 4 → 3 F 4, 3 H 6 → 3 F 4) in these crystals has been estimated.

Analysis of energy transfer processes in Yb3+-Tb3+ co-doped, low-silica calcium aluminosilicate glasses

Energy transfer processes in Yb3+-Tb3+ co-doped, low-silica calcium aluminosilicate glasses were analyzed. Luminescence and time-resolved measurements were used to study upconversion processes, such as Yb-Tb cooperative sensitization, Yb-Yb cooperative luminescence, and Yb-Tb cross relaxation. The quantum cross relaxation efficiency was evaluated as a function on the Yb3+ concentration, and the maximum estimated value was approximately 51%. In addition, the intensity of the upconversion luminescence from the Tb3+:5D4 level decreased by two orders of magnitude comparing the value at room temperature with that at 123 K. As a consequence, Yb-Yb cooperative luminescence around 500 nm became comparable with the intensity of upconversion from the Tb3+:5D4 level. Furthermore, a dependence of the upconversion kinetics luminescence on temperature was observed. The upconversion rise time was constant and equal to 65 μs for temperatures between 296 to 473 K and decreased from 65 to 19 μs, without variation in the decay part, when the temperature was lowered from 296 to 123 K. These results were explained by a phonon-assisted cooperative sensitization process for the population of the Tb3+:5D4 level.

Cross-Relaxation and Upconversion Processes in Pr3+ Singly Doped and Pr3+/Yb3+ Codoped Nanocrystalline Gd3Ga5O12: The Sensitizer/Activator Relationship

The room temperature luminescence properties of Pr3+-doped gadolinium gallium garnet (GGG:Pr3+, Gd3Ga5O12:Pr3+) nanocrystals (0.1, 1, 5 and 10 mol %) were evaluated. Increasing the Pr3+ concentration in the nanocrystals resulted in a decrease of the 3P0 emission to lower lying states via the [3P0,3H4] → [3H6,1D2] cross-relaxation (CR) process. Similarly, a decrease in the 1D2 emission was observed and was attributed to the [1D2,3H4] → [1G4,3F3,4] cross-relaxation mechanism. The increase in cross-relaxation efficiency on increasing the Pr3+ concentration was attributed to the smaller average interionic distances between the dopant ions. Dominant blue/green emission due to the 3P0 → 3H4 and 3P0 → 3H6 transitions was observed after laser excitation at 457.9 nm for Pr3+/Yb3+ codoped nanocrystalline GGG samples. The blue/green emission decreased as the sensitizer (Yb3+) concentration increased in the GGG samples. The observed near-infrared (NIR) 2F5/2 → 2F7/2 emission from the Yb3+ ion, upon 457.9 nm excitatio...

Study of the Hydrophobic Cavity of β‐Cryptogein through Laser‐Polarized Xenon NMR Spectroscopy

The interaction of xenon with beta-cryptogein, a basic 10 kDa protein belonging to the elicitin family, has been studied by using dissolved thermal and laser-polarized gas in liquid-state NMR. 13C and 1H chemical-shift-mapping experiments were unfruitful, the proton lines only experienced a slight narrowing but no significant frequency variation when the xenon concentration was increased. Nevertheless magnetization transfer from hyperpolarized xenon to protons of the protein demonstrates an undoubted interaction and enables localization of the noble-gas-binding site. Due to the proton-proton cross-relaxation efficiency, however, this experiment is subjected to important spin-diffusion. An automatic procedure that takes spin-diffusion into account when assigning the protons that interact with xenon is then used. The binding site, as defined by 30 Xe--H interactions, is situated in the inner core of the protein. The protons that interact with xenon border the channel by which sterols are known to enter into the cavity. These results support the idea that xenon is a good probe for hydrophobic protein regions.

Visible quantum cutting through downconversion in KLiGdF5:Eu3+ crystals

Visible quantum cutting through downconversion is observed for the Gd3+–Eu3+ couple in KLiGdF5:Eu3+ (KLGF:Eu3+). In this Gd3+-based fluoride, visible quantum cutting, the emission of two photons of visible light per absorbed photon, occurs upon vacuum ultraviolet excitation of Gd3+ at the GJ6 level via two-step energy transfer from Gd3+ to Eu3+ by cross relaxation and sequential transfer of the remaining excitation energy. The dependence of the efficiency of cross relaxation required for visible quantum cutting on the Eu3+ doping concentration is discussed in terms of the probability of energy transfer and the behavior of decay of the D05 emission of Eu3+, and it is found that the KLGF crystal doped with 2at.% Eu3+ exhibits the maximum cross-relaxation efficiency of 0.4. From the number of nearest neighbors around Gd3+ ions and the D05 emission intensity, energy transfer from excited Gd3+ ions is determined to extend to the fourth-nearest Eu3+ neighbors in the quantum cutting process.

Water enhances the luminescence intensity of β-diketonates of trivalent samarium and terbium in toluene solutions

The influence of water on the luminescence of toluene solutions of lanthanide β-diketonates (Ln(L) 3· nH 2O, where L is benzoyltrifluoroacetone, thenoyltrifluoroacetone, acetylacetone, dipivaloylmethane) was studied. It was shown that in toluene the luminescence of dysprosium, neodymium and ytterbium β-diketonates is quenched by water while the luminescence of samarium and terbium β-diketonates is enhanced by water at [Ln(L) 3· nH 2O]>10 −4 M. The concentration quenching of the luminescence of Ln(L) 3· nH 2O in toluene was observed due to the formation of dimers [Ln(L) 3· nH 2O] 2. The quenching in dimers is caused by deactivation in both the ligand (before the energy transfer to Ln 3+ ion takes place) and by cross-relaxation in the Ln 3+ ion. It is suggested that introduction of water in toluene causes dissociation of dimers to monomers which results in the enhancement of the luminescence intensity of Sm 3+ and Tb 3+, while the quenching of Nd 3+, Dy 3+ and Yb 3+ luminescence is due to the efficient deactivation of these ions by OH-groups. Probably addition of water enhances ligand-to-Ln 3+ energy-transfer efficiency and lowers cross-relaxation efficiency of Ln 3+.

1H magnetic cross-relaxation between multiple solvent components and rotationally immobilized protein.

Magnetic cross-relaxation spectra or Z-spectra are presented for water, acetone, methanol, dimethylsulfoxide, and acetonitrile in cross-linked bovine serum albumin gels. Each solvent studied, reports the same Z-spectrum linewidth and shape for the solid component that follows from solutions of the coupled relaxation equations. The Z-spectra demonstrate competition among solvents for specific protein binding sites. The rate of magnetization transfer in the rotationally immobilized protein environment is approximated by 1/T2 for the solid component, which is shown to account for the observed magnetization transfer rates in the systems studied. The temperature dependence of the Z-spectra are different for water compared with the organic solvents. The cross-relaxation efficiency in the organic solvents decreases with increasing temperature because molecules bind less well at high temperature. For water, the hydrogen exchange path becomes increasingly important relative to the whole molecule path with increasing temperature, which improves the net cross-relaxation efficiency.