This study is to investigate the impact of thulium concentration on the performance of in-band pumped thulium-doped silica fiber amplifiers with considering ion–ion interactions. Due to the ...fluorescence quenching in silica glass, the fluorescence lifetimes are required to compute at every Tm concentration. The theoretical model of fluorescence decay curves at in-band pumped thulium-doped silica fiber is used to determine the fluorescence lifetimes of the
and
levels. The calculated lifetimes of the commercially available thulium-doped silica fiber are 650 µs for
level and 14 µs for
level and these results are consentient well with the experimental reported results. The theoretical evaluating of the amplifier performance shows that the gain amplifier reduces with concentration increase because of the impact of both fluorescence quenching and the reverse cross-relaxation process. Thus, in contrast to pumping wavelength at 790 nm, there are negative effects of the high doping concentration of Tm ions on the amplifier performance at the in-band pumping scheme.
Abstract
Precise control of energy migration between sensitizer ions and activator ions in lanthanide-doped upconversion nanoparticles (UCNPs) nowadays has been extensively investigated to achieve ...efficient photon upconversion. However, these UCNPs generally emit blue, green or red light only under fixed excitation conditions. In this work, regulation of the photon transition process between different energy levels of a single activator ion to obtain tunable upconversion fluorescence under different excitation conditions is achieved by introducing a modulator ion. The cross-relaxation process between modulator ion and activator ion can be controlled to generate tunable luminescence from the same lanthanide activator ion under excitation at different wavelengths or with different laser power density and pulse frequency. This strategy has been tested and proven effective in two different nanocrystal systems and its usefulness has been demonstrated for high-level optical encryption.
Luminescence quench is common in overdoped upconversion nanoparticles. Various methods have been proposed to counteract the adverse effects of concentration quenching on luminescence, but in ...upconversion nanoparticles that are highly doped with both sensitizers and activators, the factors that contribute to the diminishing of the emission cannot be summarized by a single cause. Herein, a core-shell design is used to spatially separate the sensitizer (Yb
3+
) and activator (Er
3+
) and to modulate the emission by changes in the distribution position as well as the concentration of the dopant ions in order to probe the factors affecting the luminescence. When the sensitizer ions are located in the core, the luminescence intensity of the nanoparticles is significantly weaker than that of the other distribution, which implies that the effect of sensitizer and activator on luminescence in the highly doped state has a different and more complex mechanism. The intensity of the emission is more affected by Yb
3+
than Er
3+
, which includes not only the self-quenching of Yb
3+
, but also the dominance in the Yb
3+
–Er
3+
cross-relaxation. In this finding may provide new ideas for revealing the reasons for the diminished luminescence of highly doped upconversion nanoparticles and thus for enhancing luminescence.
The tetragonal-phased LiYF4:Yb3+/Er3+ nanoparticles with different core-shell nanostructures have been successfully synthesized by a modified solution-based method. The upconversion emissions of ...LiYF4: Yb3+/Er3+ nanoparticles were precisely tuned from green to red with the increase in Yb3+ concentrations under 980 nm excitation. The corresponding intensity ratios of the red and green emissions (R/G ratios) were enhanced from 0.06 to 5.33. However, the overall integral upconversion intensity of LiYF4:Yb3+/Er3+ nanoparticles clearly decreased when the Yb3+ doping concentrations were increased. For the enhancement of the red upconversion intensity and expansion of their application, two kinds of core-shell nanostructures, namely, LiYbF4:Er3+@LiYF4 and LiYbF4:Er3+@LiYbF4, were constructed through epitaxial growth at similar experimental conditions. The LiYbF4:Er3+@LiYF4 core-shell nanostructures exhibited stronger upconversion emission and higher R/G ratios than the LiYbF4:Er3+ and LiYbF4:Er3+@LiYbF4 nanoparticles. The highest R/G ratios increased to 6.27 in LiYbF4:Er3+@LiYF4 core-shell nanostructures, increasing 104-fold. The upconversion regulation and enhancing mechanism were also discussed according to the emission spectra and fluorescence lifetimes.
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Purpose
To optimize a selective inversion recovery (SIR) sequence for macromolecular content mapping in the human brain at 3.0T.
Theory and Methods
SIR is a quantitative method for measuring ...magnetization transfer (qMT) that uses a low‐power, on‐resonance inversion pulse. This results in a biexponential recovery of free water signal that can be sampled at various inversion/predelay times (tI/tD) to estimate a subset of qMT parameters, including the macromolecular‐to‐free pool‐size‐ratio (PSR), the R1 of free water (R1f), and the rate of MT exchange (kmf). The adoption of SIR has been limited by long acquisition times (≈4 min/slice). Here, we use Cramér‐Rao lower bound theory and data reduction strategies to select optimal tI/tD combinations to reduce imaging times. The schemes were experimentally validated in phantoms, and tested in healthy volunteers (N = 4) and a multiple sclerosis patient.
Results
Two optimal sampling schemes were determined: (i) a 5‐point scheme (kmf estimated) and (ii) a 4‐point scheme (kmf assumed). In phantoms, the 5/4‐point schemes yielded parameter estimates with similar SNRs as our previous 16‐point scheme, but with 4.1/6.1‐fold shorter scan times. Pair‐wise comparisons between schemes did not detect significant differences for any scheme/parameter. In humans, parameter values were consistent with published values, and similar levels of precision were obtained from all schemes. Furthermore, fixing kmf reduced the sensitivity of PSR to partial‐volume averaging, yielding more consistent estimates throughout the brain.
Conclusions
qMT parameters can be robustly estimated in ≤1 min/slice (without independent measures of ΔB0,
B1+, and T1) when optimized tI‐tD combinations are selected.
Pr3+ doped Al2O3–CaO based glasses were excellent candidate optical gain mediums for visible solid-state lasers due to its wide transmittance region and excellent mechanical properties. However, poor ...glass-forming ability limited its further application in visible fiber laser. In this work, B2O3, GeO2, Ga2O3 were employed for modifying the glass-forming ability and lowing the melting temperature of the Al2O3–CaO based glasses. By introducing 15 mol% GeO2 and Ga2O3, the thermal stability ΔT were enlarged to be 199 and 264 °C, respectively. The influence of glass composition on the fluorescence characteristic of Pr3+ were systematically investigated. For lower doping concentration, different multiphonon relaxation rare of 3P0 and 1D2 energy levels accounting for the different fluorescence spectra of Pr3+ doped Al–Ga, Al–Ge and Al–B glasses, while for the higher doping concentration, cross relaxation processes lead to the fluorescence was gradually dominated by the transition of 3P0 with increasing Pr3+ concentration.
The applications of lanthanide-doped upconversion nanomaterials are limited by unsatisfactory brightness currently. Herein, a general strategy is proposed for boosting the upconversion efficiency in ...Er
ions, based on combined use of a core-shell nanostructured host and an integrated optical waveguide circuit excitation platform. A NaErF
@NaYF
core-shell nanoparticle is constructed to host the upconversion process for minimizing non-radiative dissipation of excitation energy by surface quenchers. Furthermore, an integrated optical microring resonator is designed to promote absorption of excitation light by the nanoparticles, which alleviates quenching of excited states due to cross-relaxation and phonon-assisted energy transfer. As a result, multiphoton upconversion emission with a large anti-Stokes shift (greater than 1150 nm) and a high energy conversion efficiency (over 5.0%) is achieved under excitation at 1550 nm. These advances in controlling photon upconversion offer exciting opportunities for important photonics applications.
Dissolution dynamic nuclear polarization (dDNP) is a versatile hyperpolarization technique to boost signal intensities in nuclear magnetic resonance (NMR) spectroscopy. The possibility to dissolve ...biomolecules in a hyperpolarized aqueous buffer under mild conditions has recently widened the scope of NMR by dDNP. The water-to-target hyperpolarization transfer mechanisms remain yet unclear, not least due to an often-encountered dilemma of dDNP experiments: The strongly enhanced signal intensities are accompanied by limited structural information as data acquisition is restricted to short time series of only one-dimensional spectra or a single correlation spectrum. Tackling this challenge, we combine dDNP with molecular dynamics (MD) simulations and predictions of cross-relaxation rates to unravel the spin dynamics of magnetization flow in hyperpolarized solutions.
The static behavior of a thulium-doped fiber amplifier (TDFA) operating around 2 μm region at different pump wavelengths is investigated in this paper. A theoretical model is built up by solving a ...set of rate and propagation equations with considering the effect of cross relaxation mechanism. The developed model provides the influences of the amplified spontaneous emission noise, seed wavelength, and the thulium-doped fiber length into the TDFA performance. Simulation results indicate that the TDFA performance with pump at 1570 nm is more efficient than pump at 793 nm for core pumped thulium-doped silica fiber. Our findings show that the maximum gain reaches up to 30 dB with a 27 dBm pump power when a -10 dBm seed wavelength at 1840 nm is used. In contrast to indirect pumping at 793 nm, only 22 dB maximum gain is achieved under the same conditions. The model is also validated with previous experimental work. Our simulations are consistent with the experimental findings with small variations.
Abstract
Cross-relaxation among neighboring emitters normally causes self-quenching and limits the brightness of luminescence. However, in nanomaterials, cross-relaxation could be well-controlled and ...employed for increasing the luminescence efficiency at specific wavelengths. Here we report that cross-relaxation can modulate both the brightness of single upconversion nanoparticles and the threshold to reach population inversion, and both are critical factors in producing the ultra-low threshold lasing emissions in a micro cavity laser. By homogenously coating a 5-μm cavity with a single layer of nanoparticles, we demonstrate that doping Tm
3+
ions at 2% can facilitate the electron accumulation at the intermediate state of
3
H
4
level and efficiently decrease the lasing threshold by more than one order of magnitude. As a result, we demonstrate up-converted lasing emissions with an ultralow threshold of continuous-wave excitation of ~150 W/cm
2
achieved at room temperature. A single nanoparticle can lase with a full width at half-maximum as narrow as ~0.45 nm.
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