Phonon-assisted Cross-relaxation Research Articles

Achieving high-sensitivity dual-mode optical thermometry via phonon-assisted cross-relaxation in a double-perovskite structured up-conversion phosphor

By adjusting the phonon energy of the host crystal (Ca2+/Sr2+/Ba2+) LaLiTeO6:Yb3+, Tm3+ phosphor, combined with the cross-relaxation phenomenon, a high-sensitivity thermometer for dual-mode temperature measurement was constructed.

Cross Relaxation Channel Tailored Temperature Response in Er3+ -rich Upconversion Nanophosphor.

Recently high doping of lanthanide ions (till 100 %) is realized unprecedentedly in nanostructured upconversion (UC) phosphors. However, oddly enough, this significant breakthrough did not result in a corresponding UC enhancement at ambient temperature, which hinders the otherwise very interesting applications of these materials in various fields. In this work, taking the Er3+ -rich UC nanosystem as an example, we confirm unambiguously that the phonon-assisted cross relaxation (CR) is the culprit. More importantly, combining the theoretical modeling and experiments, the precise roles of different CR channels on UC energy loss are quantitatively revealed. As a result, lowering the temperature can exponentially enhance the relevant UC luminescence by more than two orders of magnitude. Our comprehension will play an important role in promoting the UC performance and further application of high doping rare earth materials. As a proof of concept, an Er3+ -rich core/multi-shell nanophosphor is exploited which demonstrates the great potential of our finding in the field of ultra-sensitive temperature sensing.

The spectroscopic quality factor, phase, morphological, structural, and photoluminescent analysis of ZrO2:Nd3+ coatings created by Plasma electrolytic oxidation

15 μm thick ZrO2:Nd3+ coatings were synthesized by the plasma electrolytic oxidation process from the pure zirconium substrate in an alkaline electrolyte with the addition of five different concentrations of Nd2O3 powders. SEM/EDS images show morphologies characterized by numerous pores and cracks, and uniform distribution of coating constituents. XRD revealed that the monoclinic phase is dominant and that Nd impurity stabilizes the tetragonal phase. The average crystalline size is 27.5 nm. XRF revealed a linear increase in incorporated Nd with increasing Nd2O3 concentration. Photoluminescence spectra are characterized by the NIR emissions at typical energies for Nd3+ ion. The highest emission intensity is obtained with 2.0 g/L Nd2O3. The explicit equation for branching ratios in relation to the emission intensities and Judd-Ofelt parameters is recalculated. A novel, more precise equation for the calculation of the spectroscopic quality factor from the most intense emissions is presented. The spectroscopic quality factor is the lowest in the sample with the highest photoluminescent intensity, and is one of the lowest values, in comparison with the 203 spectroscopic quality factors calculated from the Judd-Ofelt parameters in the literature. The luminescence intensity decreases with temperature by the phonon-assisted cross-relaxation mechanism. The luminescence thermometry by the lifetime of 4F3/2 level shows ∼2% K−1 relative sensitivity in the 300–380 K temperature range.

Superenhancement Photon Upconversion Nanoparticles for Photoactivated Nanocryometer

Highly doped lanthanide luminescent nanoparticles exhibit unique optical properties, providing exciting opportunities for many ground-breaking applications, such as super-resolution microscopy, deep-tissue bioimaging, confidentiality, and anticounterfeiting. However, the concentration-quenching effect compromises their luminescence efficiency/brightness, hindering their wide range of applications. Herein, we developed a low-temperature suppression cross-relaxation strategy, which drastically enhanced upconversion luminescence (up to 2150-fold of green emission) in Er3+-rich nanosystems. The cryogenic field opens the energy transport channel of Er3+ multiphoton upconversion by further suppressing phonon-assisted cross-relaxation. Our results provide direct evidence for understanding the energy loss mechanism of photon upconversion, deepening a fundamental understanding of the upconversion process in highly doped nanosystems. Furthermore, it also suggests the potential applications of upconversion nanoparticles for extreme ambient-temperature detection and anticounterfeiting.

Understanding and tuning blue-to-near-infrared photon cutting by the Tm3+/Yb3+ couple

Lanthanide-based photon-cutting phosphors absorb high-energy photons and ‘cut’ them into multiple smaller excitation quanta. These quanta are subsequently emitted, resulting in photon-conversion efficiencies exceeding unity. The photon-cutting process relies on energy transfer between optically active lanthanide ions doped in the phosphor. However, it is not always easy to determine, let alone predict, which energy-transfer mechanisms are operative in a particular phosphor. This makes the identification and design of new promising photon-cutting phosphors difficult. Here we unravel the possibility of using the Tm3+/Yb3+ lanthanide couple for photon cutting. We compare the performance of this couple in four different host materials. Cooperative energy transfer from Tm3+ to Yb3+ would enable blue-to-near-infrared conversion with 200% efficiency. However, we identify phonon-assisted cross-relaxation as the dominant Tm3+-to-Yb3+ energy-transfer mechanism in YBO3, YAG, and Y2O3. In NaYF4, in contrast, the low maximum phonon energy renders phonon-assisted cross-relaxation impossible, making the desired cooperative mechanism the dominant energy-transfer pathway. Our work demonstrates that previous claims of high photon-cutting efficiencies obtained with the Tm3+/Yb3+ couple must be interpreted with care. Nevertheless, the Tm3+/Yb3+ couple is potentially promising, but the host material—more specifically, its maximum phonon energy—has a critical effect on the energy-transfer mechanisms and thereby on the photon-cutting performance.

Effect of OH– on fluorescence properties in 1.5–1.6 μm for Er:Yb:YCOB crystal

FTIR absorption spectra indicate that H+ can easily enter the crystal structure and form OH– centers in Er:Yb:YCOB and OH bonds prefer to lie in the a-c plane. Within our current studied concentration level, crystal samples with higher OH– absorption coefficients demonstrate stronger fluorescence intensity and longer fluorescence lifetime at 1535 nm. As the stretching vibration energy of OH– group approximately corresponds to the energy gap between the 4I11/2 and 4I13/2 levels of Er3+, and thus, OH– ions can shorten the fluorescence lifetime of Er3+-4I11/2 level by the phonon-assisted cross-relaxation process between the Er3+ and OH– ions. Our current results confirm that a certain content of OH– ions can enhance the energy transfer process from Yb3+ to Er3+ and subsequently promote fluorescence output in 1.5–1.6 μm.

Up-conversion luminescence lifetime thermometry based on the 1G4 state of Tm3+ modulated by cross relaxation processes.

β-PbF2:Tm3+/Yb3+ glass ceramics (GCs) with varied Tm3+ doped concentrations were synthesized by the conventional melting-quenching method and the subsequent glass crystallization route. For the first time, the lifetime-based thermometer based on up-conversion luminescence (UCL) originating from the 1G4 state excited upon a miniaturized 976 nm diode laser excitation in β-PbF2:Tm3+/Yb3+ GCs is achieved. Furthermore, the thermosensitivity of the 1G4 state lifetime is realized by the phonon-assisted cross relaxation (CR) processes between Tm3+ which depopulate the 1G4 state and get severer with temperature. The relative sensitivity (SR) of the 1G4 state lifetime is promoted by intensifying CR processes via raising Tm3+ concentration in this work. The maximum values of SR of 1G4 state lifetime obtained in 0.0005Tm, 0.01Tm and 0.05Tm are 0.16%K-1, 0.26%K-1, and 0.46%K-1 at 488 K, respectively, which indicates the potential ability of Tm3+-doped nanoparticles to work as indicators for UCL lifetime-based thermometers.

A novel high-sensitive upconversion thermometry strategy: Utilizing synergistic effect of dual-wavelength lasers excitation to manipulate electron thermal distribution

Conventional upconversion thermometry strategy, based on the thermally coupled levels (TCL) of lanthanide ions, confronts a dilemma in simultaneously achieving high absolute/relative temperature (T) sensitivities (Sa and Sr) and good signal discriminability. Herein, a novel thermometry strategy by utilizing the synergistic effect of dual-wavelength lasers to manipulate electron thermal distribution is proposed to go beyond the limitation of the TCL-based strategy. In the case of NaGdF4: 20%Yb3+, 2%Er3+ nanoparticles upon 980 & 1530 nm dual-excitation, it is found that more efficient utilization of excited photons results in a higher electron concentration in ladder-like energy levels of Er3+, owing to the diverse pumping routes, which increases transition rate of the T-dependent phonon-assisted cross-relaxation process, and in turn establishes a thermally-sensitive electronic connection between Er3+: 2H11/2,4S3/2 and Er3+: 4F9/2. Remarkably, not only a highest record value of Sa (0.0365 K−1) for the Er3+ doped host materials in the physiological temperature range (303–343 K) is achieved, but also a high Sr (1.29% K-1) and an excellent signal discriminability (Δλ = 112 nm) are obtained. Combining a much intensified upconversion (UC) signal and a good size/shape homogeneity in nanometer scale, the investigated NaGdF4: 20%Yb3+, 2%Er3+ upon dual-wavelength excitation is potentially applicable in the intracellular thermal sensing and imaging. This work exploits an effective way to develop high-performance T-sensors, and the proposed thermometry strategy can be extended to surges of the other lanthanide ions doped systems pumped by multiple-wavelength lasers.

Amplification properties of KW Nd/Cr:YAG ceramic multi-stage active-mirror laser using white-light pump source at high temperatures

An output peak power of over 1kW for a Nd/Cr3+:YAG ceramic multi-amplifier system with white-light pumping was obtained. A four-stage active-mirror amplifier system was adapted for obtaining kW laser peak power. Output spectrum of pumping light used was close to that of solar light. Enhancement of saturated output laser power at high temperatures was observed owing to the phonon-assisted cross-relaxation effect. An output laser peak power of 1.6kW were achieved at a ceramic disk temperature of 100°C. The evaluated optical-optical conversion efficiency for volume of ceramics through which laser passed was 63% for the maximum output laser peak power of 1.6kW. The laser system will be applicable to solar-pumped lasers with high optical-optical conversion efficiency.

A clear effect of charge compensation through Na+ co-doping on the luminescence spectra and decay kinetics of Nd3+-doped CaAl4O7

We present a detailed analysis of luminescence behavior of singly Nd3+ doped and Nd3+, Na+ co-doped calcium aluminates powders: Ca1−xNdxAl4O7 and Ca1−2xNdxNaxAl4O7 (x=0.001–0.1). Relatively intense Nd3+ luminescence in IR region corresponding to typical 4F3/2→4IJ (J=9/2–13/2) transitions with maximum located at about 1079nm was obtained in all samples on direct excitation into f–f levels. The effect of dopant concentration and charge compensation by co-doping with Na+ ions on morphology and optical properties were studied. The results show that both, the Nd3+ concentration and the alkali metal co-doping affected the optical properties but had no influence on the powders morphology. The studies of luminescence spectra (298 and 77K) in a function of dopant concentration showed an increasing distortion of the local symmetry of Nd3+with raising activator content due to certain defects created in the crystal lattice. On the other hand Na+ addition led to significant narrowing of absorption and luminescence bands and also a reduction of the number of their components, showing smaller disturbance of Nd3+ ions local symmetries. Consequently, charge compensated by Na+ co-doping materials showed significantly enhanced Nd3+ luminescence. The decrease of emission intensity and luminescence lifetimes with increase of activator concentration was attributed mainly to phonon-assisted cross-relaxation processes between Nd3+ ions. Analysis with Inokuti–Hirayama model indicated dipole–dipole mechanism of ion-ion interaction. Na+ addition led to much smaller concentration quenching due to smaller clustering of dopant ions in CaAl4O7 lattice.

High-color-purity red-emitting phosphors RE2WO6:Pr3+ (RE = Y, Gd) for blue LED

Pr3+-doped RE2WO6 (RE = Y, Gd) phosphors in pure phase were synthesized via conventional solid–state reaction in air. Photoluminescence properties of those phosphors were investigated in detail. High-color-purity red emission originated from the 1D2 → 3H4, 3P0 → 3H6 and 3P0 → 3F2 transitions was observed in Pr3+-doped RE2WO6 (RE = Y, Gd) under 450 nm excitation. Fluorescence concentration quenching in Pr3+-doped Y2WO6 could be attributed to phonon-assisted cross-relaxation energy transfer between the Pr3+ ions. The Commission International de L'Eclairage (CIE) chromaticity coordinates of 0.5 at% Pr3+-doped Y2WO6 was observed to be (0.663, 0.337). However, the thermal quenching of Pr3+-doped Y2WO6 is relatively serious. By integrating this phosphor with commercial Ce3+:YAG and a 450 nm blue chip, a white LED with 5254 K correlated color temperature and high-color- rendering index (Ra = 81) was fabricated. The present work suggests that Pr3+-doped RE2WO6 (RE = Y, Gd) as pure red phosphors have a potential application in the blue-chip WLEDs.

Luminescent properties of deep red light-emitting phosphors NaGd(WO4)2:Pr3+ for blue LED

Pr3+-doped NaGd(WO4)2 phosphors with deep red emission in pure phase were prepared via high temperature solid-state reaction. Photoluminescence properties of those phosphors were investigated. Deep red emission bands with central wavelength at 649nm were observed under the excitation of 430–500nm, which is suitable for excitation of the blue LED chips. The optimum doping concentration of Pr3+ ions in NaGd(WO4)2 is 4.0at.%. The fluorescence concentration quenching could be attributed to the phonon-assisted cross-relaxation energy transfer between the Pr3+ ions. The CIE color coordinates of Pr3+-doped NaGd(WO4)2 phosphors with different Pr3+ concentration can be tuned from deep red to light red. The present work suggests that Pr3+-doped NaGd(WO4)2 exhibit great potential for using as deep red phosphors for the blue LED.

Energy transfer in Eu^3+ doped scheelites: use as thermographic phosphor

In this paper the luminescence of the scheelite-based CaGd₂(₁-x)Eu₂x(WO₄)₄ solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the ⁵D₀ - ⁷F₂ transition in Eu³⁺, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. For high Eu³⁺ concentrations the intensity ratio of the emission originating from the ⁵D₁ and ⁵D₀ levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent.

Use of thulium-sensitized rare earth-doped low phonon energy crystalline hosts for IR sources

Crystalline hosts with low phonon energies enable novel energy transfer processes when doped with rare earth ions. Two applications of energy transfer for rare earth ions in thulium-sensitized low phonon energy crystals that result in infrared luminescence are discussed. One application is an endothermic, phonon-assisted cross-relaxation process in thulium-doped yttrium chloride that converts lattice phonons to infrared emission, which raises the possibility of a fundamentally new method for achieving solid-state optical cooling. The other application is an optically pumped mid-IR phosphor using thulium-praseodymium-doped potassium lead chloride that converts 805-nm diode light to broadband emission from 4,000 to 5,500 nm. These two applications in chloride crystals are discussed in terms of critical radii calculated from Forster-Dexter energy transfer theory. It is found that the critical radii for electric dipole-dipole interactions in low phonon energy chloride crystals are comparable to those in conventional oxide and fluoride crystals. It is the reduction in multi-phonon relaxation rates in chloride crystals that enable these additional energy transfer processes and infrared luminescence.

Highly Efficient Lasing Action of Nd3+- and Cr3+-Doped Yttrium Aluminum Garnet Ceramics Based on Phonon-Assisted Cross-Relaxation Using Solar Light Source

We constructed a theory to explain the mechanism of laser generation with a high optical–optical conversion efficiency for Nd3+- and Cr3+-doped yttrium aluminum ceramics when sunlight or lamplight sources are used for pumping. As a result, a unique mechanism of laser action was found where the solar or lamp-light power could be converted to laser power with a high efficiency close to 80%, which has not previously been observed. The high conversion efficiency was not only considered to be based on one-to-one photon conversion but on two-photon excitation by a single photon with phonon assistance. Thus, the mechanism of lasing action should include a process where thermal energy is converted to photon energy. The theoretical results we obtained were consistent with those of the experiments.

1540 nm fiber laser excited upconversion luminescence in erbium-doped lead–fluoride nanocrystals

Abstract Upconversion luminescence in erbium-doped PbGeO3–PbF2–CdF2-based vitroceramic under 1540 nm infrared excitation is investigated. Luminescence signals around 410, 525, 550, 660 and 850 nm were generated and attributed to the 2H9/2, 2H11/2, 4S3/2 and 4F9/2 transitions to the 4I15/2 ground-state, and 4S3/2–4I13/2, respectively. The erbium ions excited-state emitting levels were populated through a combination of stepwise ground-state absorption, phonon-assisted excited-state absorption and cross-relaxation processes. The results also disclosed that all emission signals obtained with vitroceramic samples presented intensities three times higher when compared to the precursor glass samples. In addition, the red emission signal at 660 nm for 1540 nm pumping exhibited an expressively high intensity when compared to the green signal.

Improvement of Er 3+: 4I 11/2→Ce 3+: 2F 5/2 energy transfer rate in Er 3+/Ce 3+ co-doped TeO 2–ZnO–Na 2O–Nb 2O 5 glasses

The B 2O 3 component was introduced into Er 3+/Ce 3+ co-doped TeO 2–ZnO–Na 2O–Nb 2O 5 glass to improve energy transfer rate of Er 3+: 4I 11/2→Ce 3+: 2F 5/2 phonon-assisted cross-relaxation process. With the 6 mol% substitution of B 2O 3 for TeO 2, the energy transfer rate increased from 1300 to 1831 s −1 and the fluorescence intensity increased by about 13.4%. However, the more B 2O 3 substitution in the same glass system reduced the quantum efficiency of Er 3+: 4I 13/2→ 4I 15/2 transition due to the higher OH − group concentration. The results show that an appropriate amount of B 2O 3 component can be used to improve the phonon-assisted energy transfer rate and enhance 1.53 μm fluorescence emission by increasing the phonon energy of host glass. The effect of B 2O 3 on the energy transfer process, the lifetimes of the 4I 11/2 and 4I 13/2 levels, and the upconversion emission have also been investigated.

Power dependence of luminescence of Tb 3+-doped KYb(WO4) 2 crystal

The effect of excitation intensity on luminescence properties of Tb 3+-doped KYb(WO 4) 2 crystal is reported. In particular, emissions following the Stokes and anti-Stokes excitations were investigated. It was found that, in addition to the Tb 3+ luminescence, an intense Yb 3+ luminescence at room temperature was observed after a direct excitation of Tb 3+ ions in spite of the fact that both ions are far off-resonance. The intensities of Tb 3+ and Yb 3+ luminescences were dependent on the excitation power and changed by a sublinear function (slope<1). The mechanism of the non-resonant energy transfer from Tb 3+ to Yb 3+ is discussed in terms of the phonon-assisted cross-relaxation and the down-conversion contributing to the process. The up-conversion emission of Tb 3+ (after a direct excitation of Yb 3+ ions) was observed at room temperature and its intensity varied with the slope <2. It was found that the emission decay times were strongly dependent on the excitation power. The mechanism of this behaviour is discussed in terms of many-body energy transfer.

Interaction between excitons and rare-earth ions

Energy transfer between rare-earth (RE) ions and excitons in semiconductors has already been known for some time. In this paper we would like to direct your attention to the interaction between excitons and RE ions in ionic crystals as used, e.g. for laser applications. A suitable model substance to study these effects is RE-doped CsCdBr 3. The reasons are: (i) the low phonon frequencies due to the heavy ion masses ( ℏω max<200 cm −1); (ii) metastable excitonic states in the visible spectral range; (iii) strong exciton–ion interaction due to covalent overlap of the wavefunctions. Due to the low phonon frequencies, multiphonon processes of the RE ions are reduced drastically and ion–exciton processes can be studied in more detail. The following processes were found: (i) exciton-mediated nonradiative relaxations. These processes can bridge much larger energy gaps than direct multiphonon relaxation and are more efficient. The reason is that the nonradiative multiphonon relaxation of the RE ion is dominated by a fast multistep one-phonon relaxation of the exciton by energy transfer; (ii) exciton-mediated quantum upconversion. This effect is based on a cooperative energy transfer from two excited RE ions to an exciton and a subsequent back transfer to a single ion. This process is much faster than upconversion by phonon-assisted cross relaxation between two excited RE ions; (iii) exciton-induced changes in the crystal-field splitting of RE ions. Energy levels of RE ions in resonance with the excitons show crystal-field splittings which cannot be described by the parameters suitable for the other levels. We propose an increased covalent overlap between the wavefunction of the RE ion and of the exciton-forming ligands due to hybridization as an explanation for this effect.

Upconversion in CsCdBr3:Pr3+

Abstract In CsCdBr3 the trivalent Pr ions substitute the divalent Cd ions. Charge compensation leads to three Pr-vacancy centers. The first one is the well known symmetric pair center Pr3+-Cd vacancy—Pr3+ the second one is the asymmetric pseudopair center Pr3+-Pr3+-Cd vacancy, and the third one finally is a Pr3+ single ion with a Cs vacancy nearby. Only the Pr3+-Pr3+-Cd vacancy center showed upconversion emission from 3 P 0 and 3 P 1 under 1D2 excitation. The dynamics of the upconversion process is studied. It cannot be explained in the standard picture of phonon-assisted cross relaxation in the energy level scheme of the rare-earth ions. In our interpretation it is made up by a cooperative energy transfer from the doubly excited pair state 1D2 × 1D2 to lattice excitons followed by a back transfer of the energy exciting the pair into the states 3P1 × 3H4, 3P0 × 3H4, and 1D2 × 3H6, First experiments on CsCdBr3:Tm3+ confirm the picture of exciton-mediated upconversion.