Energy Transfer Upconversion Research Articles

Single-longitudinal-mode Nd:GdVO4 laser at 912 nm with optimum laser medium length considering the temperature distribution

A diode-end-pumped continuous-wave single-longitudinal-mode Nd:GdVO4 laser at 912 nm is reported. A ring laser resonator was designed and the laser medium length was optimized to enhance the output power. A maximum single-longitudinal-mode output power of 2 W was obtained. A theoretical model of reabsorption loss considering the temperature distribution in the laser medium has been developed from the quasi-three-level rate equation including the energy-transfer upconversion and the exited state absorption effects. The experimental results are in good agreement with the theoretical calculations.

Colour-optimized quantum yields of Yb, Tm Co-doped upconversion nanocrystals

We present here a systematic analysis of the influence of Tm3+ doping concentrations (xTm) on the excitation power (P)-dependent upconversion luminescence and -performance of hexagonal-phase NaYF4: 20% Yb3+, xTm % Tm3+ upconversion nanoparticles (UCNPs) for xTm of 0.2, 0.5, 0.8, 1.2, and 2.0, respectively. Our results reveal the influence of these different Tm3+ doping concentrations with respect to optimized upconversion quantum yield (ΦUC) values of the various Tm3+ upconversion emission bands, with the highest ΦUC values of the Tm3+ emission bands above 700 nm resulting for different xTm values as the Tm3+ emission bands below 700 nm. This underlines the potential of Tm3+ dopant concentration for colour tuning. Special emphasis was dedicated to the spectroscopic parameters that can be linked to the (de)population pathways of the various Tm3+ energy levels, like the P- and xTm-dependent slope factors and the intensity ratios of selected emission bands. The evaluation of all parameters indicates that not only energy transfer upconversion-, but also cross-relaxation processes between neighbouring Tm3+ ions play a vital role in the (de)population of the excited energy levels of Yb3+, Tm3+ codoped nanocrystals.

Multi-Watt Simultaneous Orthogonally Polarized Dual-Wavelength Pulse Generation of an Intracavity Nd:YLF/YVO4 Raman Laser

We present the first demonstration of an actively Q-switched orthogonally polarized dual-wavelength Nd:YLF/YVO4 Raman laser to our knowledge. The gain competition of the orthogonally polarized dual-wavelength fundamental laser at 1047 and 1053 nm was ultimately avoided by incorporating two Nd:YLF crystals into two laser resonators. By adjusting the gain or loss difference of two resonators, the pulse synchronization of the orthogonally polarized dual-wavelength Raman laser was realized. Benefitting from the efficient thermal stress and energy-transfer upconversion management of Nd:YLF crystal, a maximum dual-wavelength Raman output power of 4.5 W was obtained with M 2 < 1.8, which contained a 2.5 W Raman component at 1155 nm and a 2.0 W Raman component at 1162 nm, under the incident pump power of 46.1 W and pulse repetition frequency of 5 kHz. The corresponding peak powers were as high as 178.6 and 142.9 kW, respectively. As far as we know, we have increased the orthogonally polarized dual-wavelength Raman average output power by six times.

Frequency upconversion mechanism in Ho3+/Yb3+-codoped TeO2–TiO2–La2O3 glasses

Frequency upconversion from Ho3+/Yb3+-codoped glass or crystal under Yb3+ sensitization is a known phenomenon. However, inconsistencies are prevalent in the understanding of double energy transfer mechanisms for Ho3+/Yb3+-codoped systems. In this context, rate equations are proposed for Ho3+/Yb3+-codoped low-phonon TeO2–TiO2–La2O3 glass under Yb3+ sensitization with continuous and pulsed excitations. The proposed rate equations are validated with experimental results to elucidate the mechanisms responsible for populating 5(S2, F4) and 5F5 energy levels of Ho3+ ion. The solutions of rate equations with experimental results are substantiating the occurrence of both excited state absorption (ESA) and energy transfer upconversion (ETU) mechanisms in populating Ho3+:5(S2, F4) level, though higher concentration of Ho3+ ion would decrease the probability of ETU and increase of ESA. In contrast, Ho3+:5F5 level has been populated via ETU only. Numerical solutions to the rate equations are also proposed to elucidate the mechanics for populating 5(S2, F4) and 5F5 levels of Ho3+ ion. The proposed rate equation for pulsed excitation explains the characteristics of respective decay curves, which are further used to quantify energy transfer coefficient (W02) as (1.77 ± 0.12) × 10− 17cm3 s−1 for Ho3+/Yb3+-codoped TTL glass host.

Analytical model for the intensity dependence of 1500 nm to 980 nm upconversion in Er3+: A new tool for material characterization

We propose a simplified rate-equation model for the 1500 nm to 980 nm upconversion in Er3+. The simplifications, based on typical experimental conditions as well as on conclusions based on previously published more advanced models, enable an analytical solution of the rate equations, which reproduces known properties of upconversion. We have compared the model predictions with intensity-dependent measurements on four samples with different optical properties, such as upconversion-luminescence yield and the characteristic lifetime of the 4I13/2 state. The saturation of the upconversion is in all cases well-described by the model over several orders of magnitude in excitation intensities. Finally, the model provides a new measure for the quality of upconverter systems based on Er3+—the saturation intensity. This parameter provides valuable information on upconversion parameters such as the rates of energy-transfer upconversion and cross-relaxation. In the present investigation, we used the saturation intensity to conclude that the differences in upconversion performance of the investigated samples are mainly due to differences in the non-radiative relaxation rates.

Molecular Upconversion in Water in Heteropolynuclear Supramolecular Tb/Yb Assemblies.

Piling up excited states to reach upconversion (UC) is severely restricted by vibrational quenching mechanisms, especially when one looks at discrete molecular entities in solution. By carefully controlling the supramolecular assembly processes resulting from the strong electrostatic interactions between negatively charged Yb complexes and Tb3+ cations in aqueous solutions, we engineered the formation of heteropolynuclear complexes of [(YbL)2Tb x] compositions ( x = 1 and 2). These edifices display a phenomenon of cooperative photosensitization UC with green emission of the Tb cations upon NIR excitation at 980 nm in the Yb absorption band. The photophysical properties of the complexes were carefully investigated by steady-state and time-resolved luminescence experiments in D2O, allowing one to quantify the impact of the composition and pD of the solution on the emission intensity as well as clarifying the exact cooperative photosensitization upconversion mechanism. Using optimized conditions, the energy transfer UC process could be observed for the first time in nondeuterated water with discrete molecular compounds.

Electrochemically triggered upconverted luminescence for light-emitting devices.

Electrochemically triggered upconverted luminescence through triplet-triplet energy transfer (TTET) and subsequent triplet-triplet annihilation upconversion (TTA-UC) is observed for the first time in the electrochemiluminescence properties of a Ru(bpy)32+/DPA-containing electrochemical device.

Room-temperature visible upconversion luminescence of Ni2+ sensitized by Yb3+ in transparent glass ceramics

Upconversion luminescence of transition metal Ni2+ ions is seldom at room temperature (RT) due to large non-radiative transition probability. Here, a green Ni2+ upconversion luminescence at RT is obtained by the near-infrared excitation of Yb3+ 2F7/2 → 2F5/2 at 980 nm in Ni2+/Yb3+ codoped transparent wide bandgap semiconductor γ-Ga2O3 glass ceramics, which can be assigned to the Ni2+ 1T2(1D) → 3A2(3F) transition. Lifetime measurement and upconversion power dependence data reveal energy transfer upconversion as the underlying upconversion mechanism for the Yb3+-Ni2+ systems incorporated into the γ-Ga2O3 nanocrystals. It is suggested that the low thermal quenching effect of the wide bandgap semiconductor γ-Ga2O3 and resonant sensitizing of the Yb3+ 2F5/2 state to the Ni2+ upconversion 3T2(3F) intermedia state are responsible for the achievement of room-temperature upconversion luminescence of Ni2+. The results demonstrate that wide bandgap semiconductor nanocrystal (γ-Ga2O3, TiO2, SnO2, et al.) glass ceramics may be a good candidate for hosting Ni2+ room-temperature upconversion luminescence.

Up-conversion luminescence tuning in Er3+-doped ceramic glass by femtosecond laser pulse at different laser powers**Project supported by the National Natural Science Foundation of China (Grant Nos. 51132004, 11474096, 11604199, U1704145, and 11747101), the Fund from the Science and Technology Commission of Shanghai Municipality, China (Grant No. 14JC1401500), the Henan Provincial Natural Science Foundation, China (Grant No. 182102210117), and the Higher

The up-conversion luminescence tuning of rare-earth ions is an important research topic for understanding luminescence mechanisms and promoting related applications. In this paper, we experimentally study the up-conversion luminescence tuning of Er3+-doped ceramic glass excited by the unshaped, V-shaped and cosine-shaped femtosecond laser field with different laser powers. The results show that green and red up-conversion luminescence can be effectively tuned by varying the power or spectral phase of the femtosecond laser field. We further analyze the up-conversion luminescence tuning mechanism by considering different excitation processes, including single-photon absorption (SPA), two-photon absorption (TPA), excited state absorption (ESA), and energy transfer up-conversion (ETU). The relative weight of TPA in the whole excitation process can increase with the increase of the laser power, thereby enhancing the intensity ratio between green and red luminescence (I547/I656). However, the second ETU (ETU2) process can generate red luminescence and reduce the green and red luminescence intensity ratio I547/I656, while the third ESA (ESA3) process can produce green luminescence and enhance its control efficiency. Moreover, the up-conversion luminescence tuning mechanism is further validated by observing the up-conversion luminescence intensity, depending on the laser power and the down-conversion luminescence spectrum under the excitation of 400-nm femtosecond laser pulse. These studies can present a clear physical picture that enables us to understand the up-conversion luminescence tuning mechanism in rare-earth ions, and can also provide an opportunity to tune up-conversion luminescence to promote its related applications.

Numerical Modeling of in-Band Pumped Ho-Doped Silica Fiber Lasers

A theoretical model to describe in-band pumped holmium doped silica fiber lasers is presented. The model is established based upon theory and parameters obtained from published research works. Both core-pumped system and cladding-pumped system are simulated with the model. The simulation results agree well with most experimental results, and for the cases in which the simulation results show discrepancy with the experimental results the disagreement can be reasonably explained. Through numerical analysis it is found that besides common negative factors such as non-radiative decay, fiber loss, and non-optimized resonator configuration, energy transfer upconversion plays a deleterious role in the performance of in-band pumped holmium doped silica fiber lasers. In particular, inhomogeneous upconversion associated with ion clustering is found to cause significant degradation of slope efficiencies of lasers, a fact that is useful to understand the unsolved problem regarding the lower-than-expected slope efficiencies observed in previous studies of in-band pumped holmium doped fiber lasers.

Multicolor Tunable Upconversion Luminescence from Sensitized Seed-Mediated Grown LiGdF4:Yb,Tm-Based Core/Triple-Shell Nanophosphors for Transparent Displays

Red, green, blue, and natural white upconversion (UC) luminescence colors are realized from the tetragonal-structured LiGdF4-based core/triple-shell (C/T-S) upconversion nanophosphors (UCNPs) and the C/T-S UCNP-incorporated polymer composites. The LiYF4:Yb cores are used as sensitized seeds for the formation of LiGdF4:Yb,Tm UC shell followed by the growth of LiGdF4:Tb,Eu color tuning shell. Finally, LiYF4 inert shell is grown on the core/shell/shell UCNPs, and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs exhibit enhanced UC luminescence. The single tetragonal-phased C/T-S UCNPs exhibit blue, green, and red UC luminescence, which is attributed to the electronic transitions in Tm3+ via energy transfer UC process and Tb3+ and Eu3+ via energy migration UC process, respectively. The multicolor UC emissions, including natural white, medium aquamarine, purple, and thistle color, are created by fine-tuning of the ratio of Tb3+ and Eu3+ in the color tuning shell. The transparent polymer composites are prep...

Visible to NIR down-shifting and NIR to visible upconversion luminescence in Ca14Zn6Ga10O35:Mn4+, Ln3+ (Ln=Nd, Yb, Er)

A series of phosphors Ca14Zn6Ga10O35:Mn4+, Ln3+ (Ln = Nd, Yb, Er) have been prepared by conventional high temperature solid-state reaction. Mn4+ single doped Ca14Zn6Ga10O35 shows deep red emitting, and it can obtain NIR emission when co-doped with Ln3+ in Ca14Zn6Ga10O35:Mn4+via an efficient energy transfer from Mn4+ to Ln3+. This process mainly is attributed to the dipole-dipole interaction mechanism. Visible-NIR to NIR down-shifting is realized via the energy transfer from Mn4+ to Ln3+ ions. Reversely, the green emission of Er3+ can be observed upon 980 nm light excitation, and the red emitting of Mn4+ appears in Mn4+ and Er3+ co-doped Ca14Zn6Ga10O35 phosphors through the up-converted energy transfer from Er3+ to Mn4+. Two-photon processes are required to populate the 2H11/2, 4S3/2 and 4F9/2 excited states of Er3+. The dual-mode luminescence mechanism, including energy transfer from Mn4+ to Ln3+ and up-conversion energy transfer from Er3+ to Mn4+, has been discussed in detail.

Rise kinetics of up-conversion luminescence under pulsed excitation. Probabilistic model and experiment

A probabilistic model describing the luminescence kinetics in systems of interacting rare-earth ions is proposed. The basic position of the model is that any mechanism of energy transfer between ions can be represented as a sequence of elementary processes. The duration of each elementary process of energy transfer is a random variable. An analytical expression is found for the time dependence of the radiant flux spectral density of ions for any pump modulation and its special case when the durations of elementary processes have exponential distribution laws and the excitation modulation is a rectangular or short pulse. It is shown that the initial stage of this dependence is a power function. The process of energy transfer from the pump radiation to the emitting ion consists of a number of elementary successive stages which determine the exponent. Mechanisms of up-conversion energy transfer under pulsed excitation are considered in systems consisting of ions of two types: donors interacting with pump radiation and acceptors receiving energy from donors. The number of successive elementary processes, of which these mechanisms are composed, is determined. A regression analysis of the rise kinetics of up-conversion luminescence of the Y0.8Yb0.2F3: Tm3+(1 at%) crystal is made using the proposed probabilistic model. The kinetics was obtained under rectangular pulsed excitation by IR radiation of a λp=933nm semiconductor laser diode. The most important mechanisms of energy transfer from Yb3+ ions to the Tm3+ ions responsible for transitions between the ground 3H6 term and excited 3F4, 3H4, 1G4, 1D2, 1I6 terms of the Tm3+ ions are established. The durations of these energy transfer processes are determined.

Near-infrared-triggered photon upconversion tuning in all-inorganic cesium lead halide perovskite quantum dots

All-inorganic CsPbX3 (X = Cl, Br, and I) perovskite quantum dots (PeQDs) have shown great promise in optoelectronics and photovoltaics owing to their outstanding linear optical properties; however, nonlinear upconversion is limited by the small cross-section of multiphoton absorption, necessitating high power density excitation. Herein, we report a convenient and versatile strategy to fine tuning the upconversion luminescence in CsPbX3 PeQDs through sensitization by lanthanide-doped nanoparticles. Full-color emission with wavelengths beyond the availability of lanthanides is achieved through tailoring of the PeQDs bandgap, in parallel with the inherent high conversion efficiency of energy transfer upconversion under low power density excitation. Importantly, the luminescent lifetimes of the excitons can be enormously lengthened from the intrinsic nanosecond scale to milliseconds depending on the lifetimes of lanthanide ions. These findings provide a general approach to stimulate photon upconversion in PeQDs, thereby opening up a new avenue for exploring novel and versatile applications of PeQDs.

Tuning the upconversion photoluminescence lifetimes of NaYF4:Yb3+, Er3+ through lanthanide Gd3+ doping

The multiplexing capacity of conventional fluorescence materials are significantly limited by spectral overlap and background interference, mainly due to their short-lived fluorescence lifetimes. Here, we adopt a novel Gd3+ doping strategy in NaYF4 host materials, realized tuning of upconversion photoluminescence (UCPL) lifetimes at selective emissions. Time-correlated single-photon counting (TCSPC), was applied to measure the photoluminescence lifetimes accurately. We demonstrated the large dynamic range of lifetimes of upconversion nanoparticles with good upconversion quantum yields, mainly owing to the dominance of high efficient energy transfer upconversion mechanism. The exceptional tunable properties of upconversion materials allow great potential for them to be utilized in biotechnology and life sciences.

Enhanced Efficiency of Functional Smart Window with Solar Wavelength Conversion Phosphor-Photochromic Hybrid Film.

Here, the high sensitivity and enhanced photochromic efficiency of the hybrid film of solar wavelength conversion phosphor and photochromic film for functional smart window have been achieved by fabricating a double-layered hybrid structure of wavelength conversion phosphor and photochromic film. Y2SiO5:Pr3+ phosphor nanoparticles, which upconvert visible light to UV light, were synthesized by simple hydrothermal method. The synthesized Y2SiO5:Pr3+ nanoparticle was coated as layered structured film on photochromic H3PW12O40 film. The Y2SiO5:Pr3+/H3PW12O40 hybrid film showed an enhanced sensitivity and efficiency of photochromic process with solar light irradiation due to the increased UV portion of solar light through upconversion process of visible light by wavelength conversion phosphor layer. The increased UV portion by upconversion process of Y2SiO5:Pr3+ layer through excited-state absorption and energy transfer upconversion process, contributed to an enhancement of coloration rate of photochromic H3PW12O40 film by 7 times with 50 min of 1 sun irradiation due to fast conversion of W6+ state to W5+ state in H3PW12O40 film with 5 times enhanced photochromic sensitivity compared with pristine photochromic film.

Revisit of energy transfer upconversion luminescence dynamics—the role of energy migration

Upconversion is a process in which one photon is emitted upon absorption of several photons of lower energy. Potential applications include super resolution spectroscopy, high density data storage, anti-counterfeiting and biological imaging and photo-induced therapy. Upconversion luminescence dynamics has long been believed to be determined solely by the emitting ions and their interactions with neighboring sensitizing ions. Recent research shows that this does not hold for nanostructures. The luminescence time behavior in the nanomaterials is confirmed seriously affected by the migration process of the excitation energy. This new fundamental insight is significant for the design of functional upconversion nanostructures. In this paper we review relevant theoretical and spectroscopic results and demonstrate how to tune the rise and decay profile of upconversion luminescence based on energy migration path modulation.

Morphological and luminescence studies on KGdF4:Yb3+/Tb3+ up-conversion nanophosphors

Potassium Gadolinium Fluoride (KGdF4) Up-Conversion Nano Particles (UCNPs) doped with varying concentrations of Ytterbium (Yb3+) and co-doped with Terbium (Tb3+) ions were synthesized using a wet chemical route. Morphological studies like X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) were carried out on the as prepared nanoparticles. The size of the UCNPs was found to be of the order of 6–8 nm using Debye Scherrer Formula. Under 980 nm CW laser excitation, the as prepared UCNPs emit intense Up-Converted green light at 545 nm (5D4→7F5). The energy transfer for this transition between the sensitizer (Yb3+) and activator (Tb3+) ions during the Up-conversion (UC) process was found to be through Cooperative Energy Transfer (CET) mechanism. UC emission was also observed from 5D3 to 7F3 level of Tb3+ ions whose intensity was found to be increasing with Yb3+ ion concentration due to non-radiative energy transfer up-conversion (ETU) from a single excited Yb3+ ion, which is not involved in CET. The experimental lifetimes of 545 nm emission under 980 nm excitation for the as prepared UCNPs was observed to be in the range of 0.1–0.4 ms. Relatively higher experimental lifetimes and ability to emit intense visible green emission under NIR excitations allow us to contemplate that these UCNPs can be prospective host lattices that could be used for the present day bio-imaging and targeted drug delivery applications.

Photostable and efficient upconverting nanocrystal-based chemical sensors

Chemical sensing in living systems demands optical sensors that are bright, stable, and sensitive to the rapid dynamics of chemical signaling. Lanthanide-doped upconverting nanoparticles (UCNPs) efficiently convert near infrared (NIR) light to higher energy emission and allow biological systems to be imaged with no measurable background or photobleaching, and with reduced scatter for subsurface experiments. Despite their advantages as imaging probes, UCNPs have little innate chemical sensing ability and require pairing with organic fluorophores to act as biosensors, although the design of stable UCNP-fluorophore hybrids with efficient upconverted energy transfer (UET) has remained a challenge. Here, we report Yb3+- and Er3+-doped UCNP-fluorophore conjugates with UET efficiencies up to 88%, and photostabilities 100-fold greater by UET excitation than those of the free fluorophores under direct excitation. Despite adding distance between Er3+ donors and organic acceptors, thin inert shells significantly enhance overall emission without compromising UET efficiency. This can be explained by the large increase in quantum yield of Er3+ donors at the core/shell interface and the large number of fluorophore acceptors at the surface. Sensors excited by UET show increases in photostability well beyond those reported for other methods for increasing the longevity of organic fluorophores, and those covalently attached to UCNP surface polymers show greater chemical stability than those directly coordinated to the nanocrystal surface. By conjugating other fluorescent chemosensors to UCNPs, these hybrids may be extended to a series of NIR-responsive biosensors for quantifying the dynamic chemical populations critical for cell signaling.

Lanthanide-doped Y3Ga5O12 garnets for nanoheating and nanothermometry in the first biological window

Absorption and luminescence spectra in the first biological window of Nd3+ single-doped and Er3+-Yb3+ co-doped Y3Ga5O12 nano-garnets have been studied to evaluate their potential use as simultaneous optical nanoheaters and nanothermometers in biomedicine. Nd3+-doped nano-garnets uses the 808 nm laser radiation, resonant with the largest absorption peak of the 4I9/2 → 4F5/2 transition, for both heating the nanoparticle and populating the 4F3/2 emitting level. Changes in the relative intensities of different emission peaks between Stark levels of the 4F3/2 (R1,2)→4I9/2 (Z1-5) transition can be directly related to the temperature of the nano-garnet. On the other hand, the Yb3+/Er3+combination takes advantage of the large absorption cross-section of the Yb3+ ions at around 920 nm to heat the sample, while triggering the Yb3+-to-Er3+ upconversion energy transfer processes that populate the thermally coupled 2H11/2,4S3/2 emitting levels of Er3+ ions, whose relative intensity changes with temperature can be calibrated. Accurate spatially- and temperature-controlled optically stimulated heating of these nano-garnets from room temperature up to 75 °C within the first biological windows is proved.