Cooperative Energy Transfer Research Articles

Broadband flat near/mid-infrared emissions of Tm3+–Ho3+ co-doped, and Tm3+–Ho3+–Yb3+ tri-doped zinc silicate glasses under 808 and 980 nm laser diode excitations

We investigate and report the broadband flat near-infrared (NIR)/mid-infrared (MIR) emissions of Tm3+–Ho3+ co-doped, and Tm3+–Ho3+–Yb3+ tri-doped SiO2–ZnO–K2O–La2O3 zinc silicate glasses under excitation of 808 and 980 nm laser diode (LD). Broadband relatively flat MIR emissions of Tm3+–Ho3+ co-doped, and Tm3+–Ho3+–Yb3+ tri-doped with a full-width at half-maximum (FWHM) of about 420, and 390 nm corresponding to commercial of 808, 980 nm LD excitations were identified. The role of Yb3+ in enhancing the intensity of the NIR/MIR emissions of Tm3+–Ho3+ co-doped, and Tm3+–Ho3+–Yb3+ tri-doped was also investigated. The GTm(λ), GHoλ) gain coefficient spectra of Tm3+ single-doped, and Ho3+ single-doped were determined respectively. Besides, the mechanisms for cross-relaxation (CR), energy transfer (ET), cooperative ET (CET) processes between Tm3+, Ho3+, and Yb3+ are proposed and discussed.

Concentration dependence of upconversion luminescence of Er3+/Yb3+ in the fluorophosphate glasses with small phosphates content

Abstract Frequency upconversion into green and red luminescence in fluorophosphate glasses doped with Er3+ ions (1021 to 1016 ion/cm3) and Yb3+ ions (1021 ion/cm3) while pumped with a 970 nm diode laser is presented. The red upconversion emission results from a two-photon excitation process. The upconversion emission intensity dependence on the incident pump power shows that the green upconversion luminescence arising from 2H11/2 level is also contributed by the two-photon processes. The values of n-parameter for the 4S3/2 level change from 2.7 to 2.0 during low power excitation and continue decreasing at higher pumping energy. It has been shown that the green upconversion luminescence intensity dependence on the erbium ions content has a sharp maximum at the concentration of 2* 1019 ion/cm3. We attribute the high upconversion luminescence in case of low Er3+ and Tm3+ ions concentrations (about 1018–1016 ion/cm3) to the cooperative energy transfer from Yb–Yb* dimers. The formation of excited dimers is confirmed experimentally by cooperative luminescence band at 495 nm.

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.

High-entropy sesquioxide X2O3 upconversion transparent ceramics

In this study, a high-entropy approach was used to design a highly transparent (81.3% in-line transmittance) sesquioxide X2O3 ceramics, in which a single-phase solid solution, with the general formula (Lu0.2Y0.395Gd0.2Yb0.2Tm0.005)2O3, was obtained after sintering under vacuum with La2O3 and ZrO2 as the sintering additives. Upconversion emission from Tm3+ ion at 485 nm was demonstrated upon the 980 nm laser emission excitation as a result of cooperative energy transfer from the Yb3+ ions. High-resolution scanning electron microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) elemental mapping showed that the obtained transparent ceramics were pore-free and without segregation of different elements.

Energy transfer and luminescent properties of Eu3+, Tb3+, Eu3+-Yb3+and Tb3+-Yb3+ doped α-NaYF4 nanophosphors prepared by coprecipitation route

Cubic α-NaYF4 nanometer-sized crystals doped with Eu3+, Tb3+, Yb3+-Eu3+ or Yb3+-Tb3+ were synthesized by an original coprecipitation route. The obtained nanoparticles exhibited primary particles showing cubic shape with sizes ranging between 35 and 65 nm. The singly- or co-doped nanophosphors exhibited strong red (Eu3+) or green (Tb3+) fluorescence upon ultraviolet (UV) or near infrared (NIR) excitation, which resulted respectively from down- or up-conversion processes occurring in their structure. Spectroscopic properties were investigated on the basis of emission spectra as well as luminescence decays. From the emission spectra of Eu3+ doped samples, the Ω2 and Ω4 Judd-Ofelt intensity parameters were calculated. The concentration quenching of the Eu3+ or Tb3+ ion emissions in singly-doped or Yb3+ co-doped α-NaYF4 were ascribed to resonant cross-relaxations. The main derived interaction between the active ions was evidenced as an electric dipole-dipole one through fitting the decays curves with the Inokuti-Hirayama model. The critical distances and energy transfer microparameters for the transfer processes were given showing very short range interaction. The dependence of integral up-conversion intensity on the NIR energy of the beam power was measured. The results indicated a two-photon process based on a cooperative energy transfer.

Investigation of upconversion luminescence for Tb3+/Yb3+ co-doped CaLnAlO4 (Ln = Y, Gd, La) phosphors

Tb3+/Yb3+ co-doped CaGdAlO4 phosphors were synthesized by a conventional solid-state reaction method. Green UC emission was obtained from the prepared samples under 980 nm excitation via a cooperative energy transfer process. The cooperative energy transfer efficiency from Yb3+ to Tb3+ and the phonon-assisted energy transfer efficiency from Tb3+ to Yb3+ were evaluated to be ~40% and ~1%, respectively. Partial substitution of Gd3+ ions with Y3+ or La3+ ions leads to an increase of UC intensity due to the enhancement of local distortion. The results indicate that the synthesized phosphors may be used to increase the efficiency of perovskite-based photovoltaic cells excited by green light.

Up-conversion luminescence and EPR properties of KGdF4:Yb3+/Tm3+ nanophosphors

Synthesis of KGdF4:Yb3+/Tm3+ up-conversion nanoparticles (UCNPs) via wet chemical route has been reported. HR-TEM, EDAX and EPR studies were performed on the samples. The samples emit intense blue light centred at 472 nm (1G4→3H6) under 980 nm CW laser excitation. Energy transfer (ET), ground/excited state absorption (ESA/GSA) and cooperative energy transfer (CET) were established as the possible energy transfer mechanisms in the up-conversion (UC) process. Since these UCNPs, synthesized by a much cost effective route, have sizes comparable at cellular level and exhibit the ability of up-conversion, we propose to utilize these UCNPs for prospective bio-photonic applications.

Broadband downconversion in Bi3+-Yb3+-codoped transparent glass ceramics containing LaF3 nanocrystals

Broadband near-infrared downconversion is achieved in transparent glass ceramics containing Bi3+/Yb3+:LaF3 nanocrystals, which involves emitting two near-infrared photons by downconversion one incident ultraviolet photon via cooperative energy transfer. The X-ray diffraction patterns and scanning electron microscope images are recorded to demonstrate the morphology and distribution of LaF3 nanocrystals. The dependence of fluorescence spectra and decay lifetimes on Yb3+-doped concentrations is examined to verify the energy transfer mechanism. Further, the quantum efficiency is estimated and the maximal value is close to be 155.4%.

Energy transfer and spectroscopic properties of Cr3+/Yb3+ co-doped TeO2–ZnO–La2O3 tellurite glasses under different wavelength excitation lights

In this paper, the upconversion/downconversion (UC/DC), visible (VIS) and near-infrared (NIR) emissions of Cr3+/Yb3+ co-doped TeO2–ZnO–La2O3 (TZL) tellurite glasses under different wavelength excitation lights were investigated. The CIE 1931 chromaticity coordinates for UC emission of Cr3+/Yb3+ co-doped TZL tellurite glasses under different excitation wavelengths were also investigated. Through the energy transfer (ET) and cooperative energy transfer (CET) processes from Cr3+ to Yb3+ and the combination of Cr3+-Yb3+ lead to the intensity of UC-, DC-, VIS-, NIR-emissions of Cr3+/Yb3+ co-doped TZL tellurite glasses significantly increased. Under different wavelength excitation lights, the optimal concentration of Yb3+ for the UC-, DC-, and VIS- emissions of Cr3+/Yb3+ co-doped TZL tellurite glasses is different. In addition, the ET, CET processes between Cr3+ and Yb3+ are also proposed and discussed.

Semiconductor host for designing phosphors for modification of solar spectrum

Synthesis and characterization of SrCrO4 activated with rare earth Nd3+ and Yb3+ is reported for the first time. Interesting luminescence properties have been observed for the synthesized phosphors. Host sensitized near infrared (NIR) emission is observed for Yb3+ doped phosphor which occurs as a result of cooperative energy transfer. Downshifted Nd3+ emission which is again in the NIR region can be excited by f-f transitions as well as by host absorption followed by energy transfer. The excitation spectra of these phosphors cover 350–500 nm region showing excellent overlap with solar spectrum that is not efficiently converted by c-Si solar cells. Emission is around 1 μm where photovoltaic conversion of c-Si is most efficient. These properties make them suitable as downshifting/downconversion phosphors for c-Si solar cells.

JUDD-OFELT ANALYSIS OF Tb3+ AND UPCONVERSION STUDY IN Yb3+–Tb3+ CO-DOPED CALIBO GLASSES

We report on the study of the spectroscopic properties in Ytterbium-Terbium (Yb3+-Tb3+) co-doped calcium lithium borate (CaLiBO) glasses, with the study’s focus being on the upconversion process. Intensity parameters Ωλ for CaLiBO:Tb3+ are determined by the Judd-Ofelt method to be Ω2 = 15.5 × 10-20 cm2, Ω4 = 1.90 × 10-20 cm2 and Ω6 = 3.69 × 10-20 cm2. We have also obtained electric-dipole (and magnetic-dipole) radiative transition probabilities, branching ratios, and lifetime for the Tb3+:5D4 and 5D3 levels. In addition, an evaluation of the upconversion processes by luminescence and time-resolved spectroscopy were carried out. The upconversion rise and decay times, energy transfer probability from Yb3+ to Tb3+ ions, and the efficiency of the processes that depopulate the Tb3+:5D4 level after resonantly pumping the Yb3+:2F5/2 level were estimated. Our results showed that a cooperative energy transfer (CET) from two Yb3+ ions to one Tb3+ ion is the origin of the Tb3+ upconversion luminescence in the visible region. While, CET followed of the cross relaxation or/and excited state absorption is responsible for the upconversion luminescence in the ultraviolet region.

Spontaneous emission by an ensemble of dipoles coupled to a plasmonic nanoresonator

ABSTRACTSpontaneous emission from ensembles of quantum emitters (QEs) such as atoms, molecules or semiconductor quantum dots can be greatly enhanced by cooperative effects arising from electromagnetic correlations. We describe a cooperative mechanism for emission of light by an ensemble of QEs based upon cooperative energy transfer from QEs to localized surface plasmons in metal-dielectric structures followed by plasmon radiation at a rate that scales with the ensemble size. For large QE ensembles saturating the plasmon mode volume, we derive universal, i.e. independent of local field distribution, expression for cooperative Purcell factor that can by far exceed the field enhancement limits for individual QE in a hot spot. We also derive radiated power spectrum that retains the plasmon resonance lineshape and, in contrast to common cooperative mechanisms, is insensitive to natural variations of QEs' emission frequencies.

Cooperative Energy Transfer Controls the Spontaneous Emission Rate Beyond Field Enhancement Limits

Quantum emitters located in proximity to a metal nanostructure individually transfer their energy via near-field excitation of surface plasmons. The energy transfer process increases the spontaneous emission (SE) rate due to plasmon-enhanced local field. Here, we demonstrate a significant acceleration of the quantum emitter SE rate in a plasmonic nanocavity due to cooperative energy transfer (CET) from plasmon-correlated emitters. Using an integrated plasmonic nanocavity, we realize up to sixfold enhancement in the emission rate of emitters coupled to the same nanocavity on top of the plasmonic enhancement of the local density of states. The radiated power spectrum retains the plasmon resonance central frequency and line shape, with the peak amplitude proportional to the number of excited emitters indicating that the observed cooperative SE is distinct from superradiance. Plasmon-assisted CET offers unprecedented control over the SE rate and allows us to dynamically control the spontaneous emission rate at room temperature which can enable SE rate based optical modulators.

Exciton-Plasmon Energy Exchange Drives the Transition to a Strong Coupling Regime.

We present a model for exciton-plasmon coupling based on an energy exchange mechanism between quantum emitters (QE) and localized surface plasmons in metal-dielectric structures. Plasmonic correlations between QEs give rise to a collective state exchanging its energy cooperatively with a resonant plasmon mode. By defining carefully the plasmon mode volume for a QE ensemble, we obtain a relation between QE-plasmon coupling and a cooperative energy transfer rate that is expressed in terms of local fields. For a single QE near a sharp metal tip, we find analytically the enhancement factor for QE-plasmon coupling relative to QE coupling to a cavity mode. For QEs distributed in an extended region enclosing a plasmonic structure, we find that the ensemble QE-plasmon coupling saturates to a universal value independent of system size and shape, consistent with the experiment.

Cooperative emission mediated by cooperative energy transfer to a plasmonic antenna

We develop a theory of cooperative emission mediated by cooperative energy transfer (CET) from an ensemble of quantum emitters (QEs) to plasmonic antenna at a rate equal to the sum of individual QE-plasmon energy transfer rates. If the antenna radiation efficiency is sufficiently high, the transferred energy is radiated away at approximately the same cooperative rate that scales with the ensemble size. We derive explicit expressions, in terms of local fields, for cooperative Purcell factor and enhancement factor for power spectrum valid for plasmonic structures of any shape with characteristic sizes smaller than the radiation wavelength. The radiated power spectrum retains the plasmon resonance lineshape with overall amplitude scaling with the ensemble size. If QEs are located in a region with nearly constant plasmon local density of states (LDOS), e.g., inside a plasmonic nanocavity, we demonstrate that the CET rate scales linearly with the number of excited QEs, consistent with the experiment, and can be tuned in a wide range by varying the excitation power. For QEs distributed in an extended region saturating the plasmon mode volume, we show that the cooperative Purcell factor has universal form independent of the system size. The CET mechanism incorporates the plasmon LDOS enhancement as well, giving rise to possibilities of controlling the emission rate beyond field enhancement limits.

Influence of the grain sizes on Stokes and anti-Stokes fluorescence in the Yb3+ and Tb3+ ions co-doped nanocrystalline fluorapatite

In response to the need of new materials characterized by the spectroscopic properties in the near-infrared window in biological tissues (700–1100 nm), a new type of fluorapatites co-doped with Yb3+ and Tb3+ ions has been synthesized. The nanostructured fluorapatites were prepared using the so-called co-precipitation method and heat-treated in a broad temperature range (600–1100 °C). The structural and morphological properties of the obtained materials were determined by using XRPD (X-ray powder diffraction), TEM (transmission electron microscopy) and SEM (scanning electron microscopy) techniques. The crystallites size was verified and calculated by applying the Rietveld refinement. The primary size of the particles was estimated by TEM analysis and was found to be in the range from 30 nm × 15 nm for the sample annealed at 700 °C to 120 nm × 90 nm for the sample annealed at 1000 °C. Moreover, the formation of defects involving the doping process and their contribution to the charge compensation mechanism has also been discussed. Furthermore, the spectroscopic properties of the obtained materials have been investigated in detail with emission spectra, power dependence relations and luminescence kinetics. The co-doped fluorapatites demonstrated not only Stokes, but also anti-Stokes green luminescence (up-conversion (UC)) depending on the laser power. It was also found that there was weak emission from 5D3 energy level under UV irradiation, not detected under NIR excitation. The cooperative energy transfer (CET) was suggested as the main process responsible for the UC luminescence.

Upconversion luminescence, intrinsic optical bistability, and optical thermometry in Ho3+/Yb3+: BaMoO4 phosphors

Ho3+/Yb3+: BaMoO4 phosphors with different concentrations were fabricated by a gel combustion method. The upconversion (UC) luminescence, intrinsic optical bistability, and the corresponding mechanisms were reported for the present system. The optical thermometric properties based on red (5F5→5I8) and green (5F4/5S2→5I8) emissions were studied. The sensing sensitivities could be tuned by manipulating the cooperative energy transfer process. The highest absolute sensitivity was 99 × 10−4 K−1 at 573 K, which is larger than that of many previous UC materials.

Cooperative Chirality and Sequential Energy Transfer in a Supramolecular Light-Harvesting Nanotube.

By constructing a supramolecular light-harvesting chiral nanotube in the aqueous phase, we demonstrate a cooperative energy and chirality transfer. It was found that a cyanostilbene-appended glutamate compound (CG) self-assembled into helical nanotubes exhibiting both supramolecular chirality and circularly polarized luminescence (CPL). When two achiral acceptors, ThT and AO, with different energy bands were co-assembled with the nanotube, the CG nanotube could transfer its chirality to both of the acceptors. The excitation energy could be transferred to ThT but only be sequentially transferred to AO. During this process, the CPL ascribed to the acceptor could be sequentially amplified. This work provides a new insight into the understanding the cooperative chirality and energy transfer in a chiral supramolecular system, which is similar to the natural light-harvesting antennas.

Cooperative Chirality and Sequential Energy Transfer in a Supramolecular Light‐Harvesting Nanotube

AbstractBy constructing a supramolecular light‐harvesting chiral nanotube in the aqueous phase, we demonstrate a cooperative energy and chirality transfer. It was found that a cyanostilbene‐appended glutamate compound (CG) self‐assembled into helical nanotubes exhibiting both supramolecular chirality and circularly polarized luminescence (CPL). When two achiral acceptors, ThT and AO, with different energy bands were co‐assembled with the nanotube, the CG nanotube could transfer its chirality to both of the acceptors. The excitation energy could be transferred to ThT but only be sequentially transferred to AO. During this process, the CPL ascribed to the acceptor could be sequentially amplified. This work provides a new insight into the understanding the cooperative chirality and energy transfer in a chiral supramolecular system, which is similar to the natural light‐harvesting antennas.

On the efficient Te4+→Yb3+ cooperative energy transfer mechanism in tellurite glasses: A potential material for luminescent solar concentrators

Te4+/Yb3+ co-doped 80%TeO220%Li2O glasses (amounts in mol%) were prepared by the conventional melt-quenching method in an ambient atmosphere, in order to investigate their optical characteristics for application in solar cells. An efficient near-infrared down-conversion mechanism was observed in these samples, involving the emission of two near infrared photons (at around 978 nm) after the absorption of one photon in the ultraviolet-blue region. This effect occurred by means of a process of cooperative energy transfer (CET) from Te4+ to Yb3+ ions. The CET efficiency (ηCET) was calculated from radiative transitions (lifetime) and also by a new method, developed in this work, based on rate equations quantifying the non-radiative transitions (thermal effect) involved in the system. The two methods showed very good agreement, with ηCET between 82 and 100% obtained for the sample prepared using a higher Yb3+ concentration (4 mol%). The results suggested that these tellurite materials could have potential applications in improving the efficiency of silicon-based solar cells.