Excited Yb Research Articles

Investigations on the up/downconversion of Gd2O2S: Er, Yb nanophosphor with both excitation wavelength and Yb3+-dependent emission color modulation

Herein, a comprehensive investigation of photoluminescent characteristics of gadolinium oxysulphide (Gd2O2S) phosphors co-doped with erbium (Er3+) and ytterbium (Yb3+) under various excitation wavelengths and varying Yb3+-concentration is reported. Up/downconversion by characteristic Er3+-excited states exhibited a strong visible emission (525 nm, 556 nm, and 668 nm), along with the unusual presence of ultraviolet (UV ~ 360–375 nm) and near-infrared (NIR ~ 800–1000 nm) emissions, simultaneously. Maximum emission intensity in each emission region (visible, UV, NIR) and emission color (i.e., violet, green, pink, magenta, yellow, orange, red) is modulated by tuning Yb3+-doping concentration for different excitation wavelengths. The spectral engineering obtained under various incident photon energies (285 nm, 380 nm, 450 nm, 808 nm, 980 nm, and 1064 nm) is explained via multiple optical transition routes between Er3+ and Yb3+ ions. This Gd2O2S: Er, Yb (GOS) phosphor having manifold excitation and emission avenues, high immunity against ambient degradation and no toxicity, is actively proposed as an efficient energy converter for increased solar energy harvesting.

Temperature dependence of energy transfer between Nd3+ and Yb3+ ions in phosphate glass.

The temperature dependence of energy transfer between Nd3+ and Yb3+ ions in phosphate glass was investigated with emission spectra, time-resolved spectroscopy, and luminescence lifetimes. The time-resolved spectrograms vividly displayed processes of both Nd3+→Yb3+ energy transfer and Yb3+→Nd3+ energy transfer. The impact of temperature on energy transfer rate has been studied by analyzing the cumulative rise times and decay times of Yb3+ upon excitation of Nd3+ and the cumulative rise times and decay times of Nd3+ upon excitation of Yb3+ at different temperatures. It has been found that both Nd3+→Yb3+ energy transfer and Yb3+→Nd3+ energy transfer are accelerated along with increasing temperatures in a certain temperature range.

Enhancing the energy transfer from Mn4+ to Yb3+ via a Nd3+ bridge role in Ca3La2W2O12:Mn4+,Nd3+,Yb3+ phosphors for spectral conversion of c-Si solar cells

In this work, a series of Mn4+, Nd3+ and Yb3+ singly and co-doped Ca3La2W2O12 (CLWO) phosphors were prepared using a high-temperature solid-state reaction approach. In CLWO:0.016Mn4+, the emission spectrum presents a band around 709 nm attributed to Mn4+2Eg→4A2g transition under 365 nm UV excitation, which partly overlaps the excitation spectrum of Nd3+ singly doped CLWO phosphors. This indicates that a non-radiative resonant energy transfer process from Mn4+ to Nd3+ ions would like to take place in Mn4+, Nd3+ co-doped CLWO phosphors. Although scarce spectral overlap between Mn4+ emission and Yb3+ excitation in respective singly CLWO samples is observed, the energy transfer from Mn4+ to Yb3+ ions can also be deduced based on the analyses of excitation spectra, emission spectra and decay times in Mn4+, Yb3+ co-doped CLWO materials, which is proposed to be a phonon-assisted energy transfer process since the energy difference between Mn4+ lowest excited state 2Eg and Yb3+ excited state 2F5/2 is quite large. Moreover, in Nd3+, Yb3+ co-doped CLWO, the energy transfer phenomenon from Nd3+ to Yb3+ ions can be also observed. Therefore, we introduce Nd3+ ions into Mn4+, Yb3+ co-doped CLWO phosphors to realize the Mn4+→Nd3+→Yb3+ successive energy transfer process. Based on this, the Yb3+ emission intensity is highly enhanced in Mn4+, Nd3+ and Yb3+ tri-doped CLWO relative to Mn4+, Yb3+ co-doped CLWO. As a result, with the assistance of two channels Mn4+→Nd3+→Yb3+ and Mn4+→Yb3+ energy transfer processes, the broadband UV/blue light from sunlight spectrum can be effectively converted into Yb3+ near infrared (NIR), matching with the excellent spectral response of solar cells, which is highly expected to improve the c-Si solar cells spectral conversion efficiency.

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.

Efficiently Cooperative Energy Transfer Up‐Conversion Luminescence in Tb3+ /Yb3+ Co‐Doped Cubic Na5Lu9F32 Single Crystals by Vertical Bridgman Method

AbstractTb3+/Yb3+ co‐doped Na5Lu9F32 single crystals with ≈1.0 mol% Tb3+ concentrations and various Yb3+ are prepared by Bridgman method. Several visible up‐conversion (UC) luminescence are observed following the excitation of 980 nm laser diode (LD) as well as its pump power dependence on UC emission of the crystals. The quadratic dependence of pump power on UC emission reveals that two‐photon UC processes are mainly responsible for the 486 nm and 543 nm emissions of Tb3+/Yb3+ co‐doped system in Na5Lu9F32 single crystals. It is proposed that cooperative energy transfer (CET) from two excited Yb3+ ions to the ground state of Tb3+ ions is in charge of the ≈486 nm, ≈543 nm and ≈620 nm, which are attributing to 5D4 → 7F6, 5D4 → 7F4 and 5D4 → 7F3, respectively, are obtained in Tb3+/Yb3+ doped Na5Lu9F32 single crystals. Furthermore, a favorable yellowish green light performance can be achieved with certain Yb3+ concentrations and 1.0 mol% Tb3+ co‐doped samples, and its external quantum efficiency approached to 1.25% under 5.5 W cm−2 980 nm excitation which makes further research valuable in developing processable UC materials for electro‐optical devices.

Evaluation of TeO2 content on the optical and spectroscopic properties of Yb3 +-doped calcium borotellurite glasses

This paper reports the synthesis and the characterization of Yb3+-doped calcium borotellurite (CaBTeX) glasses with composition 10CaF2–(29.5−0.4x)CaO–(60−0.6x)B2O3–xTeO2–0.5Yb2O3 (x=10, 16, 22, 31 and 54mol%). The results of XRD confirm the amorphous character of all the samples. The density, molar volume, refractive index and electronic polarizability values show an increase with TeO2 content. Otherwise, the optical band gap energy shows a decrease with the increase of TeO2 content. The replacement of CaO and B2O3 by TeO2 changes the glass structure, which decreases the excited Yb3+/cm3 and, consequently, the luminescence intensity. The temperature dependence of luminescence was studied for all the samples up to 420K. The fluorescence lifetime does not change significantly due to TeO2 addition. In addition, absorption and emission cross section were calculated and present high values as compared to other tellurite and phosphate glasses.

Enhancement of Eu3+ Red Upconversion in Lu2O3: Yb3+/Eu3+ Powders under the Assistance of Bridging Function Originated from Ho3+ Tridoping.

The red upconversion (UC) emission of Eu3+ ions in Lu2O3: Yb3+/Eu3+ powders was successfully enhanced by tridoping Ho3+ ions in the matrix, which is due to the bridging function of Ho3+ ions. The experiment data manifest that, in Yb3+/Eu3+/Ho3+ tridoped system, the Ho3+ ions are first populated to the green emitting level 5F4/5S2 through the energy transfer (ET) processes from the excited Yb3+ ions. Subsequently, the Ho3+ ions at 5F4/5S2 level can transfer their energy to the Eu3+ ions at the ground state, resulting in the population of Eu3+5D0 level. With the assistance of the bridging function of Ho3+ ion, this ET process is more efficient than the cooperative sensitization process between Yb3+ ion and Eu3+ ion. Compared with Lu2O3: 5 mol % Yb3+/1 mol % Eu3+, the UC intensity of Eu3+5D0→7F2 transition in Lu2O3: 5 mol % Yb3+/1 mol % Eu3+/0.5 mol % Ho3+ is increased by a factor of 8.

Mechanism of ultraviolet upconversion luminescence of Gd3+ ions sensitized by Yb3+-clusters in CaF2:Yb3+, Gd3+

Ultraviolet (UV) luminescence under the near infrared (NIR) excitation is very important for the photodynamic therapy and NIR photocatalysis. In this paper, UV emission of Gd3+ ions in CaF2:Yb3+, Gd3+ excited by 980nm laser is studied theoretically on the basis of the cooperative energy transfer from excited lanthanide tetramers. The main mechanism for the cooperative energy transfer from an excited Yb3+-tetramer to a Gd3+ ion is dq-dq-qq (d means an electric dipole and q means an electric quadrupole) and the energy transfer rate is calculated to be 1.7 × 10−6s−1, which is well comparable with the previous calculated cooperative luminescence rate of 1.8 × 10−6s−1 from an Yb3+-trimer. This is reasonably agreed with our previous experimental result that showed UV luminescence of Gd3+ ions and Yb3+-trimers within the same order of magnitude in CaF2:Yb3+, Gd3+ under the excitation of 980nm laser. The present paper suggests that CaF2:Yb3+, Gd3+ is a potential up-conversion luminescent material for UV, and stimulates the design and development of new UV luminescent phosphors sensitized by lanthanide ion clusters.

(Invited) Up-Conversion Luminescence of Lanthanide Activated Phosphors

The major problem that limits solar cells’ efficiency is their insensitivity to the whole solar spectrum which is the so-called spectral mismatch. Therefore, several mechanisms have been explored based on photoluminescence (PL) to convert the solar cell spectrum where the spectral response of the solar cell is low to regions where the spectral response of the solar cell is high. Downconversion, up-conversion (UC) and downshifting are some of the mechanisms that may be applied to improve the spectral response. Upconversion nanoparticles (UCNPs) have shown some promising possibilities to be considered in this respect, however, low UC efficiency of UCNPs is still the most severe limitation of their applications. This study reports on the PL and cathodoluminescence (CL) behaviour of different phosphors. The vastly studied lanthanide pairs in UC are Er3+, Ho3+ and Tm3+ with Yb3+ as a sensitizer. Whereas, very few UC studies have been carried out for Yb3+/Tb3+ and Yb3+/Eu3+ pairs. The reason behind these two categories of lanthanides is the way they transfer their energies. Er3+, Ho3+ and Tm3+ co-doped with Yb3+ can be excited by ground state absorption (GSA), excited-state absorption (ESA) and energy transfer up-conversion (ETU) mechanisms whereas the energy transfer from Yb3+ to Tb3+/Eu3+ is due to co-operative energy transfer (CET). Since, Tb3+ and Eu3+ do not have energy levels which can absorb the near infra-red (NIR) light directly, Yb3+ is the best choice in order to obtain the UC luminescence in Tb3+/ Eu3+ doped system. Yb3+ has a single electronic transition within the 4f subshell. The transition from the lower level through ground state absorption can be easily achieved by using NIR radiation, and resulting energy from two or more excited Yb3+ ions can be utilized in the excitation of a single Tb3+ or Eu3+ ion, for gaining improved UC emission. Doping different host materials with these lanthanide pairs were investigated and tested for possible increase in solar cell efficiency. Power tuneable visible UC and infrared emissions were detected upon excitation with a 980 nm diode laser.

Oxyfluoride silicate glasses and glass-ceramics doped with erbium and ytterbium - An examination of luminescence properties and up-conversion phenomena

Oxyfluoride glasses doped with erbium and ytterbium have been fabricated at T=1460°C. To prepare glass-ceramic systems, the precursor glasses were annealed at temperature 750°C for 2h. The ordered fluoride-like crystalline phases were recognized applying XRD, SAED and EDS methods. The spectroscopic properties of the glass-ceramic samples were significantly different as compared to those observed in the precursor glass. Particularly, the emission spectra of glass-ceramics are evidence that some of Er3+ and Yb3+ ions reside into the ordered environment. Up-conversion phenomena attributed to visible anti-stokes emission of Er3+ has been observed under excitation of Yb3+ ions by a 980nm diode laser. Especially, the substantial enhancement of red up-converted luminescence was observed in the glass-ceramic system. Dissimilar spectral characteristic and relation of both aforementioned emissions examined in glass and glass-ceramic samples point at different contributions of processes relevant to up-conversion phenomena. The detailed analysis of the crucial Er3+ and Yb3+ excited states relaxation dynamics and their spectroscopic qualities has been performed to determine the mechanisms which lead to conversion of the infrared excitation into visible emission.

Development of novel upconversion luminescent nanoparticle of Ytterbium/Thulium–doped beta tricalcium phosphate

A novel class of upconversion luminescent nanocrystals of Yb/Tm:calcium deficient hydroxyapatite were synthesized by co-precipitation method in aqueous solution (pH adjusted to 6) and specially heat-treated with microwave radiation at different temperatures (from 900°C to 1000°C) and times (2–10min) to produce small nanocrystals of Yb/Tm:β-tricalcium phosphate (β-TCP). As a result, we report for the first time, a single-phase Yb/Tm-doped β-TCP nanocrystals with a mean crystallite size of 55.3nm. This material has an efficient visible luminescence from the 1G4 (blue emission) and 3F2 (red emission) and near infrared emission from the 3H4 excited states of Tm3+ induced by the Yb3+→Tm3+ energy transfer under laser excitation (Yb3+) at 972nm. This β-TCP activated by Yb3+ and Tm3+ ions constitutes a new nano-fluoroprobe that can be used as optical contrast agents, affording high resolution and sensitivity for visible-near infrared applications.

Enhanced high-order ultraviolet upconversion luminescence in sub-20 nm β-NaYbF4:0.5% Tm nanoparticles via Fe3+doping

Despite showing great promise in bioapplications, lanthanide-doped upconversion nanoparticles (UCNPs) are still restricted by low upconversion efficiency. In this work, ultra-small, monodisperse and uniform β-NaYbF4:0.5% Tm NPs were successfully prepared through a solvothermal route, and remarkably enhanced ultraviolet upconversion luminescence (UV UCL) had been achieved via Fe3+ doping. The UV UCL enhancement in NaYbF4:0.5% Tm NPs was investigated in detail. It was found that the UCL intensities of the UV and blue emissions of NPs doped with 5 mol% Fe3+ ions were enhanced by 10 and 6 times, respectively, compared to that of the Fe3+-free sample. The enhancement of UV UCL is mainly ascribed to the high population density of excited Yb3+ and the energy transfer from |2F7/2, 4T1g > (Yb3+–Fe3+ dimer) to 3F2,3 (Tm3+) states. Moreover, through dynamic analysis, we further demonstrate the proposed mechanisms of UC processes. The results suggest that ultra-small UCNPs with enhanced UV UCL may have potential applications in the biomedical field.

Influence of a backward reflection on low-threshold mode instability in Yb3+-doped few-mode fiber amplifiers.

An influence of backward reflection on spatio-temporal instability of the fundamental mode in Yb3+-doped few-mode polarization maintaining fiber amplifiers with a core diameter of 10 μm was studied experimentally and theoretically. The mode instability threshold was registered to decrease dramatically in the presence of a backward reflection of the signal from the output fiber end; an increase of the signal bandwidth or input power resulted in the increase of the threshold. Numerical simulation revealed a self-consistent growth of the higher-order mode LP11 and a traveling index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb3+ ions). The presence of the backward-propagating wave resulted in four-wave mixing on the common index grating induced by the interference field of pairs of the fundamental LP01 and LP11 modes.

Role of Ce in Yb/Al laser fibers: prevention of photodarkening and thermal effects.

We report on detailed investigations of ytterbium (Yb) and aluminum (Al) doped silica fiber and preform samples co-doped with cerium (Ce). The prevention of pump-induced photodarkening (PD) by temporary oxidation of Ce3+ to Ce4+ (or rather Ce3++) was proved by observed modifications in the ultraviolet (UV) spectra of transient absorption during near-infrared (NIR) pumping of thin preform slices. Only a small part of available Ce3+ ions (< 4%) was found to be involved in this process despite Yb inversions of up to 0.28. The modifications in the UV absorption spectra disappeared completely when the pump power was switched-off. From these observations we conclude that the recombination to Ce3+ takes place very fast thereby enabling these ions to capture liberated holes h+ perpetually during further pumping. We found a concentration ratio of Ce/Yb ≈0.5 to be sufficient to reduce PD loss to 10% in comparison to Ce-free fibers. Thus, the thermal load caused by absorption of PD color centers at pump (and laser) wavelength is expected to be also reduced. Unfortunately, new heat sources arise with the presence of Ce which cannot be explained by the absorption of Ce ions at the pump wavelength but must be attributed to the interaction with excited Yb ions. Fiber temperature increase of more than 200 K was observed if both, Yb2O3 and Ce2O3 concentration exceed 0.4 mol%.

Downconversion in Y2Si2O7: Pr3+, Yb3+ polymorphs for its possible application as luminescent concentrators in photovoltaic solar-cells

Polycrystalline powders α and β phases of Y2Si2O7 (yttrium disilicate) co-doped with Pr3+, Yb3+ ions have been prepared by the sol–gel method. The structural phase purity of obtained silicates has been confirmed by XRD measurements. The possible Pr3+→Yb3+ energy transfer mechanisms were analyzed in a function of Yb3+ concentration for both phases of Y2Si2O7. It was proven that the probability of downconversion (DC) process is strongly dependent on excitation power and Yb3+ concentration. From the power dependence of emission intensity analysis it was found that for low excitation power regimes DC process is responsible for Pr3+→Yb3+ energy transfer while above threshold power a cross relaxation process becomes dominant.

Efficient upconversion luminescence in Er3+/Yb3+ co-doped cubic NaYF4 single crystals by vertical Bridgman method with KF flux

Er3+/Yb3+ co-doped α-NaYF4 single crystals with various Yb3+ concentrations and ∼0.50 mol% Er3+ were grown by Bridgman method using Potassium fluoride as flux. The up-conversion (UC) luminescence and its pump power dependence on UC emission of the crystals under excitation of 980 nm laser diode were investigated. The UC emissions at green ∼522 nm, ∼541 nm and red ∼668 nm, which are attributed to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively, were obtained in Er3+/Yb3+ doped α-NaYF4 single crystals. The quadratic dependence of pump power on the UC emission suggests that a two-photon process is responsible for the transition from the excited Yb3+ to the lower state of Er3+ ions in α-NaYF4 single crystals. A favorable performance of yellowish green light can be achieved with certain Yb3+ concentrations and 0.52 mol% Er3+ co-doped samples that makes further research valuable in developing UC displays for electro-optical devices.

Down-conversion luminescence from (Ce, Yb) co-doped oxygen-rich silicon oxides

We have studied down-conversion photoluminescence (PL) from (Ce, Yb) co-doped “oxygen rich” silicon oxide films prepared by sputtering and annealing. The Ce3+ ∼510 nm PL is sensitive to the Ce concentration of the films and is much stronger for 3 at. % Ce than for 2 at. % Ce after annealing at 1200 °C. The PL emission and excitation spectroscopy results indicate that the excitation of Yb3+ is mainly through an energy transfer from Ce3+ to Yb3+, oxide defects also play a role in the excitation of Yb3+ after lower temperature (∼800 °C) annealing. The Ce3+ 510 nm photon excites mostly only one Yb3+ 980 nm photon. Temperature-dependent PL measurements suggest that the energy transfer from Ce3+ to Yb3+ is partly thermally activated.

Broadband white radiation in Yb3+- and Er3+-doped nanocrystalline powders of yttrium orthophosphates irradiated by 972-nm laser radiation

The up-conversion luminescence of Er3+ from the 2 H 11/2, 4 S 3/2, and 4 F 9/2 levels in nanocrystals of Y0.95(1‒x)Yb0.95x Er0.05PO4 (x = 0, 0.3, 0.5, 0.7, 1) orthophosphates activated with Er3+ ions has been studied under the excitation of Yb3+ ions to the 2 F 5/2 level by 972-nm cw laser radiation. Broadband radiation in the wavelength range of 370–900 nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of 972 nm at a pulse repetition frequency of 10 Hz and a duration of a pulse of 15 ns. Experimental data indicating that this radiation is thermal in nature have been presented.

Boiling effect in liquid nitrogen directly cooled Yb³⁺:YAG laser.

Liquid nitrogen (LN2) behavior on the surface of excited Yb(3+):YAG is investigated using fluorometry. From the time-resolved temperature variations and integrated fluorescence spectra intensity on this directly cooled Yb(3+):YAG surface, we observe a phase transition of LN2 from nucleate boiling to film boiling. As a result of this pool boiling, good beam quality should occur when the temperature and heat flux at an excited surface of Yb(3+):YAG are below 95 K and 15.8 W/cm2, respectively. That is, the LN2 should remain in a steady state of nucleate boiling to produce good beam quality using pool boiling.

Infrared to-visible upconversion luminescence in Er:Yb:SrFBr nanocrystals

The ability of SrFBr nanocrystals codoped with ∼1 mol% of Er3+ and Yb3+ to perform infrared-to-visible light upconversion is demonstrated. Excitation of Yb3+ at 980 nm results in two-photon upconversion and green-yellow luminescence from Er3+, highlighting the potential of alkaline-earth fluorohalides as hosts for Yb3+–Er3+ sensitizer–activator pairs.