Cross-relaxation Energy Transfer Research Articles

Near-infrared light driven highly efficient and thermally stable Gd2Ti2O7:Er3+/Yb3+ sub-microspheres for photocatalytic and plant growth LED applications

Photocatalysis is a technique that can help address various global challenges of energy and environment by utilizing solar energy. Photocatalysts that can capture light from different regions of the electromagnetic spectrum, such as ultraviolet, visible or near-infrared, are required to optimize the use of solar energy. Therefore, researchers have been developing various strategies to design such photocatalysts. Herein, we report the NIR-induced photocatalytic performance of pyrochlore-structured Gd2Ti2O7:1Er3+/10Yb3+ (GT:Er3+/Yb3+) sub-microspheres by using rhodamine B (RhB), a commom organic dye, as a probe molecule. A facile solvothermal process was used to prepare the sub-microspheres that emitted strong red light when irradiated by a 980 nm laser. The luminescence mechanism was attributed to cross-relaxation and back energy transfer processes. Furthermore, the GT:Er3+/Yb3+ sub-microspheres exhibited remarkable thermal stability and durability under 980 nm laser irradiation and the catalytic studies showed in the presence of laser light of only 1 W input power. The sub-microspheres completely broke down RhB dye into H2O and CO2 in 10 h. This study offers a novel approach to exploit NIR-driven lanthanide materials for LED and photocatalytic applications.

Efficient 1.7 µm pumping of 2 µm thulium lasers.

Solid-state 2 µm lasers based on thulium-doped active media Tm:YAG, Tm:YAP, and Tm:YLF were investigated under 1.7 µm resonant diode pumping. In contrast with standard 0.8 µm pump wavelength, a high slope efficiency was achieved, up to 80% in the case of Tm:YAP and Tm:YLF, nearing a quantum limit without relying on Tm3+-Tm3+ cross-relaxation energy transfer. Low thermal load allowed for stable continuous-wave operation with good beam quality and output power up to 6 W (Tm:YAG, Tm:YLF), and 8W (Tm:YAP).

Population Control of Upconversion Energy Transfer for Stimulation Emission Depletion Nanoscopy.

Upconverting stimulated emission depletion microscopy (U-STED) is emerging as an effective approach for super-resolution imaging due to its significantly low depletion power and its ability to surpass the limitations of the square-root law and achieve higher resolution. Though the compelling performance, a trade-off between the spatial resolution and imaging quality in U-STED has been recognized in restricting the usability due to the low excitation power drove high depletion efficiency. Moreover, it is a burden to search for the right power relying on trial and error as the underpinning mechanism is unknown. Here, a method is proposed that can easily predict the ideal excitation power for high depletion efficiency with the assistance of the non-saturate excitation based on the dynamic cross-relaxation (CR) energy transfer of upconversion nanoparticles. This allows the authors to employ the rate equation model to simulate the populations of each relevant energy state of lanthanides and predict the ideal excitation power for high depletion efficiency. The authors demonstrate that the resolution of STED with the assistance of nonsaturated confocal super-resolution results can easily achieve the highest resolution of sub-40nm, 1/24th of the excitation wavelengths. The finding on the CR effect provides opportunities for population control in realizing low-power high-resolution nanoscopy.

Visible and mid-infrared spectral features of Dy3+: SrF2 and Dy3+/Y3+: SrF2 crystals

In this work, Dy3+: SrF2 and Dy3+/Y3+: SrF2 crystals have been grown successfully by the Bridgman-Stockbarger method, and their absorption spectra, visible and mid-infrared (MIR) emission spectra, as well as fluorescence decay curves were analyzed. The advantages of co-doping Y3+ ions on the yellow and MIR emissions of Dy3+ ions were investigated. The intensity of yellow emission in the doubly-doped crystal is ∼ 4 times stronger than that of the singly-doped crystal. The absorption cross-section (0.782 × 10−21 cm2) at 452 nm and emission cross-section (1.156 × 10−21cm2) at 573 nm of Dy3+/Y3+: SrF2 crystal is higher than that of Dy3+: SrF2 crystal related values. Also, compared to Dy3+: SrF2 crystal, the emission cross-section (3.55 × 10−21 cm2) at 2880 nm and fluorescence decay time (1.062 ms) of Dy3+/Y3+: SrF2 crystal are found to be higher. When co-doped Y3+ ions, the distance between Dy3+ ions increased leading to the reduction of cross-relaxation and resonant energy transfer probabilities while enhancing the intensities of yellow and MIR emissions. From all the optical results, it can be inferred that Dy3+: SrF2 and Dy3+/Y3+: SrF2 crystals are potential materials to simultaneously realize yellow and MIR lasing action.

Tailoring multiphonon relaxation and cross relaxation energy transfer in mixed ionic-electronic 20Li2O-xBi2O3-(78-x)TeO2-1Er2O3-1Ag glasses at NIR region

A glass composed of 20Li2O-xBi2O3-(78-x)TeO2-1Er2O3-1Ag was fabricated by melt–quenching method to examine the influence of mixed ionic–electronic (MIE) effect on AC conductivity (σAC) and photoluminescence properties. The σAC decreased as the Bi2O3 content increased, until a minimum σAC at x=11 mol% attributable to the MIE effect was achieved. The photoluminescence intensity and lifetime (τmes)of the 4I13/2→4I15/2 transition were reported with a drop at x=11 mol%, coincided with the minimum σAC. Results on the analysis of the multiphonon relaxation rate (WMPR) revealed that adding low-phonon Bi2O3 concentration promoted higher photoluminescence intensity and longer τmesdue to the reduction in WMPR. However, the drop in photoluminescence intensity and τmes at x=11 mol% may be due to a blocking effect. The blocking effect may have induced nonradiative phonon-assisted Sab and cross-relaxation γ6 energy transfer, leading to a maximum WMPR and energy transfer parameter.

UV-excited blue- to green-emitting Tb3+ -activated sodium calcium metasilicate colour tunable phosphor for luminescence devices.

Tb3+ -doped Na4 Ca4 Si6 O18 (NCMS:Tb3+ ) phosphors were synthesized using a solid-state reaction route with increasing dopant concentration. The phase identification studies for NCMS:Tb3+ phosphors were carried out using an X-ray diffraction (XRD) technique. The XRD patterns for the as-synthesized phosphors showed satisfactory agreement with the standard pattern (JCPDS card no. 75-1687) for the pure phase of the Na4 Ca4 Si6 O18 compound. The morphology and size of particles were illustrated from scanning electron microscope (SEM) micrographs. The photoluminescence excitation (PLE) spectrum of Tb3+ -doped Na4 Ca4 Si6 O18 phosphor depicts the strong excitation peak obtained in the UV spectral region. The trivalent terbium-activated NCMS phosphors excited in the UV region (232 nm wavelength) exhibited intense emission in the blue (350-470) and green (470-650) spectral regions. When increasing the concentration of Tb3+ ions in the host matrix, the emission colour shifted from the blue to the green region due to cross-relaxation energy transfer (CRET) mechanisms and showed the tunable behaviour of the Tb3+ -activated as-synthesized NCMS phosphor. These results demonstrated that the Tb3+ -activated sodium calcium metasilicate phosphor has immense potential to contribute as a green- and blue-green-emitting component in lighting and display device applications.

Plasmonic induced 5D3–5D4 cross-relaxation of Tb3+ in CaF2 thin films

Au nanoparticles (NPs) interleaved between ZnO films were prepared on Si substrates and CaF2:Tb3+ thin films were then spin coated on top. The Au NPs were ∼43 nm in diameter and exhibited a plasmonic absorption band at ∼544 nm, which red-shifted with increasing ZnO layer thickness. Photoluminescence (PL) of the Si/ZnO/Au/ZnO/CaF2:Tb3+ structure exhibited characteristic Tb3+ emissions at 450 nm (5D3 – 7F3) and 489, 543, 585, 621 nm (5D4 – 7FJ, J = 3, 4, 5, 6). For the sample with no ZnO spacer layer, the PL intensity associated with the 5D4 transitions increased in the presence of the Au NPs, while that of the 5D3 transition decreased. The emission intensity showed similar trends for the sample with ∼25 nm ZnO spacer layers although the enhancement was less. The PL intensity for the sample with thicker ZnO spacer layers (∼50 nm) exhibited overall reduction of the emission intensity. Lifetime measurement indicated that the lifetime of the sample with no ZnO spacer layer slightly increased compared to their reference samples without Au NPs, but remained almost unchanged for the samples with ZnO spacer layers. These results suggest that the Au NPs enhanced cross-relaxation energy transfer between Tb3+ ions.

Tailoring Multiphonon Relaxation and Cross Relaxation Energy Transfer in Mixed Ionic-Electronic 20li2o-Xbi2o3-(80-X)Teo2-1er2o3-1ag Glasses at Nir Region

Tailoring Multiphonon Relaxation and Cross Relaxation Energy Transfer in Mixed Ionic-Electronic 20li2o-Xbi2o3-(80-X)Teo2-1er2o3-1ag Glasses at Nir Region

Upconversion Visible Light Emission in Yb/Pr Co-Doped Yttria-Stabilized Zirconia (YSZ) Single Crystals

As a development on previous research on single crystals of Pr3+-doped yttria-stabilized zirconia (YSZ), we report here the preparation and optical properties of Yb/Pr co-doped YSZ single crystals with different Yb2O3 concentrations. Results from X-ray diffraction (XRD) and Raman spectroscopy indicated that all of the crystal samples had a cubic phase structure, and transmission was ≥88% in the 550–780 nm range. Photoluminescence (PL) under excitation with a 980 nm laser showed upconversion emission, and several peaks were observed centered on 448 nm, 508 nm, 525 nm, 542 nm, 617 nm and 656 nm. The effects of excited state absorption (ESA), energy transfer upconversion (ETU), cross relaxation (CR), and cooperative energy transfer (CET) on the upconversion luminescence and energy transition mechanism in YSZ crystals were further studied. The fluorescence lifetime of the 3P0 → 3H5 transition at 542 nm reached 207 μs, which shows that the samples are of potential use for laser and fluorescence output.

Enhancement of green emission from Ca14Al10Zn6O35: Tb3+ phosphors via cross-relaxation energy transfer by Li+ ions

Tb3+ doped Ca14Al10Zn6O35 phosphors were prepared by a traditional solid-state reaction approach. The Tb3+ doped phosphors exhibit blue and green emission under the 285 nm excitation. The green to blue emission ratio is gradually enhanced as the Tb3+ concentration increases, which is ascribed to the enhanced cross-relaxation energy transfer (CRET) processes between Tb3+ ions. Li+ ions were introduced to the composition as an attempt to address the charge mismatch induced by Tb3+ for Ca2+ ion substitution. An enhancement of green emission from Tb3+ ions is observed after co-doping with Li+ ions. The lifetime measurement confirms that the CRET efficiency and energy transfer rate are simultaneously increased by Li+ ions. The results suggest that the incorporation of Li+ ions is a feasible approach to overcome charge vacancy defects and enhance CRET processes.

Orange color emitting Sm3+ ions doped borosilicate glasses for optoelectronic device applications

An intense orange color emitting Sm3+ activated Li2O–PbO–ZnO–Al2O3–B2O3–SiO2 (LPZABS) glasses were propitiously fabricated by using sudden quenching method to study the luminescent potentiality using spectroscopic techniques such as XRD, FT-IR, optical absorption, photoluminescence (PL) and PL decay. XRD and FT-IR reveals the glassy nature and various functional groups present in LPZABS host glass respectively. Judd-Ofelt parameters derived from absorption spectra are used to estimate various radiative parameters for the excited states of Sm3+ ions in LPZABS glasses. Under 400 nm excitation, the luminescence spectra in the as prepared glasses exhibit three emission bands that corresponds to 4G5/2 → 6H5/2 (562 nm), 4G5/2 → 6H7/2 (598 nm) and 4G5/2 → 6H9/2 (645 nm) transitions of Sm3+ ions. Among these three, 4G5/2 → 6H9/2 transition observed in orange region (598 nm) is relatively more intense and prominent. The PL decay curves recorded for 4G5/2 fluorescent level reveal exponential behavior and single exponential fitting is applied to evaluate the experimental lifetimes (τexp). The τexp values are found to be decreasing with Sm3+ ion content due to cross-relaxation energy transfer process. The results reveal that the as prepared glasses can be effectively used in fabricating intense visible orange color emitting optoelectronic devices.

Temperature dependent near-infrared emission and energy transfer of NaY(MoO4)2:Re (Re = Er3+/Nd3+, Tm3+) crystals

To investigate the energy transfer between Er3+ and Nd3+, Tm3+ ions, NaY(MoO4)2:Re (Re = Er3+/Nd3+, Tm3+) crystals were grown by the Vertical Bridgman method using spontaneous crystallization. The temperature-dependent luminescence properties of the crystals between 298 and 473 K under 980 nm excitation were investigated. The emission intensities decreased with the increasing temperature due to the thermal quenching effect. Compared to Er3+ doped NaY(MoO4)2, the emission intensity at 1534 nm reduced significantly. The crystals gave intense and characteristic emission of Tm3+ and Nd3+ in near-infrared emission region due to sensitization by Er3+ ions. The fluorescence lifetimes (4I13/2→ 4I15/2) of Er3+ single-doped crystals decreased with the addition of Tm3+ and Nd3+ ions. Efficient energy transfer from Er3+ to Tm3+ and Nd3+ ions were observed and the cross relaxation energy transfer efficiency between Er3+ and Nd3+,Tm3+ ions were 77.4% and 51.1%, respectively.

Multipath energy transfer and NIR luminescent properties of dual-lanthanide complexes

Near infrared (NIR)-emitting lanthanide ions, especially Sm3+, Nd3+, Yb3+, and Er3+, show special advantages in non-damage and deep-reaching biological imaging and telecommunication field. But most lanthanide ions have low luminescent efficiency in the NIR region, as a result of insufficient protection of the metal centers against non-radiative deactivation processes and/or inefficient inter- or intramolecular energy transfer. In recent years, organic ligands have been used as antennas to sensitize lanthanide ions to improve their luminescent efficiency. Because the chemical properties of lanthanide ions are very similar, mixed lanthanide complexes can be formed. At the same time, lanthanide ions have many energy levels, the energy transfer is easy to occur between different lanthanide ions. Thus, we can make use of these multi-path energy transfers to obtain efficient dual/multi-lanthanide multi-component NIR luminescent materials, which can be used for the ideal multi-channel near-infrared luminescence anti-counterfeiting bar code application. Herein, A series of NIR luminescent mono- or dual-lanthanide complexes were synthesized by TVPNTB ligand with lanthanide chloride salts. The monocrystalline structures of these rare earth complexes were obtained by X-ray single crystal diffraction. These lanthanide complexes are all hetero-isomorphic structures, crystallized in the monoclinic system and belong to P 21/ c space group. The phase purity was also proved by powder X-ray diffraction (PXRD). For the series of one component lanthanide complexes, the ligand TVPNTB can transfer energy to the excited state energy level of Sm3+, Nd3+, Er3+, Yb3+, Pr3+, Dy3+, and Ho3+, and produce NIR luminescence of the order of microsecond lifetime. Among them, the NIR luminescence studies of several lanthanide complexes such as Pr3+, Dy3+, Er3+, and Ho3+ are rarely reported. The results showed that the TVPNTB ligand has good near-infrared photosensitization effects on Sm3+, Nd3+, Yb3+, and Er3+. Moreover, by comparing the energy level difference of these ions, it is found that the energy level difference of Sm3+ from 4G2/5 to 6F5/2 is close to the energy level difference of Yb3+ from 2F5/2 to 2F7/2 in the ground state, and the energy level difference of Nd3+ from 4F3/2 to 4I11/2, which may lead to the cross relaxation energy transfer between rare earth ions and enhance the luminescence of Yb3+ and Nd3+. So we prepared a series of dual-lanthanide complexes, such as TVPNTB-Sm x Nd1 −x Cl3, TVPNTB-Sm x Yb1 −x Cl3 and TVPNTB-Sm x Er1 −x Cl3. Energy transfer from the antenna ligand can sensitize both NIR luminescence of the two lanthanide ions, which might serve as NIR polychromatic anti-counterfeiting luminescent barcode materials. We also investigated the energy transfer properties between the dual lanthanide ions, and confirmed that crossing relax energy transfer from Sm3+ to Nd3+, or from Sm3+ to Yb3+ were realized. The largest energy transfer efficiency ( η ) reaches 39% and 36%, leading to 4 or 2 times longer of the decay lifetimes of Nd3+ and Yb3+, respectively. Through the study of NIR luminescence performance of dual-lanthanide complexes with different proportions, it is clear that in addition to the antenna energy transfer from ligand to lanthanide ions, there is also a cross-relaxation energy transfer process from Sm3+ to Nd3+ and Sm3+ to Yb3+, thus achieving the effective energy transfer between dual-lanthanide complexes, thus further improving the NIR luminescence life of Nb3+ and Yb3+. The multi-path energy transfer strategy summarized in this study not only provides a basis for the preparation of novel high-performance NIR luminescent rare earth complexes, but also provides a reference for the design and synthesis of other multifunctional composite optical materials.

Tunable white light emission in zinc phosphate glasses activated with [formula omitted] clusters and Sm3+

Zinc phosphate glasses, activated with Agmn+ clusters and Sm3+, were prepared by the conventional melt-quenching method. The X-ray diffraction patterns revealed that the samples remain amorphous for Ag and Sm3+ contents up to 3.0 and 1.0 mol%, respectively. The Raman and FTIR spectra showed that the main vibrational modes are associated with P–O bonds. The absorption coefficient spectrum of the Ag singly doped glass sample displayed a broad band centered at 318 nm, related to Agmn+ clusters, whereas those co-doped with Sm3+ showed, in addition to the Agmn+ cluster absorption, the well-known Sm3+ absorptions at 343, 360, 374, 401, 415, 438, 465 and 477 nm. The photoluminescence excitation spectrum of the Ag singly doped glass sample exhibited a broadband from 3 to 6 eV (207–413 nm), assigned to superposition of the Ag+: 4 d10 → 4d95s and Agmn+ cluster: S0 → S1 transitions, being the excitation into the Agmn+ clusters attractive for W-LEDs applications. The photoluminescence emission spectra of the Ag singly doped glass sample, upon Agmn+ cluster excitations at 340, 350 and 360 nm, displayed cold white light tonality, with (0.279, 0.300) CIE1931 chromaticity coordinates of 9453 K and bluish-white light tonality with (0.264, 0.276) and (0.259, 0.270) CIE1931 chromaticity coordinates and correlated color temperature values of 12901 and 14201 K, respectively. The global emission of the Ag and Sm3+ co-doped glass samples was, upon 340, 350 and 360 nm excitations, gradually tuned from the bluish and cold white region to the warm white one, as the Sm3+ content was increased, with correlated color temperatures in the 14201-2691 K range. The Sm.3+ emission bands, under excitations at 340 and 350 nm, were attained at expense of radiative and non-radiative energy transfer from the Agmn+ clusters, as revealed respectively by the sinks mounted on the Agmn+ cluster emission bands and the emission decay profile shortening in presence of Sm3+. Analysis of the Agmn+ cluster emission intensity and decay profiles, with the Dexter and Burstein models, showed that Agmn+ cluster cross-relaxation and/or non-radiative energy transfer to Sm3+ might be dominated by an electric quadrupole-quadrupole interaction.

Study of a color-tunable long afterglow phosphor Gd1.5Y1.5Ga3Al2O12:Tb3+: luminescence properties and mechanism.

Long afterglow phosphors play a significant role in practical applications. However, due to their complex electronic structures, it is difficult to obtain tunable long afterglow phosphors. Herein, a novel long afterglow phosphor, Gd1.5Y1.5Ga3Al2O12:Tb3+, was successfully synthesized. Its detailed structural information was extracted via the Rietveld method, and its corresponding optical properties were further systematically explored via photoluminescence (PL), phosphorescence and thermoluminescence (TL) spectroscopy. Both the photoluminescence and long afterglow properties showed tunable behavior with a variation in the concentration of Tb3+. The investigation of the tunability of its photoluminescence and long afterglow properties revealed that the cross-relaxation energy transfer mechanism contributes to its tunable character. Meanwhile, long-lasting phosphorescence could be observed for a few hours by the naked eye in a dark environment after discontinuing the UV irradiation. Due to the potential existence of the photo-excited (Tb3+)+, the defect clusters were proposed, with oxygen vacancies acting as electron trapping centers and (Tb3+)+ attracting holes. Finally, an appropriate mechanism for the tunable long afterglow emission was proposed.

Structural and spectroscopic properties of phosphate–tungsten glasses doped with Nd3+ and Tb3+

Glasses of composition 90NaPO3-10WO3-xNd2O3, (0 ≤ x ≤ 1.7 mol%) and 90NaPO3-10WO3-yTbF3, (0 ≤ y ≤ 5.0 mol%) have been prepared and characterized. 31P and 23Na magic-angle spinning (MAS) NMR and 23Na{31P} rotational echo double resonance results are consistent with previously published data, indicating the presence of Q(2) and Q(1)1 W units. In addition, linear dependences of 31P MAS-NMR linewidths and spin-lattice relaxation rates and the electron spin-spin relaxation behavior (in the case of Nd2O3 doping) are consistent with a random dopant distribution, For Nd2O3- doped glass samples, absorption spectra in the visible and infrared regions were collected and analysed, and photoluminescence excitation and emission spectra were obtained under 575 nm excitation. Using the Judd-Ofelt formalism, the Ω2, Ω4 and Ω6 parameters were determined and by using them, the radiative lifetime (τr) were calculated for the 4F3/2 excited state. The fluorescence quantum efficiency value for this level (QE = τf/τr) of 13% was determined for glass with x = 1.0. The photophysical properties of Tb- doped glasses were also studied and upon UV excitation (372 nm) the glasses show emissions in the green (540 nm, 5D4 → 7F5 transition) and blue (434 nm, 5D3 → 7F4). The relative luminescence intensity ratio 5D3 → 7F5/5D4 → 7F5 is higher for lower Tb3+ content (0.5 Tb glass sample), because for higher Tb3+ contents, a cross-relaxation energy transfer process takes place favoring the green emission.

Cross-relaxation energy transfer between the Er3+ ions in vitreous arsenic chalcogenide

In this paper, we study widely investigated photoluminescent properties of [Formula: see text] compounds in the near-infrared ranges. The increase of the infrared luminescence intensity with increasing [Formula: see text] concentration can be attributed to the cross-relaxation process between the activator ions.

Enhanced 2.0 μm emission and lowered up-conversion emission in Ce3+/Yb3+/Ho3+ tri-doped Na5Y9F32 single crystals

In this paper, the Na5Y9F32 single crystals tri-doped with ~3 mol% Yb3+/~1 mol% Ho3+, and different concentrations of Ce3+ (0, 0.2, 0.25, 0.35 and 0.4 mol%) were successfully grown by the modified Bridgman method. The absorption spectra, fluorescence spectra and decay curves of co-doped Yb3+/Ho3+ Na5Y9F32 single crystals with different concentrations of Ce3+ ions were measured. The intensity parameters of Ho3+ ions were calculated according to Judd–Ofelt theory and measured absorption spectra. The emission spectra showed the up-conversion (UC) emission intensity at 545, 657, and 755 nm reduced significantly as the addition of Ce3+. When the concentration of Ce3+ reached about 0.25 mol%, the maximum fluorescence intensity of ~2.0 μm was obtained, and the emission cross section reached 2.29 × 10−20 cm2. The mechanism of up-conversion reduction and ~2.0 μm emission enhancement in Ce3+/Yb3+/Ho3+ tri-doped Na5Y9F32 crystals was discussed from their energy level diagrams and optical spectra. The cross-relaxation (CR) energy transfer efficiency between Ho3+ and Ce3+ ions were 36.3% calculated from the measured lifetime of Yb3+ /Ho3+ co-doped and Ce3+/Yb3+/Ho3+ tri-doped samples. Analysis on the fluorescence dynamics indicated that electric dipole-dipole is dominant for the energy transfer from Ho3+ to Ce3+ ions. The results show that Ce3+/Yb3+/Ho3+ tri-doped Na5Y9F32 crystals has certain application prospects in improving the performance of Ho3+ ~2.0 μm solid-state laser.

Integrating temporal and spatial control of electronic transitions for bright multiphoton upconversion

The applications of lanthanide-doped upconversion nanomaterials are limited by unsatisfactory brightness currently. Herein, a general strategy is proposed for boosting the upconversion efficiency in Er3+ ions, based on combined use of a core−shell nanostructured host and an integrated optical waveguide circuit excitation platform. A NaErF4@NaYF4 core−shell nanoparticle is constructed to host the upconversion process for minimizing non-radiative dissipation of excitation energy by surface quenchers. Furthermore, an integrated optical microring resonator is designed to promote absorption of excitation light by the nanoparticles, which alleviates quenching of excited states due to cross-relaxation and phonon-assisted energy transfer. As a result, multiphoton upconversion emission with a large anti-Stokes shift (greater than 1150 nm) and a high energy conversion efficiency (over 5.0%) is achieved under excitation at 1550 nm. These advances in controlling photon upconversion offer exciting opportunities for important photonics applications.

Double Perovskite K3InF6 as an Upconversion Phosphor and Its Structural Transformation Through Rubidium Substitution

Luminescent properties including energy upconversion on rare‐earth doped cryolite (double perovskite) structured K3InF6 have been investigated by synthesizing samples (both pure and Eu3+, Tb3+, Er3+ doped and Yb3+/Er3+ co‐doped samples) solvothermally. Cryolite structure of K3InF6 was evident in its powder X‐ray diffraction (PXRD) pattern which could be refined successfully in Fd3 space group with a lattice constant of a = 17.718(3) Å. Three bands centred at 227, 311 and 496 cm–1 were present for K3InF6 in its Raman spectrum at room temperature confirming cryolite structure and phonon energy of it was estimated to be 367 cm–1 by Lorentz fitting procedure. Emissions in red and green regions were observed for Eu3+ and Tb3+ doped K3InF6 samples, respectively. The local site symmetry and nature of bonding in Eu3+ doped samples were analyzed by Judd–Ofelt (J–O) parameters. For Er3+ and Er3+–Yb3+ doped samples, upconversion emission with the laser of λ = 980 nm was carried out in addition to normal excitation and emission spectral measurements. Intra configurational f–f transitions of Er3+ ions were noticed both in normal and upconverted spectra. Emission in red region over the green dominated both in the normal and in upconverted emission spectra. These were reasoned to arise from cross relaxation (CR) energy transfer process between two nearby Er3+‐ions. Structural transformation of cryolite K3InF6 to elpasolite has also been examined by substituting K+ with Rb+.