Energy Transfer In Yb3 Research Articles

Effect of the temperature on the luminescence profile of photonic materials for biological applications

Temperature has a significant impact on the luminescence profile of photonic materials for biological applications. This paper presents the effect of temperature on the luminescence profile of three different photonic materials in the range of the second biological window. The effect of temperature on luminescence properties is shown on glass, glass-ceramics, and phosphors co-doped with Eu3+/Nd3+/Yb3+ under UV laser diode excitation. Each sample has been analyzed for temperature effects on optical parameters in biological window ranges. Moreover the effect of temperature on mechanisms of energy transfer Eu3+ → Nd3+ → Yb3+ in all fabricated photonic materials was analyzed. Full Text: PDF References J. Zhou, J.L. Leano Jr., Z. Liu, D. Jin, K.-L. Wong et al., "Impact of Lanthanide Nanomaterials on Photonic Devices and Smart Applications", Small 14, 1801882 (2018). CrossRef Z. Li, Y. Zhang, G. Han, "Lanthanide-Doped Upconversion Nanoparticles for Imaging-Guided Drug Delivery and Therapy", Springer Series in Biomaterials Science and Engineering 6, 139 (2016). CrossRef A. Zhang, Z. Sun, M.Jia, et al., "Simultaneous luminescence in Ⅰ, Ⅱ and III biological windows realized by using the energy transfer of Yb3+→Er3+/Ho3+→Cr3+", Chemical Engineering Journal 365, 400 (2019). CrossRef M.-F. Tsai, S.-H.G. Chang, F.-Y. Cheng, V. Shanmugam, Y. Cheng et al., "Au Nanorod Design as Light-Absorber in the First and Second Biological Near-Infrared Windows for in Vivo Photothermal Therapy", ACS Nano 7, 5330 (2013). CrossRef S. Ding, L. Lu, Y. Fan, F. Zhang, "Recent progress in NIR-II emitting lanthanide-based nanoparticles and their biological applications", J. Rare Earth. 38, 451 (2020). CrossRef U. Rocha, K.U. Kumar, C. Jacinto, I. Villa, F. Sanz-Rodriguez et al., "Neodymium-Doped LaF3 Nanoparticles for Fluorescence Bioimaging in the Second Biological Window", Small 10, 1141 (2014). CrossRef M. Jia, Z. Sun, H. Xu, X. Jin, Z. Lv et al. "An ultrasensitive luminescent nanothermometer in the first biological window based on phonon-assisted thermal enhancing and thermal quenching", J. Mater. Chem. C 8, 15603 (2020). CrossRef H. Lin, T. Hu, Y. Cheng, M. Chen, Y. Wang, "Glass Ceramic Phosphors: Towards Long-Lifetime High-Power White Light-Emitting-Diode Applications–A Review", Laser Photonic Rev. 12, 1700344 (2018). CrossRef H. Su, Y. Nie, H. Yang, D. Tang, K. Chen, T. Zhang, "Improving the thermal stability of phosphor in a white light-emitting diode (LED) by glass-ceramics: Effect of Al2O3 dopant", J. Eur. Ceram. Soc. 38, 2005 (2018). CrossRef Kenry, Y.Duan, B. Liu, "Recent Advances of Optical Imaging in the Second Near-Infrared Window", Adv. Mater. 30, 1802394 (2018). CrossRef M.L. Debasu, H. Oliveira, J. Rocha, L.D. Carlos, "Colloidal (Gd0.98Nd0.02)2O3 nanothermometers operating in a cell culture medium within the first and second biological windows", J. Rare Earth. 38, 483-491 (2020). CrossRef L. Pavasaryte, A. Katelnikovas, V. Klimavicius, V. Balevicius, A. Krajnc et al., "Eu3+-Doped Y3−xNdxAl3O12 garnet: synthesis and structural investigation", Phys. Chem. Chem. Phys. 19, 3729 (2017). CrossRef K. Sadowska, P. Awramiuk, I Zgłobicka, K Rećko, J. Żmojda, "Quantum efficiency of europium doped LaPO4 phosphors for UV sensing applications", Photonics Lett. Poland 14, 28 (2022). CrossRef P. Lei, J. Feng, H. Zhang, "Emerging biomaterials: Taking full advantage of the intrinsic properties of rare earth elements", Nano Today 35, 100952 (2020). CrossRef K. Sadowska, T. Ragiń, M. Kochanowicz, P. Miluski, J. Dorosz, et al., "Analysis of Excitation Energy Transfer in LaPO4 Nanophosphors Co-Doped with Eu3+/Nd3+ and Eu3+/Nd3+/Yb3+ Ions", Materials 16, 1588 (2023). CrossRef

Structure, morphology and upconversion luminescence properties of Yb3+/Er3+ Co-doped NaY(WO4)2 phosphors

NaY(WO4)2:Yb3+/Er3+ upconversion phosphors with different Yb3+/Er3+ doping concentrations were synthesized by high temperature solid-state reaction at 1200 °C for 4 h. The phase structure, surface morphology and luminescent properties were characterized by X-ray diffraction, scanning electron microscope and fluorescence spectroscopy. The sample composed of 7% Yb3+ and 2.5% Er3+ has the best luminescent properties. At 980 nm excitation, the emission peak of all the samples locates at the green area. The energy transfer of Yb3+→Er3+ is dominant in the energy transfer. The results demonstrate NaY(WO4)2:Yb3+/Er3+ can be used as the feasible materials for biomarkers and upconversion white light-emitting diodes.

Multicolor luminescence of hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals with tunable morphology under 940 nm excitation for temperature-responsive anti-counterfeiting

In this study, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals were synthesized controllably, and upconversion luminescence excited at 940 nm and its application in temperature-responsive anti-counterfeiting are reported. It is clarified that the Ln3+ (Ln = Y + Yb + Ho + Ce) density ratio of bottom plane to side plane in the unit cell can be regulated by Ce3+ doping. It is also proved that the energy transfer of Yb3+ to Ho3+ is responsible for the activation of Ho3+ under 940 nm excitation, while the cross relaxation between Ho3+ and Ce3+ participates in the redistribution of electron population of 5S2/5F4 and 5F5 levels. Both theory and experiment confirm that the intensity ratio of red to green emission (IR/IG) as a function of temperature as an independent variable has good linear characteristics in the temperature range of 300–500 K. Due to the good responsiveness of multicolor luminescence to temperature, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ with tunable morphology is a promising candidate for advanced temperature-responsive upconversion anti-counterfeiting. Our results provide a new pathway for the controllable synthesis of hexagonal NaYF4 microcrystals as well as the regulation of upconversion luminescence that is excited by wavelengths other than 980 nm and its application in anti-counterfeiting.

Color tunability and white light generation through up-conversion energy transfer in Yb3+ sensitized Ho3+/Tm3+ doped tellurite glasses

In this study, tellurite glasses in the (50-x)TeO2-25WO3-25Li2O-xRE (RE: Ho3+, Tm3+, Yb3+) system were synthesized to investigate white light generation through up-conversion energy transfer. Thermal properties of glasses were studied, and optical absorption spectra were collected. Energy transfer mechanisms between RE ions and effect of RE concentrations on up-conversion emission and color tunability were investigated through photoluminescence analysis under 980 nm excitation. Additionally, effect of excitation power on up-conversion emission intensities were investigated. Colorimetric properties such as CIE color coordinates, correlated color temperature, color rendering index and color purity were calculated. The obtained colorimetric results under 2 watt pump power were comparable or better than other reported glass systems. H005T025Y2 sample was found to show the closest color coordinates (x = 0.32 and y = 0.33) to pure white light and suitable colorimetric properties indicating that studied glasses have high potentiality to be used as luminescent materials for white light generation.

Upconversion luminescence, optical thermometric properties and energy transfer in Yb3+/Tm3+ co-doped phosphate glass

Yb3+/Tm3+ co-doped transparent phosphate glasses were successfully synthesized by conventional melt-quenching method. The upconversion (UC) emission spectra, fluorescence lifetime and transmission spectra were characterized by using UV–visible spectrophotometer and Fluorescence Spectrometer. The results show that the Yb3+/Tm3+ co-doped phosphate glasses are highly transparent with a transmittance of about 90% in the visible light range, and the energy transfer between Yb3+ and Tm3+ occurs. Furthermore, the temperature dependent luminescence of the glasses was systematically investigated to explore the possible application as optical temperature sensors. Using the fluorescence intensity ratio (FIR) technique, the FIR of the UC emissions from 693 nm (Tm3+: 1G4→3H5) and 658 nm (Tm3+: 3F2→3H6) was studied as a function of temperature in the range of 303–653 K, under 980 nm laser excitation. Impressively, the high relative temperature sensitivity of 3.94% K−1 was obtained at 303 K in the Yb3+/Tm3+ co-doped phosphate glass, which is a very promising candidate for optical temperature sensors.

Co-operative energy transfer in Yb3+-Er3+ co-doped SrGdxOy upconverting phosphor

Upconversion nanoparticles (UCNPs) have shown considerable interest in many fields; however, low upconversion efficiency of UCNPs is still the most severe limitation of their applications. Yb3+ and Er3+ co-doped SrGd4O7/Gd2O3(SGO) upconversion (UC) phosphors were synthesized by a modified co-precipitation process. The UC properties were investigated by direct excitation with a 980nm laser. It was observed that the as prepared materials showed relatively strong green emission, while upon the incorporation of the Er3+ ion, there was an increase in the upconversion luminescence intensity for the red component. The effect of different doping concentration of Er3+on the emission spectra and X-ray diffraction patterns of the UC materials have also been studied. The luminescence lifetimes and Commission Internationale de L'Eclairage coordinates for these as prepared samples were determined to understand the energy transfer (ET) mechanisms occurring between Yb3+ and Er3+ in the SGO host matrix. The UC luminescence intensity as a function of laser pump power was monitored and it was confirmed that the UC process in SGO:Yb3+/Er3+is a two-photon absorption process. The findings reported here are expected to provide a better approach for understanding of the ET mechanisms in the oxide based Yb3+/Er3+ co-doped UC phosphors. This study might be helpful in precisely defined applications where optical transitions are essential criterion and this can be easily achieved by smart tuning of the emission properties of Yb3+/Er3+ co-doped UC phosphors.

Photon upconversion in Yb/Tb co-sensitized core-shell nanocrystals by interfacial energy transfer

We report the realization of photon upconversion through interfacial energy transfer in Yb3+/Tb3+ coupled core-shell nanostructure. The spatial separation of sensitizer and activator at different layers through the core-shell structure enables the enhancement of photon emission by minimizing the non-radiative decays of the activator. Furthermore, the energy migration among Tb3+ ions within the core matrix lattice effectively facilitates the energy transfer to a much farther distance, resulting in efficient photon upconversion by the Tb3+-mediated interfacial energy transfer. Our result offers a novel and widespread approach for achieving photon upconversion of lanthanide-doped nanocrystals that would help develop a new class of efficient upconverting materials.

Pure red upconversion emission from CaF2:Yb3+/Eu3+

Single phase cubic CaF2 microcrystals co-doped with Eu3+ and Yb3+ ions were synthesized through a coprecipitation process followed by annealing at 1200°C. Under the excitation of 980nm laser, the intensive emission at ~614nm (5D0 → 7F2) in the red region was observed at room temperature. The color coordinate of CaF2:0.1%Yb3+/0.1%Eu3+ was calculated with the Commission Internationale de I’Eclairage (CIE) chromaticity coordinate of (x=0.572, y=0.265). The excitation of Eu3+ ions came from the energy transfer of Yb3+-dimers through a cooperation sensitization process. Furthermore, not only the emissions from the 5Di (i=0, 1, 2, 3)→7Fj (j=0, 1, 2, 3) transitions but also the rarely reported emission (~398nm) from the 5L6 →7F0 transition was recorded. The cooperation sensitization process was so effective in the Yb3+/Eu3+ co-doped system that all the cooperative luminescence (CL) from Yb3+-clusters were quenched and only the red upconversion luminescence of Eu3+ ions could be seen.

Single-band near-infrared quantum cutting of Ho3+-Yb3+ codoped KLu2F7 phosphors by energy clustering

A single-band near-infrared (NIR) quantum cutting (QC) is efficiently achieved for 1190 nm fluorescence by structuring Ho3+-Yb3+ couple as energy clustering in KLu2F7 host under excitation of ultraviolet-to-green photons. In comparison with Ho3+ singly doping, the intensity of NIR emission was enhanced by at least 5 times as 75%Yb3+ is codoped into KLu2F7:Ho3+. On the basis of experimental data and theoretical analysis, the formation of Yb3+ (fractional Ho3+) energy clustering at sub-lattice level is demonstrated to account for resonant energy transfer (ET) of Ho3+ → Yb3+, back ET of Yb3+: 2F5/2 → Ho3+: 5I6, and the crucial energy preserving by confining Yb3+ energy migration. By mean of steady and dynamic spectroscopies, an internal quantum yield for the single-band 1190 nm emission is appropriately evaluated to be more than 190% for K(Lu0.24Ho0.01Yb0.75)2F7 without any obvious concentration quenching. This nanostructured design of single-band NIR QC will open up novel available way to get efficient NIR emitters, which can boost the promising applications of solar cells, optical communication, short-wavelength infrared (SWIR) bio-imaging in vivo, etc.

Red–green–blue upconversion luminescence and energy transfer in Yb3+/Er3+/Tm3+ doped YP5O14 ultraphosphates

Red, green, and blue emissions through up conversion and energy transfer processes were investigated in triply doped YP5O14: Yb3+, Er3+, Tm3+ ultraphosphates upon excitation at 980nm as a function of the Yb3+, Er3+ and Tm3+ concentrations. Highly crystalline tri-doped samples showing a monoclinic form with C2/c space group symmetry were synthesized by a flux method leading to the elaboration of single crystals which were crushed for physical-chemical mainly by X-ray powder diffraction and optical up conversion characterization.The blue, green and red emissions intensity can be adjusted by the Yb3+, Er3+ and Tm3+ amounts. The Er3+ red and green up conversion emissions were examined in the case of the Yb3+/Er3+ co-doped YP5O14 materials. Two- and three-photons processes are observed for the red and green emissions of Er3+ respectively. Three visible red, green and blue up conversion emissions corresponding to Er3+4F9/2→4I15/2 (red), Tm3+1G4→3F4; 3F2,3→3H6 (red), Er3+2H11/2, 4S3/2 →4I15/2 (green) and Tm3+1G4 → 3H6 (blue) emission bands were simultaneously observed in triply doped YP5O14: Yb3+, Er3+, Tm3+ ultraphosphates due to Yb–Er and Yb–Tm up conversion mechanisms. Moreover, a three-photon process is observed for the red emission in the tri-doped materials instead of a 2-photons mechanism observed for samples without Tm, due to a Er3+→ Tm3+ energy transfer involving the Er3+4F9/2 and the Tm3+3F2,3 states.Despite the presence of large phonons energy due to the phosphate material, around 1200cm−1, Er3+ and Tm3+ visible emissions due to up conversion phenomena can be subsequently combined in order to obtain a white light and find new applications in display devices.

1.8 μm luminescent properties and energy transfer of Yb3+/Tm3+ co-doped α-NaYF4 single crystals

This paper reports on successful preparation of α-NaYF4 single crystals co-doped with ∼1.9 mol% Tm3+ and various concentrations (3.85 mol%, 7.69 mol%, 11.54 mol%, 15.38 mol%) of Yb3+ by using a flux-Bridgman method. The fluorescence decay curve was measured to investigate the luminescent properties of the Yb3+/Tm3+ co-doped α-NaYF4, and the energy transfer process from Yb3+ to Tm3+; the J-O intensity parameters of Tm3+ were further calculated and analyzed according to the absorption spectra. Results show that, an intense 1.8 μm emission was achieved with Yb3+ as sensitizer for Tm3+ in the α-NaYF4 single crystal under the excitation of 980 nm LD (Laser Diode) because of the strong energy transfer from Yb3+ to Tm3+. The maximum emission intensity at 1.8 μm is obtained at about 15.38 mol% doping concentration of Yb3+ when the concentration of Tm3+ ions is fixed at ∼1.90 mol% in the current research. Moreover, the calculated maximum value of emission cross section at 1.8 μm is 1.63 × 10−20 cm2 for 3.85 mol% Yb3+/1.9 mol% Tm3+ sample, and the obtained energy transfer rate (WET) and energy transfer efficiency (η) are 1543 s−1 and 83.8%, respectively. Our analysis of the fluorescence dynamics indicates that electric dipole-dipole interaction is dominant for the energy transfer from Yb3+ ions to Tm3+ ions by using Inokuti-Hirayama’s model.

Blue–green cooperative upconverted luminescence and radiative energy transfer in Yb3+-doped tungsten tellurite glass

Ytterbium-doped tungsten tellurite glasses have been prepared and studied their cooperative upconverted luminescence and radiative energy transfer properties. In a 3.0mol% Yb2O3-doped glass, near-infrared emission band is peaked at around 977nm with a full width at half maximum of around 15nm. This glass emits blue–green upconverted emission under 980nm excitation due to cooperative processes involving two interacting Yb3+ ions. The upconverted emission band is centered at around 502nm with a bandwidth of around 45nm. Power dependence of cooperative emission intensity and the temporal evolutions of the near-infrared and blue–green emissions confirm the presence of cooperative luminescence. Photoluminescence and lifetimes have been measured by moving the laser excitation from one edge of the sample and found that radiative energy transfer is predominant in 3.0mol% of Yb2O3-doped glass. The absorption coefficient obtained from absorption spectrum is in good agreement with that obtained by fitting the curve of luminescence intensity versus distance from the edge of the sample.

Near-infrared downconversion through host sensitized energy transfer in Yb3+-doped Na2YMg2(VO4)3

Host sensitized near-infrared emitting phosphor Na2YMg2(VO4)3:Yb3+ was fabricated by a sol–gel method. Efficient broad emission is observed under ultraviolet excitation ranging from 240nm to −400nm and this luminescence originates from ligand–metal charge transfer of 2p of O2− to 3d of V5+. Under excitation of ultraviolet light in the broad absorption band, intense near-infrared emission of Yb3+ around 974nm is observed for Yb3+-doped Na2YMg2(VO4)3. Through study of photoluminescence spectra, fluorescence decay curves and concentration-dependent luminescence, mechanism of energy transfer from VO4 group to Yb3+ was proposed to rationalize the downconversion process. This phosphor with performance of ultra-violet to near-infrared downconversion could have potential application in improving efficiency of silicon-based solar cell.

Broadband near-infrared quantum-cutting by cooperative energy transfer in Yb3+–Bi3+ co-doped CaTiO3 for solar cells

An efficient near-infrared (NIR) quantum-cutting (QC) process which converts broadband ultraviolet-blue into NIR via downconversion (DC) has been demonstrated in CaTiO3:Yb3+,Bi3+ phosphors for the first time. These phosphors are valuable for use in crystalline Si (c-Si) solar cells, because they efficiently convert 300–480nm light that is not fully utilized by existing c-Si solar cells into ∼1000nm NIR light that can be sufficiently absorbed by solar cells. Co-doping with 15% Bi3+ ions, a significant enhancement in Yb3+ NIR integrated emission intensity by a factor of 10 is realized. The phosphors were characterized with the photoluminescence excitation (PLE) and the photoluminescence (PL) spectra evidence the presence of energy transfer (ET) processes from Bi3+-related charge transfer state to Yb3+ ions. The dependence of Yb3+ luminescent intensity on the excitation power was also measured and it demonstrates that the ET processes involve cooperative energy transfer (CET) via two photons QC processes.

Cooperative energy transfer in Yb3+–Tb3+ co-doped CaAl4O7 upconverting phosphor

Tb3+ and Yb3+ co-doped calcium aluminate powders: Ca1−x−yYbxTbyAl4O7 (x = 0.02–0.06; y = 0.02), Ca1−x−yYbxTbyAl4O7 (x = 0.03, y = 0.01–0.07), Ca1−x−yYbxTbyAl4O7 (x = 0.01–0.05, y = 0.01) were synthesized by a modified Pechini citrate process and their optical properties were investigated. Upon excitation at 245 nm all studied samples yielded relatively strong Tb3+ luminescence corresponding to the 5D4 → 7FJ and 5D3 → 7FJ transitions with the dominant component around 542 nm. The direct excitation of two Yb3+ ions at 980 nm led to cooperative sensitization of the Tb3+5D4 level giving rise to its green up-conversion luminescence. The up-conversion emission was also observed from higher 5D3 level of Tb3+, which is rather rare in high phonon frequency host materials. The effect of activator concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied. It was found that Na+ addition lowers disturbance/distribution of activator ions local symmetries and enhanced Stokes Tb3+ photoluminescence brightness, while the up-conversion emission intensity does not appear to change noticeably then.

Efficient conversion from UV light to near-IR emission in Yb3+-doped triple-layered perovskite CaLaNb3O10

Yb3+-doped triple-layered perovskite CaLaNb3O10 micro-particles were synthesized via the solid-state reaction method. The crystal structure and morphology of the polycrystalline samples were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) measurements, respectively. The reflectance spectra, photoluminescence (PL) excitation and emission spectra, the decay curves, and the absolute quantum efficiency (QE) of the near-infrared (NIR) emission (910–1100nm) were measured. Under excitation of UV light, Yb3+-doped perovskite shows an intense NIR emission attributed to the 2F5/2→2F7/2 transitions of Yb3+ ions, which could match maximum spectral response of a Si-based solar cell. This is beneficial for its possible application in an enhancement of the photovoltaic conversion efficiency of solar energy utilization. The efficient energy transfer in Yb3+-doped CaLaNb3O10 from NbO6 groups into Yb3+ ions was confirmed by the spectra and fluorescent decay measurements. Cooperative energy transfer (CET) was supposed to be the NIR emission mechanism.

Enhanced 2.0 μm emission and energy transfer in Yb3+/Ho3+/Ce3+ triply doped tellurite glass

Ce3+ induced enhancement of Ho3+ ~2.0μm emission in Yb3+/Ho3+ codoped sodium–zinc–tellurite (TNZ) glass was achieved under 980nm LD laser excitation. The spectroscopic studies show that the upconversion is remarkably reduced by the presence of Ce3+. The ~2.0μm fluorescence intensity is nearly triply enhanced, and the energy transfer efficiency from Yb3+ to Ho3+ is improved from 16.1% to 42.6% by increasing the Ce3+ concentration from 0 to 0.8mol%. The mechanism responsible for the upconversion reduction and ~2.0μm emission enhancement in Yb3+/Ho3+/Ce3+ triply-doped TNZ glass is also discussed. Our results indicate that the Yb3+/Ho3+/Ce3+ triply-doped TNZ glass is a promising candidate material for improving the Ho3+ 2.0μm fiber laser performance.

Er3+ and Yb3+ concentration effect in the spectroscopic properties and energy transfer in Yb3+/Er3+ codoped tellurite glasses

Spectroscopic properties of Yb3+/Er3+ codoped tellurite glasses as a function of Er3+ and Yb3+ concentration have been investigated. Under 970 nm excitation three strong up-conversion emission bands centred at 525, 546 and 656 nm were observed, and the characteristic near infrared emission band was centred at 1.53 µm. With fluorescence and radiative lifetime the quantum efficiency (QE) of infrared (1.53 µm) and visible upconversion (546 and 660 nm) emissions was calculated. The maximum stimulated emission cross section for 4I13/2 → 4I15/2 transition of Er3+ is 9.7 × 10−21 cm2 for 3/0.5 mol%. The energy transfer (ET) efficiency from Yb3+ to Er3+ (4F5/2) + (4I15/2) → (4 F7/2) + (4I13/2) was calculated, being the maximum ET of 69% for 0.5 mol% of Er3+ with 4.5 mol% of Yb3+. The results indicate that both ET and QE depend mostly on Er3+ rather than on Yb3+ concentration.

Spectroscopic properties and energy transfer in Yb3+/Er3+-doped phosphate glasses

Abstract The phosphate glasses with high Yb3+/Er3+ dopant were prepared. The absorption and fluorescence emission spectra were measured, and the emission cross-section for 4 I 13/2 → 4 I 15/2 transition of Er3+ was calculated on the basis of McCumber theory. The energy transfer (ET) and the effect of changing concentrations of Er3+ and Yb3+ ions on spectroscopic properties were investigated, pumping at 970 nm. The optimizations of Yb2O3 and Er2O3 concentration were found in our glasses. The influence of –OH groups on the ET and the emission at 1.54 μm were also discussed.

Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.