Two-photon Upconversion Process Research Articles

Color tunable luminescence in ThO2:Er3+,Yb3+ nanocrystals: a promising new platform for upconversion.

Lanthanide-doped luminescent nanoparticles are an appealing system for many applications in the area of biomedical, solar cell, thermometry, anti-counterfeiting, etc. due to their sensitivity, reliability, high photochemical stability, and high optical transparency in the visible-NIR range. A color-tunable upconversion-luminescence (UCL) in a new low phonon energy ThO2 host based on modulating sensitizer concentration has been realized in this work and it may work as a potential candidate to replace corrosive and toxic fluoride based hosts in the future. Er3+-Yb3+ co-doped thoria nanoparticles were prepared using a gel combustion route and their structural and luminescence properties were determined as a function of the Yb3+ concentration. Phonon dispersion measurements have established the dynamic structural stability of the thoria nanoparticles. Density functional theory (DFT) was used to calculate the defect formation energy, highlighting the feasibility of dual ion (Er3+ and Yb3+) doping in thoria. The morphology and average size of the doped thoria was studied using high resolution transmission electron microscopy (HRTEM), and any defects evolving as a result of aliovalent doping were probed using positron annihilation lifetime spectroscopy (PALS). With 980 nm laser excitation, the nanothoria emits green and near-red light. A significant enhancement of the red-to-green intensity ratio of Er3+ ions in nanothoria was observed with an increase in Yb3+ concentration which resulted in beautiful color tunability from green to yellow light in going from lower (up to ∼5 mol%) to higher (10 and 15 mol%) Yb3+ concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two-photon upconversion processes for the green and red emissions.

Intense green up/down conversion emission and high-sensitivity optical thermometry of NaYb(WO4)2:Er3+ stoichiometric phosphors

Optical thermometry based on fluorescence intensity ratio (FIR) has garnered significant interest among researchers due to its rapid, non-contact, and high sensitivity characteristics. In this study, stoichiometric NaYb(WO4)2 phosphors, doped with varying concentrations of Er3+, were successfully synthesized using the high-temperature solid-state method. Under both 980 nm and 266 nm excitation, the samples exhibited strong green and very weak red upconversion (UC) and downconversion (DC) luminescence. The structural and morphological properties of the prepared samples were thoroughly characterized using techniques such as X-ray diffraction (XRD), XRD Rietveld refinement, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. The phosphors demonstrated high-purity UC and DC emission, with maximum green-to-red ratios of 60.9 and 77.0, respectively. This discovery opens up vast possibilities for their application as pure green color converters. The UC emission of Er3+ ions in the stoichiometric NaYb(WO4)2 phosphor was identified as a two-photon UC process, as evidenced by the relationship between the luminescence intensity and the excitation power of the sample. Moreover, the temperature-dependent emission spectra of UC and DC were monitored, and investigated the fluorescence intensity ratios (FIRs) of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions to assess the optical temperature sensing behavior of NaYb(WO4)2:Er3+. The experimental results revealed that the maximum absolute sensitivities (SA) of NaYb(WO4)2 phosphors were 0.0120 K-1 (UC) and 0.0077 K-1 (DC), and the maximum relative sensitivities (SR) were 0.0089 K-1 (UC) and 0.0092 K-1 (DC), respectively. These findings highlight the promising of Er3+-doped Yb3+ stoichiometric NaYb(WO4)2 phosphors in optical temperature sensors.

Utilizing Energy Transfer in Mn2+/Ho3+/Yb3+ Tri-doped ZnAl2O4 Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry.

Lanthanide (Ln3+)-doped upconversion (UC) phosphors converting near-infrared (NIR) light to visible light hold very high promise toward biomedical applications. The scientific findings on luminescent thermometers revealed their superiority for noninvasive thermal sensing. However, only few reports showcase their potential for applications in extreme conditions (temperatures below -70 °C) restricted by low thermal sensitivity. Here, we demonstrate the tailoring of luminescence properties via introducing Ho3+-Mn2+ energy transfer (ET) routes with judicious codoping of Mn2+ ions in ZnAl2O4/Ho3+,Yb3+ phosphor. Preferentially, a singular red UC emission is required to improve the bioimaging sensitivity and minimize tissue damage. We could attain UC emission with 94% red component by a two-photon UC process. Higher temperature annealing brings the color coordinates to the green domain, highlighting the potential for color-tunable luminescence switch. Moreover, this work investigates the thermometric properties of ZnAl2O4/Yb3+, Ho3+ in the range of 80-300 K and influence of inducing extra ET pathways by Mn2+ codoping. Interestingly, the luminescence intensities for nonthermally coupled (5F4,5S2) and the 5F5 radiative transitions of Ho3+ ions display opposite behavior at 80 and 300 K, which revealed competition between temperature-sensitive decay pathways. The codoping of Mn2+ ions is fruitful in causing a fourfold increase of absolute sensitivity. Notably, the color tunability from green through yellow to red is helpful in rough temperature estimation by naked eyes. The maximum relative (Sr) and absolute sensitivities (Sa) were estimated to be 1.89% K-1 (140 K) and 0.0734 K-1 (300 K), respectively. Even at 80 K, a Sa of 0.00447 K-1 and Sr of 0.6025% K-1 were achievable in our case, which are higher than most of the other Ln3+-based systems. The above-mentioned results demonstrate the potential of ZnAl2O4/Yb3+,Ho3+ for cryogenic optical thermometry and a strategy to design new Ln3+-based UC thermometers by taking advantage of ET routes.

Structure, morphology, photonconversion and energy transfer characteristics of Er3+/Yb3+:BaYF5 nanocrystals synthesized by hydrothermal method for photovoltaics

Lanthanide doped nanocrystalline fluoride optical materials have been explored for diverse applications in the solar energy harvesting, industry, medical, forensic and biological fields because of high photoluminescence quantum yield and low phonon energy. In this manuscript, Er3+/Yb3+:BaYF5 nanocrystals were synthesized by hydrothermal process. These nanocrystals were investigated through X-ray diffraction (XRD), SEM/TEM, Raman, quantum yield, photoluminescence upconversion (UC) and downconversion (DC) studies. XRD patterns exhibited the cubic phase and no phase deviation with the addition of Er3+ and Yb3+ ions. The TEM images illustrate the cubic-like morphologies of the obtained nanocrystals. Raman spectrum unveils the characteristic vibrational bands with an intense band at 498 cm−1, which is considered the phonon energy of the BaYF5 matrix. Under 488 nm excitation, the near infrared (NIR) emission of Er3+ increases up to 5 mol% and then decreases for a 10 mol% doped sample. Photoluminescence DC quantum yield of Er3+/Yb3+:BaYF5 sample is measured by using integrating sphere in the visible region under 488 nm (Xe flash lamp) excitation, is 4.25%. Under 900 nm laser excitation, intense NIR emissions at 978 and 1550 nm corresponding to Yb3+ (2F5/2 → 2F7/2) and Er3+ (4I13/2 → 4I15/2) ions, respectively, were observed. With the increase of Yb3+ concentration, the NIR emissions of both Er3+ and Yb3+ increase to 5 mol% and then decrease to 10 mol%. The strength of green UC emission of Er3+ decreases with increase of Er3+ concentration, whereas it enhances with the rise of Yb3+ concentration under 980 nm laser excitation. Two-photon UC process is confirmed as the slopes of the green, and red intensities are 2 and 2. These nanocrystals exhibit intense UC and DC emissions under 980 nm and 480 nm, respectively, which can be suitable for improving the efficiency of solar cells.

Frequency-Domain Method for Characterization of Upconversion Luminescence Kinetics.

The frequency-domain (FD) method provides an alternative to the commonly used time-domain (TD) approach in characterizing the luminescence kinetics of luminophores, with its own strengths, e.g., the capability to decouple multiple lifetime components with higher reliability and accuracy. While extensively explored for characterizing luminophores with down-shifted emission, this method has not been investigated for studying nonlinear luminescent materials such as lanthanide-doped upconversion nanoparticles (UCNPs), featuring more complicated kinetics. In this work, employing a simplified rate-equation model representing a standard two-photon energy-transfer upconversion process, we thoroughly analyzed the response of the luminescence of UCNPs in the FD method. We found that the FD method can potentially obtain from a single experiment the effective decay rates of three critical energy states of the sensitizer/activator ions involved in the upconversion process. The validity of the FD method is demonstrated by experimental data, agreeing reasonably well with the results obtained by TD methods.

Energy transfer between optically trapped single ligand-free upconversion nanoparticle and dye

The quenching in luminescence emission of an optically trapped ligand-free hydrophilic NaYF4:Yb, Er upconversion nanoparticle (UCNP) as a function of rose Bengal dye molecule is investigated here. The removal of oleate capping of the as-prepared UCNPs was achieved via acid treatment and characterized via FTIR and Raman spectroscopic techniques. Further, the capping removed hydrophilic single UCNP is optically trapped and the emission studies were carried out as a function of excitation laser power. Compared to the studies using the bulk solution, the single UCNP luminescence spectrum exhibited additional spectral lines. The excitation laser power-dependent studies using the bulk solution yield a slope value between 1 and 2 for Blue, Green 1, Green 2, and Red emission and thus indicate that upconversion is a two-photon upconversion process. On the other hand, in the case of laser power-dependent studies on an optically trapped single-particle study, Blue and Green 1 yield a slope value of less than 1 whereas Green 2 and Red emission gave a slope value between 1 and 2. The energy transfer studies between an optically trapped ligand-free single UCNP and the rose Bengal dye show a concentration-dependent quenching in the emission of Green emissions and illustrate the potential of developing sensor platforms.

Quantum yield and energy transfer in up-conversion SrGd2O4:Yb, Er nanoparticles obtained via sol-gel assisted combustion

Nanopowders of up-conversion SrGd2O4 orthorhombic (Pnma) phase co-doped with different Yb3+ (1, 2.5 and 5 at%) and constant Er3+ (0.5 at%) ions were successfully prepared via sol-gel assisted combustion. Rietveld refinement indicated unit cell lattice parameters increase with Yb3+ and Er3+ ions doping. Scanning transmission electron microscopy with corresponding energy-dispersive X-ray spectroscopy revealed that obtained powders are composed of agglomerated nanoparticles that have a uniform distribution of all constituting elements. Photoluminescence measurements implied intensification of the up-conversion (UC) emission in the visible part of spectrum with the increase of Yb3+ content, which is followed by a significant change in the green to red ratio. Two-photon UC processes are established as a result of Er3+ f-f electronic transitions: green emission at 523 and 551 nm (2H11/2, 4S3/2 → 4I15/2) as well as a red emission at 661 nm (4F9/2 → 4I15/2). The highest value of absolute quantum yield of 0.055% is determined for SrGd2O4 nanoparticles doped with 0.5 at% of Er3+ and co-doped with 5 at% of Yb3+ (λexc = 976 nm, power density 200 W/cm2).

Luminescent (Er,Ho)2O3 thin films by ALD to enhance the performance of silicon solar cells

We have fabricated luminescent (Er,Ho)2O3 thin films by atomic layer deposition (ALD) and studied their capability to enhance the performance of state-of-the-art single-junction c-Si bifacial solar cells. The films convert IR photons (e.g. 1523 nm) by three- and two-photon upconversion process to emit visible-light in the 400–700 nm range. When the films were coupled with solar cells, ~3% improvement in the short-circuit current density (620 ± 5 to 638 ± 5 mAcm−2) was recorded under a simulated solar excitation equivalent to 16 suns. These findings highlight a potential of ALD for the design and fabrication of luminescent coatings for practical solar cell devices.

Facile synthesis of accordion-like Y2O3:Er3+ nanothermometers for ratiometric temperature sensing applications

Non-contact thermometry operating at the sub-micrometer level based on self-referencing upconversion (UC) micro-nanomaterial is an attractive pursuit for optical thermometers. Here, a novel accordion-like Y2O3:Er3+ nanothermometer with green/red UC luminescence is obtained from the solvent-free grinding and the following temperature-programmed calcination of the Er3+-doped Y-containing metal-organic framework (MOF) precursor. Upon 980 nm laser irradiation, the as-prepared Y2O3:Er3+ exhibits dual-emitting green and red emissions originating from Er3+ ions with a two-photon UC process, and the intensities are strongly dependent on temperature. The optical thermometry behaviours of Y2O3:Er3+ nanothermometer using the fluorescence intensity ratio (FIR) technique are studied in the temperature range of 298–393 K within the physiological temperature, in which the thermally coupled levels of two green emissions and another pair of coupled levels with red and green emissions are comparatively investigated. A relative large calculated energy gap ΔE (951 cm−1) for the thermally couple levels of 2H11/2 and 4S3/2 and a maximum absolute sensitivity (Sa) of 0.0027 K−1 at 393 K are determined, respectively. This novel accordion-like Y2O3:Er3+ nanothermometer exhibits excellent optical sensing stability and high temperature resolution, demonstrating its great potential as reliable and promising optical thermometers.

Critical Power Density: A Metric To Compare the Excitation Power Density Dependence of Photon Upconversion in Different Inorganic Host Materials

In photon upconversion (UC) based on triplet-triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet-triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb3+ sensitizer ions and 2 mol % Er3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

Synthesis, characterization and upconversion luminescence of core-shell nanocomposites NaYF4: Er/Yb@SiO2@Ag/Au

The improvement in the upconversion (UC) luminescence due to the surface plasmonic properties of precious metals is an effective mode to increase UC luminescence efficiency. In the present research work, pure hexagonal-phase NaYF4: Yb, Er UC nanocrystals were prepared via a reverse micro emulsion method. A single-step method was adopted to change the hydrophobic surfaces of the as-prepared NaYF4: Yb, Er nanocrystals. After the surface modification, Au/Ag nanoparticles were decorated on the surfaces of NaYF4:Yb, Er nanocrystals. The structural analysis and composition of the nanocomposites were studied in details. The XRD pattern indicated the formation of the hexagonal structure of NaYF4 crystals. The decorated UC nanocrystals exhibited a prominent UC emission in green and red bands under 980 nm laser excitation due to normal two-photon UC process. The UC intensity increased by ∼3 times than the uncoated samples. The enhancement mechanism of UC luminescence was investigated in details.

Upconversion luminescence properties and thermal quenching mechanisms in the layered perovskite La1.9Er0.1Ti2O7 towards an application as optical temperature sensor

The structural evolution between the monoclinic (layered-perovskite) and cubic (pyrochlore) structures is studied for the complete series of La2-xErxTi2O7 synthetized by solid-state reaction. A partial solid solution with layered perovskite structure is achieved until x = 0.10. The luminescence properties of these oxides show a low quantum efficiency and the concentration quenching is observed for x = 0.10. However, La1.9Er0.1Ti2O7 oxide exhibits interesting UpConversion (UC) properties at multiple excitation wavelengths (784, 808 and 980 nm). Laser power studies have demonstrated a two-photon upconversion process for 808 and 980 nm excitations. Moreover, the measurement of the lifetimes on the 4F9/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions makes it possible to identify an Energy Transfer (ET) mechanism. The thermal effect on upconversion properties is also studied under 808 nm excitation. The UC intensity shows atypical evolutions: 4F9/2 → 4I15/2 transition follows a continuous decrease as a function of temperature, while 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 transitions reach maxima at 150 and 450 K, respectively. The calculation of temperature dependent Fluorescence Intensity Ratio and CIE 1931 color coordinates indicated that La1.9Er0.1Ti2O7 oxide could serve as a temperature sensor.

RETRACTED: Near-infrared-to-visible upconverting luminescence of Er3+-doped CdSe nanocrystals grown by chemical bath

Abstract CdSe thin films were prepared by chemical bath and doped in situ with Er3+ ions. Three impurity levels were prepared by changing the relative volume of the salt solution containing Er3+ ions in the CdSe growing solution. Changes in the grain size (∼5.5–3.5 nm) and band gap energy (∼1.80–2.25 eV) were observed in the undoped and doped CdSe films, respectively. Photoluminescence studies displayed room temperature emission exhibiting NIR-to visible upconversion. The transition bands 4I13/2 → 4I15/2, 2I9/2 → 4I15/2 and 2F9/2 → 4F15/2 in the ∼700–850 nm region were investigated. Upconversion emissions were observed from the CdSeEr sample under light excitation (325 nm). The upconversion emission intensity ratio of these transitions is attributed to the variation of the local structure around Er3+ ions. These results confirm that visible upconversion emissions of Er3+ in the CdSeEr nanocrystals are mainly produced via two-photon excited-state absorption and energy transfer upconversion processes.

Upconversion luminescence of (Lu,M)NbO4:Yb3+,Er3+ (M: Al3+, Ga3+)

The effects of substitution of Al3+ and Ga3+ ions on the upconversion luminescence of (Lu,M)NbO4:Yb3+,Er3+ (M: Al3+, Ga3+) were studied for the first time. Under infrared (980nm) radiation, the upconversion spectra consisted of strong green and weak red emission bands, which were assigned to the transition of the 2H11/2/4S3/2 and 4F9/2 levels, respectively, to the ground state (4I15/2) of the Er3+ ions through an energy transfer process from Yb3+ to Er3+. A two-photon upconversion process was involved in the upconversion emission of (Lu,M)NbO4:Yb3+,Er3+. Substitution of Al3+ and Ga3+ ions increased the green (red) emission intensity to approximately 120% (120%) and 135% (130%), respectively. These behaviors were explained in terms of the crystal field asymmetry and energy transfer probability.

The upconversion luminescence and magnetism in Yb3+/Ho3+ co-doped LaF3 nanocrystals for potential bimodal imaging

Biocompatible upconversion nanoparticles with multifunctional properties can serve as potential nanoprobes for multimodal imaging. Herein, we report an upconversion nanocrystal based on lanthanum fluoride which is developed to address the imaging modalities, upconversion luminescence imaging and magnetic resonance imaging (MRI). Lanthanide ions (Yb3+ and Ho3+) doped LaF3 nanocrystals (LaF3 Yb3+/Ho3+) are fabricated through a rapid microwave-assisted synthesis. The hexagonal phase LaF3 nanocrystals exhibit nearly spherical morphology with average diameter of 9.8 nm. The inductively coupled plasma mass spectrometry (ICP-MS) analysis estimated the doping concentration of Yb3+ and Ho3+ as 3.99 and 0.41%, respectively. The nanocrystals show upconversion luminescence when irradiated with near-infrared (NIR) photons of wavelength 980 nm. The emission spectrum consists of bands centred at 542, 645 and 658 nm. The stronger green emission at 542 nm and the weak red emissions at 645 and 658 nm are assigned to 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+, respectively. The pump power dependence of luminescence intensity confirmed the two-photon upconversion process. The nanocrystals exhibit paramagnetism due to the presence of lanthanide ion dopant Ho3+ and the magnetization is 19.81 emu/g at room temperature. The nanocrystals exhibit a longitudinal relaxivity (r 1) of 0.12 s−1 mM−1 and transverse relaxivity (r 2) of 28.18 s−1 mM−1, which makes the system suitable for developing T2 MRI contrast agents based on holmium. The LaF3 Yb3+/Ho3+ nanocrystals are surface modified by PEGylation to improve biocompatibility and enhance further functionalisation. The PEGylated nanocrystals are found to be non-toxic up to 50 μg/mL for 48 h of incubation, which is confirmed by the MTT assay as well as morphological studies in HeLa cells. The upconversion luminescence and magnetism together with biocompatibility enables the adaptability of the present system as a nanoprobe for potential bimodal imaging.

Up-conversion luminescence and optical temperature sensing behaviour of Yb3+/Er3+ codoped CaWO4 material

Present article report on structural and optical properties of Er3+/Yb3+ codoped CaWO4 phosphors. Structural properties are explored using XRD and Raman technologies. The upconversion emission has been investigated with 980 nm excitation. The upconversion emission intensity is dependent on the concentrations of Yb3+ ions and reaches a maximum at 7%. Logarithmic plots of power dependencies reveal that the green and red emissions originate from a two-photon upconversion process. Based on the photon energy and the emission spectra, the possible upconversion processes and emission mechanisms are discussed. Finally, the optical temperature sensing properties has been performed using the fluorescence intensity ratio technique based on green upconversion emissions. Its temperature sensitivity is found to be above 0.0025 K-1 in the whole temperature range of 300–540 K, revealing this phosphor to be a promising optical temperature sensing material.

Two-photon upconversion affected by intermolecule correlations near metallic nanostructures

We investigate an efficient two-photon up-conversion process in more than one molecule coupled to an optical antenna. In the previous work [Y. Osaka et al., PRL 112, 133601 (2014)], we considered the two-photon up-conversion process in a single molecule within one-dimensional input-output theory, and revealed that controlling the antenna-molecule coupling enables the efficient up-conversion with radiative loss in the antenna suppressed. In this work, aiming to propose a way to enhance the total probability of antenna-photon scattering, we extend the model to the case of multiple molecules. In general, the presence of more than one molecule decreases the up-conversion probability because they equally share the energy of the two photons. However, it is shown that we can overcome the difficulty by controlling the inter-molecule coupling. Our result implies that, without increasing the incident photon number (light power), we can enlarge the net probability of the two-photon up-conversion.

Infrared-to-Visible Light Conversion in Er(3+) -Yb(3+) :Lu3 Ga5 O12 Nanogarnets.

Er(3+) -Yb(3+) co-doped Lu3 Ga5 O12 nanogarnets were prepared and characterized; their structural and luminescence properties were determined as a function of the Yb(3+) concentration. The morphology of the nanogarnets was studied by HRTEM. Under 488 nm excitation, the nanogarnets emit green, red, and near-infrared light. The decay curves for the ((4) S3/2 , (2) H11/2 ) and (4) F9/2 levels of the Er(3+) ions exhibit a non-exponential nature under resonant laser excitation and their effective lifetimes are found to decrease with an increase in the Yb(3+) concentration from 1.0 to 10.0 mol %. The non-exponential decay curves are well fitted to the Inokuti-Hirayama model for S=8, indicating that the mechanism of interaction for energy transfer between the optically active ions is of dipole-quadrupole type. Upon 976 nm laser excitation, an intense green upconverted emission is clearly observed by the naked eyes. A significant enhancement of the red-to-green intensity ratio of Er(3+) ions was observed with an increase in Yb(3+) concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two-photon upconversion processes for the green and red emissions.

Preparation and Upconversion Luminescence Characterization of Eu3+/Yb3+ Codoped 12CaO · 7Al2O3 Ceramics

Eu3+/Yb3+ codoped 12CaO · 7Al2O3 (C12A7) ceramics were prepared via high temperature solid-state reaction method, and characterized by X-Ray powder diffraction. Under the excitation of 980 nm laser radiation, efficient upconversion emissions were observed in 450–900 nm region and attributed to the 5DJ→7FJ transitions of Eu3+ ions. The proposed upconversion emission mechanism involves the two-photon cooperative upconversion process. The effects of Yb3+ ions on the upconversion emission follow a concentration-dependent manner. The Commission Internationale de I'eclairage (CIE) chromaticity coordinates (x, y) illustrate the color of emissions can be tunable to purer green by increasing the Yb3+ concentration.

Strong emission via up-conversion of Gd2O3:Yb3+, Ho3+ nanopowders co-doped with alkali metals ions

Gadolinium-oxide nanopowders doped with Yb3+ and Ho3+ ions and co-doped with alkali metal ions (Li+ and Na+) were synthesized via the polymer complex solution method and their up-conversion emission properties are analyzed. Prepared powders were well crystallized in cubic bixbyite structure, without any impurity phases and with an average crystallite size of about 25nm, as shown by X-ray powder diffraction analysis. No adsorbed species on the surface of nanoparticles were detected by infrared measurements. The particle morphology was analyzed by transmission electron microscopy, which revealed agglomerated crystalline nanoparticles with irregular, polygonal-like shapes, having a size in the range from 30 to 50nm. Up-conversion emission properties of Gd2O3:Ho3+,Yb3+ nanopowders were evaluated from photoluminescence emission and decay measurements using 980nm excitation. Two-photon up-conversion processes produced emissions in visible – near infrared spectral range, with the strongest green emission from 5F4,5S2→5I8 Ho3+ electronic transition, and weak red and near infrared from 5F5→5I8 and 5F4, 5S2→5I7 transitions, respectively. In addition, three-photon up-conversion emission was detected in ultra-violet–blue spectral region (5G4→5I8, 3D3→5I6 and 5G5→5I8 transitions). Luminescence measurements were performed on samples with different Ho3+ (0.5 and 1at%) and Yb3+ (2, 4 and 6at%) concentrations to find that 0.5at% of Ho3+ and 2at% of Yb3+ provides the most intense up-conversion emission. The influence of alkali metal (Li+ and Na+) ion co-doping is studied on the sample that showed the most intense up-conversion emission. Increase in the intensity of up-conversion emission is observed for both Li+ and Na+ co-doping. The effect is more pronounced with Li+, where addition of 5at% of Li+ led to more than one order of magnitude emission enhancement. Alkali metal ion co-doping also significantly improved up-conversion color purity through considerably larger increase of green emission compared to red and near infrared.