Intense Red Upconversion Emission Research Articles

Study of Electrical Properties and Luminescence of Conventional Furnace and Microwave-Sintered Er3+/Yb3+ Co-doped (K, Na)NbO3 Ceramics

Yb2O3 doped (K0.5Na0.5)NbO3–0.5% Er2O3 (KNNEr) ceramics were synthesized by microwave sintering. (K, Na)NbO3 co-doped Yb2O3/Er2O3 ceramic exhibited enhanced piezoelectric and ferroelectric properties owing to the donor doping effect. The optimal performances of d 33 = 115 pC/N, k p = 0.37, e r = 620 and tanδ = 4.9% were obtained for the ceramics with the addition of 0.25 mol% Yb2O3. The green and red up-conversion emission intensities of KNNEr ceramics excited under 980 nm increased with the increasing content of Yb2O3. The results indicated that the KNNEr-Yb2O3 ceramics have potential application of multifunctional device by combining its excellent luminescence and electrical properties.

Study on optical properties and upconversion luminescence of Er^3+/Yb^3+ co-doped tellurite glass for highly sensitive temperature measuring

Er3+/Yb3+ co-doped tellurite glass is prepared by conventional melt-quenching method. The thermal and optical properties of the glass are investigated for temperature sensing. The Judd-Ofelt theory is applied for spectral analysis and the obtained intensity parameters (Ω2 = 6.48 × 10−20 cm2, Ω4 = 1.82 × 10−20 cm2, Ω6 = 1.27 × 10−20 cm2) are used to estimate the spectroscopic parameters of Er3+ ions in the glass. The upconversion (UC) luminescence of the glass is investigated under the laser diode (LD) excitation at 976 nm. The absolute quantum yield for UC luminescence is determined to be 0.0049% when the pump power density is 10 W/cm2. The dependences of green and red UC emission intensities on pump power indicate that both of the red and green UC emissions of Er3+ ions are mostly contributed by the co-existing two and three-photon involved energy transfer processes between the Yb3+ and Er3+ ions. Furthermore, the temperature-dependent green UC emissions of the glass are studied and the results show that the glass is an excellent candidate for construction of temperature sensors based on the self-referenced fluorescence intensity ratio (FIR) technique.

Improvement of Green Upconversion Monochromaticity by Doping Eu3+ in Lu2O3:Yb3+/Ho3+ Powders with Detailed Investigation of the Energy Transfer Mechanism.

The monochromaticity improvement of green upconversion (UC) in Lu2O3:Yb3+/Ho3+ powders has been successfully realized by tridoping Eu3+. The integral area ratio of green emission to red emission of Ho3+ increases 4.3 times with increasing Eu3+ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb3+/Ho3+/Eu3+ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho3+5F4/5S2 level tothe Eu3+5D0 level and ET from the Ho3+5I6 level to the Eu3+7F6 level. In addition, the population routes of the red-emitting Ho3+5F5 level in the Yb3+/Ho3+ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb3+2F5/2 level to the Ho3+5I7 level and the cross-relaxation process between two nearby Ho3+ ions in the 5F4/5S2 level and 5I7 level, respectively, have been demonstrated to be the dominant approaches for populating the Ho3+5F5 level. The multiphonon relaxation process originating from the Ho3+5F4/5S2 level is useless to populate the Ho3+5F5 level. As the energy level gap between the Ho3+5I7 level and Ho3+5I8 level matches well with that between Eu3+7F6 level and Eu3+7F0 level, the energy of the Ho3+5I7 level can be easily transferred to the Eu3+7F6 level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho3+5F4/5S2 level to the Eu3+5D0 level as well as the ET from the Ho3+5I6 level to the Eu3+7F6 level, the integral area ratio of green emission to red emission of Ho3+ has been improved significantly.

Tuning upconversion fluorescence emission of -NaLuF4:Yb3+/Ho3+ nanocrystals through codoping Ce3+ ions

Rare-earth-doped up-conversion (UC) fluoride materials have been widely used in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging, due to the fact that their low phonon energy can effectively suppress the nonradiative multiphonon relaxation process. In this work, the NaLuF4:Yb3+/Ho3+ nanocrystals are successfully synthesized by a facile solvothermal method. The crystal structure and morphology of the NaLuF4 nanocrystals are characterized by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) respectively. The diffraction peaks are well consistent with those of high-purity hexagonal NaLuF4 (JCPDS No. 77-2042, P63/m space group). The TEM image reveals that the product is composed of monodisperse hexagonal rods with an average length of about 170 nm and an average diameter of 30 nm. The crystal structure and morphology do not present obvious change with the increasing Ce3+ ion concentration, which is due to the similarity in ion radius between Ce3+ and Lu3+. Under 980 nm excitation, the UC emissions of -NaLuF4:Yb3+/Ho3+ nanocrystals with different Ce3+ codoping concentrations are carefully studied. The strong green and red UC emissions of Ho3+ ions are observed in -NaLuF4 nanocrystals. It can be found that the UC emission of Ho3+ ions is tuned from green to red in -NaLuF4 nanocrystals through increasing Ce3+ ion concentrations from 0 to 12%, and the red-to-green (R/G) ratio is enhanced from 0.34 to 8.44. According to the level structure of Ho3+ ions, the red UC emission originates from the excited state 5F5. However, the population of the 5F5 excited state mainly depends on the two nonradiative relaxation processes of 5S2/5F45F5 and 5I65I7 transitions. In fact, the two nonradiative relaxation processes are very difficult to occur according to multiphonon nonradiative relaxation rate. When Ce3+ ion is introduced into the system, the red UC emission intensity and R/G ratio of Ho3+ are increased, because the energy gap from the excited state 5F7/2 to the ground state 2F5/2 is about 3000 cm-1 for Ce3+ ions, which is similar to the gaps of 5S2/5F45F5 and 5I65I7 transitions of Ho3+ ions. According to the energy conservation law, the two inefficient nonradiative processes from the 5S2/5F4 and 5I6 states of Ho3+ ions are substituted in order by resonant cross relaxation (CR) processes 5S2 (5F4) (Ho3+) + 2F5/2 (Ce3+5F5 (Ho3+) + 2F7/2 (Ce3+) and 5I6 (Ho3+) + 2F5/2 (Ce3+)5I7 (Ho3+) +2F7/2 (Ce3+) between Ho3+ and Ce3+ ions. These two resonant CR processes can transfer populations from the 5S2/5F4 state and 5I6 state to the 5F5 state and its intermediate 5I7 state, respectively. The resonant modality and the strong interaction between Ho3+ and Ce3+ ions are employed to enhance the red emission and suppress the green emission. The occurrence of CR process between Ho3+ and Ce3+ ions is further proved by the down-conversion emission spectra of Ho3+ ions under 532 and 980 nm laser excitation, respectively. We demonstrate that the highly efficient red UC emission of -NaLuF4:Yb3+/Ho3+/Ce3+ nanocrystals offers opportunities as desired optical materials for color displays, anticounterfeiting techniques and multiplexed labeling applications.

Improved luminescence behavior in Er3+ doped KNbO3 perovskite structure ceramics

To investigate the effect of Er3+ doping on the luminescence behavior of perovskite structure ceramics, the Er2O3 doped KNbO3 smart ferroelectric ceramics have been synthesized by the conventional solid-state reaction. The results of luminescence property demonstrated that the green and red up-conversion emission intensities of these ceramics excited under 980nm increased with the content of Er2O3 and achieved the maximum value with Er2O3 of 0.75mol%. The ratio of green and red emission intensity could also be adjusted by changing the content of Er2O3, and the mechanism of up-conversion processes involved a two-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNbO3, this kind of material may render potential application as a multifunctional device by integrating its up-conversion and piezoelectric property.

Color-tunable up-conversion luminescence of Er3+/Yb3+ co-doped (K,Na)NbO3 ceramics

To investigate the effect of Yb3+ doping on the up-conversion emissions, the Yb3+/Er3+ co-doped (K,Na)NbO3 smart ferroelectric ceramics have been synthesized by the conventional solid-state reaction. The results here illustrated that the green and red up-conversion emission intensities of these ceramics excited under 980nm increased with the increasing content of Yb2O3 and achieved the maximum value at x=0.5%. The ratio of green and red emission intensity could be adjusted by changing the content of Yb2O3, and the mechanism of up-conversion processes involved a two-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNN, this kind of material may have potential application as a multifunctional device by integrating its up-conversion and piezoelectric property.

Selectively enhanced up- and down-conversion emissions of Er3+ via Yb3+-Mn2+ dimer sensitizing in spinel MgGa2O4:Er3+,Yb3+,Mn2+

Selectively enhanced green upconversion (UC) and near-infrared (NIR) downconversion (DC) emissions of Er3+ by Yb3+-Mn2+ dimer sensitizing have been demonstrated in Er3+/Yb3+/Mn2+ tri-doped spinel MgGa2O4 upon excitation of 980nm laser diode (LD). By optimizing the doping rate of Mn2+, the intensity ratio of green to red UC emission intensities of Er3+ (G/R) could be enhanced about 10 times. Meanwhile, the 1.5μm NIR DC emission could also be increased about 3.6 times in this system. The DC and UC emission properties, UC mechanism, as well as the enhancing mechanism have been investigated and discussed in detail. A selectively enhancing mechanism based on Yb3+-Mn2+ dimer sensitizing is proposed to explain the selectively enhancing phenomena. This work could inspire the design of novel UC materials for various applications, such as temperature sensors and optical transmission, etc.

Effect of Mn2+ ions on the enhancement red upconversion emission and energy transfer of Mn2+/Tm3+/Yb3+ tri-doped transparent glass-ceramics

The effects of Mn2+ ions on the enhancement red upconversion emission intensity of Mn2+/Tm3+/Yb3+ tri-doped transparent glass-ceramics were investigated. The ratio I698nm/I478nm significantly increases about ten-fold with the increase of Mn2+ concentrations up to 4.0mol.%. The red upconversion emission of Mn2+/Tm3+/Yb3+ tri-doped transparent glass-ceramics is greatly enhanced, due to constitution of the energy transfer bridge. The upconversion mechanism, the energy transfer and back energy transfer processes between the (1G4→3H6), (3F2→3H6) transitions of Tm3+ ions and the ∣2F7/2, 4T1〉→∣2F7/2, 6A1〉 transitions of Mn2+–Yb3+ dimer were discussed.

Enhancement of the red upconversion luminescence in NaYF4:Yb3+, Er3+ nanoparticles by the transition metal ions doping

In recent years, the application of upconversion nanoparticles, especially in vivo, has so far largely been hampered due to the strong tissue absorption of short-wavelength light below 600nm. Therefore, it is an urgent need to carry out some new selective approaches to increase red emission efficiency. This work reports an effective strategy to enhance red upconversion emission of NaYF4:Yb3+, Er3+ nanocrystals by doping with the different concentrations of transition metal ions (Mn2+ or Fe3+). The upconversion emission spectra of the samples demonstrated that the red emissions were significantly enhanced and that different transition metal ions had a distinguishable effect on the red upconversion luminescence. Compared with the pure NaYF4:Yb3+, Er3+ nanoparticles, the red upconversion emission intensity of the nanoparticles co-doped with 10mol% Mn2+ was increased by nearly as high as 14 times. In addition, the red emission intensity was increased by about 24 times, which was observed in the sample co-doped with 30mol% Fe3+. The mechanism of the enhancement was discussed detailedly. Both the improvement of the nanoparticles׳ crystallinity and the redistribution of electron cloud around the Er3+ ions after introducing Mn2+ or Fe3+ were confirmed to be the origin of the improvement. The enhancement in red upconversion luminescence intensity makes the upconversion nanoparticles versatile imaging tools for diagnosis.

The electrical, upconversion emission, and temperature sensing properties of Er3+/Yb3+-codoped Ba(Zr0.2Ti0.8)O3–(Ba0.7Ca0.3)TiO3 ferroelectric ceramics

To investigate the Yb3+ doping effect on the upconversion emissions and temperature sensing properties of the Er3+ doped smart ferroelectric ceramics, (Ba0.85Ca0.15)0.994−xZr0.1Ti0.9O3–0.006Er3+/xYb3+ (x=0, 0.005, 0.01, 0.015, 0.025 and 0.035) have been synthesized by the conventional solid-state reaction process. The results illustrate that the ferroelectric and piezoelectric properties decrease with the increment of Yb3+ doping content. The green and red upconversion emission intensities of the ceramics excited under 980nm increase with the increasing Yb3+ content and achieve the maximal value at x=0.015. Based on the fluorescence intensity ratio of green emissions at 525 and 550nm, the temperature sensing properties of the optimized composition of the prepared ceramics (x=0.015) were studied. The ceramics as temperature sensing materials can be operated in a wide temperature range of 173–573K, and the maximum sensitivity achieves 0.0068K−1. It was demonstrated that the comprehensive temperature sensing properties achieved here is better than the previous reported results based on the same technology.

Experimental optimal design of the Er3+/Yb3+ codoped BaGd2ZnO5 phosphor and its upconversion luminescence properties

In order to obtain the Er3+/Yb3+ co-doped BaGd2ZnO5 up-conversion phosphor which has the maximum green and red emission intensity, firstly, the method of homogeneous design rooted in the experimental optimal design is employed to search optimum Er3+/Yb3+doping concentration preliminarily. Next, the quadratic general rotary unitized design is adopted to further optimize the experiment, and the regression equation in green and red emission intensity is established as a function of the doping concentration of Er3+/Yb3+. Finally, the optimal solution, that is, the doping concentration of Er3+/Yb3+ corresponding to the maximum emission intensity, is calculated by genetic algorithm. The optimal Er3+/Yb3+co-doped BaGd2ZnO5 phosphor is synthesized by the conventional high temperature solid state method. The crystal structure of as-prepared products is characterized by X-ray diffraction (XRD), and the results show that all the Er3+/Yb3+co-doped BaGd2ZnO5 phosphors we synthesized are of pure phase. The steady-state up-conversion (UC) emission spectra of products are measured under the excitation of a continuous 980 nm laser diode at different working currents. From the UC luminescent spectra of Er3+/Yb3+co-doped BaGd2ZnO5 phosphor, we can see a red emission centered at 662 nm, two green emissions centered at 551 nm and 527 nm, which are assigned to 4F9/2→4I15/2, 4S3/2→4I15/2 and 2H11/2→$4I15/2 transitions of Er3+ ion, respectively. The dependences of green and red UC emission intensities of optimal samples on working current are investigated, indicating that the red emission and green emission of optimal samples both originate from two-photon process. From the normalized green UC emission spectra, it can be concluded that the experimental laser working current induced temperature variation of samples can be omitted. According to Boltzmann distribution law and the thermal equilibrium existing between the levels of 2H11/2 and 4S3/2, the relationship between green emission and temperature in the optimal green UC emission sample is discussed in depth, and the energy level gap between 2H11/2 level and 4S3/2 level is calculated to be 926.11 cm-1. Through the study of the temperature effect on the optimal green UC emission sample, we find that the emission intensity decreases with the increasing of the temperature, owing to the thermal quenching effect. Furthermore, we calculate the activation energies of the samples, the activation energies of the green emission, red emission, and the overall emission are deduced to be 0.45, 0.46, and 0.45 eV, respectively.

Synergistic Dual-Modality in Vivo Upconversion Luminescence/X-ray Imaging and Tracking of Amine-Functionalized NaYbF4:Er Nanoprobes

In this work, the amine-functionalized NaYbF4:Er nanoparticles were developed as dual-modal nanoprobes for synergistic upconversion (UC) luminescence and X-ray imaging in a single system by a simple one-step method of simultaneous synthesis and surface modification. The water-soluble NaYbF4:Er nanoparticles present excellent green and dominant red UC emissions. The in vitro cell imaging shows that the high-contrast green and intense red UC emissions can be observed from HeLa cells treated with these nanoparticles, indicating the successful labeling of HeLa cells. Moreover, the localized spectra measured from HeLa cells and background presented significant green and dominant red UC emissions with the absence of any autofluorescence, further verifying that these nanoparticles can be successfully used as ideal probes for optical UC bioimaging with high contrast and non-autofluorescence. In addition, the amine-functionalized NaYbF4:Er nanoparticles maintained low cell toxicity in HeLa cells evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. More importantly, these amine-functionalized NaYbF4:Er nanoparticles can also be used as X-ray imaging, owing to the large X-ray absorption efficiency of the Yb ion. The synergistic in vivo UC and X-ray imaging present significant UC luminescence and X-ray signals in the same region of a nude mouse, and the two signals are matched very well, which provides direct evidence for simultaneous UC luminescence and X-ray imaging in a single compound of lanthanide-doped material. Moreover, ex vivo UC imaging shows that these nanoparticles are first accumulated in the lung and gradually translocated from the lung into the liver. These results demonstrate that the amine-functionalized NaYbF4:Er nanoparticles presented here are very attractive nanoprobes for dual-modal UC luminescence and X-ray imaging with low cytotoxicity, autofluorescence free, and synergistic combination of the advantages of the two imaging modalities.

One-step synthesis of water-soluble hexagonal NaScF4:Yb/Er nanocrystals with intense red emission.

Water-soluble hexagonal NaScF4:Yb/Er nanocrystals have been directly synthesized via a facile one-step hydrothermal route. The as-prepared nanocrystals were monodisperse and could form stable colloidal solutions in polar solvents, such as water and ethanol. The novel NaScF4:Yb/Er nanocrystals exhibited intrinsic intense red upconversion emission, in contrast to other lanthanide-doped fluoride nanocrystals. Remarkably, the upconversion emission intensity was enhanced ∼6.8 times through the coating of an active-shell (containing Yb(3+)) surrounding the NaScF4:Yb/Er core nanocrystals, owing to surface passivation effect and efficient energy transfer from Yb(3+) ions in both the core and the shell to Er(3+) ions. These water-soluble hexagonal NaScF4:Yb/Er nanocrystals with intense red emission are ideal for a variety of in vitro and in vivo biological imaging applications.

Intense red upconversion emission and temperature sensing in Er<SUP>3+</SUP>/Yb<SUP>3+</SUP> co-doped Ba<SUB>5</SUB>Gd<SUB>8</SUB>Zn<SUB>4</SUB>O<SUB>21</SUB> phosphor

Intense red upconversion emission and temperature sensing in Er<SUP>3+</SUP>/Yb<SUP>3+</SUP> co-doped Ba<SUB>5</SUB>Gd<SUB>8</SUB>Zn<SUB>4</SUB>O<SUB>21</SUB> phosphor

Hydrothermal Synthesis and Tunable Multicolor Upconversion Emission of Cubic Phase Y2O3Nanoparticles

Highly crystalline body-centered cubic structure Y2O3with lanthanide (Ln) codopants (Ln = Yb3+/Er3+and Yb3+/Ho3+) has been synthesized via a moderate hydrothermal method in combination with a subsequent calcination. The structure and morphology of Y(OH)3precursors and Y2O3nanoparticles were characterized by X-ray diffraction and transmission electron microscopy. The results reveal that the Y2O3nanoparticles possess cubic phase and form the quasispherical structure. The upconversion luminescence properties of Y2O3nanoparticles doped with different Ln3+(Yb3+/ Er3+and Yb3+/ Ho3+) ions were well investigated under the 980 nm excitation. The results show that the Yb3+/Er3+and Yb3+/Ho3+codoped Y2O3nanoparticles exhibit strong red and light yellow upconversion emissions, respectively. It is expected that these Y2O3nanoparticles with tunable multicolor output and intense red upconversion emission may have potential application in color displays and biolabels.

Effects of Bi2O3 on Up-conversion Luminescence Properties of Tm3+/Yb3+-codoped Ga2O3-GeO2-Bi2O3-PbO Glass

Abstract This paper reports on frequency up-conversion (UC) emission properties in Tm 3+ /Yb 3+ -codoped Ga 2 O 3 -GeO 2 -Bi 2 O 3 -PbO glass upon excitation of 980 nm laser diode. A close correlation is observed between the Bi 2 O 3 substituted content for PbO and UC luminescence properties of Tm 3+ . It was found that the decreasing PbO and increasing Bi 2 O 3 content improve the blue and red UC emission intensity. The maximum of UC emission intensity is obtained when substituted PbO content is at 35%. In addition, the dependence of the UC emission intensity upon the excitation power has been examined and the involved mechanisms have also been discussed.

Effect of Yb3+ Concentrations on the Upconversion Luminescence Properties of ZrO2:Er3+,Yb3+ Phosphors

ZrO2:Er3+ and ZrO2:Er3+,Yb3+ phosphors were synthesized via a solvothermal reaction method. For low concentrations of Yb3+, the crystalline structure changed from the tetragonal to monoclinic phase as sintering temperature increased. As the Yb3+ concentration increased to a value above 0.05 mol, ZrO2 phosphors displayed a very stable tetragonal phase. The green and red upconversion emissions of ZrO2:Er3+,Yb3+ phosphors were measured under the excitation with a 975 nm continuous wave diode laser, and the pump power dependence of upconversion intensity was investigated. As the Yb3+ concentration increased from 0 to 0.05 mol, the red upconversion emission intensity increased more rapidly than the green emission intensity. This is attributed to the energy transfer (4I11/2→4I15/24I13/2→4F9/2) between Er3+ ions and the energy back transfer [4S3/24I13/2(Er3+)→2F7/22F5/2(Yb3+)] between the Er3+ and Yb3+ ions. In this case, the pump power dependence of red emission intensity changed from quadratic to linear as Yb3+ concentration increased.

Intense red upconversion emission of Yb/Tm/Ho triply-doped tellurite glasses

By conventional melting and quenching methods, 3Yb2O3-0.2Tm2O3-xHo2O3 (wt%, x=0.2~1.2) was doped into an easily fiberized tellurite glass with composition of 78TeO2-10ZnO-12Na2O (mol%) to form YTH-TZN78 glasses. Under 976 nm excitation, the direct sensitizing effect of Yb ions (Yb→Ho) and indirect sensitizing and self-depopulating effects of Tm ions (Yb→Tm→Ho) were found to present intense red upconversion emission at 657 nm (Red, Ho:5F5→5I8) and were responsible for the absence of the usually observed 484 nm emission (Blue, Tm:1G4→3H36). Regardless of the dopant concentration of Ho ions, the intensity of the red emission at 657 nm (Red, Ho:5F5→5I8) is about three times stronger than that of the green one at 543 nm (Green, Ho:5S2→5I8). For this certain red emission at 657 nm, 0.4 wt% Ho2O3-doped YTH-TZN78 glass was found to present the highest emission intensity and is therefore determined as a promising active tellurite glass for red fiber laser development.

Selective enhancement of green upconversion emissions of Er3+:Yb3Al5O12 nanocrystals by high excited state energy transfer with Yb3+–Mn2+ dimer sensitizing

An innovative upconversion (UC) emissions route of Er3+ by Yb3+–Mn2+ dimer sensitizing in Er3+–Mn2+:Yb3Al5O12 (YbAG) nanocrystals is reported here, which resulted in the selective enhancement of green UC emission and suppression of red UC emission by a 976nm laser diode excitation. By codoping of Mn2+, the green UC emission intensity increased about 260 times, while the red UC emission intensity decreased about 20 times than that of Er3+:YbAG nanocrystals. It indicates that the green enhancement and red suppression arise from the high excited state energy transfer with |2F7/2, 4T1g> (Yb3+–Mn2+ dimer) to the 4F7/2 (Er3+), which partly decreases the nonradiative processes happened in the lower levels of Er3+. The proposed sensitizing route here may constitute a promising step to realize high-efficient UC emissions of rare-earth ions doped oxides and significantly extend their scope of applications.

Synthesis and upconversion luminescent properties of Er 3+ doped and Er 3+–Yb 3+ codoped GdOCl powders

Er 3+ doped and Er 3+–Yb 3+ codoped GdOCl phosphors were prepared by modified solid state reaction. X-ray diffraction, scanning electron microscope, and Raman spectrum of the samples were studied. The result of Raman spectrum shows that the cutoff phonon energy of GdOCl is only 505 cm −1, which is beneficial for upconversion luminescence. Infrared-to-visible upconversion emissions were observed under 980 nm diode laser excitation. It was found that the ratio of green to red upconversion emission intensity varies with concentration of the Er 3+ or Yb 3+ ion. Laser power and doping concentration dependence of upconversion luminescence were studied to understand the upconversion mechanisms. Excited state absorption and energy-transfer processes are proposed to be the possible mechanisms for the visible emissions.