Upconversion properties of Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glasses

The Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glass is prepared by conventional melting method, and its upconversion spectra are measured. The intense green upconversion luminescence upon excitation with a 976 nm laser diode is observed with the naked eyes. The dependence of luminescence intensity on the ratio of Yb^3＋/Er^3＋ is discussed in detail, and the relationship between the ratio of green luminescence intensity to red luminescence intensity and the ratio of Yb^3＋/Er^3＋ is also studied, The luminescence intensity increases with the ratio of Yb^3＋/Er^3＋ increasing. The ratio of Yb^3＋/Er^3＋ plays a more important role than the concentration of Er^3＋ in determining the upconversion luminescence intensity. The ratio of green luminescence intensity to red luminescence intensity reaches a maximum when ratio of Yb^3＋/Er^3＋ is 3. Thus the glass could be one of the potential candidates for LD pumping solid-state lasers.     

Intense Green Upconversion Luminescence in Er^3＋：Yb^3＋ Codoped Fluorophosphate Glass Ceramic Containing SrTe5O11 Nanocrystals

Er^3＋ ：Yb^3＋ codoped tellurite-fluorophosphate （TFP） glass ceramic exhibits much stronger upconversion luminescence. The intensity of the 540nm green light and 651 nm red light of the TFP glass ceramic is 120 times and 44 times stronger than that of the fluorophospahte （FP） glass, respectively. XRD analysis shows that the nanocrystal in TFP glass ceramic is SrTe5O11. TFP glass ceramic also displays much higher upconversion fluorescence lifetime and crystallization stability. The narrow and strong peak at 540nm is very ideal for practical upconversion luminescence realization. This work is a new trial for exploring non-PbF2 involved nanocrystal upconversion glass ceramics.     

High Efficiency Infrared-to-Visible Upconversion Emission in Er^3＋,Yb^3＋, and Ho^3＋ Codoped Tellurite Glasses

A novel method of codoping the Er^3＋, Yb^3＋, and Ho^3＋ ions in tellurite glasses is demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Three intense emission bands observed in Er^3＋, Yb^3＋, and Ho^3＋ codoped tellurite glasses centred at 525, 547, and 657nm correspond to Er^3＋： ^2H11/2 -4 ^4I15/2, Er^3＋： ^4S3/2 →^4I15/2＋Ho^3＋： ^5S2（^5F4） → ^5Is, and Er^3＋： ^4Sa/2 → ^4I15/2＋Ho^3＋： ^5F5 → ^5Is transitions, respectively. No visible upconversion quenching phenomenon is observed when three rare-earth ions are codoped together in tellurite glasses. In contrast, the upconversion intensity of red and green emissions in Er^3＋, Yb^3＋, and Ho^3＋ codoped glasses is enhanced largely when compared with Er^3＋ /Yb^3＋-codoped glasses. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms are evaluated. The three emissions are based on two-photon absorption processes.     

Upconversion Luminescence Dynamics in Er^3＋/Yb^3＋ Codoped Nanocrystalline Yttria

The upconversion luminescence and dynamics in Er^3＋ /Yb^3＋ codoped nanocrystalline yttria （7-65 nm） are studied under 980-nm pulsed laser excitation, It is found that the red emission of ^4F9/2-^4I15/2 and the green emission of ^2H11/2/^4S3/2 in nanoparticles with lower concentration of Yb^3＋ result from a two-photon excitation, In nanocrystals with higher Yb^3＋ concentration, the red emissions from a two-photon excitation, while the green emissions from a three-photon excitation, The luminescence dynamics indicates that as the particle size decreases, both the rise and the decay time constants become shorter, As the size decreases to several nanometres, the rise process nearly disappears, suggesting that the upconversion luminescence originates mainly from self-excitation of Er^3＋, instead of the energy transfer of Yb^3＋→ Er^3＋.     

Spectroscopic properties and mechanism of Tm^3＋/Er^3＋/Yb^3＋ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals

Transparent Tm^3＋/Er^3＋/yb^3＋ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals are prepared. Under excitation of a 980-nm laser diode （LD）, compared with the glass before heat treatment, the Tm^3＋/Er^3＋/yb^3＋ co-doped oxyfluorogermanate glass ceramics can emit intense blue, green and red up-conversion luminescence and Stark- split peaks; X-ray diffraction （XRD） and transmission electron microscope （TEM） results show that BaF2 nanocrystals with an average diameter of 20 nm are precipitated from the glass matrix. Stark splitting of the up-conversion luminescence peaks in the glass ceramics indicates that Tm^3＋, Er^3＋ and （or） Yb^3＋ ions are incorporated into the BaF2 nanocrystals. The up-conversion luminescence intensities of Tm^3＋, Er^3＋ and the splitting degree of luminescence peaks in the glass ceramics increase significantly with the increase of heat treat temperature and heat treat time extension. In addition, the possible energy transfer process between rare earth ions and the up-conversion luminescence mechanism are also proposed.     

Er^3＋/Tm^3＋/Yb^3＋ tridoped oxyfluoride glass ceramics with efficient upconversion white-light emission

A novel Tm^3＋/Yb^3＋ triply-doped glass ceramics containing BaF2 nano-crystals are successfully prepared. Fluoride nanocrystals BaF2 are successfully precipitated in glass matrix, which is affirmed by the X-ray diffraction results. The intense blue （476 nm）, green （543 nm）, and red （656 nm） emissions of the glass ceramics are simultaneously observed at room temperature under 980-am excitation, and the emission luminescence intensity increases significantly compared with the precursor glass, which is attributed to the low phonon energy of fluoride nanocrystals when rare-earth ions are incorporated into the precipitated BaF2 nanocrystals. Under 980-nm excitation at 400 mW, the international commission on illumination （CIE） chromaticity coordinate （X = 0.278, Y = 0.358） of the tridoped oxyfluoride glass ceramics＇ upconversion emissions is close to the standard white-light illumination （X = 0.333, Y= 0.333）. The results indicate that Tm^3＋/Yb^3＋ triply doped glass ceramics can act as suitable materials for potential three-dimensional displays applications.     

Red Up-Conversion Luminescence Process in Oxyfluoride Glass Ceramics Doped with Er^3＋／Yb^3＋

The phonon-assisted quantum cutting (PQC) model is presumed to clarify the red up-conversion luminescence process in Er^3 /yb^3 co-doped glass ceramics by the excitation and emission spectra. The red up-conversion luminescence of Er^3 ions mainly comes from three-photon absorption by the PQC process when the rare earth ions are doped in the g/ass ceramics and excited by 98Ohm pumped-laser. Er^3 ions absorb three-photons and relax to the ^4G11/2 state and then emit red up-conversion luminescence by the PQC process. The factor coefficient for the relation of pump-laser power and up-conversion intensity (P-I) is found by the analysis of excitation spectra of the red luminescence, which plays a major role to understand the true red up-conversion luminescence process. The new P-I relation is explained by the model of PQC.     

Abnormal Visible Luminescence Mechanism of Tb^3＋-Yb^3＋ Codoped SiOu-Al2O3-CaF2 Glass Studied by Time-Resolved Spectra

The upconversion energy transfer mechanism in Tb^3＋-Yb^3＋ co-doped SiO2-Al2O3-CaF2 glass is investigated by time-resolved spectra. The effect of donor ion Yb^3＋ is involved in the dynamic decay behavior of acceptor ion Tb^3＋, which provides direct proof for the energy transfer from Yb^3＋ to Tb^3＋. The pump power dependence curves show that the upconversion luminescence is a two-photon process. The measured decay curves of the 5D4 state （Tb^3＋） contain two parts： a slow decay process corresponding to its radiation, and a fast one with a decay parameter approximately twice the lifetime of the ^2F5/2 state （Yb^3＋ ）. The fast decay process is contradictory to the generally accepted cooperative sensitization upconversion rate equation model. Since the effect of the host environmental is excluded by comparative experiments, we believe that there should be another energy transfer mechanism in Tb^3＋-Yb^3＋ co-doped SiO2-Al2O3-CaF2 glass in addition to the cooperative sensitization process.     

Upconversion Luminescence of Er3+ Ions in Transparent Germanate Glass Ceramics Containing CaF2 Nanocrystals

The visible upconversion and near-infrared luminescence of Er3＋ ions in germanate glass ceramics containing GaF2 nanocrystals are investigated. The nanocrystals are characterized by x-ray diffraction （XRD） and transmission electron microscopy, showing their mean sizes less than 20hm. High transmittance of the glass ceramics is displayed by absorption spectra. The upconversion luminescence intensity in the glass ceramics increases significantly with increasing temperature. Both the shifts of the XRD peaks and the Stark-split shown in the luminescence spectra indicate the entrance of the Er3＋ ions into the CaF2 nanocrystals, which is confirmed by a Judd-Ofelt analysis. Possible mechanisms of the upconversion luminescence are analyzed.     

Enhanced NIR emission in Ce^3＋/Er^3＋-doped YAG induced by Bi^3＋ doping

Ce^3＋/Er^3＋/Bi^3＋ triply-doped yttrium aluminum garnet （YAG） is synthesized using co-precipitation method. The Bi^3＋ concentration-dependent near-infrared （NIR） emission behavior is systemically in- vestigated. The NIR emission of Er^3＋ ions at 1531 nm is enhanced threefold by the addition of 7 mol% Bi^3＋. Bi^3＋doping results in the formation of exciton in YAG and the variation in the local environment of the doped rare-earth ions. The enhancement in NIR luminescence is ascribed to the combined effects of the sensitization of exciton→Ce^3＋ →Er^3＋ and the Bi^3＋ doping-induced adjustment of the local environment for Ce^3＋ and Er^3＋ ions.     

Infrared-to-green upconversion luminescence and mechanism of Ho3+, Nd3+ and Yb3+ ions in oxyfluoride glass ceramics

New oxyfluoride glasses and glass ceramics co-doped with Nd^{3+}, Yb^{3+} and Ho^{3+} were prepared. The upconversion of infrared radiation into green fluorescence has been studied for Nd^{3+}, Yb^{3+} and Ho^{3+} in the transparent oxyfluoride glass ceramics. At room temperature very strong green upconversion luminescence due to the Ho^{3+}: ({}^5F_4, {}^5S_2)→{}^5I_8 transition under 800 nm excitation was observed in the glass ceramics. The intensity of the green upconversion luminescence in a 1mol% YbF_3-containing glass ceramic was found to be about 120 times stronger than that in the precursor oxyfluoride glass. The reason for the highly efficient Ho^{3+} upconversion luminescence in the oxyfluoride glass ceramics is discussed. The upconversion mechanism is also investigated.     

Er^3＋-doped phosphor-tellurite glass for broadband short-length Er^3＋-doped fiber amplifier

Er^3＋-doped phosphor-tellurite glass for broadband short-length Er^3＋-doped fiber amplifier （EDFA） is fabricated and characterized. The differential value （AT） of onset crystalline temperature （Tx） and glass transition temperature （Tg） is 206 ℃. The stimulated emission cross section of Er^3＋ is calculated from absorption spectrum by McCumber theory and is 0.87 × 10 ^-20 cm^2 at 1532 nm. A broad 1.5-μm fluorescence spectrum with 54-nm full-width at half maximum （FWHM） is demonstrated. Especially, the maximum phonon energy of undoped phosphor-tellurite glass is 1100 cm^- 1, which restricts the upconversion luminescence. It is possible to pump efficiently at 980 nm. These results indicate Er^3＋-doped phosphor-tellurite glass is suitable for fabricating broadband short-length EDFA.     

Blue Upconversion Luminescence in Tm^3＋／Yb^3＋-Codoped Lead Chloride Tellurite Glass

The upconversion properties of Tm^3 /Yb3 -codoped lead chloride tellurite glass under 980hm excitation were investigated. The intense blue (476nm) emission and weak red (649 nm) emission corresponding to the ^1G4→4 ^3H6 and ^1G4→^3H4 transitions of Tm^3 ions, respectively, were simultaneously observed at room temperature. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms are evaluated. The intense blue upconversion luminescence of Tm^3 /Yb^3 -codoped lead chloride tellurite glass can be used as potential host material for the development of blue upconversion optical devices.     

Influences of Mg^2＋ ion on dopant occupancy and upconversion luminescence of Ho^3＋ ion in LiNbO3 crystal

The effects of a Mg^2＋ ion on the dopant occupancy and upconversion luminescence of a Ho^3＋ ion in LiNbO3 crystal are reported. The birefringence gradient of the crystal is measured to investigate the optical homogeneity. The X-ray powder diffraction spectrum and the upconversion luminescence are used to investigate defect structure and spectroscopic properties of Mg,Ho：LiNbO3. Under 808-nm excitation, blue, red, and very intense yellow-green bands are observed. Based on the energy levels of Ho^3＋ in LiNbO3, and the pump intensity dependence of the observed emission, an excitation scheme is presented. The upconversion emission spectra reveal an enhancement of upconversion intensity when the Mg^2＋ ions are introduced into Ho：LiNbO3. The main upconversion mechanism is discussed in this work.     

Upconversion luminescence of Tm^3＋/Yb^3＋ codoped oxyfluoride glasses

Novel oxyfluoride glasses are developed with the composition of 30SiO2-15Al2O3-28PbF2-22CdF2-0.1TmF3 - xYbF3 - （4.9 - x） AlF3（x=0, 0.5, 1.0, 1.5, 2.0） in tool fraction, Furthermore, the upconversion luminescence characteristics under a 970nm excitation are investigated. Intense blue, red and near infrared luminescences peaked at 453nm, 476nm, 647nm and 789nm, which correspond to the transitions of Tm^3＋： ^1D2 →^3F4, ^1G4 →^3H6, ^1G4 →^3F4, and ^3H4 →^3H6, respectively, are observed. Due to the sensitization of Yb^3＋ ions, all the upconversion luminescence intensities are enhanced considerably with Yb^3＋ concentration increasing. The upconversion mechanisms are discussed based on the energy matching rule and quadratic dependence on excitation power. The results indicate that the dominant mechanism is the excited state absorption for those upconversion emissions.     

Structural and Luminescence Properties of Transparent Nanocrystalline ZrO2：Er^3＋ Films

The structural and luminescence properties of nanocrystalline ZrO2 ：Er^3＋ films are reported. Transparent nano-ZrO2 crystalline films doped with Er^3＋ have been prepared using a wet chemistry process. An intense roomtemperature emission at 1527nm with a full width at half-maximum of 46 nm has been observed, which is assigned to the ^4Ⅰ13/2 → ^4Ⅰ15/2 intra-4f^n electric transition of Er^3＋. Correlations between the luminescence properties and structures of the nanocrystalline ZrO2 ：Er^3＋ films have been investigated. Infrared-to-visible upconversion occurs simultaneously upon excitation of a commercially available 980-nm laser diode and the involved mechanisms have also been explained. The results indicate that the nanocrystalline ZrO2：Er^3＋ films might be suggested as promising materials for achieving broadband Er^3＋-doped waveguide amplifiers and upconversion waveguide lasers.     

Nd^3＋, Yb^3＋ and Ho^3＋ Codoped Oxyfluoride Glass Ceramics with High Efficient Green Upconversion Luminescence

New oxyfluoride g/asses and glass ceramic codoped with Nd3 , Yb3 and Ho3 were prepared. The x-raydiffraction analysis revealed that the heat treatments of the oxyfluoride g/asses could cause the precipitationof (Nd^3 , Yb^3 , Ho^3 )-doped fluorite-type crystals. Very strong green up-conversion luminescence due to theHo3 : (^5F4, ^5S2)→ ^5I8 transition under 800-nm excitation was observed in these transparent glass ceramics.The intensity of the green up-conversion luminescence in a 1-mo1% YbF3-containing glass ceramic was found tobe about 120 times stronger than that in the precursor oxyfluoride glass. The reason for the highly efficient Ho^3 up-conversion luminescence in the oxyfluoride g/ass ceramics is discussed.     

Transparent Ni^2＋-Doped MgO-Al2O3-SiO2 Glass Ceramics with Broadband Infrared Luminescence

We report on transparent Ni^2＋-doped MgO-Al2O3-SiO2 glass ceramics with broadband infrared luminescence. Ni^2＋-doped MgO-Al2O3-SiO2 glass is prepared by using the conventional method. After heat treatment at high temperature, MgAl2O4 crystallites are precipitated, and their average size is about 4.3 nm. No luminescence is detected in the as-prepared glass sample, while broadband infrared luminescence centred at around 1315 nm with full width at half maximum （FWHM） of about 300 nm is observed from the glass ceramics. The observed infrared emission could be attributed to the ^3T2g（^3F）→^3A2g（^3F） transition of octahedral Ni^2＋ ions in the MgAl2O4 crystallites of the transparent glass ceramics. The product of the fluorescence lifetime and the stimulated emission cross section is about 1.6×10^-24 s cm^2.     

White light generation of glass ceramics containing Ba_2LaF_7：Eu~（2＋）,Tb~（3＋） and Sm~（3＋） nanocrystals

The Eu 2＋ /Tb 3＋ /Sm 3＋ co-doped oxyfluoride glass ceramics containing Ba2LaF7 nanocrystals are prepared in the reducing atmosphere.The X-ray difiraction results show that Eu 2＋ ,Tb 3＋ and Sm 3＋ ions are enriched into the precipitated Ba2LaF7 nanophase after the annealing process.It deduces efficient energy transfers from Eu 2＋ to Tb 3＋ and Sm 3＋ and intenses warm white luminescence of the glass ceramics. Comparing with the glass,the luminescence quantum yield of the glass ceramics is also enlarged by about 3 times.This demonstrates the potential white light-emitting diode application of the glass ceramics produced in this letter.     

Broadband and High Efficient 1530 nm Emission from Oxyfluoride Glass Ceramics Codoped with Er3＋ and Yb3＋ Ions

The emission at 1530nm and its applications in optical communications are discussed. The efficient width of the emission band △eff, which is up to 91 nm, is larger as compared with silica-based glass, bismuth glass and ZrF4-BaF2-LaFa-AIF3-NaF （ZBLAN） glass doped by Er^3＋ ions. Under the excitation of 785 nm laser, the emission integral intensity of 153Onto increases about five times in the glass ceramics higher than that in the glass. This is explained by the quantum cutting process by two-photon emission with phonon assistance. The results indicate that the glass ceramics are a promising candidate for developing broadband optical amplifiers in wavelength-division multiplexed systems.