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Er3+-doped and Er3+/Yb3+-codoped GeO2-Nb2O5-K2O glasses were prepared by conventional melt-quenching method. Thermal and spectroscopic analysis were performed. The niobium insertion into the glass structure has decreased the OH groups resulting in a slight increase in the luminescence efficiency as the niobium content. Glass network structure is modified depending on rare earth doping concentration and niobium content. The number of bridge oxygen bonds and nonbridge oxygen bonds is altered and consequently the crystallization composition. The spectroscopic features were discussed in terms of niobium concentration and its influence on the applications as optical devices. The addition of niobium broadens the emission band in the region of 1.5 µm covering C, S and L-telecommunication bands. The gain coefficient has calculated establishing high potentiality to use the niobium germanate glasses for optical amplification. The codoping by Er3+/Yb3+ intensifies the emission in the NIR and visible (upconversion) region in relation to the doped samples and may be interesting for application as visible and near-infrared emitters. In addition, the codoped samples with variable niobium contents have potential as visible tunable phosphors emitting either green or red light. Accordingly, the niobium germanate glasses are interesting matrices for rare earth ions doping and for the development of optical devices, such as visible and NIR laser and optical amplifiers.

International audience; Complex sequential crystallization mechanisms were investigated in the glass composition 70GeO2-10Na2O-20Ta2O5 prepared by melt-quenching. In a first stage, a classical volume-nucleated crystallization of sodium tantalate perovskite Na2Ta8O21 is induced under short time heat-treatments whereas longer annealing treatments promote the formation of surface spherical structures known as spherulites. These singular glass-ceramics were deeply investigated by density measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Auger Electron Spectroscopy (EDS), correlated Raman and second harmonic generation (SHG) microscopy as well as macro-SHG measurements. These surface spherulites with average size up to 50 µm are richer in tantalum but poorer in germanium than the surrounding material. However, measured SHG signals were found to be related to quartz-like α-GeO2 grown inside the spherulites. Therefore these materials exhibit a second-order non-linear optical response, related to the symmetry of the point group D3 given by the quartz GeO2 grown for the first time in a transparent glass-ceramic.

Made available in DSpace on 2020-12-12T01:28:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-11-25 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) Financiadora de Estudos e Projetos Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Transparent Eu3+-doped tantalum phosphate glass-ceramics containing Na2Ta8O21 nanocrystals were successfully obtained by a two-step heat treatment. Structural analyses were performed using X-ray diffraction, Transmission Electron Microscopy, Nuclear Magnetic Resonance and Raman spectroscopy. Due to the high Ta/P ratio (= 2) the glass network is built from a mixed 3D PO4–TaO6 chain in which TaO6-rich regions are inserted and responsible for crystallization of an alkali tantalum oxide phase Na2Ta8O21. Eu3+ and their optical properties was used as a sensitive probe to study it site structure in the host matrix. The radiative properties of Eu3+ emission levels were predicted using the Judd-Ofelt theory. The chemical environment and local phonon energy around the rare-earth ions have been proposed based on phonon side band associated with the 7F0 → 5D2 transition, 5D0 → 7F2/7F1 relative intensity and narrowing of the emission bands. High tantalum content precursor glass exhibits high photoluminescence quantum efficiency due to low phonon energy and high refractive index (1.9461 at 633 nm). A perovskite-like bronze crystalline structure is formed in the glass-ceramic (nucleated at 925 °C for 0.5 h and grown at 940 °C for different times) and is considered promising for nonlinear optical applications and studies. Institute of Chemistry Sao Paulo State University - UNESP Sao Carlos Institute of Chemistry University of Sao Paulo, São Carlos Grupo de Química de Materiais Universidade Federal de Alfenas Campus Poços de Caldas, Poços de Caldas Institute of Chemistry Sao Paulo State University - UNESP CNPq: 156420/2018-0 FAPESP: 2013/07793-6 FAPESP: 2018/16126-7

Made available in DSpace on 2020-12-10T17:34:30Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-07-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) FINEP Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Alkali niobium germanate glasses were prepared by melt-quenching in the ternary systems 75GeO(2)-15Nb(2)O(5)-10M(2)O with M = Li, Na, K, Rb and Cs. Thermal properties of these glasses and related glass-ceramics are strongly dependent on the alkaline modifier. Structural investigations by Raman allowed to determine that the alkali radius has an influence on the number of non-bridging oxygens and consequently on the average glass network connectivity. Raman spectra of sodium, potassium and rubidium germanate glass-ceramics exhibited slight changes when compared to the pristine glass and the new Raman features can be attributed to a 3D NbO6 network similar to orthorhombic niobium oxide. Characterization of the heat-treated glasses allowed to determine the crystalline phases precipitated in the samples and it has been shown that a single perovskite bronze-like phase of general formula M2Nb5O21 can be obtained in Na, K, and Rb-containing glasses. Luminescent properties of Eu3+-doped glasses pointed out a non-regular tendency where sodium-based glass seems to be the inflexion point of luminescent data with higher asymmetry around Eu3+ ions and higher quantum efficiency. In transparent glass-ceramics, the Eu3+ luminescent properties are clearly modified in the Na- and K-based samples with lower symmetry and quantum efficiency after heat-treatment. Such optical behavior is attributed to Eu3+ migration in the perovskite crystalline phase with highly distorted Eu3+ sites and shorter Eu3+-Eu3+ distances. Univ Fed Alfenas, Grp Quim Mat, Campus Pocos De Caldas, Pocos De Caldas, MG, Brazil Univ Estadual Paulista, Inst Quim, Araraquara, SP, Brazil Univ Estadual Paulista, Inst Quim, Araraquara, SP, Brazil CNPq: 156420/2018-0

Eu3+-, Er3+-doped and Er3+/Yb3+-codoped niobium alkali germanate glasses, in the compositional host system GeO2-Nb2O5-K2O were synthesized by the conventional melt-quenching technique. Glass-ceramics were manufactured by controlled heat treatment of the precursor glasses. Homogeneous glasses and nanocrystalline glass-ceramics were investigated by DSC, XRD, TEM and spectroscopic analysis. K2Nb14O36 crystals are formed in the prepared glass-ceramics. This crystalline phase has low phonon energy as compared to the precursor glass and a variety of chemical sites for rare-earth ions. In glass-ceramics, the rare-earth ions occupy higher symmetry sites of the K2Nb14O36 nanocrystals resulting in inhomogeneous broadband emissions. Judd-Ofelt parameters were obtained for the Eu3+- and Er3+-doped samples. Glass-ceramics heat-treated for 20 h exhibited the highest photoluminescence quantum efficiency. The increased quantum efficiency is due to the decrease of nonradiative decay processes, that depopulate the excited levels, as well as the decrease in hydroxyl content and overall phonon energy. However, luminescence quenching is verified for samples with longer crystallization times, which emphasizes the importance of optimized heat treatment parameters. The gain coefficient and upconversion data were obtained and corroborate the potential application of these materials as photonic devices, such as visible and NIR laser and optical amplifiers.

The green emissions of the trivalent erbium ions exhibit promising applications in photonics, such as the development of solid-state lasers, optical sensors, and light-emitting diodes (LED). In this work, the intensity of the emissions of two thermally coupled levels 2H11/2 and 4S3/2 excited states of Er3+ ions are studied in Er3+/Yb3+ co-doped tungsten sodium phosphate oxyfluoride glasses. The results demonstrate that an increase in the content of the PbF2 leads to an enhancement in the performance of these samples as optical sensors, and amplifies the relative thermal sensitivity from 0.82% K−1 to 1.15% K−1 for glass samples containing 10 mol% and 50 mol% of PbF2, respectively. Additionally, the effect of PbF2 is discussed in terms of the modifications of the site symmetries around Er3+ ions and changes in the energy gap between the thermally coupled levels, which implied in the improvement of the absolute thermal sensitivity at higher temperatures for high PbF2 contained sample.

Glasses in the binary system KPO3-Ta2O5 were prepared by the melt-quenching technique. Highly modified phosphate glasses with up to 40 mol% Ta2O5 were obtained. The samples were characterized by Differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) analysis, Ultraviolet–Visible (UV–Vis) and infrared (IR) transmittance, Raman and luminescence spectroscopies. Thermal, structural and optical properties are strongly dependent of the tantalum oxide content. Thermal behavior and Raman data are in agreement with a progressive insertion of tantalum units in the phosphate network through P-O-Ta bonds. These bonds enhance the network connectivity due to TaO6 units cross-linking the linear phosphate chains. In addition, higher Ta2O5 contents in the glass network results in tantalum oxide-rich regions detected as Ta-O-Ta bridging bonds, apparently acting as crystallization nuclei. Emission spectra of Eu3+-doped glasses pointed out a decrease in the symmetry of the Eu3+ sites by increasing Ta2O5 content. A decrease in the excited state lifetimes was also identified due to the progressive Ta-rich environment around Eu3+ ion. Finally, photoluminescence quantum efficiency increases with tantalum insertion due to the reduction of local phonon energy around rare-earth ions. In this sense, high tantalum phosphate glasses are built up from TaOx polyhedra within the phosphate chains. The presence of TaOx clusters allows the improvement of optical and thermal properties and corroborates with the potential application of these materials photonic devices.

Made available in DSpace on 2018-11-28T15:26:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-05-01 Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) FINEP (CT-INFRA) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) New glass compositions were investigated in this specific proportion (90-x)TeO2-10GeO(2)-xPbF(2) in order to study the influence of lead fluoride content on the thermal, structural and optical properties. Glass samples with x = 5 to 30% were prepared by melt-quenching method. Thermal characterizations by DSC allowed to identify a clear decrease of the glass transition temperature as well as a very distinct crystallization behavior for higher PbF2 contents. Raman spectroscopy suggests a progressive PbF2 incorporation with conversion of TeO4 to TeO3 units related with the modifier behavior of PbF2 in the glass network. X-ray diffraction patterns of crystallized samples support these results since crystalline TeO2 was detected for lower PbF2 contents whereas lead tellurite Pb2Te3O7 was identified for the most lead fluoride concentrated sample. UV-Vis-MR absorption spectra are in agreement with a progressive increase of the optical bandgap with lead fluoride concentration. Emission spectra of Eu3+ doped samples also pointed out a progressive structural change around Eu3+ ions with more symmetric sites. Finally, experimental lifetimes, Judd-Ofelt parameters and quantum efficiencies were obtained from these samples and exhibited a strong dependency of the lead fluoride contents, suggesting that these new fluorotellurite glasses are interesting hosts for rare earth ions. (C) 2017 Elsevier B.V. All rights reserved. Univ Fed Alfenas, Grp Quim Mat, Campus Pocos de Caldas, Pocos De Caldas, MG, Brazil Univ Sao Paulo, Fac Filosofia Ciencias & Letras Ribeirao Preto, Dept Quim, Av Bandeirantes 3900, BR-14040901 Ribeirao Preto, SP, Brazil Univ Estadual Paulista, Inst Quim, Araraquara, SP, Brazil Univ Estadual Paulista, Inst Quim, Araraquara, SP, Brazil FAPEMIG: CEX-APQ-01410-12 CNPq: 552225/2011-8

Niobium alkali germanate glasses were synthesized by the melt-quenching technique. The ternary system (90-x)GeO2-xNb2O5-10K2O forms homogeneous glasses with x ranging from 0 to 20mol%. Samples were investigated by DSC and XRD analysis, FTIR and Raman spectroscopy, and optical absorption. Structural and physical features are discussed in terms of Nb2O5 content. The niobium content increase in the glass network strongly modifies the thermal, structural and optical properties of alkali germanate glasses. DSC, Raman and FTIR analysis suggest niobium addition promotes NbO6 groups insertion close to GeO4 units of the glass network. XRD analysis also pointed out that samples containing high niobium oxide contents exhibit preferential niobium oxide rich phase after crystallization after heat treatment, which is similar to orthorhombic Nb2O5. Absorption spectra revealed high transmission range between 400nm to 6.2μm, added to a considerably decreased hydroxyl group content as the addition of niobium in the alkali germanate network. The niobium oxide rich phase crystallization process was studied and activation energy was determined, as well as nucleation and crystal growth temperatures and time for obtaining transparent glass-ceramics. This article is protected by copyright. All rights reserved.

Made available in DSpace on 2018-11-28T01:36:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-12-01 Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) FINEP Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) In this work the novel glass system TeO2-Sb2O3-GeO2 was investigated and promising glass compositions were selected for further specific studies. Glass samples in the (80-0.8x)TeO2-(20-0.2x)Sb2O3-xGeO(2) molar composition were prepared by the melt-quenching method with a glass-forming domain from x = 10 to x = 90. Samples were investigated by XRD, DSC, FTIR, Raman spectroscopy and UV-visible absorption. The XRD and DSC results bring informations about the non-crystalline state and thermal properties of these materials. It has been observed that higher GeO2 contents lead to higher glass transition temperatures and thermal stabilities against crystallization. FTIR and Raman spectroscopies suggest a progressive incorporation of GeO2 in the covalent network of TeO2 with conversion of structural units TeO4 to TeO3. Absorption spectra revealed the high visible transparency of these samples and an increase of the optical band gap with GeO2 addition, in agreement with a decreasing polarizability of the glass network. Er3+ doped and Er3+/Yb3+ codoped samples were also studied with respect to their infrared emission properties and higher GeO2 contents lead to an increase in IR emission intensity at 1,5 mu m as well as longer radiative lifetimes. Finally, upconversion emission in the visible were also recorded and were shown to be strongly dependent of the composition. (C) 2016 Elsevier B.V. All rights reserved. Univ Fed Alfenas, Grp Quim Mat, Campus Pocos Caldas, Pocos De Caldas, MG, Brazil Univ Sao Paulo, Dept Quim, Fac Filosofia Ciencias & Letras Ribeirao Preto, Av Bandeirantes, BR-14040901 Ribeirao Preto, SP, Brazil Univ Estdual Paulista Julio Mesquita Filho, Inst Quim, Araraquara, SP, Brazil Univ Estdual Paulista Julio Mesquita Filho, Inst Quim, Araraquara, SP, Brazil

Crystalline lead–pyrophosphate precursor was prepared in aqueous solution from lead nitrate and phosphoric acid and characterized by X-ray diffraction, thermogravimetry and Raman scattering. This crystalline lead–phosphate was then used to prepare glass samples in the binary system Pb 2 P 2 O 7 –WO 3 . Dependence of WO 3 content on thermal, structural and optical properties were investigated by thermal analysis (DSC), Raman spectroscopy, UV–visible and near-infrared absorption and M-Line technique to access refractive index values. Incorporation of WO 3 in the lead–pyrophosphate matrix enhances the glass transition temperature and thermal stability against devitrification, favors formation of P O W bonds and WO 6 clusters. In addition, optical properties are strongly modified with a redshift of the optical bandgap with WO 3 incorporation as well as an increase of the refractive index from 1.89 to 2.05 in the visible.

Glasses were prepared by the melt-quenching method in the ternary system Pb(PO 3 ) 2 -WO 3 -PbF 2 and doped with Er 3+ in order to prepare luminescent transparent glass-ceramics. This work focused on thermal and structural characterization of tungsten lead-phosphate glasses and crystallization study for preparing transparent glass-ceramics. Thermal properties such as thermal stability and crystallization behavior upon heating were investigated by DSC in function of PbF 2 content. For low PbF 2 concentrations, only one crystallization peak due to Pb 3 (PO 4 ) 2 is observed whereas samples containing more than 15% of PbF 2 present another exothermic event at lower temperatures related with precipitation of PbF 2 , Pb 2 P 2 O 7 and Pb 2 OF 2 . Structural investigations by Raman spectroscopy suggest that PbF 2 modifies the tungsten-phosphate network through the formation of P―F and P―O―Pb bonds but the average network connectivity remains almost constant. A crystallization study has been performed by DSC to investigate the dominant crystallization mechanisms in these glasses and it has been established that Pb 3 (PO 4 ) 2 is nucleated on the surface whereas PbF 2 , Pb 2 P 2 O 7 and Pb 2 OF 2 crystallize dominantly from the glassy bulk. Transparent glass-ceramics containing nanosized PbF 2 crystallites were also prepared by suitable heat-treatment on the glass sample containing 20% of PbF 2 and Raman microscopy of these glass-ceramics supports the crystallization mechanisms determined by DSC.

Glasses were prepared in the binary system (100 − x )NaPO 3 – x MoO 3 with x varying from 0 to 50 mol%. An increase in the MoO 3 concentration promotes a strong absorption in the visible and near infrared attributed to Mo reduction during glass synthesis. X-ray absorption measurements were performed at the Mo K and L 3 edges to investigate both the coordination number and oxidation state of Mo in these glasses. The evolution of the pre-peak observed at the K edge suggests that Mo atoms are six-fold coordinated in these glasses. This hypothesis was confirmed by data obtained at the Mo–L 3 -absorption edge. Since the final electronic states at the L 3 -absorption edge are mostly orbitals of d-character which are splitted by the ligand field, the amplitude of the d-orbital splitting could be estimated and the related coordination number of Mo obtained. Finally, the oxidation state of Mo could be related with a change of the white line intensity at the Mo–L 3 edge. These results confirm that the optical absorption in the visible is due to Mo reduction and that transparent samples prepared by slow cooling contain less reduced Mo species.

Vitreous samples were prepared in the binary system NaPO 3 –MoO 3 and their characteristic temperatures were determined by Differential Scanning Calorimetry. Glasses with high amounts of MoO 3 (>45 mol%) exhibit an intense crystallization peak and the composition 50 NaPO 3 –50 MoO 3 was chosen for the crystallization study. Two different methods based on thermal analyzes were used to determine the mechanism of crystallization in these molybdenum–phosphate glasses. In the first procedure, thermal analyses by DTA were performed on samples with different grain sizes and the crystallization tendency deduced in function of superficial area. The second method used the classical non-isothermal crystallization study: DSC measurements were performed under several heating rates to access activation energy for crystallization and Avrami parameter n . Critical cooling rate was calculated and compared with experimental data obtained from DTA analysis upon cooling.

In this paper we investigate the energy transfer processes in Tm 3+ /Er 3+ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er 3+ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm 3+ ions. This is evidence of the relevant contribution of the route Er 1 ( 4 I 11/2 ) + Er 2 ( 4 I 13/2 ) → Er 1 ( 4 I 15/2 ) + Er 2 ( 4 F 9/2 ) to the process. Energy migration among pumped 4 I 9/2 level reducing the efficiency of the upconversion emission rate ( 3 H 11/2 , 4 S 3/2 , and 4 F 9/2 ) is observed for Er 3+ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.

The glass forming ability was investigated in the codoped Er3+/Yb3+ ternary system NaPO3-WO3-PbF2 with increasing amounts of PbF2. It has been found that glass samples can be obtained for PbF2 contents ranging from 0 mole% to 50 mole% for the codoped samples and an undoped sample containing 60% of lead fluoride could also be vitrified. Thermal properties of the most PbF2 concentrated glasses exhibit strong dependence of the composition with a decrease of the glass transition temperatures and thermal stabilities against devitrification with increasing the lead fluoride content. These glass samples were heat-treated near the crystallization peaks and the X-ray diffraction measurements pointed out that the dominant phase which precipitates from the glass samples containing 40% and 50% of PbF2 is the lead fluorophosphate phase Pb5F(PO4)3 whereas the sample containing 60% of PbF2 exhibits a preferential bulk crystallization of cubic lead fluoride β-PbF2.

In this work, new glass compositions were prepared in the ternary system GeO2–K2O–Ta2O5. Potassium oxide was added to reach the complete melt of the starting mixture and two composition series were investigated: the first one with a constant K2O molar content of 10% in the ternary system (90–x)GeO2–10K2O–xTa2O5 and the second one with the same molar content of K2O and Ta2O5 in the ternary system (100–2x)GeO2–xK2O–xTa2O5. Homogeneous and transparent glasses could be obtained between x = 0 and 20. X-ray diffraction analyzes of samples with x = 25 identified orthorhombic Ta2O5 in the first series and an isostructure of K3.8Ge3Nb5O20.4 in the second series where it is assumed that Ta5+ ions are inserted in the Nb5+ sites. As one of our goal with these materials is related with the preparation of glass-ceramics containing Ta2O5 nanocrystals, the first series has been selected for further characterizations. An increase in glass-transition temperatures with increasing Ta2O5 content as well as an increase of the thermal stability from x = 0 to 10 has been identified by differential scanning calorimetry. For higher contents, crystallization events were identified. Fourier transform infrared and Raman spectroscopic characterizations allowed to point out the intermediary behavior of Ta2O5 in the vitreous network where TaO6 octahedra are inserted inside the germanate network with TaO6 clusters identified at higher Ta2O5 contents. Heat-treated samples with high tantalum contents (x = 15 and 20) exhibit preferential precipitation of orthorhombic Ta2O5 with nanometric size, suggesting the possibility of obtaining transparent glass-ceramics for optical applications.

X-ray absorption spectroscopy was used to study the local environment of tungsten atoms in NaPO 3 –BaF 2 –WO 3 glasses and the results were compared with crystalline references Na 2 WO 4 and WO 3 . XANES measurements at the W-L 1 edge allowed to determine a distorted octahedral environment of tungsten atoms in these glasses similar to the local order of tungsten in monoclinic WO 3 . Extended X-ray absorption fine structure (EXAFS) has been used as a local probe to monitor the effect of WO 3 concentration on the tungsten environment. Based on an analysis of the EXAFS data, we proposed a three-shell model of oxygen atoms around tungsten as in monoclinic WO 3 . With increasing WO 3 concentration, it was found that R 2 decreases from 1.96 to 1.92 A whereas R 3 increases from 2.07 to 2.12 A.

Diode-pumped Yb-doped glass lasers have received considerable attention for applications such as high-power beam production or femtosecond pulses generation. In this paper, we evaluate the laser potential of three different glass families doped with Yb3+: alkali lead fluorborate (PbO–PbF2–B2O3), heavy metal oxide (Bi2O3–PbO–Ga2O3) and niobium tellurite (TeO2–Nb2O5–K2O–Li2O). Spectroscopic properties were studied for the samples and calculations of the minimum laser pump intensity (Imin), saturation fluence (Usat) and the theoretical limit of peak power (Pmax) are also presented. A comparison of laser properties of these three different glasses and their importance is shown and analyzed.

Temperature investigation of infrared-to-visible frequency upconversion in erbium-doped tellurite glasses excited by CW laser radiation at 1540 nm and under cryogenic temperatures is reported. Intense upconversion emission signals around 530, 550 and 660 nm corresponding to the 2H11/2, 4S3/2, and 4F9/2 transitions to the 4I15/2 ground state were generated and studied as a function of the laser intensity and temperature. The upconversion excitation mechanism of the Er3+ ions emitting energy levels was accomplished via stepwise multiphoton absorption. The green upconversion luminescence exhibited a fivefold intensity enhancement when the temperature of the sample was varied in the range between 5 and 300 K. A maximum green upconversion intensity was attained around 120 K and a steady decreasing behavior for higher temperatures up to 300 K was observed. A model based upon conventional rate equations was used to model the observed temperature evolution of the upconversion luminescence.

Red, green, and blue emission through frequency upconversion and energy-transfer processes in tellurite glasses doped with Tm 3+ and Er 3+ excited at 1.064 μm is investigated. The Tm 3+ /Er 3+ -codoped samples produced intense upconversion emission signals at around 480, 530, 550 and 660 nm. The 480 nm blue emission was originated from the 1 G 4 → 3 H 6 transition of the Tm 3+ ions excited by a multiphoton stepwise phonon-assisted excited-state absorption process. The 530, 550 nm green and 660 nm red upconversion luminescences were identified as originating from the 2 H 11/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 transitions of the Er 3+ ions, respectively, populated via efficient cross-relaxation processes and excited-state absorption. White light generation employing a single infrared excitation source is also examined.

Upconversion luminescence and thermal effects in Pr3+/Yb3+- and Er3+/Yb3+-codoped 60TeO2–10GeO2–10K2O–10Li2O–10Nb2O5 tellurite glasses excited by CW infrared radiation at 1.064 μm is reported. Generation of intense green and red fluorescence emission in Er3+/Yb3+-codoped samples and appreciable upconversion luminescence in the wavelength region of 450–680 nm in Pr3+/Yb3+-codoped samples is observed. Temperature-induced enhancement of ×12 in the upconversion efficiency in Er3+/Yb3+- and ×10 in the Pr3+/Yb3+-doped samples is demonstrated.

Glass samples with the composition (mol%) 80TeO2–10Nb2O5–5K2O–5Li2O, stable against crystallization, were prepared containing Yb3+, Tm3+ and Ho3+. The energy transfer and energy back transfer mechanisms in samples containing 5% Yb 3+ –5% Tm 3+ and 5% Yb 3+ –5% Tm 3+ –0.5% Ho 3+ were estimated by measuring the absorption and fluorescence spectra together with the time dependence of the Yb 3+ 2 F 5/2 excited state. A good fit for the luminescence time evolution was obtained with the Yokota–Tanimoto's diffusion-limited model. The up-conversion fluorescence was also studied in 5% Yb–5% Tm, 5% Yb–0.5% Ho and 5% Yb–5% Tm–0.5% Ho tellurite glasses under laser excitation at 975 nm. Strong emission was observed from 1 G 4 and 3 F 2 Tm 3+ energy levels in all samples. The 5 S 2 Ho 3+ emission was observed only in Yb3+Ho3+ samples being completely quenched in Yb3+/Tm3+/Tm3+ samples.

Alkali niobium tellurite glasses have been prepared and some of their properties measured by differential scanning calorimetry and Raman scattering. The vitreous domain was established in the pseudo ternary phases diagram for the system TeO2–Nb2O5–(0.5K2O–0.5Li2O). Raman scattering shows that for samples in the TeO2 rich part of the phase diagram the vitreous structure is composed essentially of (TeO4) units connected by the vertices, as in the α-TeO2 crystal. The addition of alkali and niobium oxides causes depolymerization to occur with structures composed essentially of (TeO3) and (NbO6) units. Samples with the composition (mol%) 80TeO2–10Nb2O5–5K2O–5Li2O, stable against crystallization, were prepared containing up to 10% mol Nd3+. The addition of this oxide increases the rigidity of the vitreous network shifting characteristic temperatures to higher temperatures. For the 10% Nd3+ sample amorphous phase separation is assumed to exist from the observation of two glass transition temperatures. Spectroscopic properties such as Judd–Ofelt Ωλ intensity parameters, radiative emission probabilities, and induced emission cross sections were calculated. From these results and also from the emission quenching observed as a function of Nd3+ concentration, we suggest that these glasses could be utilized in optical amplifying devices.

We investigated near-infrared-to-blue upconversion from thulium (Tm3+) doped in tellurite glasses upon continuous wave excitation near 800 nm. We observed an enhancement of over two orders of magnitude of the upconverted emission at ∼480 nm when neodymium (Nd3+) ions were codoped with Tm3+ ions. For comparison, using a Tm3+:Nd3+ codoped fluorozirconate glass as a reference material we observed a 40-fold enhancement of the blue emission. Analysis of the blue emission for samples with different doping levels of Nd3+ ions showed that energy transfer between Nd3+ and Tm3+ is the mechanism responsible for the enhancement in upconversion.

Infrared-to-visible upconversion luminescence emission in Nd 3+ -doped 60TeO 2 –10GeO 2 –10K 2 O–10Li 2 O–10Nb 2 O 5 tellurite glasses under cw excitation around 800 nm is investigated. Blue (430, and 475 nm), green (530 nm) and yellow-orange (590 nm) energy upconversion emission owing to the 2 P 1/2 → 4 I j ( j =9/2, 11/2, 13/2 and 15/2) transitions of the Nd 3+ ions, respectively, was recorded. The dependence of the upconversion intensity upon the excitation wavelength and pump power is also studied. The upconversion excitation mechanism responsible for the observed emission signals is attributed to stepwise multiphoton absorption.

Efficient energy upconversion of cw radiation at 1.064 μm into blue, red, and near infrared emission in Tm3+-doped Yb3+-sensitized 60TeO2-10GeO2-10K2O-10Li2O-10Nb2O5 glasses is reported. Intense blue upconversion luminescence at 485 nm corresponding to the Tm3+ 1G4→3H6 transition with a measured absolute power of 0.1 μW for 800 mW excitation power at room temperature is observed. The experimental results also revealed a sevenfold enhancement in the upconversion efficiency when the sample was heated from room temperature to 235 °C yielding 0.7 μW of blue absolute fluorescence power for 800 mW pump power. High brightness emission around 800 nm (3F4→3H6) in addition to a less intense 655 nm (1G4→3H4 and 3F2,3→3H6) fluorescence is also recorded. The energy upconversion excitation mechanism for thulium emitting levels is assigned to multiphonon-assisted anti-Stokes excitation of the ytterbium-sensitizer followed by multiphonon-assisted sequential energy-transfer processes.

In this work, bright visible upconversion emission excited through cooperative energy-transfer and thermal effects in Tb/sup 3+//Yb/sup 3+/-codoped 60TeO/sub 2/-10GeO/sub 2/-10K/sub 2/O-10Li/sub 2/O-10Nb/sub 2/O/sub 5/glasses excited at 1.064 /spl mu/m is investigated. Bright upconversion luminescence around 485, 550, 590, 625 and 655 nm, identified as due to the /sup 5/D/sub 4//spl rarr//sup 7/F/sub J/(J= 6, 5, 4, 3, and 2) transitions of the terbium-ions, respectively, was generated.