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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.

High photoluminescence efficiency, unusual NIR broadband and tunable emission are possible in erbium-doped and tantalum germanate glasses and nanostructured glass-ceramics. The chemical microenvironment around erbium ions and their influence on optical properties are dependent on the TaO6 clusters formation and tantalum perovskite-like bronze crystallization, which are strongly related to the tantalum oxide content and annealing. Erbium ions are preferably distributed close to tantalum-rich environment, providing an intense emission with minimized nonradiative processes and inhomogeneous broadband NIR emission with increasing site numbers. A more pronounced broadening in the nanocrystalline samples is due to the multi-sites of the tantalum perovskite-like bronze crystalline structure. Green/red tunable emission from up conversion processes was observed since the erbium ions occupy a chemical environment with low phonon energy in the tantalum germanate glass and with close neighbors in the tantalum germanate glass-ceramics. Accordingly, the highly tantalum germanate glasses and glass ceramics are promising for photonic applications in optical amplification, lighting and lasing, as well as for nonlinear optical applications and studies considering the perovskite-like bronze crystalline samples.

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 2019-10-04T13:15:57Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-10-01 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) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Niobium alkali germanate glasses doped with Er3+ up to 4.0 mol% were synthesized by the melt-quenching technique. Glass-ceramics were obtained by controlled heat treatment of the pristine glass. Thermal and optical properties were investigated and are shown to depend on the erbium content. The main radiative parameters were obtained based on the Judd-Ofelt theory. The change of radiation energy processes and modification of the environment of the earth-rare ion occurs in samples with high erbium concentrations. In general, strong enhancement of upconversion emission occurs due to the efficient process of excited-state absorption for the high erbium content samples. Visible tunable phosphors emitting either red-to-green light depends on the erbium content and heat treatment. In addition, these glasses and glass-ceramics samples have high thermal stability and wide transmission window. For these reasons, Er3+-doped niobium germanate glasses and glass-ceramics may be suitable for photonic device applications, especially as visible laser. Univ Fed Alfenas, Grp Quim Mat, Campus Pocos Caldas, Pocos De Caldas, MG, Brazil Univ Sao Paulo, Fac Filosofia, Dept Quim Ciencias & Letras Ribeirao Preto, Ribeirao Preto, SP, Brazil Univ Sao Paulo, Inst Fis Sao Carlos, Sao Carlos, SP, Brazil Univ Sao Paulo, Inst Quim Sao Carlos, Sao Carlos, SP, Brazil Univ Estadual Paulista, Inst Quim, Araraquara, SP, Brazil Univ Estadual Paulista, Inst Quim, Araraquara, SP, Brazil FAPESP: 2013/07793-6

The main objective of this work has been to prepare and characterize the thermal and structural properties of glasses in the pseudo binary system TeO 2 –Pb(PO 3 ) 2 with respect to the composition. Homogeneous and transparent glass samples were obtained by the melt-quenching method in a large glass forming range in the pseudo binary system (100 − x )TeO 2 – x Pb(PO 3 ) 2 with x varying from 5 to 100. Thermal properties investigated by DSC pointed out an increase of the glass transition temperature from x = 5 to x = 40 and further decrease of Tg for higher Pb(PO 3 ) 2 concentrations. A similar tendency has been observed for the thermal stability against devitrification measured using the stability parameter Tx–Tg. FTIR together with Raman spectroscopies allowed building a structural model for these glasses with the contribution of distinct phosphate and tellurite units depending on the composition. Identification of crystalline phases obtained after the glasses heat-treatments obtained by X-ray diffraction support the structural evolution suggested by vibrational spectroscopies. These data suggest that incorporation of TeO 2 in the lead metaphosphate glass results in tellurium conversion from TeO 4 seesaw geometry to TeO 3 trigonal pyramids and consequent conversion of well-known metaphosphate units Q 2 to modified pyrophosphate units Q 2 1Te in which the phosphorus PO 4 tetrahedron is linked to another PO 4 unit and one TeO 3 pyramid. These tellurite trigonal units cross-link the modified metaphosphate chains with a resulting increase of the glass network connectivity. For high TeO 2 concentrations, all Q 2 were converted to Q 2 1Te and the additional tellurium atoms are incorporated in the glass network as TeO 4 seesaw units with a glass network built from a tridimensional network of TeO 3 , TeO 4 and Q 2 1Te . Finally, this work pointed out the possibility to use this pseudo-binary system for the preparation of phosphate glass-ceramics containing a TeO 2 crystalline phase or tellurite glass-ceramics containing a pyrophosphate or metaphosphate crystalline phase.

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.

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.

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb 3+ /Yb 3+ -codoped tellurite glasses excited at 1.064 μm is investigated. Bright luminescence emission around 485, 550, 590, 625 and 655 nm, identified as due to the 5 D 4 → 7 F J ( J =6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the 5 D 4 emitting level of the Tb 3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300–500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb 3+ -sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb–Yb pair and Tb 3+ ground-state depopulation via multiphonon excitation of the 7 F J excited states describes quite well the experimental results.

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.

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.