Incorporation of PbF2 into Heavy Metal Oxide Borate Glasses. Structural Studies by Solid State NMR

A series of heavy metal oxide (HMO) glasses with composition 26.66B(2)O(3)-16GeO(2)-4 Bi2O3-(53.33-x)PbO-xPbF2 (0 <= x <= 40) were prepared and characterized with respect to their bulk (glass transition and crystallization temperatures, densities, molar volumes) and spectroscopic properties. Homogeneous glasses are formed up to x = 30, while crystallization of beta-PbF2 takes place at higher contents. Substitution of PbO by PbF2 shifts the optical band gap toward higher energies, thereby extending the UV transmission window significantly toward higher frequencies. Raman and infrared absorption spectra can be interpreted in conjunction with published reference data. Using B-11 and F-19 high-resolution solid state NMR as well as B-11/F-19 double resonance methodologies, we develop a quantitative structural description of this material. The fraction of four-coordinate boron is found to be moderately higher compared to that in glasses with the same PbO/B2O3 ratios, suggesting some participation of PbF2 in the network transformation process. This suggestion is confirmed by the F-19 NMR spectra. While the majority of the fluoride ions is present as ionic fluoride, similar to 20% of the fluorine inventory acts as a network modifier, resulting in the formation of four-coordinate BO3/2F- units. These units can be identified by F-19{B-11} rotational echo double resonance and B-11{F-19} cross-polarization magic angle spinning (CPMAS) data. These results provide the first unambiguous evidence of B-F bonding in a PbF2-modified glass system. The majority of the fluoride ions are found in a lead-dominated environment. F-19-F-19 homonuclear dipolar second moments measured by spin echo decay spectroscopy are quantitatively consistent with a model in which these ions are randomly distributed within the network modifier subdomain consisting of PbO, Bi2O3, and PbF2. This model, which implies both the features of atomic scale mixing with the network former borate species and some degree of fluoride ion clustering is consistent with all of the experimental data obtained on these glasses.

PbO-GeO2 rare earth doped glasses with silver nanoparticles as materials for IR laser triggers

We have shown the possibility of operation by the piezooptical response of PbO-GeO2 glasses doped with rare earth ions and silver nanoparticles by illumination of double frequency CO2 nanosecond laser. Substantial influence of thermoannealing on the output photoinduced elastooptical susceptibilities was established. The effect is very sensitive to temperature and to the corresponding tensor components. The effect of thermoannealing leads to enhanced long-range ordering with the occurrence of corresponding trapping levels within the forbidden gaps. The discovered effects may be used for creation of low-temperature IR laser triggers.

Photoluminescence from germanate glasses containing silicon nanocrystals and erbium ions

We report the first observation of photoluminescence enhancement in Er3+ doped GeO2-Bi2O3 glasses containing silicon nanocrystals (Si-NCs) excited by a laser operating at 980 nm. The growth of approximate to 200% in the intensity of the Er3+ transition S-4(3/2) -> I-4(15/2) (545 nm) and of approximate to 100% for transitions H-2(11/2) -> I-4(15/2) (525 nm), F-4(9/2) -> I-4(15/2) (660 nm), and I-4(5/2) -> I-4(13/2) (1530 nm) was observed in comparison with a reference sample that does not contain Si-NCs. The results open a new road for obtaining efficient Stokes and anti-Stokes emissions in germanate composites doped with rare-earth ions.

Frequency upconversion properties of Tm3+ doped TeO2-ZnO glasses containing silver nanoparticles

Frequency upconversion (UC) properties of Tm3+ doped TeO2-ZnO glasses containing silver nanoparticles (NPs) were investigated. Infrared-to-visible and infrared-to-infrared UC processes associated to the Tm3+ ions were studied by exciting the samples with a cw 1050 nm ytterbium laser. The luminescence intensity as a function of laser intensity was also measured using a pulsed 1047 nm Nd3+:YVO laser in order to determine the number of photons participating in the UC processes. Enhancement of the UC signals for samples heat-treated during various time intervals is attributed to the growth of the local field in the vicinity of the NPs. PL enhancement by one-order of magnitude was observed in the whole spectrum of the samples heat-treated during 48 h. On the other hand PL quenching was observed for the samples heat-treated more than 48 h. (c) 2011 Elsevier B.V. All rights reserved.

Efficient plasmonic coupling between Er3+:(Ag/Au) in tellurite glasses

We report a systematic study of the localized surface plasmon resonance effects on the photoluminescence of Er3+-doped tellurite glasses containing Silver or Gold nanoparticles. The Silver and Gold nanoparticles are obtained by means of reduction of Ag ions (Ag+ -> Ag-0) or Au ions (Au3+ -> Au-0) during the melting process followed by the formation of nanoparticles by heat treatment of the glasses. Absorption and photoluminescence spectra reveal particular features of the interaction between the metallic nanoparticles and Er3+ ions. The photoluminescence enhancement observed is due to dipole coupling of Silver nanoparticles with the I-4(13/2) -> I-4(15/2) Er3+ transition and Gold nanoparticles with the H-2(11/2)-> I-4(13/2) (805 nm) and S-4(3/2) -> I-4(13/2) (840 nm) Er3+ transitions. Such process is achieved via an efficient coupling yielding an energy transfer from the nanoparticles to the Er3+ ions, which is confirmed from the theoretical spectra calculated through the decay rate. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.

Cation Distribution and Local Order in Mixed Sodium Metaphosphate Glasses

Two structural properties in mixed alkali metal phosphate glasses that seem to be crucial to the development of the mixed ion effect in dc conductivity were systematically analyzed in Na mixed metaphosphates: the local order around the mobile species, and their distribution and mixing in the glass network. The set of glasses considered here, Na1-xMxPO3 with M = Li, Ag, K, Rb, and Cs and 0 <= x <= 1, encompass a broad degree of size mismatch between the mixed cation species. A comprehensive solid-state nuclear magnetic resonance study was carried out using P-31 MAS, Na-23 triple quantum MAS, Rb-87 QCPMG, P-31-Na-23 REDOR, Na-23-Li-7 and Li-7-Li-6 SEDOR, and Na-23 spin echo decay. It was observed that the arrangement of P atoms around Na in the mixed glasses was indistinguishable from that observed in the NaPO3 glass. However, systematic distortions in the local structure of the 0 environments around Na were observed, related to the presence of the second cation. The average Na-O distances show an expansion/compression When Na+ ions are replaced by cations with respectively smaller/bigger radii. The behavior of the nuclear electric quadrupole coupling. constants indicates that this expansion reduces the local symmetry, while the compression produces the opposite effect These effects become marginally small when the site mismatch between the cations is small, as in Na-Ag mixed glasses. The present study confirms the intimate mixing of cation species at the atomic scale, but clear deviations from random mixing were detected in systems with larger alkali metal ions (Cs-Na, K-Na, Rb-Na). In contrast, no deviations from the statistical ion mixture were found in the systems Ag-Na and Li-Na, where mixed cations are either of radii comparable to (Ag+) or smaller than (Li+) Na+. The set of results supports two fundamental structural features of the models proposed to explain the mixed ion effect: the. structural specificity of the sites occupied by each cation species and their mixing at the atomic scale.

The influence of SiO2 content on spectroscopic properties and laser emission efficiency of Yb3+-Er3+ co-doped calcium aluminosilicate glasses

We have studied the influence of SiO2 content on the spectroscopic properties and laser emission efficiency of Yb3+-Er3+ co-doped calcium aluminosilicate glasses. An increase in SiO2 content resulted in higher phonon energy, which reduced the up-conversion emission, enhanced the energy transfer efficiency up to 70 % from Yb3+ to Er3+, and enhanced the optical quality. All these results led to an increase from 20 to 30 % in the laser emission efficiency.

Infrared-to-visible upconversion emission in Er3+ doped TeO2-WO3-Bi2O3 glasses with silver nanoparticles

The influence of silver nanoparticles (NPs) on the frequency upconversion luminescence in Er3+ doped TeO2-WO3-Bi2O3 glasses is reported. The effect of the NPs on the Er3+ luminescence was controlled by appropriate heat-treatment of the samples. Enhancement up to 700% was obtained for the upconverted emissions at 527, 550, and 660 nm, when a laser at 980 nm is used for excitation. Since the laser frequency is far from the NPs surface plasmon resonance frequency, the luminescence enhancement is attributed to the local field increase in the proximity of the NPs and not to energy transfer from the NPs to the emitters as is usually reported. This is the first time that the effect is investigated for tellurite-tungstate-bismutate glasses and the enhancement observed is the largest reported for a tellurium oxide based glass. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754468]

Femtosecond third-order nonlinear spectra of lead-germanium oxide glasses containing silver nanoparticles

This work reports on the spectral dependence of both nonlinear refraction and absorption in lead-germanium oxide glasses (PbO-GeO2) containing silver nanoparticles. We have found that this material is suitable for all-optical switching at telecom wavelengths but at the visible range it behaves either as a saturable absorber or as an optical limiter. (C) 2012 Optical Society of America

Electrical conductivity of silicate glasses with tetravalent cations substituting Si

The effects of substituting Si by M4+ cations in soda-lime silica glasses were analyzed by impedance spectroscopy in the frequency range of 1 Hz-1 MHz. The glass composition was (mol%) 22Na(2)O center dot 8CaO center dot 65SiO(2)center dot 5MO(2), M = Si, Ti, Ge, Zr, Sn, and Ce. Although the Na+ concentration in the glasses is constant, the Zr-containing glass exhibits the highest dc conductivity and the lowest activation energy, while the Ce-containing glass exhibits the lowest conductivity. The activation energies obtained experimentally agree with those obtained by a theoretical equation proposed by Anderson and Stuart. The differences in electrical conductivity presented by the several M-containing glasses are attributed to the effect that the M4+ ion has on the mobility of the diffusing Na+ ion. (C) 2012 Elsevier B.V. All rights reserved.

Quantification of Short and Medium Range Order in Mixed Network Former Glasses of the System GeO2-NaPO3: A Combined NMR and X-ray Photoelectron Spectroscopy Study

Glasses in the system xGeO(2)-(1-x)NaPO3 (0 <= x <= 0.50) were prepared by conventional melting quenching and characterized by thermal analysis, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and P-31 nuclear magnetic resonance (MAS NMR) techniques. The deconvolution of the latter spectra was aided by homonuclear J-resolved and refocused INADEQUATE techniques. The combined analyses of P-31 MAS NMR and O-1s XPS lineshapes, taking charge and mass balance considerations into account, yield the detailed quantitative speciations of the phosphorus, germanium, and oxygen atoms and their respective connectivities. An internally consistent description is possible without invoking the formation of higher-coordinated germanium species in these glasses, in agreement with experimental evidence in the literature. The structure can be regarded, to a first approximation, as a network consisting of P-(2) and P-(3) tetrahedra linked via four-coordinate germanium. As implied by the appearance of P-(3) units, there is a moderate extent of network modifier sharing between phosphate and germanate network formers, as expressed by the formal melt reaction P-(2) + Ge-(4) -> P-(3) + Ge-(3). The equilibrium constant of this reaction is estimated as K = 0.52 +/- 0.11, indicating a preferential attraction of network modifier by the phosphorus component. These conclusions are qualitatively supported by Raman spectroscopy as well as P-31{Na-23} and P-31{Na-23} rotational echo double resonance (REDOR) NMR results. The combined interpretation of O-1s XPS and P-31 MAS NMR spectra shows further that there are clear deviations from a random connectivity scenario: heteroatomic P-O-Ge linkages are favored over homoatomic P-O-P and Ge-O-Ge linkages.

Down-conversion process in Tb3+-Yb3+ co-doped Calibo glasses

The down-conversion process in Tb3+-Yb3+ co-doped Calibo glasses was studied. The emission, excitation and time-resolved measurements indicated the existence of an energy conversion through the excitation of Tb3+ ions to near-infrared emission by Yb3+ ions. The emission intensity dependence on excitation power confirms that the one-photon process is responsible for the Yb3+ emission. An enhanced Yb3+ emission was observed with Yb3+ doping and an optimal energy transfer efficiency of 32% was obtained before reaching near-infrared emission quenching. The mechanism of the non-resonant energy transfer from Tb3+ to Yb3+ is discussed in terms of the Tb3+-Yb3+ cross-relaxation and multiphonon decay processes. (C) 2012 Elsevier B.V. All rights reserved.

Near-infrared quantum cutting through a three-step energy transfer process in Nd3+-Yb3+ co-doped fluoroindogallate glasses

The Nd3+-Yb3+ couple was investigated in fluoroindogallate glasses using optical spectroscopy to elucidate the energy transfer mechanisms involved in the downconversion (DC) process. Upon excitation of a Nd3+ ion by an ultraviolet photon, DC through a three-step energy transfer process occurs, in which the energy of the ultraviolet photon absorbed by the Nd3+ ion is converted into three infrared photons emitted by Yb3+ ions, i.e. quantum cutting (QC). In addition, with excitation in the visible, our results confirm that the DC process occurs through a one-step energy transfer process, in which the energy of a visible photon absorbed by the Nd3+ ion is converted into only one infrared photon emitted by an Yb3+ ion. Time-resolved measurements enabled the estimation of the efficiencies of the cross-relaxation processes between Nd3+ and Yb3+ ions.

Linking origin of the electric field-assisted beta-PbF2 crystallization in lead oxyfluoroborate glasses below T (g) to simultaneous cathode/anode-compensated electrochemical reactions

Full validation of the electrochemical mechanisms so far postulated as driving force of electric field-assisted non-spontaneous crystallization development in given glasses has suffered experimental restrictions. In this work, we looked into origin of this phenomenon in lead oxyfluoroborate glasses, resulting in beta-PbF2 growth even below the corresponding glass transition temperatures, through achieving a systematic study of not only Pt,Ag/Glass/Ag,Pt- but also Pt,Ag/Glass/YSZ:PbF2/Ag,Pt-type cells, where YSZ:PbF2 represents a two-phase system (formed by Y2O3-doped ZrO2 and PbF2). It is demonstrated that crystallization induction in these glasses involves Pb2+ ions reduction at the cathode, the phenomenon being, however, confirmed only when the F- ions were simultaneously also able to reach the anode for oxidation, after assuring either a direct glass-anode contact or percolation pathways for free fluoride migration across the YSZ:PbF2 mixtures. A further support of this account is that the electrochemically induced beta-PbF2 phase crystallizes showing ramified-like microstructure morphology that arises, accordingly, from development of electroconvective diffusion processes under electric field action.

Spectroscopic investigation and heat generation of Yb3+/Ho3+ codoped aluminosilicate glasses looking for the emission at 2 μm

Energy transfer (ET) and heat generation processes in Yb3+/Ho3+-codoped low-silica calcium aluminosilicate glasses were investigated using thermal lens (TL) and photoluminescence measurements looking for the emission around 2.0 μm. Stepwise ET processes from Yb3+ to Ho3+, upon excitation at 0.976 μm, produced highly efficient emission in the mid-infrared range at around 2.0 μm, with high fluorescence quantum efficiency (η1 ∼ 0.85 and independent of Ho3+ concentration) and relatively very low thermal loading (<0.4) for concentration up to 1.5% of Ho2O3. An equation was deduced for the description of the TL results that provided the absolute value of η1 and the number of emitted photons at 2.0 μm per absorbed pump photon by the Yb3+ ions, the latter reaching 60% for the highest Ho3+ concentration. These results suggest that the studied codoped system would be a promising candidate for the construction of photonic devices, especially for medical applications.