Carl W. Ponader

Clustering of rare earths in GeAs sulfide glass

Abstract Clustering of rare-earth dopants in GeAs sulfide glasses was studied by fluorescence spectroscopy of Pr-doped glasses and by EPR measurements of Gd-doped samples. The linewidth of the gxa0∼xa02 resonance of Gd3+, as well as the relative intensity of emission from the 1D2 level of Pr3+, was used as a relative measure of rare-earth clustering. Rare earths were found to have low solubility in uncodoped GeAs sulfide glasses, which also displayed poor fluorescence efficiency due to severe clustering. Codoping such glasses with Ga greatly enhanced rare-earth solubility and dispersal, particularly for Ga:rare earth ratiosxa0≥xa010:1, as evidenced by the narrower EPR resonances and more intense luminescence of Gd- and Pr-doped glasses, respectively. In, P and Sn were also observed to ‘decluster’ rare earths, although less efficiently than Ga, whereas codoping with I was found to have no effect on clustering. These phenomena are explained by a structural model in which (1) rare-earth dopants and codopants are spatially associated and (2) rare-earth dispersal is accomplished by a statistical distribution of codopants in tetrahedral network sites.

Physical properties and Raman spectroscopy of GeAs sulphide glasses

Ternary GeAsS glasses have been investigated to determine the compositional dependence of selected physical properties. Although the latter show a linear variation with Ge:As ratio at a constant S content, many exhibit a non-linear dependence on the S content at a fixed Ge:As ratio. The trend of optical properties tracks the increasing formation of metal-metal bonds as the S concentration of these glasses is reduced below stoichiometry. Molar volume and thermal expansion coefficient display maxima near an average coordination number of 2.6 for which there is no apparent indication in the Raman spectra. The spectra of stoichiometric and S-excess glasses are dominated by a band at 345 cm -1 due to the symmetric stretching of both AsS 3 and GeS 4 groups; those of S-deficient glasses display bands associated with metal-metal bonds between 210 and 240 cm -1 whose intensity increase with increasing S deficiency.

Origin of the refractive-index increase in laser-written waveguides in glasses

We present firm evidence that the mechanism for the refractive-index increase in fused silica caused by irradiation with ultrafast intense laser pulses is the densification of glass. This conclusion is based on the correlation observed between the refractive-index values in waveguides in silica produced by focused femtosecond laser pulses and the shift of the central frequency of ω4 (TO) band (Si–O stretching mode) in micro-Raman spectra. These data were compared with the relation of the Raman shift to density and to refractive index changes in glasses modified by high pressure or irradiation. We conclude that the measured refractive-index increase in silica waveguides can be explained by densification of glass and exclude other hypothesis such as fictive-temperature effect, color center formation, etc.

Property extrema in GeAs sulphide glasses

Abstract GeAsS glasses with Ge:As ratios of 2.5:1, 1:1 and 1:2 with S concentrations ranging from 35% to 80%, corresponding to an average coordination number (〈r〉) ranging from 2.35 to 2.98 were synthesized. For glasses with fixed Ge:As ratio, the absorption edge and the glass transition temperature vary monotonically with S content. The thermal expansion coefficient and molar volume, however, attain maxima at 〈r〉 near 2.6, which extrema correspond to a maximum in the relative intensity of a narrow polarized Raman band at 275 cm−1 and a minimum in the frequency of the boson peak. These features shift to smaller 〈r〉 with decreasing Ge:As ratio and are attributed to the development of molecular As4S3-like species in GeAsS glasses with intermediate S deficits.

X-ray absorption study of the coordination of titanium in sodiumtitaniumsilicate glasses

Abstract The local structure around titanium in sodiumue5f8titaniumue5f8silicate glasses was determined as a function of the silica content and the Na Ti ratio. The samples covering the range 16.7–33 mol% Na2O and 16.7–33 mol% TiO2 were prepared by melting predetermined mixtures of Na2CO3, TiO2, and SiO2 at 1400°C. They were studied using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies around the Ti K-edge at 4966 eV. The data indicate that titanium is found in two sites in the glass. In one site, titanium is coordinated by six oxygens; in the other, titanium is coordinated by five oxygens. The number of five-coordinated titanium atoms increases relative to the number of six-coordinated titanium atoms as the silica content of the glass decreases. The ratio of five- to six-coordinated sites decreases as the Na Ti ratio of the glass decreases at a fixed silica content.

Atomic structure and chemical order in Ge-As sulfide glasses : a combined Ge and As K-edge EXAFS study

Abstract The coordination environments of Ge and As atoms in Ge x As x S 100−2 x glasses with 13.3⩽ x ⩽32.5 have been studied with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold- and threefold-coordinated, respectively, in all glasses. The atomic structures of the stoichiometric and S-excess glasses are found to consist of GeS 4 tetrahedra and AsS 3 trigonal pyramids implying the preservation of chemical order at least over the length scale of the first coordination shell. As–As homopolar bonds are found to appear at low and intermediate levels of S-deficiency. Ge–Ge bonds are formed in extremely S-deficient glasses only after all As atoms participate in homopolar As–As bonds, implying clustering of like metal atoms and violation of chemical order in S-deficient glasses. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physical properties in these glasses.

Analysis of residual stress in optical fiber

Residual stress in an optical fiber impacts several fiber properties, including reliability and geometry. The residual stress profile arises from a thermal expansion mismatch of the constituent materials, the tension applied during fiber forming, and the thermal profile experienced by the fiber during formation. The thermal profile of the fiber is determined by measuring the intensities of the two `defect bands in the silica cladding of the fiber using Raman spectroscopy. Other studies have shown that the intensities of these bands increase with increasing fictive temperature and that these changes can be used to map changes in fictive temperatures across the fiber diameter. Optical birefringence is used to measure the residual stress profile of the fiber. The relationship between fictive temperature and fiber residual stress is explored for both straight fibers and fibers with a natural radius of curvature (curled fibers).

The effect of phosphorus on the properties and structure of GeAsS glasses

Stoichiometric Ge-poor (<50% GeS2) GeAsPS glasses, as well as GeAsPS glasses with a fixed Ge:As:P ratio of 5:1:1 and S contents ranging from 56% to 74%, were synthesized. The compositional dependence of the physical properties and nuclear magnetic resonance (NMR) spectra of the stoichiometric glasses indicates that tetrahedral Sue605PS3/2 groups constitute the dominant P species. On reducing the S content, the absorption edge of variable S GeAsPS glasses remains constant until S deficits of about 25% are attained. Raman spectra of stoichiometric and S-deficient glasses show bands attributed to As2P2S8 and (As,P)4S3 species, respectively, whereas those of variable S glasses with S deficits ⩽15% show a high frequency band indicating the presence of tetrahedral Sue605PS3/2 groups. NMR spectra of glasses with S deficits of 7–15% are dominated by a 31P resonance at about 117 ppm that is assigned to trigonal PS3/2 groups. NMR spectra of glasses with S deficits ⩾10% have a 31P resonance near −100 ppm, indicating that tetrahedral Sue605PS3/2 and trigonal PS3/2 groups are replaced by species containing P–P and, ultimately, P–(Ge, As) bonds as the S content of these glasses decreases. Comparison of these results with those for GeAsS glasses demonstrates that P–P bonds form preferentially to metal–metal bonds containing either Ge or As, and that P tends to associate with As instead of Ge as next nearest neighbors.

Refractive-index increase in laser-written waveguides in glasses

We present estimates of the refractive-index change in waveguides in silica produced by focused femtosecond laser pulses. The estimates are based on the shift of the central frequency of ω4 (TO) band (Si-O stretching mode) in micro-Raman spectra. These data were compared with the relation of this parameter to density and to refractive index changes in seen in glasses modified by high pressure or irradiation. We conclude that the measured refractive-index increase in the waveguides can be explained by densification of glass.