S. Turrell

Raman spectroscopic investigations of the effect of the doping metal on the structure of binary tellurium-oxide glasses

Raman studies over the range 10 to 1000 cm−1 have been performed on binary tellurium-oxide glasses (1-x)TeO2-xMO (M = Pb,Zn or Mg) prepared using a conventional melt technique. The intensities and positions of Raman bands observed in the range above 250 cm−1 were found to depend both on the compound oxide and on the amount of doping. Indeed, the ratio of the intensity of the band around 680 cm−1 (assigned to vibrations of TeO4 trigonal bipyramids) with respect to that of the component around 750 cm−1 (related to stretching-vibrations in TeO3+1, TeO3 and MO groups) are affected in different ways for the glass-modifier MgO and for the intermediate glass-formers PbO and ZnO. Concurrently, an explicit dependence on the compound oxide and amount of doping was also observed on the maximum of the boson peak (BP) in the low-frequency region around 40 cm−1. The structural correlation lengths in the glasses, calculated using the model described by Shuker and Gammon, were found to be about 0.50 nm (1-x)TeO2-xMgO glasses and around 0.65 nm for (1-x)TeO2-xMO (M = Pb or Zn) glasses. All these results are interpreted in terms of the effect of the metal oxide on the changes induced in the structural arrangements of 1χ[TeO4-TeO3] chains.

Enhanced fluorescence from Eu3+ in low-loss silica glass-ceramic waveguides with high SnO2 content

We report on the sol-gel fabrication and characterization of (100−x)SiO2–xSnO2 (x=8, 16, and 25mol%) glass-ceramic waveguides doped with 1mol% Eu3+. A suitable top-down thermal process led to the formation of SnO2 nanocrystals ∼4nm embedding Eu3+ ions. The excitation spectra evidence the role of interband electronic transition of SnO2 nanocrystals on the luminescence of Eu3+. Monitoring the D05→F27 Eu3+ emission, we observe about 15 times increase in the intensity of SnO2 absorption band, moving from x=8to25mol%. These waveguides also exhibit low losses, making them quite promising for development of high-gain integrated optical amplifiers.

Alteration of lead silicate glasses due to leaching in heated acid solutions

Leaching processes were studied for lead glasses having a composition similar to that of certain archaeological materials (66 wt% PbO) to determine the mechanism of the structural evolution. The glasses were leached in two static acid media (HNO3 and CH3COOH, pH 2) at 90 °C for 35 days. Analyses were undertaken of the leaching solution (pH, inductive coupled plasma–atomic emission spectrometry) and of the bulk glass (scanning electron microscopy–energy dispersive X-ray spectrometry, Rutherford backscattering spectrometry, Raman). Results show that in both acids, up to 1 day a silicon-rich surface layer is formed via typical diffusion processes. This process continues up to 35 days in the case of acetic acid. In nitric acid, one observes a stabilisation of this layer and an increase in the metal content. In both cases, Raman data are used to interpret the structural evolutions which occur in the ‘gel’ phase.

Kinetics of densification of porous silica gels: a structural and textural study

The present work is devoted to the structural analysis of silica xerogel samples containing controlled pore sizes ranging from 25 to 270 A. It focuses on the kinetics of densification processes. Porous xerogels were treated in air at the fixed temperature of 1050 °C for periods varying from 10 min to 3 h and were allowed to cool down in open air. They were characterized by density and nitrogen adsorption–desorption measurements, as well as by Raman spectroscopy. The structural studies were based on the evolution of the intensities and profiles of certain characteristic bands in the Raman spectra. Among these are the two defects bands, D1 and D2 associated with the vibrations of four- and three-membered silica rings, respectively, and the ν(Si–O–Si) band centered at 430 cm−1. Analyses allowed a characterization of the structural changes occurring with densification of the amorphous matrix. In addition it was found that all modifications are brought to a halt after a characteristic time which depends on the initial porosity. It is important to note that the final microscopic structure is the same for all samples regardless of the initial pore size.

Spectroscopic investigations of CdS nanoparticles in sol-gel derived polymeric thin films and bulk silica matrices

Absorption, photoluminescence and Raman scattering spectroscopies have been performed on two kinds of SiO2 bulk matrices and sol-gel polymeric thin films, containing CdS nanoparticles. Waveguiding in the polymeric CdS films has been demonstrated for wavelengths in the visible region. A simple surface energy diagram, including defect levels, has been used to explain the size-selective luminescence of the nanocrystals. The luminescence spectrum is composed of an excitonic recombination band and a broader red-shifted band, imputed to the transition between the surface trap levels. The particle interaction with the silica xerogels appears to enhance these surface states. Resonance Raman scattering shows that the excitons are coupled to LO-phonon modes, but the spectra exhibit no matrix effect on the frequency vibration of the fundamental LO phonon.

Preparation of SiO2—GeO2: Eu3+ planar waveguides and characterization by waveguide Raman and luminescence spectroscopies

Abstract SiO2—GeO2 planar waveguides, doped with Eu3+ ions, have been prepared using the dip-coating technique. Optical characterization of the waveguides has been performed by m-line spectroscopy. The structural modification occurring during the densification process has been followed by waveguide Raman and luminescence spectroscopies. A strong rearrangement in the glass occurs after annealing at 900°C and the final structure appears more ordered.

Optical spectroscopy of Pr3+ ions in sol-gel derived GeO2-SiO2 planar waveguides

A GeO 2 -SiO 2 planar waveguide, activated with 1% mol Pr 3+ , has been prepared using a dip-coating technique. The GeO 2 -SiO 2 film was deposited on a substrate consisting of a silicon wafer with a silica buffer layer. After annealing at 800°C and 900°C the waveguide luminescence and Raman spectra were measured. The Raman spectra show that complete densification is achieved after annealing at 800°C. Analysis of the emission spectra and the decay curves from the 3 P 0 and 1 D 2 states of Pr 3+ indicates that the non-radiative relaxation due to the rare-earth clustering is less important than in massive silica xerogels.

Femtosecond laser direct writing of gratings and waveguides in high quantum efficiency erbium-doped Baccarat glass

The femtosecond laser direct writing technique was employed to inscribe gratings and waveguides in high quantum efficiency erbium-doped Baccarat glass. Using the butt coupling technique, a systematic study of waveguide loss with respect to input pulse energy and writing speed was performed to achieve the best waveguide with low propagation loss (PL). By pumping at 980 nm, we observed signal enhancement in these active waveguides in the telecom spectral region. The refractive index change was smooth and we estimated it to be ~10−3. The high quantum efficiency (~80%) and a best PL of ~0.9 dB cm−1 combined with signal enhancement makes Baccarat glass a potential candidate for application in photonics.

Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Abstract Undoped and Mn 2+ -doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and fluorescence and Raman spectroscopies. Raman measurements over the range 4–1200 cm −1 showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn 2+ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn 2+ hampers the destruction of three- and four-membered rings. In the low-wave number region (4–100 cm −1 ), doping with manganese ions was found to affect the position of the boson peak. The boson peak profiles were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm).

Spectroscopic study of pH and solvent effects on the structure of Congo red and its binding mechanism to amyloid-like proteins

Abstract The pH and solvent effects on the structure of the azo dye Congo red were investigated. The absorption, resonance Raman and NMR spectra give evidence of the existence of different protonated dye molecules in acidic solutions. There exist a red form with an azoic structure and at least three blue forms with quinoid structures. The position of the equilibrium between the azoic and quinoid structures depends on the solvent and the pH and is related to the capability of the solvent to form three and two dimensional solvation structures, respectively. Spectral variations similar to those observed for Congo red dissolved in organic solvents accompany binding of the dye to amyloid-like proteins. This observation leads to the suggestion that the dye molecules must experience similar changes in environment in both cases.