M. Bouazaoui

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.

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.

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.

Application of molecular dynamics techniques and luminescent probes to the study of glass structure: the SiO2–GeO2 case

In this paper, we report on the results obtained from molecular dynamic simulation of a Eu3+-doped germanosilicate glass. This simulation provides further information on the structure. In particular it reveals a homogeneous distribution of SiO4 and GeO4 units, a decrease of defects compared to SiO2 and GeO2 glasses, and a trend to clustering of the doping ions. Using the modified crystal-field theory, the luminescence spectroscopic properties have been computed and comparison with experimental data has allowed a correlation of the spectral features with two main types of local environment depending on the coordination number and on the medium-range arrangement around the doping ions.

Raman spectroscopic characterization of Er3+-doped tellurite-based glasses

Er3+-doped tellurite-based glasses have been examined with the aim of determining the structural characteristics of systems with broad Er3+ emission bands. Raman spectroscopy was used for the characterization of the (100−x)TeO2–xZnO, and (100−x)TeO2–xPbO systems. Results show that in both cases, the addition of even small quantities of the metal oxide leads to a decrease in intensity of the ratio I(TeO4)/I(TeO3), with greater changes observed for the PbO systems. However, for higher concentrations (x>20% for ZnO and x>30% for PbO), concentrations at which these oxides behave as glass-formers, the I(TeO4)/I(TeO3) ratio becomes nearly constant, indicative of a blockage of the spatial orientation of the metal chains. The addition of a small amount of erbium ions (<1%) results in significant changes in the intensity of the low-frequency region and of the distribution of TeO3 and TeO3+1 groups in the case of PbO. Luminescence spectra of the Er3+-doped glasses show that the best luminescence properties are obtained when the TeO4 network is most intact, i.e. for low concentrations of the metallic oxide.

Effects of the sol-gel solution host on the chemical and optical properties of PbS quantum dots

PbS nanoparticles were prepared via a colloidal route using an organic capping agent, and were incorporated in both a silica and a titania sol-gel solution with a concentration of 10 mol%. The stability of the particles embedded in the sols was lengthened from 2 h to several days and their growth was considerably slowed down. The kinetics of the optical absorption allowed us to quantify the growth rate of the particles. Analysis of the optical emission in the three solutions (colloid, SiO2/PbS and TiO2/PbS) showed that the PbS nanocrystals are partially passivated by the silica chains and completely by the titania chains in the sols. Conjoint study of the optical absorption and emission measurements revealed a correlation between the presence of a structured exciton peak in the absorption spectra and the absence of shallow surface states.

UV-induced permanent gratings in sol–gel germanosilicate thin films

Abstract Germanosilicate thin films have been elaborated by the sol–gel process and the dip-coating technique. Pulsed or continuous wave UV laser (244 nm) was used to write permanent gratings in these films. In the case of exposure to cw laser, the grating diffraction efficiencies were measured using a focused beam from a He–Ne laser at 633 nm and photo-induced changes in refractive index as high as 4×10 −3 have been obtained. The thermal behaviour of these gratings has been investigated showing a good stability up to 400°C. Exposure to pulsed fringe pattern led to a glass photo-expansion modulated by a strong corrugation which can be due mainly to photo-ablation at the places of the bright fringes. The waveguide surface at the grating places was investigated through Atomic Force Microscopy (AFM) and microscopic profilometry techniques. Preliminary results on the kinetics of the grating growths are also reported.

Waveguide Raman spectroscopy: a non-destructive tool for the characterization of amorphous thin films

Abstract It is shown that waveguide Raman spectroscopy (WRS) can be used to study structural changes occurring in GeO 2 –SiO 2 and Al 2 O 3 –SiO 2 sol–gel derived planar waveguides as a result of variations both in composition and in annealing temperature. Bands assigned to ring defects characteristic of pure silica are absent in the germano-silicate systems indicating the destabilizing effect of the inclusion of GeO 2 in the matrix. In all cases, WRS shows that densification resulting from thermal treatment is accompanied by T–O–T (T = Si, Ge or Al) angle changes which cause an appreciable rearrangement of the glass network. Data for the low-wavenumber region show that the densification process is correlated to the position of the boson peak. The fact that this band shifts towards higher values with annealing temperature, but towards lower wavenumbers for rare-earth doped systems is interpreted in terms of changes in the diameter of cohesive domains and consequently to the degree of the densification process. Finally, for the alumino-silicate system, WRS spectral changes indicate the initiation of crystallization at high temperatures. The position of the low-wavenumber band is used to estimate the average size of the micro-crystallites formed in the matrix.

Densification and crystallization processes of aluminosilicate planar waveguides doped with rare-earth ions

Aluminosilicate planar waveguides have been prepared using a sol–gel process and the dip-coating technique. The structure of Er3+- and Ce3+-doped as well as undoped films has been investigated as a function of the annealing temperature. Low-wavenumber Raman scattering and opto-geometrical properties of these thin films have demonstrated the role of Er3+ and Ce3+ ions in the densification and nucleation processes. Indeed, doping with rare-earth ions first hinders the densification process in the amorphous phase, while crystallization begins at a lower annealing temperature for the doped waveguides. It was found that the two behaviors are intimately linked to the doping effect.