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

Etude des effets d'une explosion externe sur des réservoirs cylindriques : caractérisation du chargement

Ce travail vise à fournir des données de référence pour l'évaluation de la vulnérabilité de réservoirs métalliques soumis à une explosion externe. Des expérimentations ont été réalisées à échelle réduite satisfaisant des conditions de similitudes énergétiques et mécaniques. Des essais sur réservoirs rigides et déformables permettent de caractériser : le chargement issu d'une détonation (répartition spatio-temporelle de la surpression et de l'impulsion), la réponse au chargement des réservoirs (flambement). Des modélisations simplifiées sont confrontées aux résultats expérimentaux

Fluorescence of Eu3+ ions in TiO2 optical planar waveguides fabricated by the sol-gel method

In recent years, rare-earth-doped optical planar waveguides have attracted considerable attention owing to their potential use in the field of integrated optics. To this end, several processes have been employed to produce rare-earth-doped waveguides. Here, we report on the fluorescence of europium trivalent ions in TiO2 planar waveguides fabricated by using the sol-gel method and the dip-coating procedure. 2-mol % Eu3+ : TiO2 films were deposited on optically polished pyrex substrates. These films are 80 nm thick and their refractive index is ranging from 1.8 to 2.1 depending on the annealing temperature (100°C-600°C). The fluorescence of Eu3+ ions was obtained by the excitation in the 5D2 level with an Argon ion laser (wavelength = 465.8 nm). At room temperature, the fluorescence spectra were recorded for different annealing temperatures. A correlation was seen between the luminescence spectra and the structural evolution of the gel network. Indeed, it was observed that heat treatment increases the Stark splitting which partially removes the transitions J-degeneracy, indicating that Eu3+ ions occupy low-symmetry sites and are more embedded in the TiO2 gel (densification of the TiO2 gel). Furthermore, it was observed that increasing the annealing temperature entails a high frequency shift and a broadening of the [MATH] transition, which shows that Eu3+ ions occupy more distorted sites. these observations are in good agreement with results obtained by waveguide Raman spectroscopy

Tailoring strong CW supercontinuum in PCFs with two zero dispersion wavelenghts

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Tailoring CW supercontinuum generation in microstructured fibers with two-zero dispersion wavelengths

International audienceWe theoretically study broadband supercontinuum generation in photonic crystal fibers exhibiting two zero dispersion wavelengths and under continuous-wave pumping. We show that when the pump wavelength is located in between the zero-dispersion wavelengths, a wide and uniform spectral broadening is achieved through modulation instability, generation of both blue-shifted and red-shifted dispersive waves and subsequently through soliton self-frequency shift. This supercontinuum is therefore bounded by these two dispersive waves which allow the control of its bandwidth by a suitable tuning of the fiber dispersion. As a relevant example, we predict that broadband (1050-1600 nm) continuous-wave light can be generated in short lengths of microstructured fibers pumped by use of a 10-W Ytterbium fiber laser

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

Direct Access to L-Azetidine-2-carboxylic Acid

International audienceA straightforward synthesis of L-azetidine-2-carboxylic acid is described, leading to both orthogonally protected versions or totally deprotected L-Aze. The starting material is a commercially available aspartic acid derivative, whose chirality is conserved. The (2-trimethylsilyl)ethaneslfonyl protecting group (SES) acts as a leaving group on the hydroxy function and serves as an activator for the amine function, which is the key-step of the reaction

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

Synthesis and optical properties of MPTMS-capped CdS quantum dots embedded in TiO2 thin \ufb01lms for photonic applications

CdS nanoparticles have been prepared via a colloidal route using 3-mercatopropyltrimethoxysilane as a capping agent. The stability of the particles in solution and embedded in TiO2 matrices has been followed by optical absorption. Both the size and the size distribution of the particles are well controlled, thus allowing a tunable emission. Raman and photoluminescence spectroscopies have been used to characterize colloidal CdS, TiO2:CdS solutions and thin films deposited on soda-lime slides. The Z-scan technique has been used to measure the non-linear refractive indices of the solutions and the non-linear absorption coefficients of the thin films