Investigation of the local environment of Eu3+ in a silicophosphate glass using site-selective spectroscopy and Molecular Dynamics simulations

Silicophosphate glasses (SiO2-P2O5) doped with Eu3+ ions were synthesized by the sol-gel process. Optical properties of these glasses were investigated by means of emission spectra and lifetime measurements. The Fluorescence Line Narrowing (FLN) technique was also used to explore the local structure around the Eu3+ ions in this host and to understand the role of phosphate as a codopant. As it is the case for aluminum, the ability of phosphate to avoid the rare earth clustering was investigated, and the role of this codopant in modifying the local order around the rare earth ion was evidenced. The analysis of the FLN spectra and lifetime measurements is consistent with this interpretation. Molecular Dynamics simulations were performed to evaluate and confirm these structural features. Two classes of europium sites were distinguished in agreement with the experimental characterization

Fluctuations in active membranes

Active contributions to fluctuations are a direct consequence of metabolic energy consumption in living cells. Such metabolic processes continuously create active forces, which deform the membrane to control motility, proliferation as well as homeostasis. Membrane fluctuations contain therefore valuable information on the nature of active forces, but classical analysis of membrane fluctuations has been primarily centered on purely thermal driving. This chapter provides an overview of relevant experimental and theoretical approaches to measure, analyze and model active membrane fluctuations. In the focus of the discussion remains the intrinsic problem that the sole fluctuation analysis may not be sufficient to separate active from thermal contributions, since the presence of activity may modify membrane mechanical properties themselves. By combining independent measurements of spontaneous fluctuations and mechanical response, it is possible to directly quantify time and energy-scales of the active contributions, allowing for a refinement of current theoretical descriptions of active membranes.Comment: 38 pages, 9 figures, book chapte

A NGR STUDY OF IRON CATALYSTS IN CO HYDROCONDENSATION REACTIONS

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Modeling of Rare Earth-doped Silicate Glasses: Codoping Effects on the Luminescent Sites Structures and Formation of Dielectrics Nanoparticles

International audienceRare-earth (RE) containing glasses are known to be good candidate materials for devices such as optical fiber amplifiers, upconversion lasers and glass lasers. In such materials, strong changes in the luminescent properties can be induced by changes in the distribution and local environment of the RE ions. The investigation of the local structure around the RE ions in glasses is therefore a key issue to interpret their luminescent properties in terms of local structure. Our group is for a long time involved in the study of these properties by means of an original approach which combines photolumines-cence measurements and Molecular Dynamics (MD) modeling [1]. We use the Fluorescence Line Narrowing (FLN) as a site-selective technique to determine the different types of site occupied by the Eu 3+ ions. Structural information on these sites is deduced from FLN spectra and from the structures modeled by MD simulations. Using this methodology, we especially study the effective role of a codoping in the enhancement of the fluorescence of RE ions, which is commonly attributed to the ability of certain cations to avoid the RE clustering. The presented work focuses on the comparison between Al 3+ and P 5+ codoping of a RE-doped silica glass. Surprisingly, both cations do not act to disperse significantly the clustered RE ions but strongly modify the local structure of the luminescent ions. Another unexpected result is the striking difference between Al 3+ and P 5+ in the way they modify the first and the second coordination shell of RE 3+ [2][3]. Mg-codoping of a RE-doped silica glass is another route to tailor the spectroscopic features of optical fibers. Since it was experimentally evidenced that the RE ions can be embedded in nanoparticles (NP) formed in situ in silica through phase separation, we have developed an adaptive and transferable MD potential to model and track the formation of Mg-rich amorphous NP in a Si-rich matrix [4]. We present here our results on the dependence between the RE environment and the size of the containing NP. [1] S. Snapshot of a modeled structure: Er 3+-doped nanoparticles of Mg-rich phase in a silica matrix

Molecular Dynamics Simulations of Rare-Earth-Doped Nanoparticles in Silica Matrix: Drawing of a Preform to a Fiber

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RARE-EARTH DOPED MG-SILICATE NANOPARTICLES IN SILICA FIBER: MOLECULAR DYNAMICS SIMULATIONS FROM THE PREFORM TO THE FIBER

International audienceAn enhancement of the spectroscopic performance of rare-earth-doped silica optical fibers is still required for new photonics applications. An interesting route to tailor their optical behavior consists in embedding rare-earth ions within dielectric nanoparticles in the core of optical fibers. Experimentally, such nanoparticles can be produced in situ through spontaneous phase separation phenomenon within a MgO-SiO2 binary melt, during melt/quench sequences of MCVD fabrication process of the preform 1,2. Then, fibers are obtained by drawing at high temperature a preform containing nanoparticles. First report on the drawing process reveals an elongation of the nanoparticles in the drawing direction as well as a breakup of the larger ones 3. In this Molecular dynamics study, we use a new simple transferable model 4 to show that phase separation occurring in the MgO-SiO2 binary melt leads to the separation of liquid phases with mixed composition: Si-rich Mg-poor phases on one hand, Mg-rich Si-poor phases on the other hand. These latter phases, the so-called nanoparticles, are amorphous, non-spherical and exhibit a wide range of sizes. Mg-O coordination and MgO content increase with the nanoparticle size. With rare-earth doping, the larger nanoparticles are over-concentrated in luminescent ions. We show that the rare-earth clustering effect is prevented, compared with a pure silica matrix. Finally, at high temperature, we apply a uniaxial elongation to the nanostructured preform to mimic the experimental drawing step leading to the fiber. We report here on the effects of this drawing process on the nanoparticles characteristics

Cell protrusions and contractions generate long-range membrane tension propagation

Membrane tension is thought to be a long-range integrator of cell physiology. Membrane tension has been proposed to enable cell polarity during migration through front-back coordination and long-range protrusion competition. These roles necessitate effective tension transmission across the cell. However, conflicting observations have left the field divided as to whether cell membranes support or resist tension propagation. This discrepancy likely originates from the use of exogenous forces that may not accurately mimic endogenous forces. We overcome this complication by leveraging optogenetics to directly control localized actin-based protrusions or actomyosin contractions while simultaneously monitoring the propagation ofmembrane tension using dual-trap optical tweezers. Surprisingly, actin-driven protrusions and actomyosin contractions both elicit rapid global membrane tension propagation, whereas forces applied to cell membranes alone do not. We present a simple unifying mechanical model in which mechanical forces that engage the actin cortex drive rapid, robust membrane tension propagation through long-range membrane flows

Tm-doped nanoparticles in optical fibers

International audienceThe success of silica-based optical fibers are many: transmission fibers and fiber amplifiers for telecommunications, high-power fiber lasers or sensors. These key applications rely on the qualities of silica glass: mechanical and chemical stability, high optical damage threshold, low cost, etc. New lasers and amplifiers based on rare-earth (RE)-doped silica optical fibers need improved spectroscopic performances : gain curve engineering, photodarkening, spectral coverage, etc. In this context, a route of interest consists of embedding the RE ions within nanoparticles of composition and structure different from those of silica. In this work, we study the properties of silica-based, MCVD-prepared fibers using LaF3:Tm3+ nanoparticles. The nanoparticles with 10-20 nm of diameter were produced by precipitation methods and were incorporated by solution doping. Through SEM analyses on preform and fiber, nanoparticles were observed across the core. As F-ions evaporate, the new phase is a La-rich silicate and its composition will be discussed based on the comparison with similar samples prepared by sol-gel. The first e-folding time of the 810-nm emission band (3H4 level) increases with the concentration of La. The best compromise between lifetime enhancement and optical attenuation corresponds to 58 μs and 0.05 dB/m, respectively. Yet, to further improve the optical properties, there is a need to limit Rayleigh scattering induced by the presence of nanoparticles. In the frame of these optical losses reductions, we propose to take advantage of the fiber drawing to tailor the size of nanoparticles. Indeed, we will report evidences that this step permits the deformation and break-up of elongated particles. The possibility of considering break-up as a way to implement size tailoring of nanoparticles will be discussed. These results clearly offer new possibilities for the control of the luminescent properties and the development of optical fibers with augmented properties