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The development of new applications based on glass doped with nanoparticles is growing in interest during the last years. The structure and properties of Ca-based silicate nanoparticles formed in situ in a silica matrix through a phase separation mechanism were investigated by using Molecular Dynamics simulations and compared to nanoparticles formed from MgO-codoping. We showed that such nanoparticles have non-spherical shape, are amorphous and inhomogeneously distributed in the host glass. In this modeled structure, a release of non-bridging oxygen atoms, due to a depolymerization phenomenon of the nanoparticles’ silica network, was observed. Besides, we demonstrated that nanoparticles’ composition is size-dependent. Compared to Mg-silicate nanoparticles, Ca-based nanoparticles are larger, less concentrated in Ca, and we observed a steeper concentration change during the phase separation process. Those differences are related to the diffusion coefficients of Ca and Mg. This numerical analysis informs on the alkaline-earth nanoparticles’ properties within a host silica glass, which can be a relevant guide for the development of new materials for applications such as nanoparticle-doped optical fibers.

The development of new applications based on glass doped with nanoparticles is growing in interest during the last years. The structure and properties of Ca-based silicate nanoparticles formed in situ in a silica matrix through a phase separation mechanism were investigated by using Molecular Dynamics simulations and compared to nanoparticles formed from MgO-codoping. We showed that such nanoparticles have non-spherical shape, are amorphous and inhomogeneously distributed in the host glass. In this modeled structure, a release of non-bridging oxygen atoms, due to a depolymerization phenomenon of the nanoparticles' silica network, was observed. Besides, we demonstrated that nanoparticles' composition is size-dependent. Compared to Mg-silicate nanoparticles, Ca-based nanoparticles are larger, less concentrated in Ca, and we observed a steeper concentration change during the phase separation process. Those differences are related to the diffusion coefficients of Ca and Mg. This numerical analysis informs on the alkaline-earth nanoparticles' properties within a host silica glass, which can be a relevant guide for the development of new materials for applications such as nanoparticle-doped optical fibers.

International audience; Europium (Eu)-doped silica nanoparticles have attracted great interest for different applications, in particular in biomedicine as biosensors or for tissue regeneration. Sol-gel is the most common process used to prepare those particles, with size varying from tens to hundreds of nanometers. In this article, we focus our attention on the comparison between two commonly used sol-gel derived methods: reverse microemulsion (for particles smaller than 100 nm) and Stöber method (for particles larger than 100 nm). Europium concentration was varied between 0.2 and 1 mol%, and the nanoparticle diameters were 10, 50 and 100 nm. The link between the local environment of europium ions and their optical properties was investigated and discussed. Using Transmission Electron Microscopy, nitrogen sorption, X-ray diffraction, Fourier-Transform Infra-Red and pulsed doubled Nd:YAG laser, we confirmed that fluorescence lifetime was improved by thermal treatment at 900 °C due to the elimination of aqueous environment and modification of structure disorder. The size of nanoparticles, the amount of europium and the thermal treatment of obtained materials influence the emission spectra and the decay curves of Eu3+.

International audience; Developing new rare-earth-doped optical glasses with "enhanced" spectroscopic properties requires the elaboration of new glass compositions. To overcome some typical limitations of silica glass, a strategy consists in encapsulating rare-earth (RE) ions within oxide nanoparticles (NPs) through a phase separation mechanism. In this paper, Molecular Dynamics simulations were performed using an interatomic potential reproducing the phase separation within a MgO–SiO2 binary melt to obtain RE-codoped glass models with RE = Eu or Er. In these structures, we observed that Mg-rich regions, included into a silica-rich matrix and identified as NPs, are amorphous and exhibit a large range of sizes. We showed that such nanoparticles are the host of a depolymerization phenomenon of the NPs’ SiO4 tetrahedral network leading to a release of non-bridging oxygen atoms. In the NPs, the MgO concentration increases and the doping RE ions are mainly located into the NPs where they are over-concentrated compared with the nominal doping concentration. However, the induced clustering effect is limited because of the non-bridging-oxygen-rich environment encountered in the NPs. This numerical analysis allows to give an insight on the chemical composition of the NPs, and especially on the local environment of the encapsulated rare-earth ions.

Rare-earth (RE) doped optical fibers are extensively used in lasers and optical amplifier devices. These key applications rely on the qualities of silica glass: mechanical and chemical stability, high optical damage threshold, low cost, etc. However, silica glass has certain characteristics which may make it less efficient compared to other types of glass, particularly in some potential applications using RE ions: high phonon energy, low solubility of RE ions, etc. To overcome these limitations, one recent strategy consists of developing a fabrication method which triggers RE encapsulation in phase-separated nanoparticles. The development of this family of optical fibers was driven by this requirement: the particles must be as small as possible to avoid light scattering. However, recent studies discussed in this article tend to disapprove this doxa. First, we present the fabrication process of such fibers, emphasizing the drawing step as a process to control the shape and size of the nanoparticles. Then, we discuss on the characterization of the composition of these nanoparticles at the nm-scale. To reach this goal, we took advantage of a recent technology: Atom Probe Tomography. These results will be compared with molecular dynamics simulations. We demonstrate that the phase-separated nanoparticle composition and therefore the chemical environment of the encapsulated RE ions is nanoparticles size dependent. As a consequence, the smallest nanoparticles, promoted by the doxa, would offer limited alteration of the luminescent properties. Finally, light scattering is not only an issue but is also an opportunity to develop new temperature, strain, refractive index multiplexed optical fiber sensors.

Doping material with nanoparticles is increasingly used as an effective method for improving their mechanical, optical, and sturdiness properties in many fields. More specifically, effective materi...

International audience; 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.

International audience; In this work, we investigate the effect of co-doping with CdS nanoparticles on the photoluminescence properties of Eu3+ doped silicophosphate glass prepared via the sol gel method. Infrared spectroscopy (FTIR) revealed the insertion of phosphorus within the silicate network. XRD and TEM analyses revealed the presence of CdS nanoparticles dispersed in the glass matrix. Based on the optical study and the effective mass theory for spherical quantum dots, it was found that CdS nanocrystals have a gap of nearly 3.53 eV and a size of 2.42 nm. The enhancement of Eu3+ emission induced by CdS nanocrystals and thermal annealing was assigned to either an energy transfer via defect states or structural alteration of the glass network around the rare earth ions.

The success of glass photonics are many, with a large spectrum of applications covering Information and Communication Technology, Health and Biology, Structural Engineering, and Environment Monitoring Systems, etc. A paradigmatic example is the development of the Erbium-Doped Fiber Amplifier (EDFA) allowing the raise of optical telecommunications and high speed internet. Silica glass is the most common glass used for photonics applications. However, certain of its characteristics may be detrimental for luminescence properties (low rare-earth solubility, high phonon energy, etc). To overcome these limitations, the use of nanoparticles in optical waveguides has gained large interest in modern photonic technology nowadays. Such waveguides would combine the advantages of silica (transparency, cost, chemical and mechanical durability, etc.) and the specific properties provided by the nanoparticles which encapsulate the rare-earth ions. In this paper, we discuss on the opportunities offered by the nanoparticles addition in glasses for lasers and amplifiers thanks to the engineering of the spectroscopic properties. We present also an emerging application: sensors based on high scattered fibers.

By means of Molecular Dynamics simulations, rare-earth-doped nanoparticles are formed in situ through phase separation mechanism within MgO-SiO 2 binary melt using a simple adaptive interatomic potential. The simulated preforms are doped with Eu3+ and Er3+ ions, and are then deformed into a fiber while glass flow effects (thermodynamical and rheological) leads to modifications of the nanoparticle's morphology. In this study, we highlight a rare-earth encapsulation in Mg-silicate nanoparticles more efficient for Er3+ ions than for Eu3+ ions. Moreover, the MgO concentration seems to converge toward a maximum value when the nanoparticles grow. Finally, preliminary results concerning the influence of the drawing parameters (temperature, elongation velocity) on the size’s distribution and on the morphology of the nanoparticles are also presented.

International audience; Rare-earth-doped oxide nanoparticles in the core of silica optical fibers are becoming well studied as they yield enhanced and tailorable spectroscopic and optical properties. In this paper, the evolution of particle morphology , induced by the drawing step, is studied. Indeed, during the fiber draw process, the glass flows and particles can elongate and even break-up into smaller particles through Rayleigh-Plateau instabilities. The shape of elongated particles is related to the composition as it depends on the viscosity ratio between the particle and the matrix. Moreover, a lower drawing temperature enhances the break-up phenomenon. These observations offer new possibilities for the control of the size and the shape of particles, hence performance of active optical fibers.

International audience; A novel aspect of the medium-range structure of silica drawn into fibers is studied. The network of silica glass structure is composed of corner-shared SiO 4 tetrahedra, and it can be seen as a structure of interconnected rings (Si-O) n of various size, denoted nMR (n-Membered Ring). Molecular Dynamics simulations show that small-sized silica rings get a preferential orientation during the drawing, either during the high-temperature stage for 3MR, or during the cooling for 4MR and 5MR, and they persist in this state in the fiber at ambient temperature. This leads to a structural anisotropy, more specifically a " transverse isotropy " , because of different longitudinal and transversal physical properties. This anisotropic structural rearrangement during the drawing process induces a non-Newtonian behavior of the modeled glass melt, with strain-rate dependent properties. Highlights: Anisotropy in silica glass comes from the orientation that small silica rings acquire during the deformation. The model is in agreement with experiments (non-Newtonian behavior of the melt, anisotropic elasticity of the fiber). The anisotropy in silica fiber is a " transverse isotropy " .

A Molecular Dynamics study of rare-earth doped Mg-silicate nanoparticles in vitreous silica: from the preform to the fiber. New lasers and amplifiers still require an enhancement of the spectroscopic performance of rare-earth-doped silica optical fibers. In order to tailor their optical behavior, a route of interest consists in embedding rare-earth ions within dielectric nanoparticles in the core of optical fibers. Nanoparticles are formed 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 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. Due to an oxygen-rich environment in the nanoparticles, we show that the rare-earth clustering effect is greatly 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 results on the effects of this drawing process on the nanoparticles characteristics.

International audience; The 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.

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International audience; The Eu2O3-PbO-SiO2 glassy system submitted to a long annealing time (100ps) has been obtained by molecular dynamics calculations. The average of 87/86 sites of the Eu3+ ion with six/seven nearest neighbours are used to discuss the number of lines and the local symmetry of the luminescent site through the crystal field parameters, using the simple overlap model in the frame of the method of equivalent nearest neighbours. The magnitude of the [View the MathML source] Bq2(q=0, 1, 2) and the non-negligible [View the MathML source] B44 and [View the MathML source] B46( [View the MathML source] B34, [View the MathML source] B36 and [View the MathML source] B66) lead to the indication of distorted C4h and C3i site symmetries of the six and seven nearest neighbours, respectively. We have then compared very satisfactorily our 7F1 sublevels calculations with those observed in the emission spectra of an Eu-borate glass annealed for 30min and 17h. This comparison is justified because the emission behaviour of europium ions in different glassy systems are honestly very similar. Further, the decrease observed in the 5D0→7F2/5D0→7F1 transition intensity ratio is a clear indication that the Eu3+ ion are nucleating a crystalline phase. Such satisfactory comparisons indicate that we have obtained a transparent glass-ceramics.

International audience; Eu3+-doped SiO2–HfO2 glasses for optical applications were prepared using sol–gel technology. The effect of heat treatment on structure, emission properties and decay kinetics was investigated. The results show that the energy splitting of the 7F2 state of Eu3+ ions occurs for the samples sintered at temperatures >1000°C. This behavior can be explained by the fact that crystal phases are formed in those samples, which can be confirmed by X-ray diffraction analysis. Moreover, fluorescence line narrowing spectra for the samples sintered at different temperatures were measured, and the results were interpreted in terms of structural changes in the glass matrix and the Eu3+ bonding environment.

International audience; The SiO2–HfO2 binary system is recognized as a promising candidate for Erbium-doped waveguides amplifiers fabrication. Recently, it was demonstrated that Er3+-activated 70SiO2–30HfO2 planar waveguides with valuable optical and structural properties can be prepared by sol–gel technique with dip-coating processing. The important role played by hafnium in the silica network was evidenced by the particular spectroscopic properties presented by Er3+-ions in the silica–hafnia planar waveguides. In this work we present preliminary results on HfO2–SiO2 bulk xerogels doped with Eu3+ ions, with the aim to go inside the role of hafnium on the rare earth ions local environment. Spectroscopic measurements of the Eu3+ photoluminescence emission are given. Numerical simulations by the molecular dynamics method have been performed showing clearly a phase separation for the HfO2 richer samples. Moreover it is found than the rare earth-doping ions stay preferentially in hafnium rich domains, thus explaining why the rare earth spectroscopic properties are strongly modified.

The point charge electrostatic model (PCEM) and the simple overlap model (SOM) are applied to a series of oxide crystals (Gd2O3, Y2O3, Lu2O3, In2O3 and Sc2O3) and a silicate glass (Al2O3–SiO2) doped with the Eu3+ ion. The SOM factor ρ(2β)k+1 is input as the shielding factor for all B q k expressions, which enters in the calculations of the crystal-field strength parameter, NV. The maximum splitting ΔE of the 7F1 manifold of the Eu3+ ion is then obtained as a function of NV. It has been developed another way to calculate alpha, which is an expansion factor in the ΔE expression. For the glass, as the mean metal-ligating ions distances are larger than for the crystals, NV and ΔE are smaller, as expected. The prediction of the PCEM shows a linear dependence between ΔE and NV, even though the known mismatch in respect to the experimental splitting is kept. In the case of the SOM, two situations have been analyzed: firstly the charge factor varies in order to reproduce the experimental splitting (a phenomenological procedure); secondly the charge factor is the valence of the oxygen ions. The agreement between the experimental results and theoretical predictions for all investigated systems is very satisfactory in respect to both the linearity between ΔE and NV and the ΔE splitting.

A simple transferable adaptive model is developed and it allows for the first time to simulate by molecular dynamics the separation of large phases in the MgO-SiO2 binary system, as experimentally observed and as predicted by the phase diagram, meaning that separated phases have various compositions. This is a real improvement over fixed-charge models, which are often limited to an interpretation involving the formation of pure clusters, or involving the modified random network model. Our adaptive model, efficient to reproduce known crystalline and glassy structures, allows us to track the formation of large amorphous Mg-rich Si-poor nanoparticles in an Mg-poor Si-rich matrix from a 0.1MgO-0.9SiO2 melt.

International audience; The optical properties of Eu3+-doped glass and transparent glass-ceramic with composition (in mol%) 70.2ZrF4–23.4LaF3–0.6AlF3–5.8GaF3 were investigated by site selective spectroscopy in order to study the effect of ceramization on the symmetry of Eu3 + sites. The glass-ceramic obtained after thermal treatment of the glass ZLAG contains a unique crystalline phase of unknown structure. The analysis and comparison of FLN spectra in these materials are presented and different classes of sites are discussed. We found the presence of two main site distributions for Eu3+ ions of equivalent symmetry (C2v or lower) in the glass and two sites of different symmetry (C2v or lower and C4v or lower) in tthe glass-ceramic (GC). The estimated average crystal field strength for glass and GC decreases with the 5D0 → 7F0 energy. The results suggest that the two kinds of sites identified in the glass correspond to Eu3+ ions in and out of the former network. Decay-time measurements of 5D0 level of Eu3 + evidenced energy transfer between high and low energy sites and showed an increase of lifetime from the glass to the glass-ceramic.

Eu2O3-Al2O3-SiO2 monolithic xerogels doped with 2000 ppm Eu/Si and 5000 ppm Eu/Si, were prepared with different Al/Eu molar ratios: (a) 0, (b) 0.25, (c) 0.5, (d) 0.75, (e) 1, (f) 3, (g) 6, (h) 10, and (i) 15. The samples were investigated by photoluminescence spectroscopy. When we compare the spectra of the Al3+ -co-doped xerogels with those of the samples without aluminum, several significant differences of the spectroscopic features appear: (i) the D-5(0) -> F-7(0) transition exhibits a blue shift; (ii) the emission bands broaden; (iii) the relative amplitude among the different transitions changes as well as (iv) the splittings of the Stark components of the D-5(0) -> F-7(J) transitions, and (v) the D-5(0) level decay curves present a stretched exponential like profile. The results are discussed with the aim of assessing the effect of the AI/Eu molar ratio on the structural and spectroscopic properties of Eu2O3-Al2O3-SiO2 xerogels.

In the recent years the use of the molecular dynamics technique has become very common in the study of glass. The purpose of the present paper is to focus on recent advancements on the use of this method to investigate rare-earth-doped glasses. We report an overview of the use of simulations to study their specific structural features and luminescence properties.

Optical transitions of Er 3þ ion in two tellurite glasses of molar composition 75TeO2:12ZnO:10Na2O:2PbO:1Er2O3 and 75TeO2:12ZnO:10Na2O:2GeO2:1Er2O3 were investigated. The measured absorption and emission spectra were analysed by Judd–Ofelt and McCumber theories, in order to obtain radiative transition rates and stimulated emission cross sections. It was found that these glasses have high and broadband absorption and stimulated emission cross sections at 1.5 lm. For the metastable 4 I13=2 level, by comparing the measured lifetime with the calculated radiative decay time, quantum efficiency higher than 80% was found.

Rare-earth doped silicate-based glasses are widely used in telecommunication technology. A limitation in the performance of active optical devices such as amplifier comes from the rare-earth clustering. Simulation methods give complementary informations to the spectroscopic usual investigations. We present a molecular dynamics simulation of a SiO2-TiO2:Er3+ system in varying its composition. Different distribution functions have been analyzed in order to have information about the clustering of erbium. To this purpose, the titanium distribution has been studied for concentration 7.8 and 15.7% Ti/(Ti + Si). We have simulated the structure of the system doped with 1 and 2.3 mol%. A calculation of the crystal field strength for the different kinds of erbium site has been interpreted in the light of the local structure. The important trend of erbium for clustering in our modeled system has been explained by the energetic requirements of the rare-earth.

We have performed molecular dynamics simulations of a TiO2–SiO2: Er3+ system for various compositions. Different distribution functions have been analysed to determine whether titanium ions are homogeneously distributed. A similar analysis has been carried out to characterize the erbium clustering ability. In particular, because of the simulation technique, the microscopic structure around erbium ions has been investigated. We discuss the role of both erbium and titanium concentrations in the structural properties of the simulated samples. For this purpose, the titanium distribution has been studied for [Ti]/[Si + Ti] concentrations ranging from 7.8 to 50% in undoped samples, and the erbium clustering trend has been characterized from three doped samples in which both erbium and titanium concentrations vary. A calculation of the crystal-field strength for the different kinds of erbium site has been interpreted in the light of the associated local structures.

A comparative study of the spectroscopic properties at 1.5 μm of erbiumdoped fluoride and tellurite glasses has been performed. Emission in the third telecommunications window has been observed upon excitation at 514.5 nm and the lifetime of the 4I13/2 level has been measured. Infrared-to-visible upconversion emission upon continuous-wave laser excitation at 976 nm has been detected in the different hosts. The upconversion results in green and red emissions, whose intensity has a quadratic dependence on the excitation power. Lifetime measurements have been performed in order to clarify the contributions of excited-state absorption and energy transfer to the upconversion process.

International audience; Using classical molecular dynamics applied to crystalline PbF2, we have simulated transitions between cubic and orthorhombic phases and reciprocally. These transitions were induced by pressure and temperature treatments. The orthorhombic phase was evidenced by analysis of different distribution functions and XRD spectra drawn from the ions positions in the simulated samples. (C) 2014 Elsevier B.V. All rights reserved.

A review of the research that has been devoted to the simulation of the structure and spectroscopic properties of rare-earth doped glasses is presented. Since the seminal papers of Brawer and Weber who have applied Monte-Carlo or molecular dynamics techniques, some other very important results have been reported concerning the local structure of the dopants. As a result, crystal field models have been applied and several pecularities of the optical spectra of rare earth ions in glasses have been understood.

The Eu and Eu/Tb doped silicate glasses are good candidates for light emitting diode (LED) applications. But the optical performance of these glasses is sensitive to variations in chemical composition. In this paper we report our recent findings about the effect of addition of minor components such as B2O3, Al2O3 and CaF2 on the luminescent properties of the above-mentioned glasses. We explore the role of Eu3+ ions as a structural probe of the glasses by determining the asymmetry factor, i.e., the ratio of the emission intensity of the 5D0→7F2 transition to that of the 5D0 →7F1 transition. The results show that the asymmetry factor and luminescence lifetimes of as-prepared materials are dependent on composition. White fluorescence is achieved in Eu/Tb co-doped glasses, which can be attributed to the simultaneous generation of red, green and blue wavelengths from Eu3+ and Tb3+ ions. The variation of the excitation wavelength can tune the emission spectra as well as Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of Eu/Tb co-doped glasses for specific applications. The energy transfer from Tb3+ to Eu3+ ions is investigated by analyzing fluorescence spectra and decay curves.

As a prelude to a spectroscopic investigation of doped wet gels, we have made molecular dynamics simulations of a very dilute EuCl3 solution. The symmetry properties of the first hydration shell of Eu3+ have been determined. The 8 water molecules of this shell are roughly arranged according to the square antiprism D4d pseudo‐symmetry. Though the arrangement of oxygens is little distorted from regular positions, the orientation of the water molecules themselves is found to be not so well organized at room temperature.

International audience; Rare earth (RE) doped silica-based optical fibres with transparent glass ceramic (TGC) core were fabricated through the well-known modified chemical vapor deposition (MCVD) process without going through the commonly used stage of post-ceramming. The main characteristics of the RE-doped dielectric nanoparticles (DNP), their density and mean diameter in the fibres are dictated by various parameters. This paper reports on progresses in the fine characterization of the nanopaticles, particularly their dimensions and composition, using nanosclae mass spectroscpy and molecular dynamics modeling, and alteration of the spectroscopic properties of the erbium ions embedded within the phospho-silicate DNP. These results permit to get more insight into the complex process of the DNP self-nucleation and growth during the fibre fabrication process. This fabrication route could have important potentials in improving rare-earth-doped fibre amplifiers and laser sources.

International audience; We report on using nanometric mass spectroscopy and molecular dynamics modelling to characterize the composition and structure of self-grown erbium-doped nanoparticles in transparent glass ceramic optical fibres fabricated by modified chemical vapour deposition without post-ceramming.

International audience; Molecular dynamics simulation is applied to an europium doped sodosilicate glass containing silver [(Na–Ag)2O–SiO2–Eu2O3]. The silver is implanted in substitution of Na, simulating an ionic exchange. For ionic interactions a modified Born–Mayer–Huggins potential was employed. For the Ag–Ag interaction, a Lennard-Jones (LJ) potential is applied, while for the Eu–Ag interaction, a modified LJ potential is introduced. The particle size increases with the annealing treatment and follows a lognormal law. After 75 ps the average particle size reaches 5.8 atoms (4.8 for Ag and 1.0 for Eu), and it is found that the europium is preferentially situated on these nanoclusters.

8 páginas, 10 figuras, 6 tablas.-- et al., This paper reports on the spectroscopic properties of the 3H4 and 3F4 Tm3+ levels in a TZN glass host with a concentration range from 0.82 to 22 × 1020 cm−3. Precise refractive index measurements are performed on five different wavelengths by the prism coupling method. Judd–Ofelt intensity parameters have been determined in order to obtain transition rate, branching ratio and radiative lifetime. Spectroscopic measurements show the most promising concentration for the1.8 µm short cavity laser emission at 6.84 × 1020 cm−3 Tm3+ with a 24% quantum efficiency and upper limit concentration of 11 × 1020 cm−3 Tm3+. Energy transfer microparameters and critical ion distance are determined for both emission levels within the framework of diffusion-limited regime and dipole–dipole interaction., The authors wish to thank the Regione Piemonte Converging Technologies (‘Hipernano’ research project). R Balda and J Fern´andez acknowledge the financial support from the Spanish Ministerio de Ciencia e Innovacion under project MAT2009-14282-C02-02 and from the Basque Country Government (IT-331-07).

Molecular dynamics simulation has been used to study the effect of hydrostatic pressure on the structural and spectroscopic properties of Eu 3+ -doped Na 2 O·2SiO 2 glass. The short- and medium-range order has been investigated. The evolution of angular and radial distribution has shown the dependence of the structure with pressure. It was found that pressure induces an increase of the coordination number of the Eu 3+ ion and a shortening in the Eu–O bond distance. The pressure effects on the crystal-field parameters and on the luminescent spectra are computed and discussed in comparison with experimental data.

International audience; Glasses with compositions (100−x)SiO2−xHfO2:0.3Eu3+ (molar ratio, x=0,10,20,30) for optical applications were prepared using the sol-gel route. The introduction of hafnium into the glass matrix induced the energy splitting of the F27 state of Eu3+ ions. Furthermore, fluorescence line narrowing (FLN) spectra indicated that Eu3+ clustering occurred in glasses containing no hafnium. The addition of hafnium promoted better dispersion of Eu3+ ions in the glass matrix. The role of hafnium on modifying the properties of glasses was discussed with respect to x-ray diffraction and FLN analysis.

Simplified inactive rare-earths doped nuclear waste glasses have been obtained by molecular dynamics (MD) simulation in order to investigate the local structure around the rare-earth by luminescence studies. MD calculations were performed with modified Born–Mayer–Huggins potentials and three body angular terms representing Coulomb and covalent interactions. Atomic positions within the glasses are then determined. Simulations of luminescence spectra were then obtained by calculation of the ligand field parameters affecting each luminescent ion. Considering the C 2 v symmetry, it is possible to calculate the radiative transition probabilities between the emitter level, 5 D 0 , and the splitted receptor levels, 7 F J ( J = 0–3) for each Eu 3+ ion. The simulated emission spectra are obtained by convolution of all the Eu 3+ ions contributions. A comparison with the experimental data issue from fluorescence line narrowing and microluminescence spectroscopies allowed us not only to validate the simulation of luminescence spectra from simulated environments, but also to confirm the presence and the identification of two major Eu 3+ sites distribution in the nuclear glasses thanks to spectra-structure correlations.

Using molecular dynamics simulations, we have studied the environment of Eu3+ salts in aqueous solution. In the case of hydration number of 8, the symmetry of the first hydration shell is found to be very close to the D4d. We have therefrom calculated the ligand field parameters according to the point charge model. The luminescence spectra were reconstructed from this set of computed Bkq, and finally compared with experimental spectra.

Ultra-transparent glass–ceramics activated with thulium ions were fabricated employing different heat treatments of precursor oxy-fluoride glass. The spectroscopic properties of the thulium ions, studied through UV–Vis–NIR absorption and luminescence spectroscopy, showed that most of the active ions were embedded in the crystalline phase. The size of nucleated nanocrystals, controlled by the temperature and time of annealing and measured by low-frequency-Raman spectroscopy, was found to range from 7 to 18 nm. From the absorption measurement we extracted the cross-sections of Tm3+ ions embedded in the crystalline and glassy phases. Judd–Ofelt analysis was carried out on precursor glass and crystal. Comparing calculated and experimental lifetimes, before and after the heat treatment procedure, we found that the ceramming increases the quantum efficiency of the Tm3+ electronic transitions.

Recent results obtained for Er2O3-SiO2 monolithic xerogels and erbium activated SiO2-TiO2 planar waveguides are presented. Monolithic erbium-activated silica xerogels with erbium content ranging from 0 up to 40000 ppm were prepared by the sol-gel technique. Samples were densified by thermal treatment in air at 950 degrees C for 120 hours. The densification degree and the relative content of hydroxyl groups were studied by NIR absorption and Raman spectroscopies. Emission at 1.5 micrometers , characteristic of the 4I13/2 yields 4I15/2 transition of Er3+ ions, was observed at room temperature for all monolithic samples upon continuous wave excitation at 980 nm. For the 5000 Er/Si ppm doped xerogel, an intense photoluminescence was observed with a lifetime of 8 ms for the metastable 4I13/2 level. Passive and erbium-activated SiO2-TiO2 planar waveguides, monomode at 632.8 nm, were prepared by a dip-coating technique. Some parameters such as H2O content, intermediate and final thermal treatments, and the molar ratio TiO2/SiO2 were modified during the preparation of the solution in order to minimize the final content of residual hydroxyl groups and the phase separation between silica and titania. Raman spectroscopy was used to check the structural properties of the waveguides. A lifetime of 7 ms was measured for the metastable 4I13/2 level in a 93SiO2-7TiO2 planar waveguide activated by 10000 ppm Er/(Si + Ti). The best value of the attenuation coefficient was of 0.5 dB/cm at 632.8 nm.

The SiO 2 -TiO 2 binary system is of significant technological importance because it offers the possibility of producing planar waveguides with a controlled refractive index depending on the TiO 2 /SiO 2 molar ratio. Moreover, titaniasilica films are suitable hosts for doping by rare-earth ions. Recently, the preparation of erbium and europium-activated SiO 2 -TiO 2 planar waveguides by radio-frequency-sputtering as well as its characterization by means of different spectroscopic techniques have been reported by some of us. Raman scattering measurements and optical properties suggest an essentially homogeneous structure of the film with a network consisting of intermingled TiO4 and SiO 4 units. However, quenching of the luminescence, a signature of rare-earth clustering, has been found effective especially for erbium-doped films. Here, we use Molecular Dynamics simulation to reconstruct a silica-titania glass with a Ti/Si atomic ratio of 8.5% activated by 0.7 at% of erbium. These quantities are chosen because they give both refractive index and optically ions concentration suitable for applications. We use a modified Born-Mayer-Huggins potential taking into account a three-body interaction. The distribution of TiO 4 and SiO 4 units as well as the bridging to non-bridging oxygen ratio are evaluated. The local environment of rare-earth ions is also analyzed. In particular, the clustering of erbium is discussed. From the simulated structure, the crystal-field strength is computed and discussed according to the Er 3+ local environment. Finally, results are compared with information obtained by Raman and photoluminescence spectra.

Because doping ions are mainly surrounded by solvent molecules in the early stages of sol--gel processing, the luminescence of rare-earth ions in such an environment is known to be very similar to that observed in aqueous solutions. Thus we performed a molecular dynamics simulation of the trivalent europium in aqueous solution in order to produce a statistical set of environments and to calculate the spectroscopic features thereof. From radial and angular ordering studies, the hydration number of Eu3+ was found to be between eight and nine. The eight-hydrated configurations are nevertheless predominant and present a specific arrangement, according to the geometry of a square antiprism polyhedron. From the whole set of simulated configurations, we computed the ligand field parameters without considering any a priori symmetry. We then calculated the energy splitting of each J manifold and the transition probabilities from the 5D0 emitting state. A simulated luminescent spectrum was finally obtained and compared with the experimental sectrum. In addition to overall good agreement, we were able to analyse the 5D0 -> 7F1 transition shape, which revealed a sensitive dependence on the local structure, and especially on the hydration number.

International audience; A strong dependence of the interaction induced light scattering integrated intensity with the cutoff radius was previously reported in noble liquids molecular dynamics simulations (A.J.C. Ladd, T.A. Litovitz, J.H.R. Clarke, L.V. Woodcock, J. Chem. Phys. 72 (1980) 1759; and J.H.R. Clarke, L.V. Woodcock, Chem. Phys. Lett. 78 (1981) 121). It was concluded that the uncertainties arising from this effect were much larger than can be tolerated in any evaluation of the light scattering mechanism. No real explanation or solution ever appeared in the literature on this problem. Here we report a very strong dependence of the interaction induced spectra calculated by molecular dynamics simulations on the number of atoms used in a liquid argon simulation. We show that this effect is proportional to the square of the density involved in the simulation-4 and that it obeys to a Rcutoff power law with the cutoff radius Rcutoff. To obtain accurate spectra up to high frequency shifts, large cutoff radii have then to be used corresponding to thousands of atoms in the simulation. The time of the simulation is then prohibited. An explanation for both effects is proposed and two different methods to solve this problem are tested. @ 1997 Elsevier Science B.V.

Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were carried out. This technique appears to be a relevant way to perform systematic analysis of the system structure and to study the influence of defects on PbF2 crystallization. We modeled the total enthalpy, the radial distribution functions, and the diffracted intensities of systems containing different amounts of Er(3+) ions. We demonstrated by means of different simulations that Er(3+) ions lowered the devitrification temperature of PbF2, in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF2 has been investigated. Er(3+) ions have an unquestionable influence of the crystallization of PbF2. Although the latter does not start in the nearest neighborhood of Er(3+) ions, the presence of Er(3+) ions in a close environment may favor the lead fluoride crystallization.