On inversion sets and the weak order in Coxeter groups

In this article, we investigate the existence of joins in the weak order of an infinite Coxeter group W. We give a geometric characterization of the existence of a join for a subset X in W in terms of the inversion sets of its elements and their position relative to the imaginary cone. Finally, we discuss inversion sets of infinite reduced words and the notions of biconvex and biclosed sets of positive roots.Comment: 22 pages; 10 figures; v2 some references were added; v2: final version, to appear in European Journal of Combinatoric

Impact of the capping layers on lateral confinement in InAs/InP quantum dots for 1.55 um laser applications srudied by magneto-photoluminescence.

We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs/InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs/InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed. ©2005 American Institute of Physic

Impact of the annealing temperature on the optical performances of Er-doped Si-rich silica systems Impact of the annealing temperature on the optical performances of Er-doped Si-rich Silica systems

International audienceSeries of Er-doped Si-rich silicon oxide (SRSO:Er) layers were grown by magnetron sputtering at different temperatures from ambient to 500°C and then annealed between 600°C and 1100°C. They were characterized by spectroscopic and time-resolved photoluminescence (PL) measurements. Significant PL was detected at 1533 nm from the as-grown samples at T≥300°C excited by a non-resonant wavelength (476 nm), hence indicating the formation of Si-based sensitizers during the growth process. The PL intensity and the decay lifetime of Er 3+ ions were both greatly increased with the annealing temperature. An optimum temperature of annealing is obtained at 800°C, which is expected to favor the formation of very dense and small sensitizers. The fraction of Er coupled to sensitizers was found nearly 6-7 times higher than that reported so far in the literature. 1. Introduction The effective excitation cross section of Er 3+ ions in SiO 2 is increased by 10 3-10 4 with the insertion of Si-based sensitizers in the matrix. It was shown that an indirect excitation of Er 3+ ions occurs through Si nanoclusters (Si-nc) [1,2]. Such an energy transfer allows one to benefit from the broadband high absorbance of Si-nc for optical excitation and from the improved transport of carriers injected by electrical excitation. This paves the way to the achievement of Er 3+ population inversion by either optical or electrical pumping for integrated photonics, such as planar amplifier, laser, etc. Recent studies reported that, in conventional SRSO:Er materials, only a small fraction (<2%) of Er 3+ ions are effectively benefiting from the Si-nc-mediated excitation [3]. Such a low value is far from ensuring an inversion population of Er 3+ , but requires, on the contrary, more efforts and studies to enhance significantly the proportion of coupled Er in SRSO:Er samples. These latter are usually submitted to annealing processes to form Si nanoclusters (Si-nc) as sensitizers, to remove non-radiative defects and to activate (optically) the Er 3+ ions. Such a treatment may change, however, the structure of either the Si-based sensitizers or the Er 3+ ions. This work aims at investigating the influence of growth and annealing temperature on the optical properties of the SRSO:Er layers and to correlate it to the proportion of Er coupled to sensitizers

Asymptotical behaviour of roots of infinite Coxeter groups

Let W be an infinite Coxeter group. We initiate the study of the set E of limit points of "normalized" positive roots (representing the directions of the roots) of W. We show that E is contained in the isotropic cone of the bilinear form B associated to a geometric representation, and illustrate this property with numerous examples and pictures in rank 3 and 4. We also define a natural geometric action of W on E, and then we exhibit a countable subset of E, formed by limit points for the dihedral reflection subgroups of W. We explain that this subset is built from the intersection with Q of the lines passing through two positive roots, and finally we establish that it is dense in E.Comment: 19 pages, 11 figures. Version 2: 29 pages, 11 figures. Reorganisation of the paper, addition of many details (section 5 in particular). Version 3 : revised edition accepted in Journal of the CMS. The number "I" was removed from the title since number "II" paper was named differently, see http://arxiv.org/abs/1303.671

Performances de protocoles transactionnels en environnement mobile

National audience. L'omniprésence d'unités mobiles et le développement des réseaux sans fil motivent des avancées en matière de supports d'exécution pour une grande variété d'applications en envi-ronnement mobile. Des efforts importants sont faits pour offrir une bonne gestion des données malgré les caractéristiques limitées de tels environnements. La notion de transaction a été ré-étudiée pour proposer des modèles et des protocoles permettant d'assurer certaines propriétés transactionnelles. Les algorithmes et protocoles proposés tentent d'optimiser l'utilisation des ressources des unités mobiles et de surmonter les limitations du réseau mobile. Dans cet article nous nous intéressons spécifiquement aux protocoles de validation de transactions réparties sur plusieurs unités dont certaines mobiles. Nous présentons ici les résultats d'un travail qui iden-tifie des propriétés qualitatives et des indices de performance quantitatifs, et étudie ceux-ci sur quatre protocoles (2PC, UCM, CO2PC et TCOT). Trois de ces protocoles sont des propositions spécifiques pour des environnements mobiles et sont représentatifs des propositions actuelles. Nous nous intéressons à la phase de validation des transactions, ainsi qu'à l'influence des dif-férents protocoles sur les performances du déroulement de l'ensemble des transactions. ABSTRACT. This is an abstract. The omnipresence of mobile devices and wireless networks lead to a growing interest in supporting a wide variety of applications in mobile environments. Numerous efforts in providing appropriate data managements for such environments are made. Transaction supports have been revisited to propose adapted transaction models and properties. Proposed algorithms and protocols try to optimise the use of mobile units resources and to overcome wireless network limitations. This work concerns protocols to commit transactions distributed over several mobile and fixed units. Results presented here concern the identification of qualitative properties and quantitative performance indices that are studied on four protocols (2PC, UCM, CO2PC et TCOT). Three of them are specifics propositions for mobile environments and are representative of current proposals. The analysis concerns the transaction validation phase as well as the impact of the protocols on the performances during a transaction execution itself. MOTS-CLÉS : transactions mobiles , evaluation de performances, protocoles de validation

Undoped and Nd3+ doped Si-based single layers and superlattices for photonic applications

International audienceThis work presents the benefits of the superlattice approach to control light emission properties of materials with Si nanoclusters and rare‐earth ions. The undoped and Nd3+‐doped both Si‐rich‐SiO2 single layers and Si‐rich‐SiO2/SiO2 superlattices were grown by radio frequency magnetron sputtering. Their properties were investigated by means of spectroscopic ellipsometry, Fourier infrared transmission spectroscopy, transmission electron microscopy, and photoluminescence (PL) methods versus deposition conditions, annealing treatment, and superlattice design (doping and thickness of alternated sublayers). An intense Nd3+ emission from as‐deposited single layers and superlattices was observed. The lower annealing temperature (below 900 °C) of the single layers and superlattices favors the formation of amorphous Si clusters that act as effective sensitizers of rare‐earth ions. The highest Nd3+ PL intensity was achieved after a conventional annealing at about 600–800 °C in nitrogen flow for all samples. Crystallized Si‐nanoclusters were formed in Si‐rich‐SiO2 single layers upon annealing at 1000–1100 °C, whereas their formation in the superlattices occurred at higher temperatures (1100–1150 °C). The mechanism of Nd ions' excitation via energy transfer from Si‐nanoclusters and/or matrix defects, if any, is discussed

Si wafer bonded of a-Si/a-SiNx distributed Bragg reflectors for 1.55-µm wavelength vertical cavity surface emitting lasers

International audienceAmorphous silicon (a-Si) and amorphous silicon nitride (a-SiNx) layers deposited by magnetron sputtering have been analyzed in order to determine their optical and surface properties. A large value of ~1.9 of index difference is found between these materials. Distributed Bragg reflectors (DBR) based on these dielectric materials quarter wave layers have been studied by optical measurements and confronted to theoretical calculations based on the transfer matrix method. A good agreement has been obtained between the experimental and expected reflectivity. A maximum reflectivity of 99.5% at 1.55 µm and a large spectral bandwidth of 800 nm are reached with only four and a half periods of a-Si/a-SiNx. No variation of the DBR reflectivity has been observed with the time nor when annealed above 240°C and stored during few months. This result allows to use this DBR in a metallic bonding process to realize a vertical cavity surface emitting laser (VCSEL) with two dielectric a-Si/a-SiNx DBR. This bonding method using AuIn2 as the bonding medium and Si substrate can be performed at a low temperature of 240°C without damaging the optical properties of the microcavity. The active region used for this VCSEL is based on lattice-matched InGaAs/InGaAsP quantum wells and a laser emission has been obtained at room-temperature on an optically pumped device

Thickness-dependent optimization of Er3+ light emission from silicon-rich silicon oxide thin films

This study investigates the influence of the film thickness on the silicon-excess-mediated sensitization of Erbium ions in Si-rich silica. The Er3+ photoluminescence at 1.5 μm, normalized to the film thickness, was found five times larger for films 1 μm-thick than that from 50-nm-thick films intended for electrically driven devices. The origin of this difference is shared by changes in the local density of optical states and depth-dependent interferences, and by limited formation of Si-based sensitizers in "thin" films, probably because of the prevailing high stress. More Si excess has significantly increased the emission from "thin" films, up to ten times. This paves the way to the realization of highly efficient electrically excited devices

A new method for qualitative multi-scale analysis of bacterial biofilms on filamentous fungal colonies using confocal and electron microscopy

Bacterial biofilms frequently form on fungal surfaces and can be involved in numerous bacterial-fungal interaction processes, such as metabolic cooperation, competition, or predation. The study of biofilms is important in many biological fields, including environmental science, food production, and medicine. However, few studies have focused on such bacterial biofilms, partially due to the difficulty of investigating them. Most of the methods for qualitative and quantitative biofilm analyses described in the literature are only suitable for biofilms forming on abiotic surfaces or on homogeneous and thin biotic surfaces, such as a monolayer of epithelial cells.While laser scanning confocal microscopy (LSCM) is often used to analyze in situ and in vivo biofilms, this technology becomes very challenging when applied to bacterial biofilms on fungal hyphae, due to the thickness and the three dimensions of the hyphal networks. To overcome this shortcoming, we developed a protocol combining microscopy with a method to limit the accumulation of hyphal layers in fungal colonies. Using this method, we were able to investigate the development of bacterial biofilms on fungal hyphae at multiple scales using both LSCM and scanning electron microscopy (SEM). This report describes the protocol, including microorganism cultures, bacterial biofilm formation conditions, biofilm staining, and LSCM and SEM visualizations

Microstructure and optical properties of Pr3+-doped hafnium silicate films

International audienceIn this study, we report on the evolution of the microstructure and photoluminescence properties of Pr 3+-doped hafnium silicate thin films as a function of annealing temperature (T A). The composition and microstructure of the films were characterized by means of Rutherford backscattering spectrometry, spectroscopic ellipsometry, Fourier transform infrared absorption, and X-ray diffraction, while the emission properties have been studied by means of photoluminescence (PL) and PL excitation (PLE) spectroscopies. It was observed that a post-annealing treatment favors the phase separation in hafnium silicate matrix being more evident at 950°C. The HfO 2 phase demonstrates a pronounced crystallization in tetragonal phase upon 950°C annealing. Pr 3+ emission appeared at T A = 950°C, and the highest efficiency of Pr 3+ ion emission was detected upon a thermal treatment at 1,000°C. Analysis of the PLE spectra reveals an efficient energy transfer from matrix defects towards Pr 3+ ions. It is considered that oxygen vacancies act as effective Pr 3+ sensitizer. Finally, a PL study of undoped HfO 2 and HfSiO x matrices is performed to evidence the energy transfer