Analyse spectrale paramétrique et non-paramétrique du signal de précession libre en RMN

Ce travail consiste à dresser une étude comparative entre les méthodes d’estimation spectrale non paramétrique basées essentiellement sur la technique du périodogramme avec ses variantes et les méthodes d’estimation spectrale paramétrique notamment la modélisation autorégressive (AR), à moyenne mobile (MA), et la modélisation hybride autorégressive à moyenne mobile (ARMA). Cette panoplie de méthodes est appliquée à l’analyse spectrale du signal de précession libre (FID; Free Induction Decay) dans les expériences de résonance magnétique nucléaire (RMN). Dans cet article, nous montrons clairement les avantages des méthodes paramétriques. En ce sens qu’elles se caractérisent par une très bonne résolution spectrale et une bonne stabilité statistique. Ces deux paramètres sont indispensables pour estimer la constante de relaxation spin-spin de l’échantillon, ainsi que les déplacements chimiques. Lesrésultats obtenus par la modélisation ARMA sont meilleurs par rapport à ceux des modèles AR en terme d’ordre de prédiction.Mots-clés : Résonance Magnétique Nucléaire (RMN), Décroissance Libre de l'Induction Magnétique (FID), Densité Spectrale de Puissance (PSD), Autorégressive à Moyenne Ajustée (ARMA), Critère d'Information d'Akaike (AIC), Erreur Finale de Prédiction (FPE)

Investigations about the modelling of acoustic properties of periodic porous materials with the shift cell approach

The main advantage of designing sound packages with periodic arrangements is that they can provide a combination of absorption effects, resonance effects and wave interferences effects. This offers different applications in transportation (aeronautics, space, automotive, railway), energy and civil engineering sectors, where both weight and space, as well as vibroacoustic quality of performance and comfort, still remain as critical issues. The application of shift cell technique is presented and discussed for periodic porous media described with equivalent fluid models: it consists in a reformulation of classical Floquet-Bloch (F-B) conditions, whose major advantage stands in allowing the introduction of any frequency dependence of porous material behavior, through the resolution a quadratic eigenvalue problem, providing an efficient way to compute the dispersion curves of a porous material modelled as an equivalent fluid. The central part of this work shows the results, in terms of absorption coefficient and transmission loss curves, obtained through a numerical test campaign involving different melamine and polyurethane foams. The 48 test cases involve a cubic unit cell of porous material with a cylindrical inclusion. Furthermore, some absorption coefficient and transmission loss comparisons are shown, between a homogeneous unit cell and a unit cell with a perfectly rigid inclusion; the comparisons are carried out at fixed dimensions, then at fixed mass and then at fixed performance in the periodicity peak range. The results clearly point out the advantage of designing foam layer with periodic inclusion patterns in order to improve the performances in a specific range of frequencies, allowing a save both in terms of thickness and, most of all, mass, respect to a classical homogeneous foam layer

Design guidelines for the acoustic performance improvement of a periodic porous material

In this paper, some guidelines are provided in order to predict at which frequency the 1st performance peak (related to periodicity effects: half of the wavelength = periodicity dimension) appears, together with its amplitude, as functions of the unit cell dimensions. Conversely, also the link between the unit cell dimensions and the 1st performance peak amplitude as functions of the design frequency is shown. Furthermore, some additional guidelines are provided in order to predict at which frequency the 1st performance peak appears, together with its amplitude, as functions of the foam airflow resistivity

Modulation of the extraordinary optical transmission by surface acoustic waves

International audienceThe numerical study of periodically nanostructured metallic films exhibiting extraordinary optical transmission (EOT) deposited onto the top of a piezoelectric material is reported. Surface acoustic waves are generated in the piezoelectric substrate and their influence in the transmission spectrum of the EOT structure is studied. It is shown that low frequency acoustic waves can significantly tune the resonance frequency of the EOT structure

Modified ultrafast thermometer UFT-M and temperature measurements during Physics of Stratocumulus Top (POST)

A modified UFT-M version of the ultrafast airborne thermometer UFT, aimed at in-cloud temperature measurements, was designed for the Physics of Stratocumulus Top (POST) field campaign. Improvements in its construction resulted in the sensor's increased reliability, which provided valuable measurements in 15 of the 17 flights. Oversampling the data allowed for the effective correction of the artefacts resulting from the interference with electromagnetic transmissions from on-board avionic systems and the thermal noise resulting from the sensor construction. The UFT-M records, when averaged to the 1.4 and 55 m resolutions, compared to the similar records of a thermometer in a Rosemount housing, indicate that the housing distorts even low-resolution airborne temperature measurements. Data collected with the UFT-M during the course of POST characterise the thermal structure of stratocumulus and capping inversion with the maximum resolution of ~1 cm. In this paper, examples of UFT-M records are presented and discussed

Lymphome t/nk primitif du larynx : localisation inhabituelle de lymphome extranodal

Nous rapportons un cas de lymphome extranodal T/Nk primitif du larynx. il s’agit d’un patient âgé de 22 ans qui s’était présenté pour une dysphonie chronique associée à une dyspnée inspiratoire. une chimiothérapie protocole SMiLE avec radiothérapie ont été institués avec obtention d’une rémission complète. Huit mois plus tard, le patient présentait une récidive de la tumeur avec un oedème palpébral et une obstruction du canal lacrymonasal. L’évolution a été fatale suite à une aspergillose invasive en cours de chimiothérapie.Mots clés : lymphome, larynx.A rare case of primary laryngeal T/Nk- cell lymphoma, nasal type is reported. The patient was 22-year old male who presented with dysphonia, dyspnea. Chemotherapy protocol SMiLE and radiotherapy were instituted with complete remission of the tumor. Eight months afterward, he presented with tumor recurrence, palpebral edema, obstruction of the lacrymonasal duct. He died within few days with invasive aspergillosis while he was receiving chemotherapy SMiLE regimen.Keyswords : lymphoma, larynx

Raman-like light scattering from acoustic phonons in photonic crystal fiber

International audienceRaman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core

A user-centred evaluation framework for the Sealife semantic web browsers

Background: Semantically-enriched browsing has enhanced the browsing experience by providing contextualised dynamically generated Web content, and quicker access to searched-for information. However, adoption of Semantic Web technologies is limited and user perception from the non-IT domain sceptical. Furthermore, little attention has been given to evaluating semantic browsers with real users to demonstrate the enhancements and obtain valuable feedback. The Sealife project investigates semantic browsing and its application to the life science domain. Sealife's main objective is to develop the notion of context-based information integration by extending three existing Semantic Web browsers (SWBs) to link the existing Web to the eScience infrastructure. / Methods: This paper describes a user-centred evaluation framework that was developed to evaluate the Sealife SWBs that elicited feedback on users' perceptions on ease of use and information findability. Three sources of data: i) web server logs; ii) user questionnaires; and iii) semi-structured interviews were analysed and comparisons made between each browser and a control system. / Results: It was found that the evaluation framework used successfully elicited users' perceptions of the three distinct SWBs. The results indicate that the browser with the most mature and polished interface was rated higher for usability, and semantic links were used by the users of all three browsers. / Conclusion: Confirmation or contradiction of our original hypotheses with relation to SWBs is detailed along with observations of implementation issues

Coherent coupling between radio frequency, optical, and acoustic waves in piezo-optomechanical circuits

The interaction of optical and mechanical modes in nanoscale optomechanical systems has been widely studied for applications ranging from sensing to quantum information science. Here, we develop a platform for cavity optomechanical circuits in which localized and interacting 1550 nm photons and 2.4 GHz phonons are combined with photonic and phononic waveguides. Working in GaAs facilitates manipulation of the localized mechanical mode either with a radio frequency field through the piezo-electric effect, or optically through the strong photoelastic effect. We use this to demonstrate a novel acoustic wave interference effect, analogous to coherent population trapping in atomic systems, in which the coherent mechanical motion induced by the electrical drive can be completely cancelled out by the optically-driven motion. The ability to manipulate cavity optomechanical systems with equal facility through either photonic or phononic channels enables new device and system architectures for signal transduction between the optical, electrical, and mechanical domains

Demonstration of suppressed phonon tunneling losses in phononic bandgap shielded membrane resonators for high-Q optomechanics

Dielectric membranes with exceptional mechanical and optical properties present one of the most promising platforms in quantum optomechanics. The performance of stressed silicon nitride nanomembranes as mechanical resonators notoriously depends on how their frame is clamped to the sample mount, which in practice usually necessitates delicate, and difficult-to-reproduce mounting solutions. Here, we demonstrate that a phononic bandgap shield integrated in the membrane's silicon frame eliminates this dependence, by suppressing dissipation through phonon tunneling. We dry-etch the membrane's frame so that it assumes the form of a cm-sized bridge featuring a 1-dimensional periodic pattern, whose phononic density of states is tailored to exhibit one, or several, full band gaps around the membrane's high-Q modes in the MHz-range. We quantify the effectiveness of this phononic bandgap shield by optical interferometry measuring both the suppressed transmission of vibrations, as well as the influence of frame clamping conditions on the membrane modes. We find suppressions up to 40 dB and, for three different realized phononic structures, consistently observe significant suppression of the dependence of the membrane's modes on sample clampingif the mode's frequency lies in the bandgap. As a result, we achieve membrane mode quality factors of 5×106 with samples that are tightly bolted to the 8 K-cold finger of a cryostat. Q×f -products of 6×1012 Hz at 300 K and 14×1012 Hz at 8 K are observed, satisfying one of the main requirements for optical cooling of mechanical vibrations to their quantum ground-state. © 2014 Optical Society of America