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Search Results: 1 - 10 of 6316 matches for " Nicolas Mercadier "
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Steady-state signatures of radiation trapping by cold multilevel atoms
Quentin Baudouin,Nicolas Mercadier,Robin Kaiser
Physics , 2012, DOI: 10.1103/PhysRevA.87.013412
Abstract: In this paper, we use steady-state measurements to obtain evidence of radiation trapping in an optically thick a cloud of cold rubidium atoms. We investigate the fluorescence properties of our sample, pumped on opened transitions. The intensity of fluorescence exhibits a non trivial dependence on the optical thickness of the media. A simplified model, based on rate equations self-consistently coupled to a diffusive model of light transport, is used to explain the experimental observations in terms of incoherent radiation trapping on one spectral line. Measurements of atomic populations and fluorescence spectrum qualitatively agree with this interpretation.
Lévy flights of photons in hot atomic vapours
Nicolas Mercadier,William Guerin,Martine Chevrollier,Robin Kaiser
Physics , 2009, DOI: 10.1038/nphys1286
Abstract: Properties of random and fluctuating systems are often studied through the use of Gaussian distributions. However, in a number of situations, rare events have drastic consequences, which can not be explained by Gaussian statistics. Considerable efforts have thus been devoted to the study of non Gaussian fluctuations such as L\'evy statistics, generalizing the standard description of random walks. Unfortunately only macroscopic signatures, obtained by averaging over many random steps, are usually observed in physical systems. We present experimental results investigating the elementary process of anomalous diffusion of photons in hot atomic vapours. We measure the step size distribution of the random walk and show that it follows a power law characteristic of L\'evy flights.
Anomalous photon diffusion in atomic vapors
Martine Chevrollier,Nicolas Mercadier,William Guerin,Robin Kaiser
Physics , 2010, DOI: 10.1140/epjd/e2010-00053-4
Abstract: The multiple scattering of photons in a hot, resonant, atomic vapor is investigated and shown to exhibit a L\'evy Flight-like behavior. Monte Carlo simulations give insights into the frequency redistribution process that originates the long steps characteristic of this class of random walk phenomena.
Microscopic characterization of Lévy flights of light in atomic vapors
Nicolas Mercadier,Martine Chevrollier,William Guerin,Robin Kaiser
Physics , 2013, DOI: 10.1103/PhysRevA.87.063837
Abstract: We investigate multiple scattering of near-resonant light in a Doppler-broadened atomic vapor. We experimentally characterize the length distribution of the steps between successive scattering events. The obtained power law is characteristic of a superdiffusive behavior, where rare but very long steps (L\'evy flights) dominate the transport properties.
Threshold of a random laser based on Raman gain in cold atoms
William Guerin,Nicolas Mercadier,Davide Brivio,Robin Kaiser
Physics , 2009, DOI: 10.1364/OE.17.011236
Abstract: We address the problem of achieving a random laser with a cloud of cold atoms, in which gain and scattering are provided by the same atoms. In this system, the elastic scattering cross-section is related to the complex atomic polarizability. As a consequence, the random laser threshold is expressed as a function of this polarizability, which can be fully determined by spectroscopic measurements. We apply this idea to experimentally evaluate the threshold of a random laser based on Raman gain between non-degenerate Zeeman states and find a critical optical thickness on the order of 200, which is within reach of state-of-the-art cold-atom experiments.
A cold-atom random laser
Quentin Baudouin,Nicolas Mercadier,Vera Guarrera,William Guerin,Robin Kaiser
Physics , 2013, DOI: 10.1038/nphys2614
Abstract: Conventional lasers make use of optical cavities to provide feedback to gain media. Conversely, mirrorless lasers can be built by using disordered structures to induce multiple scattering, which increases the effective path length in the gain medium and thus provides the necessary feedback. These so-called random lasers potentially offer a new and simple mean to address applications such as lighting. To date, they are all based on condensed-matter media. Interestingly, light or microwave amplification by stimulated emission occurs also naturally in stellar gases and planetary atmospheres. The possibility of additional scattering-induced feedback (that is, random lasing) has been discussed and could explain unusual properties of some space masers. Here, we report the experimental observation of random lasing in a controlled, cold atomic vapour, taking advantage of Raman gain. By tuning the gain frequency in the vicinity of a scattering resonance, we observe an enhancement of the light emission of the cloud due to random lasing. The unique possibility to both control the experimental parameters and to model the microscopic response of our system provides an ideal test bench for better understanding natural lasing sources, in particular the role of resonant scattering feedback in astrophysical lasers.
Towards a random laser with cold atoms
William Guerin,Nicolas Mercadier,Franck Michaud,Davide Brivio,Luis S. Froufe-Pérez,Rémi Carminati,Vitalie Eremeev,Arthur Goetschy,Sergey E. Skipetrov,Robin Kaiser
Physics , 2009, DOI: 10.1088/2040-8978/12/2/024002
Abstract: Atoms can scatter light and they can also amplify it by stimulated emission. From this simple starting point, we examine the possibility of realizing a random laser in a cloud of laser-cooled atoms. The answer is not obvious as both processes (elastic scattering and stimulated emission) seem to exclude one another: pumping atoms to make them behave as amplifier reduces drastically their scattering cross-section. However, we show that even the simplest atom model allows the efficient combination of gain and scattering. Moreover, supplementary degrees of freedom that atoms offer allow the use of several gain mechanisms, depending on the pumping scheme. We thus first study these different gain mechanisms and show experimentally that they can induce (standard) lasing. We then present how the constraint of combining scattering and gain can be quantified, which leads to an evaluation of the random laser threshold. The results are promising and we draw some prospects for a practical realization of a random laser with cold atoms.
The Future of Benthic Indicators: Moving up to the Intertidal  [PDF]
Nicolas Spilmont
Open Journal of Marine Science (OJMS) , 2013, DOI: 10.4236/ojms.2013.32A008

The use of benthic indicators has increased dramatically during the last decades. The number of articles published on the subject, as well as the number of citations, has been particularly increasing since the early 90’s, notably in relation with the implementation of directives for the management of aquatic/marine ecosystems such as the Water Framework Directive and the Marine Strategy Framework Directive. Current benthic indicators suffer from severe drawbacks and their practical use is still discussed and might have reached a dead end. Indicators based on species composition are not totally satisfactory, mainly because they exhibit a high spatio-temporal variability (e.g. variable at both seasonal and pluri-annual scales) and are user-dependent (e.g. divergent results from US orEuropeexperts.) In turn, modifications of behaviour, metabolism, phenotype or stable isotopes composition in invertebrates usually occur at short time scales, compared to detectable changes in community composition, and makes their use particularly relevant as indicators of perturbation. It is argued in this paper that these functional indicators might be relatively quickly implemented in the intertidal, and represent an effective alternative to current benthic indicators.

Gender-Specific Potential Inhibitory Role of Ca2+/Calmodulin Dependent Protein Kinase Phosphatase (CaMKP) in Pressure-Overloaded Mouse Heart
Miresta Prévilon, Mylène Pezet, Laurent Vinet, Jean-Jacques Mercadier, Patricia Rouet-Benzineb
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0090822
Abstract: Background Ca2+/calmodulin-dependent protein kinase phosphatase (CaMKP) has been proposed as a potent regulator of multifunctional Ca2+/calmodulin-dependent protein kinases (i.e., CaMKII). The CaMKII-dependent activation of myocyte enhancer factor 2 (MEF2) disrupts interactions between MEF2-histone deacetylases (HDACs), thereby de-repressing downstream gene transcription. Whether CaMKP modulates the CaMKII- MEF2 pathway in the heart is unknown. Here, we investigated the molecular and functional consequences of left ventricular (LV) pressure overload in the mouse of both genders, and in particular we evaluated the expression levels and localization of CaMKP and its association with CaMKII-MEF2 signaling. Methodology and Principal Findings Five week-old B6D1/F1 mice of both genders underwent a sham-operation or thoracic aortic constriction (TAC). Thirty days later, TAC was associated with pathological LV hypertrophy characterized by systolic and diastolic dysfunction. Gene expression was assessed by real-time PCR. Fetal gene program re-expression comprised increased RNA levels of brain natriuretic peptide and alpha-skeletal actin. Mouse hearts of both genders expressed both CaMKP transcript and protein. Activation of signalling pathways was studied by Western blot in LV lysates or subcellular fractions (nuclear and cytoplasmic). TAC was associated with increased CaMKP expression in male LVs whereas it tended to be decreased in females. The DNA binding activity of MEF2 was determined by spectrophotometry. CaMKP compartmentalization differed according to gender. In male TAC mice, nuclear CaMKP was associated with inactive CaMKII resulting in less MEF2 activation. In female TAC mice, active CaMKII (phospho-CaMKII) detected in the nuclear fraction, was associated with a strong MEF2 transcription factor-binding activity. Conclusions/Significance Gender-specific CaMKP compartmentalization is associated with CaMKII-mediated MEF2 activation in pressure-overloaded hearts. Therefore, CaMKP could be considered as an important novel cellular target for the development of new therapeutic strategies for heart diseases.
Bias correction in multivariate extremes
Anne-Laure Fougères,Laurens de Haan,Cécile Mercadier
Statistics , 2015, DOI: 10.1214/14-AOS1305
Abstract: The estimation of the extremal dependence structure is spoiled by the impact of the bias, which increases with the number of observations used for the estimation. Already known in the univariate setting, the bias correction procedure is studied in this paper under the multivariate framework. New families of estimators of the stable tail dependence function are obtained. They are asymptotically unbiased versions of the empirical estimator introduced by Huang [Statistics of bivariate extremes (1992) Erasmus Univ.]. Since the new estimators have a regular behavior with respect to the number of observations, it is possible to deduce aggregated versions so that the choice of the threshold is substantially simplified. An extensive simulation study is provided as well as an application on real data.
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