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Femtosecond filamentation of optical vortices for the generation of optical air waveguides
No full textInternational audienceWe study the filamentation in air of multi-millijoule optical vortices and compare them with the classical filamentation regime. The femtosecond vortex beam generates multiple plasma filaments organized in a cylindrical geometry. This plasma configuration evolves into a meter-scale tubular neutral gas column that can be used as a waveguide for nanosecond laser pulses at 532 nm. It appears that optical vortices produce a more uniform heating along the propagation axis, when compared with Gaussian or super-Gaussian beams, and that the resulting low-density channel is poorly sensitive to the laser input power thanks to the combination of filamentation intensity clamping and phase vorticity
Incipit Gaia the Grotesque Garlanded with Satellites: Re-Imagining the Anthropocene as a Post-Planetary Age
No full textInternational audienceOver the last decades environmentally engaged literary critics and historians have begun to embrace a characterization of our current age as planetary. This essay presents a somewhat contrary view. It argues that to fully appreciate the gravity of our current situation we must also attend to what lies beyond the planet, acknowledging the degree to which our current scientific understanding of the Earth comes from extraterrestrial remote sensing technologies, and so historically is a product of the Space Age. Drawing on this insight, and in light of the increasing degradation of near space environments as a result of New Space capitalism, it argues that the Anthropocene ought to be re-framed in extra-planetary terms so as to include anthropogenic environmental degradation taking place on planet Earth but also beyond the limits of the atmosphere. Embracing the Anthropocene as post-planetary involves shifting consciousness and care outwards to include the extended critical zone of the impact of our artifices as opposed to limiting this environmental consciousness to the natural limits of our planet and its atmosphere
Minimizing recovery cost of network optimization problems
No full textInternational audienceWe propose a two-stage recoverable robustness approach that minimizes the recovery cost. In many applications, once the uncertainty ξ is revealed, it can be more important to recover a solution x ξ which is as similar as possible to the nominal solution x nom than to minimize the nominal objective value of x ξ. This for example occurs when the nominal solution is implemented on a regular basis or when the uncertainty is revealed late. We define the proactive problem which minimizes the weighted recovery costs over a discrete set of scenarios while ensuring optimality of the nominal objective value of x nom. We model the recovery cost of a scenario by a distance between the first-stage nominal solution and the second-stage solution recovered for this scenario. We show for two different solution distances d val and dstruct that the proactive problem is N P-hard for both the integer min-cost flow problem with uncertain arc demands and for the integer max-flow problem with uncertain arc capacities. For these two problems, we prove that once uncertainty is revealed, even identifying a reactive solution x r with a minimal distance to a given solution x nom is N P-hard for dstruct, and is polynomial for d val. We highlight the benefits of the proactive approach in a case study on a railroad planning problem. First, we compare it to the anchored and the k-distance approaches. Then, we show the efficiency of the proactive solution over reactive solutions. Finally, we illustrate the recovery cost reduction when relaxing the optimality constraint on the nominal objective of the proactive solution x nom. We also consider the min-max version of the proactive problem where we minimize the maximal recovery cost over all scenarios. We show that the same complexity results hold for this version. We also exhibit a class of problems for which the set of extreme points of the convex hull of a discrete uncertainty set always contain a worst-case scenario. We show that this result does not hold for three distinct classes deduced from the first one
Groupes stratégiques, mobilité et performance des entreprises: le cas des grandes entreprises non financières américaines au tournant du XXIe siècle
No full textInternational audienceL’objectif de cet article est d’explorer la proposition suivante : la hauteur des barrières à la mobilité et l’étendue de la concurrence multimarchés ont une action complémentaire et positive sur la performance des entreprises. Sur la période 1992-2007, nous montrons que la capacité des grandes entreprises non financières américaines à adopter une configuration de ressources spécifique à son groupe stratégique est valorisée à la fois économiquement et financièrement
Analyse de convergence pour des méthodes d’inversion multi-étapes de type one-shot
No full textIn this work we are interested in general linear inverse problems where the corresponding forward problem is solved iteratively using fixed point methods. Then one-shot methods, which iterate at the same time on the forward problem solution and on the inverse problem unknown, can be applied. We analyze two variants of the so-called multi-step one-shot methods and establish sufficient conditions on the descent step for their convergence, by studying the eigenvalues of the block matrix of the coupled iterations. Several numerical experiments are provided to illustrate the convergence of these methods in comparison with the classical usual and shifted gradient descent. In particular, we observe that very few inner iterations on the forward problem are enough to guarantee good convergence of the inversion algorithm.Dans ce travail nous nous intéressons à des problèmes inverses linéaires généraux où le problème direct correspondant est résolu de façon itérative en utilisant des méthodes de point fixe. Ainsi, les méthodes de type one-shot, qui itèrent en même temps sur la solution du problème direct et l’inconnue du problème inverse, peuvent être appliquées. Nous considérons deux variantes des méthodes multi-étapes de type one-shot et nous établissons des conditions suffisantes et nécessaires sur le pas de descente pour leur convergence, en étudiant les valeurs propres de la matrice par blocs des itérations couplées. Plusieurs tests numériques sont présentés pour illustrer la convergence de ces méthodes par rapport aux méthodes de descente de gradient usuelle et décentrée. En particulier, nous observons que très peu d’itérations internes pour le problème direct sont suffisantes pour garantir une bonne convergence de l’algorithme d’inversion
Les leviers de la résilience organisationnelle : cas de la crise sanitaire vécue par le secteur du tourisme polynésien
No full textInternational audienceLa résilience organisationnelle se développe dans les pratiques de travail quotidiennes à travers des choix de management. En effet, la gestion de l’événement imprévisible et destructeur entre directement en résonance avec ces choix. Nous avons retenu trois leviers sur lesquels s’appuie le développement de la résilience organisationnelle : le sens, le lien et le lead. L’événement crisique est la pandémie du COVID-19 et notre terrain est composé de responsables de structures touristiques. Notre objectif est de comprendre et qualifier les défenses immunitaires des organisations et du secteur d’activité étudiés sous le prisme des trois leviers retenus de la résilience organisationnelle. Et pour comprendre quels sont les processus déclenchés et vécus par les acteurs interviewés pour faire face à la crise, nous avons choisi de nous positionner 18 mois après l’entrée en crise. L'originalité de notre approche est d'appréhender la résilience, non pas au niveau d'une organisation isolée, mais pour un secteur économique sur un territoire délimité : le secteur du tourisme de la Polynésie française
La place des ODD dans les programmes de formation des ingénieurs : perceptions croisées des enseignants et des élèves ingénieurs
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Maxwell's equations with hypersingularities at a conical plasmonic tip
No full textInternational audienceIn this work, we are interested in the analysis of time-harmonic Maxwell's equations in presence of a conical tip of a material with negative dielectric constants. When these constants belong to some critical range, the electromagnetic field exhibits strongly oscillating singularities at the tip which have infinite energy. Consequently Maxwell's equations are not well-posed in the classical framework. The goal of the present work is to provide an appropriate functional setting for 3D Maxwell's equations when the dielectric permittivity (but not the magnetic permeability) takes critical values. Following what has been done for the 2D scalar case, the idea is to work in weighted Sobolev spaces, adding to the space the so-called outgoing propagating singularities. The analysis requires new results of scalar and vector potential representations of singular fields. The outgoing behaviour is selected via the limiting absorption principle