Evolution of vortices created by conical diffraction in biaxial crystals versus orbital angular momentum

Light states evolution versus their fractional orbital angular momentum (OAM) has been analyzed in the conical diffraction process occurring through biaxial crystals. Experimental results are provided by a non-degenerate cascade of KGd(WO$_2$)$_4$ and Bi$_2$ZnOB$_2$O$_6$ biaxial crystals. The continuous $0\to 1\to 2$ $\hbar$/photon increasing of the fractional OAM in passing through integer values was operated with the help of the spin-orbit coupling in the Bi$_2$ZnOB$_2$O$_6$ crystal. The phase of the state light and its vortices were visualized by interference patterns with a reference beam. The evolution of the fractional OAM value is accompanied by a continuous evolution of pairs of vortices with opposite signs and linked by a $-\pi/+\pi$ discontinuous phase line. The phase pattern evolution around half-integer OAM is observed to be continuous. In other cases, the evolution can be interrupted by the breaking of a $-\pi/+\pi$ discontinuous phase line and a new pair of vortices with opposite charges is born.Comment: Optical Materials, Elsevier, 202

Young Measures Generated by Ideal Incompressible Fluid Flows

In their seminal paper "Oscillations and concentrations in weak solutions of the incompressible fluid equations", R. DiPerna and A. Majda introduced the notion of measure-valued solution for the incompressible Euler equations in order to capture complex phenomena present in limits of approximate solutions, such as persistence of oscillation and development of concentrations. Furthermore, they gave several explicit examples exhibiting such phenomena. In this paper we show that any measure-valued solution can be generated by a sequence of exact weak solutions. In particular this gives rise to a very large, arguably too large, set of weak solutions of the incompressible Euler equations.Comment: 35 pages. Final revised version. To appear in Arch. Ration. Mech. Ana

Participatory assessment of the Toliara Bay reef fishery, southwest Madagascar

In order to ensure the sustainable management of reef fisheries, it is necessary to obtain data about the impacts of these fisheries on both fish  resources and the ecosystems that sustain them. Ecosystem - based surveys provide such information, but are difficult to implement because of high technical, financial and human resource requirements. In this regard participatory assessment methods have the potential to increase the amount of data collected at low cost, while taking advantage of local traditional ecological knowledge. In order to investigate the reef fishery of Toliara Bay, southwest Madagascar, we used participatory fish survey and interview data collected on site. These methods included: (i) monitoring of catch landings over six months by wholesale fish merchants, (ii) household surveys of fishing catch and effort, and fish consumption conducted by school children, and (iii) semi - structured interviews of reef users. The landings of 1,586 fishing trips were sampled between September 2006 and February 2007, 326 households were surveyed by trained school children in January 2007, and 70 reef users were interviewed in July/August 2006. Data collected by participants were compiled and compared to reference values when available, allowing an assessment of the sustainability of the reef fishery. The results of this study confirm the unsustainable nature of resource exploitation and underline the need for rapid management responses in order to reverse this trend. It also highlights the great potential of participatory assessment methods for gathering large amounts of relevant information on the status and evolution of the ecosystem upon which the fishery depends, while promoting education and awareness about the conservation and sustainable use of natural resources.Bien que les pêcheries récifales ne contribuent que marginalement aux captures de pêche mondiales, elles restent une source majeure de revenus et de protéines pour des millions de personnes, en particulier dans les pays en voie de développement. Afin de s’assurer de la bonne gestion de ces pêcheries, il est nécessaire de disposer d’informations sur l’état des ressources et des écosystèmes dont elles dépendent. Mener de telles études est d’autant plus compliqué que les pêcheries en milieu corallien portent sur un large éventail de stocks et d’espèces de poissons, concernent de nombreux pêcheurs et supposent diverses méthodes de captures, et empruntent un grand nombre de canaux de distribution. De plus, ces pêcheries sont souvent considérées de moindre valeur par les gouvernements qui leur octroient donc peu de moyens humains et financiers. Face à ces problèmes, l’implication de la société civile dans les programmes de suivi, appelé suivi participatif, semble en mesure d’apporter des solutions, d’autant que les suivis participatifs présentent l’avantage d’accroître le nombre de données collectées à moindre coût, tout en profitant des connaissances écologiques empiriques qui sont disponibles localement. Afin d’évaluer la pérennité de la pêcherie récifale de la baie de Toliara dans le sud ouest de Madagascar, des méthodes de suivi participatif ont été expérimentées. Ces méthodes consistent à mettre en oeuvre : (i) des suivis des débarquements pendant six mois impliquant des mareyeuses, (ii) des enquêtes auprès des ménages, réalisées par des écoliers préalablement formés, afin de recueillir des informations sur l’effort de pêche, les captures et la consommation de poisons, (iii) des entretiens semi - directifs avec des usagers du milieu marin. Ainsi, entre 2006 et 2007, 1586 sorties de pêche ont été échantillonnées, et 326 ménages et 70 usagers ont fait l’objet d’enquêtes. Les données collectées par les membres de la communauté locale ont été analysées afin d’évaluer la pérennité de la pêcherie dans la baie de Toliara. Les résultats de l’étude confirment une exploitation non pérenne et soulignent la nécessité de mettre en place des mesures de gestion afin d’inverser la tendance de détérioration des écosystèmes de la baie. Il ressort également de cette étude que l’implication de la société civile s’avère être pertinente pour recueillir des informations sur les pêcheries récifales à faible coût. En effet, de part le nombre, la qualité, la variété et la pertinence des informations produites, les suivis participatifs contribuent utilement à l’évaluation des pêcheries récifales. Cette expérience met également en évidence le fort potentiel des suivis participatifs à contribuer au renforcement des connaissances et des capacités des communautés locales dans le domaine de la gestion des ressources marines. Ainsi, face au difficile contexte socio - économique et politique qui prévaut actuellement à Madagascar, cette approche s’avère particulièrement prometteuse pour améliorer la gestion des pêcheries traditionnelles grâce à la production d’informations sur ces pêcheries et une plus grande implication des communautés locales dans le processus de gestion.Correction to: Madagascar Conservation & Development (2011) 6, 2: 60–67. http://dx.doi.org/10.4314/mcd.v6i2.4; published online 22 December 2011On p. 63, ‘mt/y’ should be metric ton per year and not million ton per year.Sentence should read “With the results of the household survey we were able to estimate a total production for the Toliara Bay fishery at 2,700 ± 436 metric tons per year (mt/y), and the annual yield at 14.2±2.3mt/y/km2.&#8221

Rank-(n – 1) convexity and quasiconvexity for divergence free fields

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L^2 stability estimates for shock solutions of scalar conservation laws using the relative entropy method

We consider scalar nonviscous conservation laws with strictly convex flux in one spatial dimension, and we investigate the behavior of bounded L^2 perturbations of shock wave solutions to the Riemann problem using the relative entropy method. We show that up to a time-dependent translation of the shock, the L^2 norm of a perturbed solution relative to the shock wave is bounded above by the L^2 norm of the initial perturbation.Comment: 17 page

Ion-beam mixing induced by atomic and cluster bombardment in the electronic stopping-power regime

Single crystals of magnesium oxide containing nanoprecipitates of sodium were bombarded with swift ions (∼GeV-Pb, U) or cluster beams (∼20 MeV-C60) to study the phase change induced by electronic processes at high stopping power (≳10 keV/nm). The sodium precipitates and the defect creation were characterized by optical absorption and transmission electron microscopy. The ion or cluster bombardment leads to an evolution of the Na precipitate concentration but the size distribution remains unchanged. The decrease in Na metallic concentration is attributed to mixing effects at the interfaces between Na clusters and MgO. In addition, optical-absorption measurements show a broadening of the absorption band associated with electron plasma oscillations in Na clusters. This effect is due to a decrease of the electron mean free path, which could be induced by defect creation in the metal. All these results show an influence of high electronic stopping power in materials known to be very resistant to irradiation with weak ionizing projectiles. The dependence of these effects on electronic stopping power and on various solid-state parameters is discussed

Scattering defect in large diameter titanium-doped sapphire crystals grown by the Kyropoulos technique

International audienceThe Kyropoulos technique allows growing large diameter Ti doped sapphire for Chirped pulse amplification laser. A scattering defect peculiar to Kyropoulos grown crystals is presented. This defect is characterized by different techniques: luminescence, absorption measurement, X-ray rocking curve. The impact of this defect to the potential application in chirped pulse amplification CPA laser is evaluated. The nature of this defect is discussed. Modified convexity of the interface is proposed to avoid the formation of this defect and increase the quality of the Ti sapphire crystal

Mass transportation with LQ cost functions

We study the optimal transport problem in the Euclidean space where the cost function is given by the value function associated with a Linear Quadratic minimization problem. Under appropriate assumptions, we generalize Brenier's Theorem proving existence and uniqueness of an optimal transport map. In the controllable case, we show that the optimal transport map has to be the gradient of a convex function up to a linear change of coordinates. We give regularity results and also investigate the non-controllable case