École normale supérieure de Cachan

About: École normale supérieure de Cachan is a education organization based out in Cachan, Île-de-France, France. It is known for research contribution in the topics: Decidability & Nonlinear system. The organization has 2717 authors who have published 5585 publications receiving 175925 citations.

A Method for Proving Observational Equivalence

Abstract: Formal methods have proved their usefulness for analyzing the security of protocols. Most existing results focus on trace properties like secrecy (expressed as a reachability property) or authentication. There are however several security properties, which cannot be defined (or cannot be naturally defined) as trace properties and require the notion of observational equivalence. Typical examples are anonymity, privacy related properties or statements closer to security properties used in cryptography.In this paper, we consider the applied pi calculus and we show that for determinate processes, observational equivalence actually coincides with trace equivalence, a notion simpler to reason with.We exhibit a large class of determinate processes, called simple processes, that capture most existing protocols and cryptographic primitives. Then, for simple processes without replication nor else branch,we reduce the decidability of trace equivalence to deciding an equivalence relation introduced by M. Baudet. Altogether, this yields the first decidability result of observational equivalence for a general class of equational theories.

The Euler-Poincare theory of Metamorphosis

Abstract: In the pattern matching approach to imaging science, the process of ``metamorphosis'' is template matching with dynamical templates. Here, we recast the metamorphosis equations of into the Euler-Poincare variational framework of and show that the metamorphosis equations contain the equations for a perfect complex fluid \cite{Ho2002}. This result connects the ideas underlying the process of metamorphosis in image matching to the physical concept of order parameter in the theory of complex fluids. After developing the general theory, we reinterpret various examples, including point set, image and density metamorphosis. We finally discuss the issue of matching measures with metamorphosis, for which we provide existence theorems for the initial and boundary value problems.

Size and Shape Resonances in Second Harmonic Generation from Silver Nanocavities

Abstract: The nonlinear response of subwavelength nanocavities in thin silver films are investigated. We report on significant enhancements of the second harmonic generation (SHG) when the fundamental wavelength matches dimensional resonances within the nanocavities. The nonlinear polarization properties of the nanocavities are studied as well and found to be correlated with the cavity shape and symmetry. In some nanocavities with internal nanocorrugations, giant field enhancements are observed, making them excellent candidates for high sensitivity spectroscopy.

Deviation of a machined surface in flank milling

Abstract: The flatness defects observed in flank milling with cutters of long series are mainly due to the tool deflections during the machining process. This article present the results of an identification procedure of the coefficients of a force model for a given tool workpiece couple for the prediction of the defects of the tool during the cutting. The calibration method proposed meets a double aim: to define an experimental protocol that takes the industrial constraints of time and cost into account and to work out a protocol which minimizes uncertainties likely to alter the interpretation of the results (environmental, software or mechanical uncertainties). For that, the procedure envisages the machining of a simple plane starting from a raw part formed by a tilted plane, allowing for the variation of the tool engagement conditions. The tool deviation during the cutting process is indirectly identified by measuring the machined surface. The observed straightness defect conditions can be explained by the evolution of the cutting pressures applied to the cutting edges in catch during the cutter rotation. The precision was considerably improved by the taking into account of the cutter slope defect in the calculation of the load applied to the tool. After identification of the tool-workpiece couple, the prediction model was applied to some examples and allowed to determine the variations of form and position of the surface points with a margin of 5%.

Hybrid organic-inorganic exciton-polaritons in a strongly coupled microcavity

Abstract: We characterize strongly coupled microcavities in which optical hybridization occurs between the organic-dye zinc tetraphenyl-porphrin (ZnTPP) and a thin film of a self-assembled perovskite. Position-resolved reflectivity measurements demonstrate the formation of hybrid organic-inorganic exciton polaritons corresponding to a three-way admixture of the cavity photon with the electronic transitions of the ZnTPP and the perovskite. We find that such hybrid-polariton states can undergo radiative emission, despite the existence of a competing decay channel to lower-lying $Q$-band states. We argue that such hybrid polaritons can in principle act as an intermediate energy-transfer state between the organic and inorganic excitations.