On présente des expériences de propagation d’ondes à la surface de l’eau dans des contextes de propagation en milieux complexes. La mesure du champ d’élévation de ces ondes de gravité est effectuée à l’aide d’une technique de mesure de Profilométrie par Transformée de Fourier, qui permet une mesure globale et instantanée. Outre leur intérêt propre (propagation océanique, tsunami, vagues scélérates, ...), les ondes à la surface de l’eau présentent une analogie avec la propagation d’ondes acoustiques (équation de Helmholtz) qui motive l’utilisation de concepts développés en électromagnétisme (métamatériaux, cristaux photoniques/photoniques); cependant, dans le cas des ondes à la surface de l’eau, les effets de viscosité, de capillarités (par exemple aux ménisques) peuvent modifier les phénomènes attendus. Des exemples seront donnés: le cloacking par un milieu homogénéisable en couche, l’hyperfocalisation par un réseau de tiges et la résonance d’un réseau périodique d’obstacles. Des études plus spécifiques, concernant le comportement des ondes non linéaires, seront également présentées: turbulence d’ondes de gravité/capillarité, tsunami, retournement temporel des ondes à la surface de l’eau.

We will present the physics of hydrodynamic waves in stratified fluids and in particular in the ocean. Starting with simple experiments emphasizing the generation of  localized  nonlinear  internal  waves at the interface of layers with different  densities,  we will consider the formation  of  hydrodynamics solitons, also called tsunamis. More sophisticated experiments in a continuously stratified fluid allow to understand the behavior of these waves, often very unusual. We will present in particular interesting nonlinear  effects, such  as  parametric  subharmonic  instabilities or reflection on sloping  boundaries which are among the dominant mechanisms leading to dissipation.

Polaritons, half-light half-matter mixed states arising from the strong coupling between excitons and photons in semiconductor microcavities, are composite bi-dimensional interacting bosons. They can manifest macroscopic quantum coherence effects at high temperatures (5-300 K) due to their very low mass. In particular, polaritons behave like a quantum fluid with specific properties coming from its intrinsic out of equilibrium nature, determined by the short polariton lifetime (some picoseconds). At the same time, microcavity polaritons, due to the strong non-linear polariton-polariton interactions inherited from their excitonic component, are very promising for the realization of integrated optoelectronic devices operating at the quantum level. In the first part of the talk, I will briefly review the superfluid and Cerenkov regimes in these systems, and I will discuss the formation of quantized vortex and dark solitons in a polariton quantum fluid interacting with a large obstacle. The possibility to generate vortex lattices in a confined geometry will be discussed. Finally, I will show how the non linear polariton-polariton interactions can be exploited to achieve for the first time quantum noise reduction on a light beam transmitted by semiconductor micropillar, relying on a Kerr-like effect. These results show the potential for the generation of non-classical states of light with these nanostructures. In the second part of the talk I will present our recent results on efficient room temperature single photon emitters based on core/shell colloidal semiconductor nanocrystals. In such structures, the competition between radiative and non-radiative recombination channels induces photoluminescence fluctuations between on and off states known as blinking. The shell engineering is a suitable strategy to control recombination paths and has been used to produce almost non-blinking nanocrystals, although accompanied by undesired increasing of multi-excitonic emission probability. By using asymmetric core/shell nanoparticles (dots-in rods) with a spherical CdSe core surrounded by a rod-like CdS shell, blinking effects, multi-excitonic emission and polarization of the emitted photons can be separately controlled by tuning the shell dimensions. This allows an unprecedented capability in radiative channels engineering, making dot-in-rods very efficient blinking-free sources of polarized single photons on-demand.

End tethered polymer chains have been recognized to be excellent adhesion promoters at polymer or are known to be able to modify the friction at interfaces. In this talk, we will present 2 related sets of experiments on the dynamics of polymer close to an interface. We will present results of polymer chains in contact with a melt and in contact with an elastomer. In a first part, we will present neutron reflectivity experiments characterizing the density profiles and dynamics of interdigitation between H-PS brushes, in contact with d-PS thick layers, heated above their glass transition temperature. This interdigitated brushes will be shared and disentangled from the melt leading to a large slip at the interface. The density profiles of the sheared brushed will be presented. In a second part we will present some friction measurement at a PDMS chains – PDMS elastomer interface and we will show that the grafted chains penetration and dynamics allows to control the friction at such elastomer interfaces.

Many solids around us are disordered : glassy or semi crystalline polymers, oxide or metallic glasses, nanocomposites, are common examples. In disordered systems, the link between macroscopic mechanical properties and the nanostructure of the materials is far from being understood. I will describe some of the efforts that are currently made in improving our understanding of this connection using computer simulations of microscopic or mesoscopic models of materials that range from simple metallic glasses to more complex polymer nanostructures. In particular, I will emphasize the heterogeneous nature of elastic response in these materials, and how it eventually gives rise to complex collective aspects in the plastic deformation, such as structure formation or strain localisation.

Improvements in both the photonic confinement and the emitter design have led to a steady increase in the strength of the light-matter coupling in cavity quantum electrodynamics experiments, to the point that the light matter coupling, Ω, can become a non-negligible fraction of the bare dipole frequency ω0. In such an interaction-dominated regime, usually referred to as ultrastrong coupling regime, the state of the system is usually well described in terms of mixed light-matter excitations, called polaritons. I will show that, when the coupling between light and matter becomes strong enough, this picture breaks down. In particular, in the non-perturbative deep strong coupling regime, Ω > ω0, light and matter degrees of freedom totally decouple. A striking consequence of such a counterintuitive phenomenon is that the Purcell effect is reversed and the spontaneous emission rate, usually thought to increase with the light-matter coupling strength, plummets instead for large enough couplings. This phenomenon, which should become observable already for resonant normalized couplings, Ω /ω0, of the order of 1, that is less than a factor 2 from present day experiments, poses an intrinsic upper bound to the efficiency of strongly coupled light emitters. Nevertheless, it could be exploited to dynamically control the electromagnetic field distribution in optoelectronic devices, by modulating the intensity of the light-matter interaction by optical or electrical means.

The description of the flow of granular media still represents a challenge due to the difficulty in writing constitutive laws. However, recent advances have shown that a simple hydrodynamical description based on a frictional visco-plastic law may help in understanding the properties of granular flows. In this talk, the successes and limits of this continuum description will be discussed, before revisiting the classical problem of the drag and lift forces experienced by an object moving in a granular medium. It will be shown that the peculiar rheology of granular media leads to striking behaviours when a cylinder is dragged through a packing of grains.

ATTENTION: CHANGEMENT DE SALLE EXCEPTIONNEL **** AMPHI 23.01 **** L'existence d'un fond d'ondes gravitationnelles primordiales est une prédiction fondamentale des modèles inflationnaires. Or ces modèles semblent être à l'origine de la formation des grandes structures et de toutes les inhomogénéités observables dans notre univers. Ils ont la faveur des observations existantes. Nous expliquerons l'enjeu scientifique des mesures de la polarisation du fond diffus par les expériences BICEP2 et Planck. Tous ces enjeux seront abordés de manière compréhensible pour non-spécialistes.

Why are plant leaves flat? How does a plant achieve a well-defined shape? More generally, what are the physical mechanisms behind morphogenesis in living systems? We are combining various experimental and theoretical approaches to address these questions, which has led us to study the coupling between differential geometry and elasticity, to build models of active elastic media, and the develop methods to measure the mechanical properties of live plant tissues. In this talk, I will give a broad overview of our results on the physics of plant development.