We will discuss about the coupling between the classical thermal environment and a mesoscopic system. Around this theme, we will deal with several questions : Is the heat bath always in equilibrium? How can we compare the fluctuation-dissipation relations of the 1st kind and the 2nd kind? How the energy is exchanged between the heat bath and the system? Does this energy, or heat, depend on the scale of description? What aspects are the Langevin equation unable to describe? How can we see the spatio-temporal non-locality of the heat bath? Are the `Casimir' type reversible forces related to the frictional memory forces ?

Les azopolymères sont une classe de matériaux photo-actifs qui peuvent être utilisés pour former des structures ordonnées. Ces polymères sont sensibles à la variation du champ électrique de la lumière qui crée une force au niveau de la couche. En utilisant un interféromètre de type Mach-Zehnder, il est possible de produire des réseaux d’interférence sur l’échantillon qui forme la base de la structure à fabriquer. Typiquement, ces structures ont la forme de réseaux unidimensionnels où la hauteur dépend du temps d'exposition et de la fluence de la lumière incidente. Nous avons récemment utilisé cette méthode pour fabriquer des couches minces ferromagnétiques structurées avec une anisotropie magnétique induite contrôlée par la structure. La méthode pompe - sonde est une technique optique basée sur l'utilisation de lasers ultra-rapides et permet des mesures physiques résolues en temps. Dans la deuxième partie du séminaire, je présenterai la mise en place d’un spectromètre pompe - sonde ainsi que des mesures de la dynamique magnétique avec une résolution temporelle de moins de 8 fs. Je décrirai ensuite le montage expérimental et donnerai quelques exemples de réalisations récentes.

The Institut Laue-Langevin (Grenoble, France) provides neutrons for the whole scientific community. In order to transport those particles tens of meters from the reactor, neutron guides have been implanted around the core. In most cases, they consist in 4 glass plates, coated with Nickel or Nickel-Titanium, assembled in a rectangular shape. As the reflectivity of the mirror is below 100%, the glass faces irradiation and may end up cracking. The microscopic mechanisms of this phenomenon will be exposed, as well as the future experiments that will be conducted to explain the behaviour dissimilarities among the different glasses used at ILL.

The temporal behavior of polarized reflection spectra of a microcavity with a 20-nm quantum well is experimentally studied by means of polarized pump-probe technique. Kerr rotation and ellipticity are obtained for different detunings between exciton and photon modes. The Kerr rotation signal was determined in our work by means of measurement and following analysis of the polarized reflection spectra of the probe beam that allowed us to find a maximum Kerr rotation of 180 degrees in special point on detuning. As it follows from theoretical analysis, the main role in achievement of maximum Kerr rotation plays the asymmetry of the microcavity and the position of the impedance matching point but not the Q-factor of the microcavity.

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.

Amphibians are animals that need two different kinds of habitats - water and forests - to achieve their life cycles. When water and forest are dissociated, their populations tend to decline. I present a simple population model based on diffusion equations aimed to get insight into this phenomenon. I show that there exists a critical split distance between the water and forest above which the population goes locally extinct.

Malaria outbreaks and epidemics are usually related to the presence of forests. In some cases however, like in the Brazilian Atlantic Forest, cases are very rare. We investigate into the causes of this by means of an epidemiological model, parameterized with data, which takes into account several factors of ecological nature: the presence of non-competent hosts; and the diffuse competition of vector and non-vector mosquitoes. This model is a first attempt to assess the relative importance of different factors related to biodiversity in the causes of vector-borne epidemics.

Non-equilibrium systems have been studied extensively in the last 20 years. Unlike equilibrium, these systems are characterized by a breaking of reversibility at the microscopic level leading to the presence of currents at the macroscopic level. In one dimension, exact results have been obtained for the fluctuations of macroscopic quantities in several such systems, using tools including Bethe ansatz, the replica method, and random matrices. Two prominent examples are a discrete microscopic model of classical hard core particles hopping with a preferred direction (asymmetric simple exclusion process) and a continuous equation describing the stochastic evolution of a growing surface at large scales (Kardar-Parisi-Zhang equation).

How long does it take a "searcher" to reach a "target" for the first time? This first-passage time is a key quantity for evaluating the kinetics of various processes, and in particular chemical reactions involving "small" numbers of particles. A striking example is given by gene transcription, where specific proteins search for target sequences on DNA. I will present asymptotic results which enable the evaluation of the distribution of the first-passage time to a target site for a wide range of random processes in confined domains. This approach quantifies explicitly the amplitude of fluctuations of the first-passage time and reveals its general dependence on the geometry of the problem, which can become a key parameter that controls the kinetics. I will show how these results apply to reactions involving « small" numbers of particles, and highlight their implications in transcription kinetics.