The random packing of spherical objects has fascinated researchers for centuries. When hard or soft spheres are quenched above a sphere volume fraction of about sixty per cent, a supercooled fluid of spheres enters an amorphous solid phase. The state of matter of this phase is peculiar since it has both fluid- and solid-like attributes: it has a liquid-like structure, yet mechanically responds to an applied stress as a solid. This called "jamming” phenomenon is not only of fundamental interest but also of prime importance in materials science since it governs the elasticity and flow of many soft materials such as emulsions, particle dispersions, polymeric thickeners as well as granular materials (sand, powders, etc.). After a general introduction I will discuss experiments and concepts derived from jammed assemblies of microscale hard and soft spheres. In the first set of studies we investigate deformable particles suspended in water either consisting of oil droplets or microgel-particles that are driven into a glassy and jammed state. We probe both translational and rotational motion using confocal microscopy, spatially resolved light scattering and optical birefringence. Our results show clear evidence of spatial and temporal heterogeneities, both thermally activated and driven by applying external fields. In a second set of experiments we used jammed assemblies of spheres as templates for photonic structures. We follow a protocol based on mapping the hyperuniform point coordinates of the spheres into tessellations for disordered photonic materials design. Using direct laser writing in a polymer photoresist we succeeded in fabricating such structures with features on the submicron scale.

Horaire Exceptionnel - 10h

Surface-enhanced Raman scattering (SERS) describes the giant enhancement of the Raman cross section close to a metal surface. SERS can detect single molecules by optical spectroscopy. Its variant, tip-enhanced scattering (TERS) increases the resolution of Raman imaging down to the molecular limit. Despite the wide-spread use of plasmon-enhanced Raman spectroscopy in imaging and materials characterization its fundamental principles remain debated. The dominant mechanism for SERS is thought to be plasmonic enhancement; the large Raman cross section is explained as an enhancement by the localized surface plasmons of metal nanoparticles.
Here we present our results on plasmon-enhanced Raman scattering in graphene and carbon nanotubes. The enhancement is induced by the plasmons of well-controlled lithographically prepared gold nanodimers. Mechanically exfoliated graphene is deposited in a dry transfer process, whereas carbon nanotubes are placed by dieelectrophoresis. This setup allows to study the fundamental of SERS. We present results of the polarization and wavelength-dependence of plasmonic enhancement. The enhancement is extremely strongly localized in space (10 nm) and frequency (15 meV). We propose a new theory of SERS, where the plasmon forms an integral part of a higher-order Raman process and discuss its consequences for SERS in molecules and artificial nanostructures.

A multimode optical fiber acts as a versatile tool to probe the spatial properties of waves in systems in which the rays' dynamic is complex in the geometrical limit. Indeed, the transverse cross section of a truncated fiber is analogous to a 2D-billiard, the paradigm of Ray Chaos. By playing on the level of truncation, one can explore a variety of complex dynamics and thereafter their effects on the waves properties. Strongly affected by the nature of the underlying rays' dynamics, the modes in such systems present surprising properties as strong spatial localizations along stable or unstable periodic orbit as well as singular spectral signatures for some family of modes. In this presentation, I will report experiments and numerical investigations on the observation and analysis of these singular signatures. I will discuss also the way these can be exploited to control the modes in a highly multimode optical fiber for telecommunications and optical imaging.