Changes in the structure of network-forming glasses occur in response to applied pressure. Moreover, different amorphous states with distinct short- and/or intermediate-range orders as well as contrasting physical properties can be produced following separate thermomechanical paths. Even if these phenomena, termed polyamorphism, have been extensively studied during the last decades, their complete understanding still remains an ongoing challenge. It relates to the difficulty in carrying out conclusive experiments at high pressure. Vitreous silica is the archetypal tetrahedral network-forming glass whose behavior under pressure is of long-standing interest due to its primary importance as the analog material of silicates in geophysics. At room temperature, the hydrostatic compression is reversible up to ~10 GPa. When compressed above this limit, the recovered glasses after complete unloading exhibit residual densification. The latter saturates at about 20% for maximum pressures around 20-25 GPa.
In this talk, I will present how In situ high-pressure Brillouin light scattering experiments along pressure cycles are used to investigate the compressibility of vitreous silica. And how combined with density data we provide a series of quantitative results related to the transformations along loading-unloading paths. I will also show how, contrary to common wisdom, the residual densification of the recovered silica samples can be figured out from changes in elastic properties along pressure cycles, ruling out a plastic description of the latter process.

Elemental nanomaterials, such carbon nanotubes (CNTs), graphene and more recently exfoliated black phosphorus (bP), are studied in my group for their low-dimensional semiconducting properties. Graphene is a semi-metal, whereas bP and CNTs are semiconductors having tunable band gaps in the near infrared, which depend on sample thickness for bP (Eg≈0.3-1.9 eV) and diameter for CNTs (Eg≈0.6-1.1 eV). Not present in bulk materials, the confinement effects also induce unexpected chemical, electrical, and optical properties, which are of interest for applications.
An overview of the general characteristics of quantum confinement will be presented first to highlight the impact on the optical and electrical properties of bP, graphene and NTs. This talk will more specifically present recent studies on exfoliated black phosphorus prepared in a glovebox as mono-, bi- and multilayered bP. Similar to the case of graphene, thin bP layers exhibit strong and unexpected optical resonances in Raman, the so-called D-modes, which can be understood in the context of coherent scattering involving defects and phonons. Because the process relaxes the optical selection rule forbidding access to q ≠ Γ phonons, the mechanism can produce strong and unexpected spectral features in different conditions, giving for instance anti-resonances in the mid-IR part of the spectrum and second-order Raman modes. Our work show that the defect-mediated resonances are ubiquitous in exfoliated bP, CNTs, and graphene and of interest for potential applications, such as THz wave modulators, nanoprobes and photodetectors. They are also useful to nanoscience because they help analyze interface properties, reactivity, and solve processing hurdles for making electronic nanodevices.

I will present a technique on how to inspect causal links between time-series based on dynamical systems theory, mainly, on the Takens theorem. As an application, I will show how to determine climatic drivers of malaria epidemics in a particular site (Northern Argentina). A discussion of new applications and extensions closes the talk.

Monolayers of semiconducting transition metal dichalcogenides are direct band gap semiconductors for which the direct band gap is located in the two inequivalent K and K' valleys. Because of the lack of inversion symetry and of the large spin orbit interaction, the spin and valley degrees of freedom are strongly coupled and a particular valley exciton population can be initialized using circularly polarized light excitation. As two dimensional materials, monolayers can be stacked on top of one another in the form of van der waals heterostructures to create artifical materials. Heterostructure of monolayer of TMD encapsulated in hBN offer a rich plateform to study the properties of 2D excitons.

In this talk, I will describe optical properties of bright and dark excitons in monolayers of TMD encapsulated in hexagonal boron nitride. Because of their 2D nature, excitons in monolayers of TMDs arze tighly bound and their propertie are strongly influenced by the dielectric environment. The relevent spectroscopic signature of such modifications is inscribed in their excitation spectrum and I will present a model to describe such influence. Dark excitons are activated by magnetic fields applied in different configurations. The exchange interaction mixes dark excitons in the two valleys and I will show how this mixing can be monitored by external magbetic fields.

Dense colloidal suspensions are paramount in many industrial applications. However, as they are governed by Brownian motion and particle interactions, the ability to quantify their dynamics in flowing systems remains challenging. Using nanocelluloses as model systems, we focus on the dynamics of a colloidal nanorod suspension forming a thread in a flow-focusing channel. I will first demonstrate an experimental method for characterization of the nanorod orientation dynamics using a flow-stop technique coupled with a thorough analysis of the orientation relaxation. The coupling of rotational diffusion coefficients to particle size distributions as well as flow-induced orientation mechanisms in the nanorod network will be discussed. Then, I will focus on the shape of the thread formed by the suspension. Using experiments performed with Optical Coherence Tomography and numerical simulations with a Volume of Fluid method, I will exhibit that the development of the thread in the channel is controlled by an effective interfacial tension between the colloidal suspension and water.

Optical resonators are basic elements for a huge variety of devices, from lasers to sensors. They are also at the basis of important quantum electrodynamic effects, such as tailoring the spontaneous emission via the Purcell effect. Optical resonators are therefore of the utmost relevance in many different fields and they have been extensively investigated during the last century, still some important revisions of their understanding are recently emerged in the framework on non-Hermitian theories.
The basis of the novel models is the open nature of any optical resonator, since photons cannot be confined in a finite volume for infinite time. This simple and well known fact implies complex eigenvalues and eigenvectors of the photonic master equations, leading to a deep revision of the famous expression for the Purcell factor, with counterintuitive effects, such as complex modal volume, Fano-like local density of states and even negative radiative rate contributions [1-3].
We experimentally demonstrate all these predictions in cavities on photonic crystal slabs with relatively low optical loss. Near-field hyperspectral imaging via quantum dot photoluminescence is proved to be a direct tool for detecting the counterintuitive effects of non-Hermitian theories. The experimental results fully agree with the predictions [4,5].
References
[1] C. Sauvan, PRL 110, 237401 (2013)
[2] E. A. Muljarov, W. Langbein, Phys. Rev. B 94, 235438 (2016)
[3] P. Lalanne et al., Laser Photonics Rev. 12, 1700113 (2018) 
[4] K-G. Cognée et al. Optica, 6, 269 (2019)
[5] D. Balestri et al, submitted to PRL.

Single chain nanoparticles (SCNPs) are an emergent class of synthetic nano-objects, of size 5-20 nm, obtained through intramolecular cross-linking of single polymer chains. They are the basis of the so-called single-chain technology, a rapidly growing research area of enormous possibilities and applications in fields as biosensors, catalysts, drug delivery or rheological agents. In this presentation we summarize the state-of-the art and our most recent contributions to the field through the combination of synthesis, simulations, scattering (neutron and X-ray) and relaxation techniques (dielectric spectroscopy and rheology), and discuss the main challenges for the next years.

SCNPs are topologically polydisperse. The standard synthesis protocols produce SCNPs with open sparse topologies consisting of tightly linked domains connected by flexible linkers, displaying analogies with intrinsically disordered proteins (IDPs) and suggesting SCNPs as a model system -free of specific interactions- to discriminate steric effects on IDPs in crowded cellular solutions As opposite to the gaussian conformations adopted by linear chains, sparse SCNPs in concentrated solutions and bulk exhibit an entropically driven collapse to fractal globular conformations. This observation is independent of the specific architecture of the macromolecular crowders (linear or complex) surrounding the SCNP [3,4] and on the lifetime of the SCNP bonds (irreversible or dynamic).

Simulations can be used to envisage a broad set of realistic synthesis routes to control the compactness of SCNPs, leading to globular cross-linked objects much smaller than microgels. Concentrated solutions of globular SCNPs exhibit, in contrast to hard spheres, structural and dynamic anomalies, as soft caging, reentrant behaviour (ordering and speeding up by increasing concentration) and absence of dynamic heterogeneities. This constitutes a realization in a real macromolecular system of the qualitative scenarios proposed for a long time by generic models of structureless ultrasoft particles.

Finally we explore the possibility of using SCNPs as nanofillers for linear matrixes in fully-polymeric nanocomposites and even for designing bulk materials based only in SCNPs. We characterize several dynamic scales going from the alpha-relaxation to the reorientation of the SCNPs. Whereas the glass transition is just slightly affected with respect to their linear counterparts, sparse SCNPs exhibit a much faster relaxation of the chain modes, reflecting a strong suppression of entanglements. When SCNPs are used in nanocomposites, their specific topology has a different effect on the dynamics of the linear chains. By characterizing the entanglement network in the nanocomposite we find that sparse and globular SCNPs lead respectively to mild dilation and strong narrowing of the tube for the reptational motion of the linear chains.

La matière active regroupe tous les systèmes physiques qui ne sont pas à l’équilibre thermodynamique et qui consomment de l’énergie à l’échelle de leurs composant individuels. La plupart des systèmes biologiques et notamment les tissus peuvent être considérés comme de la matière active. Au niveau théorique, les approches hydrodynamiques se sont révélées tres utiles pour l’étude de la matière active.

La première partie de l'expose donnera une introduction à la matière active et aux écoulements spontanés qui apparaissent dans la matière active. A forte activité, ces écoulements peuvent devenir turbulents et définissent un nouvelle classe de turbulence active à bas nombre de Reynolds.

La deuxième partie de l'expose sera consacrée aux tissus considérés comme des matériaux actifs. Je présenterai des expériences effectuées dans l’équipe de Pascal Silberzan a l'Institut Curie sur une monocouche de cellules allongées qui ont un ordre orientationnel nématique et je discuterai l’instabilité d’écoulement des cellules qui pourrait être importante pour certains tissus cancéreux.