Actin, a major cytoskeletal biopolymer in eukaryotic cells, is crosslinked into networks of filaments and bundles. These networks are largely responsible for the maintenance of cellular shape, rigidity and mechanical stability. Other assemblies of actin are involved in a myriad of cellular processes, such as cell migration, division, intracellular transport and morphogenesis. In these processes, the spatial and temporal regulation of the network structure, their dynamics, and force generation due to myosin motors are crucial. Experimentally, one of the challenges is to measure force transmission across such networks, which is vital to properly understand function, failure and repair mechanisms beyond the linear regime. 

In this talk, I will focus on our recent work, where we have developed a FRET-based, reversible DNA force sensor to measure forces across the cytoskeletal network. Discussing the use of these DNA constructs as flexible crosslinkers across semiflexible actin, thereby reconstituting model networks of cytoskeletal structures in order to map force distributions and stress relaxations across the resulting network.  Where, the rheology and frequency response of these networks was probed via macro-rheology. I shall also describe how the DNA force sensor was characterized in solution and across actin networks through fluorescence lifetime imaging microscopy (FLIM) measurements. Bringing us to how we estimated the FRET efficiency of our DNA sensor and finally, testing of the DNA sensors for their function in living cells.

Many materials that form polycrystalline solids can also form glasses, e.g. metallic glasses. In this talk, I will primarily discuss the physics behind the competitions between crystallization and vitrification. I will introduce a new model, termed as the Arrow-Potts model, that combines the physics behind glassy dynamics with crystallization kinetics and arrive at a new analytical formula to understand and predict the time scales of crystallization. The analytical formula is obtained by deriving a unified theory that combines the field theory of nucleation with the dynamical facilitation theory of glasses, and Kolmogorov's statistical theory of crystal coarsening.

Colloidal systems - collections of micrometer-sized particles suspended in a solvent - constitute a versatile and important class of soft materials. On one hand, they serve as optically visible analogues of atoms and molecules, providing valuable information on local structure and dynamics associated with a variety of condensed matter phenomena such as crystal nucleation and glass formation. On the other hand, they serve as mesoscopic building blocks that can be self-assembled into functional materials. In this talk, I will illustrate both of these applications of colloidal systems through specific examples. In the first part, I will demonstrate how information on particle dynamics from experiments on dense colloidal suspensions can be harnessed to critically compare and contrast microscopic predictions of the Random First-Order Transition Theory and Dynamical Facilitation, which are two prominent competing theories of glass formation. In the second part, I will show that purely repulsive colloidal particles undergo dynamic clustering when immersed in an active bath composed of motile bacteria. The clustering dynamics are governed by the interplay between short-ranged effective attraction between colloids mediated by collisions with bacteria and long-ranged hydrodynamic interactions. These experiments open a new route to nonequilibrium self-assembly at the micrometer scale driven by the “active noise” generated by self-propelled particles.

Electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) consists in sending fast electrons onto a nano-material and analyzing the energy losses undergone by these electrons. During the last decade, STEM-EELS became a privileged technique to study optical or electronic excitations at the nano-scale (e.g. plasmons [1], excitons [2] or surface-phonons [3]) as it provides both the spectral and spatial resolutions required to explore these type of phenomena. However, conventional STEM-EELS suffers from different drawbacks: contrary to optical spectroscopies, its spectral resolution is limited to few meV at the very best, it does not provide any temporal information and it is blind to the phase of the optical excitation. All these issues can be circumvented by structuring the wavefunction of the probe electron in the TEM, either in the spatial domain (e.g. generation of vortex electron [5]) or in the temporal domain (e.g. generation of ultrashort electron wavepacket [6]).

[1] Nelayah et al., Nat. Phy. 3 (2007)
[2] Tizei et al., PRL 114 (2015)
[3] Lagos et al., Nature 543 (2017)
[4] Bourrellier et al., Nano Lett. 16 (2016)
[5] Verbeeck et al., Nature 467 (2010)
[6] Feist et al., Nature 521 (2015)

We study dual strong coupling description of an integrable deformation of the $OSp(N|2m)$ sigma model. This description is based on the set of screening charges, which we present and which describes the integrable structure of this theory. We find one-parametric Yang-Baxter deformation of the $OSp(N|2m)$ sigma model and study its one-loop renormalization group flow. This action is conjectured to appear from the mentioned system of the screening charges.

Large-scale growth of high-quality hexagonal boron nitride (h-BN) has been a challenge in two-dimensional (2D)-material-based electronic and energy devices. In this talk, I will demonstrate wafer-scale and wrinkle-free epitaxial growth of multi-layer h-BN on a sapphire substrate by using high-temperature and low-pressure chemical vapor deposition. Microscopic and spectroscopic investigations and theoretical calculations reveal that synthesized h-BN has a single rotational orientation with Bernal stacking order. A facile method for transferring h-BN onto other target substrates were developed, which provides the opportunity for using h-BN as a substrate in practical electronic circuits. A graphene field effect transistor fabricated on our h-BN sheets shows highly improved carrier mobility, because the ultra-flatness of the h-BN surface can reduce the substrate-induced degradation of the carrier mobility of 2D materials. And, I will show some potential applications of h-BN for a substrate to grow transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2 which can be used for electrocatalysts, a shell layer capping Au nanoparticles in surface-enhance Raman scattering, an encapsulation (or passivation) layer to protect unstable TMDs, and a proton exchange membrane to replace the Nafion film in a polymer electrolyte membrane (PEM) fuel cell. Lastly, I will demonstrate spatially controlled conversion of h-BN to graphene on an array of Pt nanoparticles to realize an array of uniform graphene quantum dots (GQDs) embedded in an h-BN sheet.

Cette thèse est consacrée à l’étude des propriétés optiques d’un défaut ponctuel lié au carbone dans le silicium: le centre G. Ce dernier, identifié dès les années 60, peut-être comparé à un atome artificiel et émet à 1.3 um; il présente donc de forts potentiel dans le cadre des technologies quantiques, à la fois pour la réalisation de capteurs quantiques ultra-sensibles ou pour le développement de sources de photons uniques adaptés aux longueurs d’onde de télécommunication par fibre optiques sur plateformes silicium.

Durant ce travail de thèse nous avons dans un premier temps caractérisé le signal de photoluminescence d’un ensemble de centres G. Le spectre d'émission a été mesuré et analysé quantitativement. Le temps de vie du centre G a aussi été mesuré pour la première fois. Ces deux types d'études (spectrales et temporelles) ont aussi été réalisées à diverses températures afin de sonder la dynamique de désexcitation des centres G. Dans un deuxième temps nous avons pu sonder la dynamique associée à la présence d’un état métastable du centre G, dont l’existence a été prédite dès les années 80. Enfin nous avons pu isoler un centre unique qui se caractérise par une émission de photons uniques. Les propriétés de ce centre sont très similaires à celle mesurées sur des ensembles, néanmoins on ne peut pas affirmer qu’il s’agit d’un centre G unique.

Water is cohesive, so that surface tension naturally sculpts it in spheres - as we can see for instance with falling rain. We discuss how this spherical shape can be preserved on solids, and how water behaves dynamically in such spheroidal states, where we anticipate and indeed observe ultra-mobile behaviors.