I'll give a review of the role that modular symmetry plays in description of BPS black holes and counting of their microstates.

We study the superradiance of optical phonons during the two- to three-dimensional (2D-3D) crossover of the light-matter interaction in multilayers of atomic crystals. We show the emergence of a superradiant regime with a mode having a linewidth first increasing linearly with the number N of monolayers, and then decreasing as N^{−3} to zero because of the formation of stationary phonon polaritons. The linewidth culminates to values of the order of the longitudinal-transverse splitting. We estimate the extremum of the radiative efficiency for various 2D materials in the superradiant regime. We predict radiative efficiencies larger than 50% for optical phonons emitting between 6 and 165 μm. Superradiance appears as a key resource for mid- and far-infrared optophononics and advanced thermal management using multilayers of 2D materials as the active medium.

The aim of this study was to perform a structural characterization of dental pulp tissue using multiphoton microscopy and to compare this to classical histological coloration. Materiel & Method:Sample preparation: Freshly extracted teeth with healthy pulp were used in this study. All teeth were fixed in PFA 4% for 24 hours, rinsed in PBS and decalcified. The samples were then embedded in paraffin. The paraffin blocks were cut with a Microm HM340E microtome with Niagara system to obtain 3 μm thick sections. The samples were then dewaxed with xylene, rehydrated and stained with Hematoxilin/eosin and Masson’s trichrome. Multiphoton Microscopy (MPM) images were recorded using a custom-built multiphoton microscope based on a SliceScope microscope (MPSS-1000P, Scientifica) upright microscope. MPM images were obtained recording the non- linear emission spectra after laser scanning of samples. Two-photon fluorescence (2-PEF) signal was used to image autofluorescent structures. Second harmonic generation signal (SHG) was performed to image collagen. The recorded images were processed by ImageJ software.Results: At the dentin/pulp junction, we observed the dentin, which emitted an important SHG signal due to the abundance of type 1 collagen fibers. The palisade of odontoblasts and odontoblast extensions (2PEF) were imaged towards the dentinal tubules. We imaged blood vessels due to the 2-PEF signal of blood cells. A fibrous structure representing the extracellular matrix emitted a SHG signal, showing the presence of type I collagen fiber. At the apical 1/3 of the root fibers were observed at the periodontal ligament emitting both in SHG and 2PEF, coming in contact to the pulp. The MPM images were then compared to the histological stained samples. Conclusion: Multiphoton microscopy is a convenient label-free minimally invasive technique to image live, vascularized and dynamic tissue such as the pulp. It provides valuable additional information with respect to histological sections observed under an optical microscope.

Euclidean D-branes wrapped on non-trivial cycles of a Calabi-Yau threefold are known to affect the metric on the hypermultiplet moduli space determining the effective action of type II strings on such manifolds. After reviewing the existing results on the instanton corrected metric following from a combination of constraints imposed by supersymmetry and dualities, I'll show how the D-instanton effects can be computed by a direct worldsheet approach. This required solving several conceptual and technical problems, which also paved the way for extension of this approach to compactifications with lower supersymmetry. The talk is based on joint work with A. Sen and B. Stefanski.

As shown by T. Bridgeland, a Riemann-Hilbert problem determined by Donaldson-Thomas invariants naturally gives rise to the so-called Joyce structure. It can be characterized by a function known as Plebanski potential, or its close cousin Joyce potential. I'll show that a twistorial solution to the RH problem provides a simple integral expressions for both potentials. Then I'll explain the relation of this solution to the conformal limit of the twistor spacesappearing in gauge and string theories, and physical interpretation acquired bythe two potentials in these setups. For the case of the resolved conifold, I'll present a recipe to make the twistorial solution well-defined despite an infinite BPS spectrum, and trace out the emergence of a tau-function, its relation to topological strings and its behavior under S-duality to the twistorial implementation of instantons in string theory.

Les propriétés germicides des rayonnements ultraviolets (UV) suscitent un intérêt de plus en plus important dans les domaines d’applications stratégiques que sont l’environnement et la santé. Les diodes électroluminescentes (LEDs) semi-conductrices à base de nitrure d’aluminium et de gallium représentent les nouvelles sources d’émission UV qui pourront répondre à cette demande. Leurs performances, bien qu’encore modestes, progressent chaque jour et nous vivons le point de bascule entre le laboratoire et le domaine grand public. Pour définitivement parvenir à s’imposer, elles doivent associer efficacité, forte capacité de développement, ainsi que souplesse et facilité d’utilisation, performances qu’aucune source UV n’a encore atteint jusqu’à présent.