Influence des particules fines sur la prise des liants hydrauliques dans les sols argileux

La grande surface spécifique des particules fines, ainsi que leur grande réactivité dérivent directement de leur morphologie et structure cristallographique et varient selon les conditions du sol. Le but de ce projet est donc de connaître l’influence de ces particules sur la formation et la stabilité des espèces liantes issues de l’hydratation d’un liant hydraulique routier. Pour cela, les effets des échanges cationiques des particules fines avec le milieu réactionnel ont été étudiés, l’objectif étant la détermination des mécanismes par lesquels les différents processus se produisent : la compréhension de l’ensemble des réactions de la prise d’un liant hydraulique et la connaissance de la composition et la morphologie des espèces liantes produites permet de comprendre les différentes interactions qui peuvent avoir lieu. Ayant découvert la grande variété en composition des particules fines existantes, les composantes étudiées ont été réduites à l’ensemble des particules argileuses de petite taille, étant donnée leur forte capacité d’échange de cations. D’après la recherche bibliographique : 1) le gonflement est le mécanisme de réduction de résistance le plus important à court terme (premiers jours de la réaction de prise) dans le cas des argiles de type smectite, néanmoins il s’agit d’un phénomène déjà connu, donc il ne sera pas traité de façon détaillée. 2) L’adsorption des ions Ca2+ influe sur la cinétique des réactions de prise du ciment, soit les favorise, soit les empêche de former des espèces liantes, en fonction de la cinétique de la réaction de prise. 3) Les conditions de la solution : pH, température, force ionique, teneur en eau… peuvent minimiser ou maximiser les perturbations. Des expérimentations ont été effectuées avec des mélanges modèles mélanges eau-liant-sable-argiles, qui ont été caractérisés et soumis à des tests de résistance à la compression. Les mélanges avec le plus fort taux de smectite ont montré une résistance moindre. Ces résultats seraient à confirmer par l’étude plus approfondie de l’influence des différents paramètres

Exceptional Bound States in the Continuum

Bound states in the continuum (BICs) and exceptional points (EPs) are unique singularities of non-Hermitian systems. BICs demonstrate enhancement of the electromagnetic field at the nanoscale, while EPs exhibit high sensitivity to small perturbations. Here, we demonstrate that several BICs can be merged into one EP, forming an EP-BIC. The resulting state inherits properties from both BICs and EP, namely, it does not radiate and shows extremely high sensitivity to perturbations. We validate the developed theory with numerical simulations and demonstrate the formation of second and third-order EP-BICs in stacked dielectric metasurfaces. We also show that the losses of the resulting leaky resonances exhibit an anomalous behavior when the unit cell is broken, which differs from the asymptotics commonly attributed to BICs

Magnetoelectric Exceptional Points in Isolated All-Dielectric Nanoparticles

We consider the scattering of electromagnetic waves by non-spherical dielectric resonators and reveal that it can be linked to the exceptional points underpinned by the physics of non-Hermitian systems. We demonstrate how symmetry breaking in the shape of an isolated dielectric nanoparticle can be associated with the existence of an exceptional point in the eigenvalue spectrum and formulate the general conditions for the strong coupling of resonances, illustrating them for the example of the electric dipole and magnetic dipole modes supported by a silicon nanoparticle. We argue that any two modes of a dielectric nanoparticle can lead to an exceptional point provided their resonant frequencies cross as a function of a tuning parameter, such as, e.g. its aspect ratio, and their field distributions should have opposite signs after a reflection in the transverse plane of the structure. The coupled modes radiate as a mixture of electric and magnetic dipoles, which result in a strong magnetoelectric response, being easily controlled by the symmetry breaking perturbation. We also investigate the influence of a dielectric substrate, demonstrating how the latter provides an additional mechanism to tune the position of exceptional points in the parameter space. Finally, we discuss applications of magnetoelectric exceptional points for refractive index sensing

Impact of coordinate frames on mode formation in twisted waveguides

Off-axis twisted waveguides possess unique optical properties such as circular and orbital angular momentum (OAM) birefringence, setting them apart from their straight counterparts. Analyzing mode formation in such helical waveguides relies on the use of specific coordinate frames that follow the twist of the structure, making the waveguide invariant along one of the new coordinates. In this study, the differences between modes forming in high-contrast off-axis twisted waveguides defined in the three most important coordinate systems - the Frenet-Serret, the helicoidal, or the Overfelt frame - are investigated through numerical simulations. We explore modal characteristics up to high twist rates (pitch: 50 $\mu$m) and clarify a transformation allowing to map the modal fields and the effective index back to the laboratory frame. In case the waveguide is single-mode, the fundamental modes of the three types of waveguides show significant differences in terms of birefringence, propagation loss, and polarization. Conversely, the modal characteristics of the investigated waveguides are comparable in the multimode domain. Furthermore, our study examines the impact of twisting on spatial mode properties with the results suggesting a potential influence of the photonic spin Hall and orbital Hall effects. Additionally, modes of single-mode helical waveguides were found to exhibit superchiral fields on their surfaces. Implementation approaches such as 3D-nanoprinting or fiber-preform twisting open the doors to potential applications of such highly twisted waveguides, including chip-integrated devices for broadband spin- and OAM-preserving optical signal transport, as well as applications in chiral spectroscopy or nonlinear frequency conversion.Comment: Main text (15 pages, 7 figures) and Supplemental Material (29 pages, 18 figures

Quasi-Babinet principle in dielectric resonators and Mie voids

Advancing resonant nanophotonics requires novel building blocks. Recently, cavities in high-index dielectrics have been shown to resonantly confine light inside a lower-index region. These so-called Mie voids represent a counterpart to solid high-index dielectric Mie resonators, offering novel functionality such as resonant behavior in the ultraviolet spectral region. However, the well-known and highly useful Babinet's principle, which relates the scattering of solid and inverse structures, is not strictly applicable for this dielectric case as it is only valid for infinitesimally thin perfect electric conductors. Here, we show that Babinet's principle can be generalized to dielectric systems within certain boundaries, which we refer to as the quasi-Babinet principle and demonstrate for spherical and more generically shaped Mie resonators. Limitations arise due to geometry-dependent terms as well as material frequency dispersion and losses. Thus, our work not only offers deeper physical insight into the working mechanism of these systems but also establishes simple design rules for constructing dielectric resonators with complex functionalities from their complementary counterparts.Comment: 6 pages, 4 figure

Quantitative Analysis and Optimization of Nb3_3Sn Wire Designs Toward Future Circular Collider Performance Targets

In the context of the Future Circular Collider (FCC) study, industrial and academic partners are developing novel N$_3$Sn superconducting wires with a wide variety of layouts, production methods, and compositions, with the aim of achieving challenging performance targets including a non-copper critical current density of 1500 A mm–2 at 16 T and 4.2 K. There is a clear need for a systematic and quantitative approach to analyze these conductors, identifying correlations between performance, microstructure, and wire design, in order to optimize designs and heat treatments, and to identify the most promising directions for future trials. Image analysis methods have been developed to provide a quantitative description of key geometrical characteristics of a wire with an impact on N$_3$Sn phase formation and superconducting performance. In this paper, these methods are introduced, examples are presented of their application to prototype conductors produced for the FCC study, and opportunities for improving the performance of these prototype conductors are identified. Finally, initial steps toward models of diffusion and phase transformations are reported, and the potential for establishing a quantitative, analytical approach to wire design is evaluated, identifying topics requiring further research.In the context of the Future Circular Collider (FCC) study, industrial and academic partners are developing novel Nb3Sn superconducting wires with a wide variety of layouts, production methods, and compositions, with the aim of achieving challenging performance targets including a non-copper critical current density of 1500 A mm-2at 16 T and 4.2 K. There is a clear need for a systematic and quantitative approach to analyze these conductors, identifying correlations between performance, microstructure, and wire design, in order to optimize designs and heat treatments, and to identify the most promising directions for future trials. Image analysis methods have been developed to provide a quantitative description of key geometrical characteristics of a wire with an impact on Nb3Sn phase formation and superconducting performance. In this paper, these methods are introduced, examples are presented of their application to prototype conductors produced for the FCC study, and opportunities for improving the performance of these prototype conductors are identified. Finally, initial steps toward models of diffusion and phase transformations are reported, and the potential for establishing a quantitative, analytical approach to wire design is evaluated, identifying topics requiring further research