A dark sector of particles interacting with the Higgs boson might reveal the origin of dark matter or facilitate electroweak baryogenesis. At particle colliders, the direct search for such Higgs-portal interactions is difficult if the dark particles lie above the Higgs threshold. In scenarios with electroweakly interacting mediators, two alternative signatures can facilitate the search: resonant production of mediators decaying into dark particles, and indirect effects of virtual mediators in visible Higgs decays. I will discuss the complementarity of these two approaches and present new results on virtual effects of dark fermions in Higgs decays to electroweak gauge boson pairs.

Photonic crystals have reshaped light-based technologies during the last few decades. Remarkably, some years ago Raghu and Haldane discovered that the light propagation in photonic crystals may depend on some global topological properties determined by the manner how the light states are entangled in the spectral domain. The standard topological classification of photonic crystals relies on the fact that these structures have a periodic structure. The periodicity is of vital importance to guarantee that the underlying wave vector space is a closed surface with no boundary.
In this talk, I will show that it is possible to calculate topological invariants of a wide class of bianisotropic nonreciprocal electromagnetic continua with no intrinsic periodicity. The nontrivial topology of the relevant materials will be linked to the emergence of topological edge states immune to back reflections. Furthermore, I will discuss rather general conditions under which a photonic platform can support propagation channels insensitive to backscattering, showing that systems invariant under the action of a PTD operator-the composition of the parity, time-reversal, and duality transformations-may enable bidirectional waveguiding totally insensitive to deformations, analogous to the spin-Hall effect, but for light waves. Finally, time permitting, I will discuss the consequences of a topological transport of light in the context of thermally generated radiation in a closed cavity at a thermodynamic equilibrium.

Dans la gamme des radiations Terahertz, l’absence de dispositifs optiques passifs performants tels que des modulateurs, des isolateurs ou des convertisseurs de polarisation est un facteur limitant pour le développement d’applications. Le graphène présente, dans cette gamme de fréquence, des effets non-réciproques qui font de lui un candidat prometteur pour combler ce manque.

En effet, dans le graphène, lorsque la symétrie d’inversion du temps est brisée par un champ magnétique, les conductivités optiques pour la lumière polarisée circulairement σ+ et σ- deviennent différentes, donnant ainsi naissance à des phénomènes tels que le dichroïsme circulaire et l’effet Faraday. Nos travaux ont permis de démontrer que ces effets permettent un contrôle sans précèdent de l’intensité et de la polarisation de la lumière THz. De plus, le contrôle ambipolaire du dopage par une tension de grille permet de moduler l’effet en intensité et de l’accorder en fréquence.

La gamme d’énergie dans laquelle ces effets peuvent être mis à profits dans des dispositifs peut être étendue en utilisant le couplage entre la résonance cyclotron et des modes plasmoniques apparaissant dans des métasurfaces à base de graphène. Ces différents travaux permettent de mettre en avant la viabilité d’éléments non-réciproques compacts, ajustables et modulables basés sur le graphène et fonctionnant à température ambiante.

The negatively charged Nitrogen Vacancy (NV−) center in diamond has emerged as a very efficient source of single photons and a promising candidate for quantum control and sensing via its electron spin. Recently, there has been much interest in the electronic spin of the NV− center in levitating diamonds. This interest is partly motivated by proposals for hybrid optomechanics, and implications in ultrahigh force sensitivity where the NV center’s spin response to magnetic fields is exploited to read-out the motion of the diamond with high spatial resolution under ambient conditions. Amongst the many levitation schemes, optical traps are the most widely used. They provide efficient localisation for neutral and charged particles and can work under liquid or atmospheric environnements. However the trap light that is scattered from the object means that excessive heating can be at work.

A solution to particle heating is to use scattering-free traps, such as ion traps. In this seminar, a study of the properties of diamonds that are levitating in a Paul trap will be presented. Using the inherent quantization axis of NV centers, we have shown that the angle of the diamond is stable over long time scales, a prerequisite for hybrid-optomechanics. Working under low vacuum pressures, we used the spin of NV centers to measure the diamond temperature and demonstrated trapping down to 10^-2 mbar. These important characterisation steps show the potential of NV centers as probes of levitating objects and demonstrate the potential of Paul traps for hybrid-opto-mechanical studies.

A venir

Colour centres in large bandgap materials are leading candidates for a great variety of applications ranging from biophysics to quantum information. Their optical properties and ability to optically control their electronic and spin degrees of freedom is at the core of their potential for metrology. Therefore, understanding their optical properties is crucial for the successful implementation of these applications. In this talk, I will present an example of how the fluorescence stability of nitrogen-vacancy colour centres in diamond can be used for the fluorescent marking of a biological specimen, and also discuss on the large variety of emission spectra that a large bandgap material can host. In particular, we will discuss the cases of the nitrogen-vacancy and the silicon-vacancy colour centres in diamond and show how symmetry rules the interaction between the electronic degrees of freedom and phonons resulting in dramatically different emission spectra even though their atomic configuration differ just by one atom.