Measuring physical quantities at the nanometer scale is essential for investigating the properties of materials, which requires a versatile microscopy technique. In this context, scanning NV center microscopy emerges as an ideal candidate, as it exploits the unique spin and optical properties of the nitrogen-vacancy center in diamond to achieve quantum sensing with high spatial resolution and sensitivity. Importantly, the NV center is sensitive to multiple physical quantities, including magnetic and electric fields, temperature, and pressure. This approach has already proven to be efficient for magnetometry, capable of imaging static magnetic field with a sensitivity of about 5 μT/√Hz and a spatial resolution of 50 nm. In this thesis, the technique was first applied to the investigation of the effect of confinement in the van der Waals ferromagnet Fe5GeTe2 by performing quantitative maps of the stray field generated by patterned microstructures. This study revealed that geometrical confinement does not affect the Curie temperature, but favors the stabilization of magnetic vortices in microstructures. The second part of this thesis focuses on extending scanning NV center microscopy to nanoscale thermometry, specifically for microelectronic characterization, with the objective of determining its ultimate performance in terms of sensitivity and spatial resolution. This work shows that the thermal sensitivity of the technique can reach a few hundred mK/√Hz. Nevertheless, the measurements also revealed several limitations. These include perturbations from parasitic magnetic fields generated by current-carrying structures, imperfect thermal contact with the sample leading to non-local temperature measurements under ambient conditions, and thermal dissipation associated with the large diamond volume of the probes. Finally, COMSOL simulations show that the most effective configuration remains a nanodiamond in direct contact with the heat source. This explains why, to date, scanning NV thermometry using nanodiamonds attached to the probe remains the most efficient approach.

Cathodoluminescence (CL) spectroscopy in a scanning electron microscope is particularly suited for the characterisation of the local emission properties of ultra-wide bandgap materials due to the achievable spatial resolution and as there is no restriction to the bandgap energies that can be excited by the electron beam. A unique time-resolved CL system equipped has been recently installed at our institute: with rigorously UV-optimized optics and detectors, a He-cryo stage, as well as the capability for operation at low acceleration voltages achieving a particularly high spatial resolution. The combination of high spatial, spectral and temporal resolutions allows to investigate in particular the role of extended or point defects on charge carrier dynamics in widegap semiconductor layers or heterostructures, e.g. for the (Al,Ga)N material system or hBN. Therefore, such measurements can contribute to highly relevant scientific questions. For homoepitaxial AlN layers, we will show that the combination of the high spectral (1 meV) and temporal (currently 20 ps, in the future 2 ps) resolution with temperature-dependent measurements can contribute to the attribution of excitonic emission lines, the origin of which is disputed in the literature [1]. On dedicated 1-nm-thin (Al,Ga)N quantum wells covered only by a 5 nm top barrier, we can push the spatial resolution limit to reveal dark spot features with a spatial extent of only 30 nm and a density of LF^16 cm^-3, which would be consistent with an attribution to point defects. Spectrally-resolved maps indicate that these dark spots are not correlated to Al-content fluctuations. Supplemented by temperature- and time-resolved luminescence maps, we will discuss whether these features can be attributed to individual point defects acting as non-radiative centers. In analogy to studies on (In,Ga)N [2], imaging of individual point defects in quantum wells as a function of the Al content or growth parameters could notably advance the understanding of the efficiency limits of (Al,Ga)N-based UV emitters. Finally, we present first time-resolved and excitation-dependent data for the emission of excitons bound to different types of stacking faults in AlN [3]. [1] L. van Deurzen et al., APL Materials 11, 081LF9 (2023). [2] T. Weatherley et al., Nano Letters 21, 5217 (2021). [3] C. Guérin et al., J. Appl. Phys. CR7, 174303 (2025).

Morphogenesis, the process by which tissues acquire their shape, hinges on a finely orchestrated collective motion of cells. Accumulating evidence shows that many biological tissues behave as active nematics, both in vitro and in vivo. The collective motion of cells is controlled by the nematic order, and topological defects have been proposed as morphogenic organizers via active stresses [1]. However, the generation and control of tissue-scale forces involved in morphogenesis remain poorly understood, in particular within 3D surfaces. To investigate how geometry and topology control morphogenesis, I grow myoblast cells on the surface of alginate microspheres and monitor the nematic field, cellular flows, and tissue growth. When the tissue reaches confluency, four equidistant +1/2 defects are observed in the actin network, consistent with the topological charge imposed by the sphere (see image below). Subsequent growth of the monolayer due to continuous proliferation shows the formation of a multilayered tissue with two main orthogonal orientation. Upon further growth, the half defects fuse by pair, forming two +1 defects, and the thickness of the tissue becomes heterogeneous with the presence of two mounds co-localizing with the +1 defects. Together, the defect fusion and mound formation form a first spontaneous symmetry breaking event. Finally, the two +1 defects migrate towards one another and form a +2 region, accompanied by the fusion of the two mounds into one main protrusions. In this synthetic model system, a complex and spontaneous morphogenesis emerges from the interplay between the topological defects and cellular flows, illustrating the role of physical principles in a fundamental biological process.

Twisting two graphene monolayers creates a moiré superlattice that hosts diverse exotic phases [1]—such as superconductivity, correlated insulators, nematic order, Chern insulators, magnetic polarization, and strange-metal behavior. These arise from strong band reconstruction caused by interlayer tunneling and twist. At the “magic” angle (~1°), the moiré bands flatten, amplifying Coulomb interactions and enabling many interaction-driven quantum phases [2]. Moiré superlattices have also been realized in other 2D materials, including twisted MoTe₂, WSe₂, and multilayer graphene aligned with hexagonal boron nitride. Recent experiments have, for the first time, observed fractional Chern insulating phases in twisted MoTe₂ [3] and rhombohedral pentalayer graphene [4]. These remarkable states, theoretically predicted in the early 20LFs, represent fractional quantum Hall phases that emerge without an external magnetic field. In this talk, I will first give a broad introduction to moiré materials, then discuss two topics: the predicted Chern mosaic in trilayer graphene [5] and the physical origin of a fictitious magnetic field in moiré fractional Chern insulators [6]. [1] E. Y. Andrei, A. H. MacDonald, Nature Materials 19, 1265 (2020) [2] Cao et al., Nature 556, 80 (2018) [3] Kang, Shen, Qiu, Zeng, Xia, Watanabe, Taniguchi, Shan, and Mak. Nature 628, 522–526 (2024) [4] Lu, Han, Yao, Reddy, Yang, Seo, Watanabe, Taniguchi, Fu and Long, Nature (2023) [5] D. Guerci, Y. Mao, C. Mora, PRR 6, L022025 (2024) [6] K. Kolář, K. Yang, F. von Oppen, C. Mora, PRB 1LF, 115114 (2024)

The team “La physique autrement” explores different ways of talking about physics, drawing inspiration from practices coming from the world of design. Whether in the context of public outreach or teaching, it develops numerous projects in collaboration with various partners, graphic designers, web designers, illustrators, and more. A selection of the team’s projects will be presented to illustrate our approach and spark discussion.

The coupling between light and matter lies at the heart of many fundamental physical phenomena and serves as a stepping stone for countless device applications. A particularly interesting regime is the so-called strong and ultrastrong light-matter coupling happening in optical cavities, where new quasi-particles called cavity polaritons emerge. New and intriguing quantum optical phenomena have been predicted in the ultrastrong coupling regime, when the coupling strength becomes comparable to the unperturbed frequency of the system [1]. Strong light-matter coupling has been recently successfully explored in the GHz and THz range with on-chip platforms, where metallic resonators with small cavity volumes are combined with high electron density materials to exploit the collective enhancement of the coupling [2–4]. Our laboratory developed a new platform to study ultrastrong light-matter coupling using the inter-Landau-level transitions in 2 dimensional electron gases hosted by semiconductor heterostructures strongly coupled to metallic split-ring resonators, the so-called Landau polaritons [2, 4]. In this talk we will review the state-of-the-art of Landau polaritons and we will discuss recent experimental results where a single strongly subwavelength resonator is coupled to a gated, micron-sized graphene flake and spectroscopically investigated employing a system of immersion lenses [5]. The sample features an electrical gate in order to modulate the electron density and, as a consequence, the coupling strength. We observe clear ultrastrong coupling tunable in strength and magnetic field anticrossing as a function of the electron density. These measurements open the way for the engineering of many-body phenomena and study the fundamental nature of light–matter interactions in quantum materials. [1] C. Ciuti, G. Bastard, and I. Carusotto, Phys. Rev. B, 72, 11, 115303(2005). [2] G. Scalari et al., Science, 335, 6074, CR23 (2012). [3] Y. Todorov et al., Phys. Rev. Lett., LF5, 19, 196402 (20LF). [4] A. Bayer et al., Nano Lett., 17, LF, 6340 (2017). [5] S. Rajabali et al., Nat. Commun., CR, 1, 2528 (2022).

Nanographenes synthesized by bottom-up chemistry are tunable emitters with promises in optoelectronic, biosensing, and quantum technologies [1-2]. Recent investigations on the intrinsic properties of these nanographenes classify them as stable and bright single photon sources at room temperature [3-6]. The next step towards using nanographenes as quantum emitters is to reach lifetime-limited linewidth. When embedded in a polystyrene matrix, the spectral linewidth is reduced to ~ two meV at low temperature [6]. Despite this reduction, the linewidth is still a few orders of magnitude higher than the radiative limit, estimated to be a few μeV [7]. Inspired by pioneer works on small organic molecules [8], we designed a new guest/host system to decouple as much as possible the nanographene from its local environment [9]. In the first part of this presentation, we will show our recent results on the low-temperature spectroscopy of new nanographenes embedded in a new molecular crystal host [9]. In the same vein, we will show preliminary results on the spectroscopy of nanographenes deposited on the surface of h-BN. We will show that nanographene-hBN interaction can modify a lot the optical response of our quantum emitter [LF]. Finally, some perspectives on using nanographenes to build arrays of entangled emitters will be presented. References [1] Lavie, Julien, et al. "Bottom‐Up Synthesis, Dispersion and Properties of Rectangular‐Shaped Graphene Quantum Dots." Helvetica Chimica Acta LF6.6 (2023): e202300034. [2] X. Yan, X. Cui, and L.-s. Li, J. Am. Chem. Soc. CR2, 5944 (20LF) [3] Levy-Falk, Hugo, et al. "Investigation of Rod‐shaped Single Graphene Quantum Dot." physica status solidi (b) (2023). [4] S. Zhao et al, Nature Communications, 9, 3470 (2018) [5] T. Liu et al, Nanoscale, 14, 3826 – 3833 (2022) [6] T. Liu et al, Journal of Chemical Physics 156, LF4302 (2022) [7] D. Medina-Lopez et al, Nature Communications 14, 4728 (2023) [8] WP. Ambrose et al J. Chem. Phys. 95 (LF), 7150–7163 (1991) [9] Huynh Thanh Trung et al, in preparation [LF] S. Sarkar et al, in preparation

The breakup of liquid filaments plays a crucial role in both industrial processes (such as inkjet printing and spraying) and biomedical contexts, particularly in the airborne transmission of pathogens via saliva droplets generated during coughing, sneezing or speech. In this latter case, the droplets emitted carry viruses and thus represent a key vector for the transmission of diseases. To better assess and mitigate this risk, it is essential to understand how these droplets are formed in the first place. Their origin lies in the deformation and breakup of thin liquid filaments of saliva, stretched between the lips and disrupted by airflow during speech. This thesis investigates the dynamics of uniaxially stretched liquid filaments as they thin and break under the combined influence of viscous, inertial, and capillary forces, as well as under a controlled perpendicular airflow. In the first part, we examine how purely viscous filaments stretch in an original experimental setup designed to reproduce violent extensions, and how this thinning process is modified by airflow. We then focus on the filament shape under perpendicular airflow. Using a quasistatic approximation (neglecting inertia), we model the geometry with either slender-body viscous friction (low Reynolds number) or aerodynamic drag (Taylor model, high Reynolds number), and from these descriptions we compute the internal tension within the filament by simply fitting the shape of deformed filament in order to compare it to the inner viscous tension. Numerical simulations further confirm the validity of this quasistatic framework. The aim of the comparisons of tension is to validate one of the model and to have a quantitative way to study filament behaviour. These approaches are extended to viscoelastic fluids, in particular human saliva, showing that filament geometries can still be quantitatively characterized. From these measurements, it becomes possible to define an effective Ohnesorge number for saliva, allowing comparison with simpler Newtonian systems. The second part of the work addresses filament breakup and the variety of modes through which they destabilize and fragment, depending on viscosity, initial volume, and airflow conditions. We demonstrate that airflow can either accelerate or delay rupture, and even modify the breakup mechanism itself. Finally, we investigate model viscoelastic fluids (wormlike micelles) and saliva to show how elasticity influences thinning and breakup. These experiments reveal new behaviours, such as delayed rupture, extended lifetimes, and the appearance of instabilities along the filament. The main findings are: (i) the quasistatic model is reliable during pure deformation, between the end of bead extension and the onset of non-uniformities, but breaks down when drainage and inertia become significant; (ii) viscosity strongly stabilizes filaments at low Ohnesorge numbers, whereas airflow inertia promotes deformation and shifts breakup regimes; (iii) salivary elasticity enables extreme stretching and delays rupture over long timescales. Limitations remain: the study focuses mainly on glycerol as a model fluid, with fewer experiments on saliva and micelles, and no quantification of droplet number and size after breakup. Nevertheless, these results open the way to a more comprehensive understanding of complex biological fluids and their atomization, ultimately linking laboratory findings to the deformation and fragmentation of saliva filaments in the mouth and respiratory tract.