Arnaud Gloppe

Layered magnetic and strongly correlated materials, such as NiPS ₃ , present a rich platform for condensed matter physics with intrinsic properties intertwined by magnetism and low-dimensionality. A suspended light-emitting 2D antiferromagnetic membrane forms a highly controllable hybrid system in which the interplay between spin ordering, optical and mechanical degrees of freedom can be uniquely explored. NiPS ₃ hosts excitons responsible for a puzzling luminescence down to a few atomic layers, linked to its zigzag antiferromagnetic order. The exact nature of these excitons remains unclear. Here we report on the magnetic excitons of a suspended few-layer NiPS ₃ membrane. We reveal nematic zigzag states and study the optical transitions induced by the magnon-mediated motion of the excitation in the magnetic lattice, resulting in magnon-dressed excitons. We observe a strain tuning of these emission lines, dependent on their microscopic origin, with rates that indicate a strong localization of the magnetic excitons, fading with the number of magnon-mediated hops.

Cette thèse s'inscrit dans la thématique de la nano-optomécanique et de l'emploi de nanorésonateurs mécaniques comme sonde de force ultrasensible pour étudier leur interaction avec la lumière. Pour cela, un nanofil de carbure de silicium est positionné dans un faisceau laser fortement focalisé. Cela permet, en mesurant les fluctuations de l'intensité transmise, d'observer avec grande dynamique et une sensibilité proche de la limite quantique standard le mouvement Brownien du nanorésonateur. La grande sensibilité en force des nanofils, inhérente à leur très faible masse, permet d'étudier l'action en retour de la mesure, c'est-à-dire la force exercée par le laser focalisé sur le nanofil. L'exploitation de la légère levée de dégénérescence observée entre les deux polarisations mécaniques transverses permet de réaliser une cartographie vectorielle bidimensionnelle du champ de force optique, avec une sonde de diamètre sub-longueur d'onde. Cette mesure permet également de mettre en évidence le caractère non-conservatif de l'interaction lumière-matière, dont la signature emblématique est l'existence de vorticité dans le champ de force mesuré. Ce dernier présente de très fortes variations spatiales, qui modifient profondément la dynamique du nanofil. Cette action en retour de la mesure est responsable d'un fort couplage entre les deux polarisations mécaniques du mode fondamental du nanofil. Le caractère bidimensionnel du couplage ainsi que la topologie non-conservative du champ de force conduisent à une bifurcation et à une instabilité dynamique du nanofil. Cette nouvelle instabilité optomécanique est observée avec des forces optiques instantanées, qui suivent instantanément les variations d'intensités vues par le nanofil. En présence d'absorption, le cas plus général d'un champ de force partiellement retardé par les constantes de temps thermiques est également étudié, conduisant à un refroidissement, spécifique et accordable en position, des deux polarisations mécaniques. Enfin l'interaction lumière-matière entre le laser et le nanofil et la grande variété des propriétés optomécaniques accessibles à cette approche sont développés. Ces développements démontrent la possibilité d'observer et de contrôler optiquement des nanorésonateurs mécaniques de très grande sensibilité, proche de l'attonewton, pour des mesures vectorielles ultrasensibles de champ de force.

Quantized lattice vibrations (i.e., phonons) in solids are robust and unambiguous fingerprints of crystal structures and of their symmetry properties. In metals and semimetals, strong electron-phonon coupling may lead to so-called Kohn anomalies in the phonon dispersion, providing an image of the Fermi surface in a nonelectronic observable. Kohn anomalies become prominent in low-dimensional systems, in particular, in graphene, where they appear as sharp kinks in the in-plane optical phonon branches. However, in spite of intense research efforts on electron-phonon coupling in graphene and related van der Waals heterostructures, little is known regarding the links between the symmetry properties of optical phonons at and near Kohn anomalies and their sensitivity towards the local environment. Here, using inelastic light scattering (Raman) spectroscopy, we investigate a set of custom-designed graphene-based van der Waals heterostructures, wherein dielectric screening is finely controlled at the atomic-layer level. We demonstrate experimentally and explain theoretically that, depending exclusively on their symmetry properties, the two main Raman lines of graphene react differently to the surrounding environment. While the 2D line, which is due to near-zone-edge optical phonons, undergoes changes due to the neighboring dielectric environment, the in-plane, zone-center optical phonons are symmetry protected from the influence of the latter. These results shed new light on the unique electron-phonon coupling properties in graphene and related systems and provide invaluable guidelines to characterize dielectric screening in van der Waals heterostructures and moiré superlattices. Published by the American Physical Society 2025

We demonstrate the structural imaging of magnetostatic spin-wave modes hosted in a millimeter-sized ferromagnetic sphere. Unlike for low-dimensional magnetic materials, there is no prior technique to image these modes in bulk magnetized solid of revolution. Based on resonant magnetic induction tomography in the microwave range, our approach ensures the robust identification of these nontrivial spin-wave modes by establishing their azimuthal and polar dependences, starting point of magnonic fundamental studies and hybrid systems with complex spin textures well beyond the uniform precession mode.

We demonstrate the structural imaging of magnetostatic spin-wave modes hosted in a millimeter-sized ferromagnetic sphere. Unlike for low-dimensional magnetic materials, there is no prior technique to image these modes in bulk magnetized solid of revolution. Based on resonant magnetic induction tomography in the microwave range, our approach ensures the robust identification of these non-trivial spin-wave modes by establishing their azimuthal and polar dependences, starting point of magnonic fundamental studies and hybrid systems with complex spin textures well beyond the uniform precession mode.

A ferromagnetic sphere can support optical vortices in the form of whispering gallery modes and magnetic quasivortices in the form of magnetostatic modes with nontrivial spin textures. These vortices can be characterized by their orbital angular momenta. We experimentally investigate Brillouin scattering of photons in the whispering gallery modes by magnons in the magnetostatic modes, zeroing in on the exchange of the orbital angular momenta between the optical vortices and magnetic quasivortices. We find that the conservation of the orbital angular momentum results in different nonreciprocal behavior in the Brillouin light scattering. New avenues for chiral optics and optospintronics can be opened up by taking the orbital angular momenta as a new degree of freedom for cavity optomagnonics.

Summary form only given. The exploration of the interaction of light with spin waves in ferromagnets within an optical cavity might lead to new chiral photonic devices and be a stepping stone towards the coherent optical manipulation of magnons in the quantum regime [1]. The developments made so far in cavity optomagnonics have been focused on the fundamental magnetostatic mode of an yttrium iron garnet (YIG) sphere, so-called `Kittel mode' [2, 3] and lead to the observation of a magnon-induced Brillouin scattering between optical whispering gallery modes following selection rules dictated by the conservation of the orbital momentum. Higher-order magnetostatic modes, with reduced mode volume and different orbital angular momentum, could couple more efficiently with the optical whispering gallery modes hosted by the YIG sphere, lying in the vicinity of the sphere surface equatorial plane. Though the resonance frequencies of these modes can be predicted theoretically [4], small deviations due to the actual sample properties or environment, as well as possible hybridization of the modes, could cause strong misinterpretations. Hence, unambiguously experimentally identifying higher-order magnon modes in a spheroid is required to properly scrutinize their interaction with light.We demonstrate a scheme to map the magnon mode structures of a ferrimagnetic spheroid. Using two small loop coils in close proximity with a millimeter-sized YIG sphere, we perform microwave transmission measurements at various latitude-longitude coordinates. These signals are found to be very sensitive to the magnon mode structures. This makes possible the identification of the modes, supported by the predicted dependence of their resonance frequencies in the dc magnetic field. This work paves the way to the systematic investigation of the optomagnonic interaction.

Efficiently transducing microwave signals into optical signals is one of the major demands in modem telecommunication. Electro-optical crystals hosting optical whispering gallery modes incorporated in a properly designed microwave cavity emerge as a promising candidate towards much more efficient microwave-to-light transducer [1, 2]. Here, we propose to use a ferroelectric crystal close to its paraelectric-ferroelectric phase transition temperature to take advantage of its large relative permittivity due to the softened optical phonon mode, which may further enhance the electro-optic coupling.