Thèse

Jeudi 15 Octobre 2026 à 14h00.

Collective Vibrational Dynamics of Nanoparticle Assemblies Probed by Brillouin Spectroscopy

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Directeur de thèse / thesis director :
Jérémie MARGUERITAT

Membres du jury / jury members :
Lucien SAVIOT — Rapporteur, Directeur de recherche au CNRS, Université Bourgogne Europe
Samuel RAETZ — Rapporteur, Maître de conférences, Le Mans Université
Marie FORET — Examinatrice, Professeure des universités, Université de Montpellier
Anne PILLONNET — Examinatrice, Professeure des universités, Université Claude Bernard Lyon
1 Jérémie MARGUERITAT — Directeur de thèse, Chargé de recherche au CNRS, Université Claude Bernard Lyon 1
Benoît MAHLER — Invité, Chargé de recherche au CNRS, Université Claude Bernard Lyon 1

Résumé / Abstract :

This thesis was conducted within the VIRUSong project, to detect and identify viral particles through their low-frequency acoustic vibrations. A virion is a finite biological nanoparticle whose vibrational response depends on size, architecture, composition, and surrounding medium. Accessing these gigahertz modes by spontaneous inelastic light scattering is difficult because biological nanoparticles scatter weakly and the modes lie close to the intense Rayleigh line. Interpretation is complicated by particle structure and environmental coupling. This thesis addresses two prerequisites for future virus measurements: a rapid, high-contrast spontaneous Brillouin approach for resolving weak acoustic modes, and a nanoparticle reference system for determining how confinement, contacts, disorder, and connectivity shape this response.

The first contribution is the development and validation of a VIPA-based Brillouin microscope combining rapid acquisition and strong elastic-light suppression near the Rayleigh line. Its implementation required optical optimization, interferometric filtering, spectral reconstruction, and frequency calibration. Validation on reference liquids and solids, with tandem Fabry–Perot spectroscopy as a high-contrast reference, established the VIPA approach as a reliable route for measuring gigahertz modes in nanoparticle films. The second contribution is a systematic study of polystyrene assemblies. Their spherical geometry, size range, soft character, and simple composition separate particle-scale confinement from assembly-level effects before moving toward biological nanoparticles. The nanospheres formed multilayer films, where adhesion, packing disorder, and connectivity modify the low-frequency response. Electron microscopy verified particle size, film morphology, and structural organization, while Brillouin spectroscopy probed the acoustic dynamics. Together, these measurements related the vibrational response to particle diameter and mechanical organization. The central question is whether the size-dependent collective response can be described within a classical adhesive-contact framework over the investigated diameter range. The analysis focuses on the interaction-induced f(s,1,1) mode. In the isolated-sphere limit, the corresponding rigid translation has zero frequency. Mechanical coupling between neighbouring particles introduces a restoring force and converts this motion into a finite-frequency collective mode. Tracking f(s,1,1) with particle diameter assesses how interparticle coupling and structural organization influence the collective response. The evolution of f(s,1,1) reveals two regimes. For larger particles, the response remains consistent with a contact-controlled description in which adhesive contacts provide the dominant restoring stiffness. As the diameter decreases, the size dependence becomes weaker than predicted by classical contact mechanics. This change coincides with reduced positional order observed by electron microscopy, indicating that the response is increasingly governed by the contact network rather than by local contact stiffness alone. The results point to a crossover from contact-dominated behaviour at larger sizes to a connectivity-controlled response in smaller assemblies.

Together, these contributions establish a framework for studying collective acoustic vibrations in soft nanoparticle assemblies. The VIPA-based microscope provides access to weak gigahertz modes, while the polystyrene system clarifies the evolution from isolated-particle confinement to contact- and connectivity-mediated collective dynamics. It provides a reference for interpreting biological nanoparticles, where structure and environmental coupling may contribute simultaneously. The thesis concludes with extensions toward spatially resolved Brillouin mapping of heterogeneous assemblies and sensitive optical approaches, including stimulated Brillouin scattering, for virus-relevant measurements.

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