Exploring Chirality at the Nanoscale: Multiscale Modelling of Molecule-Chiral Nanostructure Systems

Chirality plays a fundamental role in chemical and biological processes, yet traditional chiroptical spectroscopy suffers from intrinsically weak signals, caused by the dimensional mismatch between the wavelength of light and the nanometric size of molecules. Plasmonic nanostructures, by confining electromagnetic fields to the nanoscale, provide a solution to this problem, enhancing light-matter interactions, even to the strong-coupling regime, where hybrid light-matter states are formed. This is the rapidly growing field of Chiral Plasmonics, where plasmonic nanoparticles are employed both to enhance weak chiroptical signals of molecules and to induce plasmon-mediated chiroptical effects. However, further elucidating these complex plasmon-molecule interactions requires advanced theoretical methodologies based on multiscale approaches. A limitation of current methods is the lack of a theoretical description of the chiroptical response of metallic nanoparticles within the Quasi-Static Approximation (QSA), upon which a variety of classical continuum models are built. First, the thesis presents a theoretical derivation of the Circular Dichroism (CD) for isolated chiral metallic nanoparticles within the quasi-static limit. We demonstrate that the chiroptical response can be reconstructed from induced magnetic dipole and electric quadrupole moments generated by the external excitation. In the second part, we move to the strong light-matter coupling regime. By deriving a plasmon-consistent density matrix and extending the SC-QED-HF framework to plasmonic cavities, we provide theoretical tools that contribute to a deeper understanding of polaritonic chemistry. Finally, we investigate whether the strong coupling regime can fundamentally alter chiral-related molecular properties, showing that asymmetric plasmonic cavities, generated by a chiral nanotip, can break the spatial symmetry of intrinsically achiral species (benzene), and exhibit enantioselective discriminatory power when interacting with chiral targets (alanine). Ultimately, this work provides a rigorous foundation for capturing plasmon-mediated chiroptical effects, paving the way toward a complete set of theoretical tools for the field of Chiral Plasmonics.