Optical Properties of Matter: from Plasmonic Nanoparticles to Airborne Microplastics

This thesis presents a multi-scale study of how light interacts with small particles, a phenomenon central to a wide range of environmental and technological issues. The first part of the work examines the optical properties of coloured airborne microplastics to assess how they absorb and scatter solar radiation and, consequently, how these particles contribute to global warming. The second part focuses on the interaction of light with nanoscale metallic particles, exploring their light-induced resonances and how they can be exploited in applications such as photochemistry and molecular sensing. To establish the optical properties of coloured polymers, we measured the light reflection and transmission of various coloured plastic films and applied Kramers-Kronig analysis to retrieve the corresponding complex refractive indices. The analysis of matte and opaque plastics required adapting the bihemispherical reflectance measurement technique using an integrating sphere. This method demonstrated adequate accuracy for various materials and can potentially be applied to more complex substances. Collecting the optical properties of coloured polymers and calculating their scattering and absorption provided the basis for further climate simulations completed by our collaborators. The results indicated that coloured microplastics exert a net atmospheric warming effect in contrast to the predominant cooling influence of clear polymer particles. At the nanoscale, two case studies were considered: light interactions in core-satellite nanostructures relevant to photochemical and water-splitting applications, and the coupling between metallic nanoparticles and adsorbed dye molecules, affecting the localised surface plasmon resonances of the core particle. The validation of the generalised coupled dipole model established a computationally efficient and physically accurate approach for simulating light absorption in complex core-satellite nanostructures. The analysis of dye-nanorod systems using both simulations and experimental data allowed us to reveal processes involved in the interactions between dye molecules and a metallic surface. Collectively, this thesis advances the understanding of light-matter interactions across environmental, material, and nanophotonic contexts, providing new experimental datasets, validated modelling frameworks, and physical insight relevant to optical material science, global climate assessments, and nanotechnology.