Measuring and modelling the absolute optical cross-sections of individual nano-objects

Nanoparticles are ubiquitous in nature, and the number of technological applications exploiting nano-objects, either synthesized chemically or fabricated lithographically, is in steady rise. In particular, metal nano-objects exhibit resonant modes corresponding to an enhanced coupling to electromagnetic radiation. The interaction of light with a nano-object is wholly described by its cross-sections for absorption and elastic scattering. In this thesis we present a method to measure the absolute amplitude of the cross-sections. Differently from currently available techniques, we account for the finite angular collection of the objective via an analytical model of the scattering process, thereby rendering our method accurate also for objects dominated by scattering and high numerical aperture detection. The model of scattering assumes that the nano-object is placed at a planar dielectric interface, representing the substrate, and a homogeneous optical environment is obtained as a limiting case. The accuracy of the quantitative method was tested on several model systems using two widespread experimental techniques: Micro-spectroscopy and widefield imaging, which are both implemented with a simple experimental set-up, constituted by a commercial microscope equipped with an imaging spectrometer or a camera. In order to quantitatively simulate microscopy experiments, a realistic description of the excitation must be included in numerical models. In this thesis we describe novel modelling practices which reproduce typical coherent or incoherent microscope illumination. Comparison of quantitative experimental and numerical results is used to estimate parameters describing the geometry of a nano-object, such as the diameter or the aspect ratio. In conjunction with the high-throughput capabilities of widefield image analysis, quantitative cross-section measurements and optical characterization of the geometry can provide a thorough statistical appraisal of the dispersity of the structural and optical properties of a sample. Therefore, this thesis represents a significant step towards an ‘all-optical’ characterization of nano-objects, complementing costly and time-consuming electron microscopy techniques.