The light-matter interaction in nanoscale electronic systems is a fundamental question with relevance in numerous modern applications of plasmonic nanostructures. This thesis approaches the topic from two different directions. In the first part, a quantum model is proposed to describe a plasmonic nanostructure as a fundamental platform to investigate the optical response of the electrons. In the second part, the complex properties of plasmonic nanocavities are exploited to control the quantum dynamics of a single emitter placed in its near-field environment.
With recent experiments investigating progressively smaller nanoparticles, quantum effects come into play and a microscopic description of the electron structure is required. In the proposed model, a density matrix formalism is used to analyse the photoexcited few-electron dynamics in a small plasmonic nanosphere. Starting from the standard derivation of the bulk plasmon one aims particularly at elucidating the role of the Coulomb interaction. The ability to calculate the linear and non-linear response of the system under various excitation conditions is demonstrated. The linear response is characterised by discrete resonances modified by a Coulomb interaction mediated collective response. In the strong excitation regime, there are optically induced oscillations of the occupations which strongly deviate from Rabi oscillations in a two-level system due to the non-linearity of the Coulomb interaction.
The fundamental limit of a single quantum emitter coupled to a plasmonic mode constitutes another example of light-matter interaction on the nanoscale. In this thesis, it is shown that an electron beam exciting a nanometre-size plasmonic nanocavity induces the strong coupling of a single emitter placed in the gap and the plasmon mode. It is further shown that the interaction of the swift electrons with the metal is changing non-intuitively with their velocity for different geometries due to the different coupling channels. This behaviour is then explored to actively control the quantum dynamics of the emitter.