In this thesis we investigate the response of plasmonic systems in a quantum optics
setting. This work can be grouped into two sub-investigations, the study of
macroscopic and microscopic responses. The narrative of the thesis comprises three
principal parts. First, we give an in-depth review of the field of quantum plasmonics
as it is an important theme that runs through the work contained in this thesis. In
particular, we focus on outlining the cutting edge research that is being done on the
intense interactions between plasmonic systems and quantum emitters. This leads
naturally to the first investigation into the macroscopic response of quantum plasmonic
systems in a metamaterial setting. We outline how complex hybrid systems
of plasmonic metal nanoparticles (MNP) and two-level quantum dots (QD) can be
used to create a quantum plasmonic metamaterial. Metamaterials are structures
composed of periodic lattices of identical subwavelength unit cell scatterers, each of
which governs completely the electromagnetic properties of the entire bulk material.
We theorize the use of MNP-QD nanorings as a unit cell in order to control
the macroscopic magnetic properties of the metamaterial. We outline how such a
metamaterial can have a tunable, and saturable, magnetic permeability. In the last
part of the thesis we consider the model of a single light mode interacting ultrastrongly
with a collection of emitters, in the anticipation that quantum plasmonic
systems can be brought into this ultrastrong-coupling regime (USC). In particular
we study the emission of the system after the coupling between the light mode and
the emitters is non-adiabatically switched-on. We find evidence that for both two-level,
and multi-level, emitters in the USC, both the counter-rotating terms and the
diamagnetic term must be included to prevent qualitative errors.