The work reported in this thesis concerns how light can be coupled to plasmonic nanocavities, increasing its electric field by orders of magnitude. Variations of a popular NanoParticle (NP) on Mirror (NPoM, or patch antenna) structure were used, which localizes visible light in 3 dimensions to an area of contact between the NP and a Self-Assembled Monolayer (SAM), within a defined facet area. This enables strong Surface-Enhanced Raman Scattering (SERS) from the molecules at the facet.

The energy deposited in the NP through laser irradiation was exploited to perform all-optical thermal measurements of SAMs in stable junctions. The deficiencies in our understanding of light coupling to such nanocavities are highlighted through this work.

The Quasi-Normal Modes (QNMs) of lossy plasmonic nanocavities were investigated across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. We show that the gap thickness t and refractive index n are spectroscopically indistinguishable, accounted for by a single gap parameter $G=n/t0.47$. Simple tuning of mode resonant frequencies and strength is found for each QNM, including important ‘dark’ modes, revealing a spectroscopic “fingerprint” for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. Selection rules based on QNM symmetry are extracted, and differences in mode brightness are accounted for by Poynting analysis on the scattered field. These insights are then applied to a range of NPoM measurements to explain the findings, and a spectroscopic method of imaging ‘dark’ modes is achieved.