Optical properties of a quantum emitter are drastically modified inside a nanometer-sized gap between two plasmonic nanostructures. At such a nanoscale gap, plasmonic resonances confine light far beyond the diffraction limit and form a nanoscopic optical cavity, called a plasmonic nanocavity. This thesis investigates the optical properties of plasmonic nanocavities and highlights their ability to facilitate strong light-matter interaction.

We first study plasmonic nanocavities by treating their resonances as leaky modes with complex eigenfrequencies. By varying their gap morphology, several bright and dark gap plasmonic resonances are discovered, which are essential for understanding how a nanocavity optically influences a quantum emitter. Near-field and far-field investigations also reveal intricate multiple-mode interaction with a quantum emitter.

Next, this thesis tackles the misconception that fluorescence emission from a quantum emitter is always quenched through non-radiative decay channels when the emitter is placed closer than 10 nm to a plasmonic nanostructure. We demonstrate the suppression of fluorescence quenching in plasmonic nanocavities due to plasmon hybridization, which enhances the excitation and radiation from the emitter. This enhancement is shown to be strong enough to facilitate single-molecule strong coupling, as evident in its dynamic Rabi oscillations.

Finally, an innovative sensing scheme is proposed, which combines immunoassay sensing with strong coupling in plasmonic nanocavities. By chemically linking the antibody-antigen-antibody complex with a quantum emitter label, we show that the proposed scheme provides 1500\% sensitivity enhancement compared to plasmonic sensors with conventional labels. This scheme could lead to the development of plasmonic bio-sensing for single molecules and new pathways towards room-temperature quantum sensing.