Thesis advisor: Michael J. Naughton / In this dissertation I present studies of plasmonic interactions in different coupling regimes, from zero to strong coupling and approaching ultrastrong coupling. Different physics are manifest in each regime, with different possible applications. The first project uses finite element electromagnetic simulations to model plasmonic waveguides that couple near field light into the far-field for sub-diffraction limited microscopy. Wavelength/32 resolution is shown by minimizing coupling between adjacent waveguiding nanowires, with minimal attenuation over a few microns. The next two projects, by contrast, seek to maximize coupling between plasmons and excitons into the strong coupling regime where the optoelectronic properties are modified and quantum coherent phenomena may be observed. Strong exciton–plasmon coupling in MoS2 is shown experimentally at room temperature and found to be a general phenomenon in other semiconducting transition metal dichalcogenides using transfer matrix modeling. A semiclassical oscillator model is fit to the experimental data to discover coherent hybridization between the ground and first excited states of MoS2. Enhanced coupling is found at the third excitonic transition, approaching the ultrastrong coupling regime where exotic properties are predicted to emerge, such as ground state virtual photons. Our strong coupling studies motivate further studies of the TMDCs as a platform for coherent quantum physics with possible applications in quantum computing and cryptography. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.