In the framework of the quasi-static approximation (QSA), some theoretical studies were conducted within the local response approximation (LRA). In these studies, certain plasmonic and plexcitonic systems were proposed, and their spectroscopic properties investigated. The QSA allows us to study metal nanoparticles (MNPs) and inter-particle distances that are small compared to the wavelength of light in the medium surrounding the MNPs, while the LRA enables us to utilize the bulk dielectric response of the metal in consideration. We have studied the following properties in detail: localized surface plasmon resonances (LSPRs), plasmon-induced transparency (PIT), and plasmon-enhanced fluorescence (PEF), while exciton-induced transparency (EIT) has only been partly studied. LSPR and PIT are properties of plasmonic systems while PEF and EIT are properties of plexcitonic systems. Both PIT and EIT are forms of electromagnetically-induced transparency. We started by constructing a geometry-based theoretical model that predicts the LSPR formula of any member of a certain group of single MNPs, using the LSPR for the most complex MNP geometry in the group. The model shows that from the LSPR of a nanorice, one could predict the LSPRs of concentric nanoshells, solid and cavity nanorods and nanodisks, respectively, and solid and cavity nanospheres. These formulae serve as quick references for predicting LSPRs since they can easily be compared to LSPRs obtained from spectral analysis. Likewise, we studied LSPR in addition to PIT in a nanoegg-nanorod dimer. We proposed this dimer in order to investigate how the interplay between plasmon coupling and MNP sizes affects PIT in complex geometries such as nanoeggs. Our result shows that the formation of PIT dips — regions in the dimer spectra where little or no incident radiation is absorbed by the dimer — are strongly-dependent on the nanorod size, due to the dependence of the plasmon coupling strength on the half-length of the nanorod. We investigated the phenomenon of PEF using a nanoegg-emitter system and a nanorod-emitter system, respectively. Emitters are organic or inorganic materials whose radiative decay rates increase dramatically when placed near a MNP subjected to plasmon excitation. Our theoretical results show that the choice of the MNP-emitter system to use depends on both the intrinsic quantum yield of the emitter and the antenna efficiency of the MNP. Theory shows that PEF is more substantial when the former is very low, and it will always occur if the latter is greater than the former. A nanorod-emitter system should serve as the preferred choice, due to the relatively easier synthesis of nanorods compared to nanoeggs, and the large longitudinal polarizability of nanorods as a result of the lightning rod effect. However, our theoretical model also shows that a nanoegg-emitter system can rival the PEF parameters obtained in a nanorod-emitter system, due to an increase in the Purcell factor of the emitter with increasing core-offset of the nanoegg, resulting from the presence of dipole-active modes in the nanoegg. / Thesis (PhD (Physics))--University of Pretoria, 2020. / University of Pretoria / National Research Foundation (NRF) / Physics / PhD (Physics) / Unrestricted