Characterisation of hemispherical nanostructures

The wonder of plasmonics has fascinated the scientific world in the past few decades due to an astounding phenomenon named Localised Surface Plasmon Resonance (LSPR), which is primarily the collective oscillation of conduction electrons confined in a nanostructure, with coherence to an incident electromagnetic radiation. These non-propagating excitations are capable of confining optical energy in subwavelength dimensions, exhibiting exotic properties ranging from profound energy absorption and narrowband scattering to resonance induced near field enhancement that disclose unprecedented openings in a vast range of fields including medicine, photovoltaic and optoelectronics. Resonance conditions of a nanoensemble can be modified by altering the structure in terms of size, shape, composition and aggregation or by varying the properties of the incident field and host medium. Therefore, when exerting light interaction with nanoparticles in a particular application, a proper mixture of the aforementioned parameters needs to be employed. In this regard, a wide range of nanostructures are being investigated extensively. However despite its simplicity, the hemispherical shape of nanoparticles is rarely studied in nanoplasmonics. Therefore, the primary objective of the research outlined in this thesis is to elucidate the optical behaviour of hemispherical shape nanoparticles and explore the plausibility of using them in Surface enhanced Raman scattering (SERS) and other sensing applications. To this end, a parametric analysis of the resonance features of a single nanohemisphere and a hemispherical nanodimer, in terms of nanoparticle size, configuration, inter-particle distance, polarisation direction and host medium, is conducted based on numerical simulations. A theoretical model that describe the behaviour of a hemispherical nanodimer upon interaction with light is presented, and the accuracy of the model is evaluated compared to the numerical simulations. Further, the electric field enhancement in the vicinity of the structure is studied, followed by an investigation about the optimum configuration and polarisation for SERS applications. The effectiveness of hemispherical dimers in these applications is analysed in relation to spherical dimers. Finally, the influence of fabrication deficiencies on the aforementioned optical characteristics is evaluated, while the thesis is wound up with a discussion of the important contributions of this study and valuable recommendations for future research.