Coupled Plasmonics for TERS and Understanding of Quantum Effects in Plasmonic Junctions

Over the past few decades, plasmonics has emerged as a technology that spans across several fields including laser spectroscopy, material science and solid-state physics. The near-field coupling between light and the surface plasmons of metal nanostructures has transformed single-molecule detection and imaging, tracking of chemical reactions and boosting catalytic efficiencies. Surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS), by taking advantage of the enormous field enhancements arising from the excitation of the localized surface-plasmon resonances (LSPR), have both shown great potential in ultra-sensitive detection. In this dissertation, we focus on the effect of coupled plasmonics in TERS for the detection and nanoscale imaging of biologically relevant molecules such as proteins, in a non-invasive, label-free and selective manner. The other aspect of this dissertation will be devoted to the effect of quantum tunneling in ultra-small plasmonic nano-junctions, as evidenced by its gap plasmon shifts and time-dependent spectral changes.

The thesis will begin with an introduction to coupled plasmonics: reviewing the related backgrounds of plasmonics & plasmonic coupling, a topic of close relevance to the research conducted in this thesis, after which a brief description of SERS and TERS will be given. This will then be followed by the introduction of VSE and its applications in determining local electric fields in various environments. A short survey of literatures that report the quantum effect (electron tunneling) as observed in plasmonic gaps and will be presented at the end of the introduction. The results of the dissertation research can be categorized into two major parts: I. Coupled plasmonics for TERS; II. Coupled plasmonics for the understanding of quantum effects in plasmonic junctions.