Plasmonics for surface-enhanced Raman scattering: from classical to quantum

Metallic nanostructures that employ localized surface plasmon resonances to capture or radiate electromagnetic waves at optical frequencies are termed "plasmonic optical antennas". These structures enhance light-matter interactions in an efficient manner, enabling unique linear and nonlinear optical applications. One such application is surface-enhanced Raman scattering (SERS), which employs plasmonic antennas to enhance Raman cross-section of molecules by orders of magnitude. SERS has attracted a significant amount of research attention since it enables molecules to be identified through their characteristic vibrational spectra, even at the single molecule level.;In this thesis, we investigate the mechanisms underlying electromagnetic enhancement in SERS, and optimize plasmonic optical antenna designs to allow efficient SERS detection. We first demonstrate a top-down fabrication procedure to reproducibly fabricate plasmonic dimers with controllable gap widths that can be as small as 3 nm. We experimentally demonstrate that SERS enhancements increase as the gap size is reduced. The method we introduce is capable of routinely delivering reproducible SERS substrates with high enhancement factors. We then investigate a technique termed "energy-momentum spectroscopy" to measure Raman emission patterns, i.e. the angular distribution of Raman scattering. In particular, we demonstrate how they are modified by plasmonic optical antennas. It is found that the Raman scattering from molecules on plasmonic dimers (pairs of gold rods) forms two beams into the substrate which supports the dimers. This would normally necessitate the use of an objective lens with a large numerical aperture for efficient collection. We investigate the abilities of two alternative optical antenna designs to modify the angular distribution of Raman scattering. We term this effect "beamed Raman scattering". The first antenna design is that of Yagi-Uda antennas. The second design consists of plasmonic dimers formed above a gold film integrated with a one-dimensional array of gold stripes. For both antenna types, beamed Raman scattering is observed.;In most cases, the electromagnetic enhancement mechanism of SERS can be understood by classical electromagnetic theory. Only recently has it become well-appreciated that quantum mechanical effects such as nonlocality and electron tunneling emerge as the feature sizes of metallic nanostructures approach atomic length-scales. We unambiguously demonstrate the emergence of electron tunneling at optical frequencies for metallic nanostructures with gap-widths in the single-digit angstrom range. Moreover we experimentally demonstrate, for the first time the best of our knowledge, that the emergence of electron tunneling limits the maximum achievable SERS enhancement.