Terahertz Raman Spectroscopy of Organic Molecules in Plasmonic Nanogaps

Abstract

This thesis contributes to the extension of the technique of surface-enhanced Raman spectroscopy (SERS) from the infrared to the terahertz (THz) region of the electromagnetic spectrum. SERS was first discovered in 1977, and has since become a widespread, powerful analytical tool for chemistry and has led to the development of many other surface-enhanced spectroscopies. However, its exploration of THz-frequency modes of materials was limited to measurements using low-throughput and expensive equipment until 2012. Recent advances in laser and notch filter technology have made the THz region accessible to Raman spectroscopy, accelerating the publication of THz Raman studies on a wide array of bulk materials. While THz SERS measurements are likewise possible, progress has been much slower than for bulk Raman, impeded by characteristics of the spectra that make the analysis challenging. Two predominant characteristics were observed of all the measured THz SERS spectra of several different sets of systems, varying analyte and nanostructure parameters, and of the few THz SERS spectra reported in literature: a continuum underlying molecular SERS peaks which rises super-exponentially at decreasing Raman Shifts on both sides of 0 cm-1 and broadening of molecular SERS peaks at decreasing Raman Shifts. These complicate the interpretation of THz SERS spectra. This thesis presents investigations of both. The result is an approach to analyzing SERS data. As an example, this is applied to early diagnosis of Alzheimer’s Disease (AD), the archetype of amyloid diseases. The polymerization of the peptide β-amyloid is considered to be central to the pathogenesis of AD. Because of the delocalized nature of THz-frequency molecular vibrations, THz SERS has the potential to directly distinguish different structures of the peptide.