Molecular Plasmonics: Directional Optical Antennas and Single Molecule SERS

Abstract

Interaction of light with metal nanoparticles creates large electric field intensity in a small region because of surface plasmons. This interaction also leads to the heating of metal particles because of Joule heating caused by interactions between electrons and phonons. Electric field enhancement provided by metallic structures have been used for understanding the fundamentals of enhanced light-molecular interactions and also for applications such as single molecule detection and strong coupling physics. Metal structures have also been shown to work as a subwavelength heating sources because of enhanced absorption and other thermoplasmonic properties. Concurrently harnessing the field enhancement and heating capabilities will open new avenues in utilization of metallic structures for fundamental studies as well for applications purposes. In this thesis, first, we will discuss how field confinement is achieved by optical cavities formed in the gaps of two metallic structures. We study emissions such as fluorescence and surface enhanced Raman scattering (SERS) from molecules confined in optical cavities. Specifically, we image the molecular emission wavevectors using Fourier plane and energy-momentum imaging. We show that well designed optical cavities can be used to tune light-matter interaction for designing directional optical antennas for elastic or inelastic light scattering. Next, we present how light induced heating of subwavelength metal particles can be harnessed for trapping a large number of metallic nanostructures at extremely low input power density. We utilized heat, and temperature gradients produced by a single heated metal nanoparticle to create a large-scale reversible assembly of metallic nanoparticles in a fluidic environment. Furthermore, we study SERS signatures from molecules in such assemblies, down to the level of single molecule limit. We apply bi-analyte technique to statistically confirm the spectroscopic signatures of single molecule SERS from the molecules in the assembly.