Directional Optical Antennas, Wafer-Scale Metasurfaces, and Single Molecule Surface-Enhanced Raman Scattering

In Chapter 1, we lay out the fundamentals of surface plasmon polaritons (SPPs), localized surface plasmons (LSPs), Raman scattering, and surface-enhanced Raman scattering (SERS), in order to place contributions of this thesis into context.;In Chapter 2, we investigate the optimal design of optical antennas. These refer to antennas that operate at optical frequencies, and are employed throughout this thesis due to their ability to concentrate light into nanoscale regions. We primarily investigate these with the application of surface-enhanced Raman scattering in mind. Optical antenna designs comprising pairs of metallic nanoparticles with different shapes are studied, using electromagnetic simulations performed via the finite difference time domain (FDTD) method. The concepts of charge and current reservoirs are introduced.;In Chapter 3, we continue our investigations on the optimal design of optical antennas, with an emphasis on improving directionality. We specifically consider the case of a device consisting of a pair of gold nanoparticles surrounded by concentric gold rings, all above a gold mirror. We perform simulations showing that the SERS enhancement in this design is two orders of magnitude higher than that of the basic design, consisting solely of a pair of gold nanoparticles. These simulations take the realistic case of focused illumination.;In Chapter 4, we experimentally demonstrate a substrate that achieves very high SERS performance. It consists of an array of pairs of gold nanoparticles with gaps well below 10 nm and a one-dimensional array of gold strips. These formed above a gold film, with a dielectric spacer layer between them. We perform simulations that show that the device achieves local electric field enhancements that are far higher than those of the basic design, consisting solely of pairs of gold nanoparticles on a glass substrate. Our experimental results reveal average SERS enhancement factor of 1.2x1010, representing an increase of about two orders of magnitude over the basic design.;In Chapter 5, we demonstrate, for the first time to the best of our knowledge, single molecule SERS using a top-down fabricated chip that contains multiple hot-spot producing nanostructures (optical antennas). Our chip consists of pairs of closely-spaced silver nanoparticles surrounded by silver rings. These are formed above a silver film, with a dielectric spacer layer between them. We perform electromagnetic simulations that predict very strong electric field intensity enhancement that is ~two orders of magnitude higher than that of the basic design, consisting of pairs of silver nanoparticles on an SiO 2 substrate. We experimentally verify that the chip achieves single molecule SERS (SMSERS) sensitivity via the isotopologue method. The experimental results show that the fraction of optical antennas on the chip that achieve single molecule sensitivity is near unity.;In Chapter 6, we present experimental results on metasurfaces. One of key developments in optics over the past decade has been that of metamaterials. These typically comprise dielectric or metallic nanostructures in a regular array throughout a three-dimensional region that enable the realization of a bulk electromagnetic behavior that may be difficult or impossible to obtain otherwise. The term "metasurface" has come to refer to the case where these nanostructures are formed at a surface. We demonstrate a metasurface that achieves the near-total absorption of light at visible wavelengths. We demonstrate the SERS detection of single molecules on the metasurface, enabled by the strong electric field enhancement it generates. The metasurface consists of silver islands formed above a silver mirror, with an SiO2 spacer layer between them. Because these are formed via the standard fabrication techniques of evaporation and sputtering, the metasurfaces are formed on a wafer scale in highly economical fashion. (Abstract shortened by UMI.).