Electromagnetic interactions in nanoscale systems are a driving force of research in the field of nanophotonics. The basic properties of these systems set the groundwork for understanding complex optical phenomena useful for the development of new technological devices. Metallic nanospheres are among the most simple and canonical structures that produce plasmon resonances, which are collective oscillations of valence electrons excited by the electric field component of an incident light wave. Much like electron orbitals in molecules, plasmon resonances hybridize to form new modes when two or more structures combine. If a broad and narrow mode overlap in energy, they will interfere to produce a Fano resonance indicated by an asymmetric line shape in the extinction spectrum. Hybridized modes can also focus light into subwavelength volumes with intensities several orders of magnitude greater than that of the incident beam. These strong near-field enhancements can be used to detect extremely small quantities of molecules in a variety of chemical sensing methods, even at the single-molecule level.

The goals of this thesis are to explore the optical properties of asymmetric nanoparticle systems and to design antennas with strong near-field enhancements for infrared molecular spectroscopy. The first part will discuss plasmonic heterodimers composed of Au nanoparticles differing in size and shape. These simple geometries give rise to a remarkably rich set of properties. For incident polarization parallel to the dimer axis, the hybrid plasmon modes produce a Fano resonance and demonstrate avoided crossing behavior. For incident polarization perpendicular to the dimer axis, the structure exhibits an optical nanodiode effect, where the scattering profile changes depending on the direction of the incident beam. The second part of this thesis will introduce two Au nanoantenna designs having strong near-field intensities in the mid-infrared range. Zeptomole quantities of molecules are detected through Surface-Enhanced Infrared Absorption (SEIRA), in which a Fano resonance exists between the antenna plasmon mode and the molecular vibration of interest. The first structure, called a cross antenna, consists of four nanorods oriented perpendicularly with a common junction, such that all polarizations of light are simultaneously absorbed. The second structure, called a fan antenna, incorporates a semicircular portion on the outer end of each rod that increases both the overall scattering cross section and the near-field enhancement at the junction. Further enhancement is achieved by placing the antenna above a Au mirror to maximize constructive interference between the incident and scattered light. Using these designs, we demonstrate enhanced detection of several classes of analytes, and we approach the limit of sensitivity for conventional spectroscopic methods in combination with standard lithographic techniques. Such findings are essential for gauging the conditions required for single-molecule infrared spectroscopy and for furthering the development of near-field chemical sensing.