Nanophotonics with metals and dielectrics [electronic resource] : exploiting resonances for applications in energy, optics, and communications
Keywords: Electrical Engineering.

Nanophotonics is a field of study on the interactions between light and nano-scale matter. When light impinges upon matter smaller than the light's wavelength, what occurs does not follow a macroscopic ray- optics description. Rather, we require a picture including the electromagnetic near-fields to accurately characterize the optical interactions. A confluence of the development of tools to deterministically manufacture nanostructures and the computational power to simulate the optical response of complex geometries has led to a recent explosion in nanophotonics research. In particular, nano-scale optical resonances offer an opportunity for scientific and technological advancement. Both nanoscale metals and dielectrics support optical resonances. For metals, the collective oscillation of free electrons in response to a driving electromagnetic field results in a plasmonic resonance. For dielectrics, refractive index contrast between the material and its background leads to a Mie resonance. In this work, we pair a nano-scale optical resonance with a scientific or technological application across a variety of domains. We begin by studying how nano-structuring the transparent contact on a thin-film solar cell enables efficient solar light-trapping. By patterning the transparent contact layer of a cell into an array of nano-beams, we can exploit the nano-beams' Mie resonances to boost the amount of sunlight captured by the absorbing semiconductor. We first describe a model system to elucidate the essential physics, then demonstrate how such patterning improves the performance of an optimized amorphous silicon thin-film solar cell. Second, we combine the electrical and optical properties of nanostructured metals to enable an electrically-tunable nonlinear light source. We theoretically and experimentally show how a metallic nano-cavity, filled with an optically nonlinear medium, concentrates an incident light field enough to drive nonlinear optical interactions, generating second harmonic waves. We go on to show how the efficiency of this second harmonic generation can be tuned by applying a voltage between the very metals that comprise the optical cavity. We find that such a structure can modulate the frequency doubling of 1.55 micron light with a normalized magnitude of ~7\% per volt. Next, we explore highly-doped indium arsenide as a plasmonic metal for mid-infrared frequencies. Unlike gold and silver, indium arsenide can be manufactured to have a modestly negative real permittivity in the mid-infrared, allowing propagating surface plasmons with sub- wavelength mode sizes. We use attenuated total reflectance spectroscopy to excite and characterize mid-infrared surface plasmons on highly-doped indium arsenide. In doing so, we develop a technique to accurately extract the optical properties of indium arsenide, as well as explore how doping affects the indium arsenide plasma frequency. We go on to characterize the length scales of indium arsenide surface plasmons and benchmark their properties. Last, we exploit the unique "epsilon-near-zero" properties of a metal interacting with light of frequency near the metal's plasma frequency to enable electro-optical modulation of a silicon waveguide. The electromagnetic boundary condition enforced on an electric field at the interface of a material with its real permittivity close to zero results in a large electric field enhancement. We show how capacitively accumulating electrons in indium tin oxide induces triggers such an electric field enhancement, modulating the output light intensity of a silicon waveguide at 1.55 microns. We design a simple structure with 3 dB modulation is achievable in a device with a footprint smaller than 30 micron. We also demonstrate a potential for 100 fJ/bit modulation, with a sacrifice in modulator performance.