Broadband nanophotonics [electronic resource] : plasmonic band theory and light trapping for solar cells

Remarkable progress has been made over the past two decades in controlling light propagation and absorption in compact devices using nanophotonic, plasmonic structures and metamaterials. From sensing and modulation, to on-chip communication and light trapping for solar cells, new device applications and opportunities now motivate the need for a richer understanding of the optical properties of these devices over a broad range of frequencies. In this dissertation, we present two strands of original, fundamental and applied work on the broadband behavior of nanophotonic devices: a plasmonic band theory and nanophotonic light trapping. First, we introduce a photonic band theory that rigorously models the behavior of plasmonic nanostructures or metamaterials made of dispersive materials such as metals. The theory extends traditional photonic band theory for periodic dielectric structures by coupling the mechanical motion of electrons in the metal directly to Maxwell's equations, and formulates the band structure of plasmonic structures as a Hermitian eigenvalue equation. We then construct a perturbation theory to physically explain and predict the effect of dielectric refractive index modulation or metallic plasma frequency variation in a plasmonic nanostructure. Furthermore, we provide an intuitive physical picture of the source of modal material loss in such devices, and derive an upper bound on the modal material loss rate of any plasmonic structure. This in turn places fundamental limits on the broadband operation of such devices for applications such as photodetection and absorption. Next, we discuss a nanophotonic light trapping theory that rigorously describes light absorption enhancement in nanoscale solar cells. Conventional light trapping techniques increase the path light of incident solar light in active material, and are subject to a fundamental absorption enhancement factor limit. We show that, at the nanoscale, it is possible to exceed this limit for all absorption regimes, and explain the mechanisms for this enhancement using our theory. We next apply these principles to organic solar cells by designing and optimizing a light trapping scheme where the transparent conductor is patterned to enhance absorption in the organic semiconductor layer below. We numerically demonstrate that such a design can provide up to 10% photocurrent enhancement relative to an optimized planar organic solar cell. Finally, we investigate the potential of plasmonic light trapping schemes to exceed conventional limits of light trapping at the nanoscale. In particular, we use principles and results from our plasmonic band theory to probe the role of parasitic loss in the metal on achievable absorption enhancement factors in the active photovoltaic material. Opportunities to exceed conventional limits using such plasmonic schemes exist but are subject to the material damping rates of the metal used, and therefore necessitate careful design consideration.

Keywords: Applied Physics.
Advisor: Fan Shanhui
Harris J. S. (James Stewart)
Brongersma Mark L. advisor. Thesis advisor