Transformation Optics Applied to Plasmonics

Abstract

Lately, transformation optics (TO) has driven the development of metamaterial science, providing a direct link between a desired electromagnetic phenomenon and the material response required for its occurrence. However, this powerful framework is not restricted to the metamaterial design, and it has recently been exploited to study surface plasmon assisted phenomena. In this thesis, we mainly focus on the general strategy based on TO to design and study analytically plasmonic devices capable of efficiently harvesting light over a broadband spectrum and achieving considerable field confinement and enhancement. Using TO, we show that a finite nanoparticle with sharp geometrical features can behave like an infinite plasmonic system, thereby allowing simultaneously a broadband interaction with the incoming light as well as a spectacular nanofocusing of its energy. Various plasmonic structures are designed and studied, such as 2D crescents, groove/wedge like nanostructures, overlapping nanowires, and rough metal surfaces. Comprehensive discussions are also provided on practical issues of this problem. First, we discuss how the edge rounding at the sharp boundary affects the local field enhancement as well as the energy and bandwidth of each plasmonic resonance. In particular, the necessary conditions for achieving broadband light harvesting with blunt structures are highlighted. The TO approach is then applied to study the interaction between plasmonic nanoparticles. We demonstrate that the energy and spectral shape of the localized surface plasmon resonances can be precisely controlled by tuning the separation between the nanoparticles. Finally, we consider the extension of the TO framework to 3D geometries, and show that the 3D structure is more robust to radiative loss than its 2D counterpart. The physical insights into sharp and blunt plasmonic nanostructures presented in this thesis may be of great interest for the design of broadband light-harvesting devices, invisible and non-invasive biosensors, and slowing-light devices.