Control of Fano Resonance in Plasmonic and Optical Systems for Highly-Sensitive Spectral Responses

Abstract

The concept of Fano resonance from quantum physics has originally been applied to describe the configuration interaction in the rare gas excitation, which is one of the universal phenomena caused by the wave interference. The main feature of Fano resonance is the sharp, asymmetrically shaped spectrum, which is due to the interference between two resonant modes of different life-time. To exploit this intriguing spectral feature, a variety of Fano resonant nanostructures implementing optical analogue of quantum-mechanical Fano resonance have been proposed and studied, including clusters of plasmonic nanoparticles and metamaterial platforms. During my Ph.D study I have been trying to bring the concept of Fano resonance into the field of optical and plasmonic systems to provide some novel paths for the existing applications such as on-chip plasmonic devices and net optical spin excitation. In this dissertation, the characteristics of asymmetrically shaped spectral responses for the Fano resonant structures are investigated in the plasmonic stub-waveguide systems and optical chiral system. In the theoretical part of this dissertation, the theoretical descriptions about the Fano resonance phenomena are provided in several different formalism, such as the original quantum perturbation analysis, analogy with classical oscillators, and the simple and general coupled mode theory. Based on the coupled mode theory model, I also establish the Fano asymmetry parameter, which is the key physical parameter for the quantification of the Fano resonance phenomenon. Through the numerical and theoretical studies, we firstly propose a plasmonic sensor based on the control of the degree of Fano asymmetry, with the plasmonic metal-insulator-metal waveguide-stub junction structure. we also introduce an ultra-efficient plasmonic on-chip modulator based on the concept of plasmonic induced transparency. Excellent performance with 12dB transmission contrast at ~60% throughput is achieved due to the extremely sharp spectral profiles, which is caused by the Fano interference using a pair of plasmonic metal-insulator-metal stubs. A novel path for the conservative and nonmagnetic optical spin excitation is introduced utilizing the spin handedness-dependent anti-symmetric Fano resonances, with Hermitian material parameters. Utilizing the pure optical spin excitation density and its extreme spectral sensitivity in the suggested design, optical spin switching is also introduced with experimentally accessible material parameters.