Novel wave manipulation of designer surface plasmons

Abstract

Due to its subwavelength field confinement and low propagation loss, designer surface plasmon (DSP) holds considerable promise in microwave- to infrared- frequencies device applications. This thesis studies the novel wave manipulation of DSP and its various applications in the microwave frequency range. We first explore the band gap nature of surface-wave band-gap crystal implemented on a single metal surface. By merging the forbidden bandgap of photonic crystal and the deep subwavelength nature of surface plasmons, the concept of surface-wave band-gap crystal with subwavelength-scale unit cell was developed with superior properties including high-efficiency pulse routing around sharp corners and multi-directional splitting of surface waves with full bandwidth isolation. Then we extend the concept of surface-wave band-gap crystal to the well-known Sievenpiper “mushroom” structure and demonstrate that this thin high-impedance surface is a new type of ultrathin surface-wave band-gap crystal with thickness much smaller than the operating wavelength. After the study of propagating DSP, we transfer to localized DSP and demonstrate the forward/backward switching of plasmonic waves propagation using sign-reversal coupling between localized DSP resonators. Since the properties of DSP can be fine-tuned by simply altering the underlying structural parameters and the mode profiles of designer surface plasmons can be imaged directly, this highly tunable platform allows us to explore the recently emerged topological photonics. In last part of this thesis, we extend the concept of topological photonic into DSP and demonstrate a flexible photonic topological insulators that can be bent, folded, or even twisted to mold the flow of topological edge states. Chapter 1 introduces the basic concept of surface plasmon polaritons (SPPs) that exist on the metal/dielectric interface and localized surface plasmons (LSPs) that exist on a close metal surface at optical frequencies, and how to mimic SPPs and LSPs at low frequencies based on the concept of designer (or spoof) surface plasmons and designer localized surface plasmons. Chapter 2 studies a class of surface-wave band-gap crystal implemented on a single metal surface which exhibits a complete band gap for surface waves. Surface waves can be tightly guided along the line defect and near-perfect transmission around sharp corners can be achieved over a broad frequency band. Moreover, functional components including a T-shaped splitter and a square open resonator are also demonstrated. Chapter 3 studies a multi-directional plasmonic surface-wave splitter with full bandwidth isolation based on a new waveguiding mechanism of surface modes through weak couplings between tightly localized defect cavities in an otherwise gapped surface-wave band-gap crystal. The dispersion of coupled defect surface modes is tight-binding-like dispersion, rather than the polaritonic dispersion of conventional designer surface plasmons which starts from light line and approaches to the asymptotic frequency at the first Brillouin zone boundary. Therefore, the waveguiding bands of different branches of the multidirectional plasmonic splitter can be completely isolated which cannot be achieved using conventional designer surface plasmon waveguides. Chapter 4 studies an ultrathin surface-wave band-gap crystal with thickness much smaller than the operating wavelength implemented on a well-known Sievenpiper “mushroom” structure. Functional devices such as a sharp bend and a T-shaped splitter are demonstrated with good performance. Chapter 5 studies the forward/backward switching of plasmonic waves propagation using sign-reversal coupling. By directly measuring the tight-binding Bloch waves on a periodic array of coupled designer surface plasmon resonators in the microwave regime, we demonstrate multi-band forward/backward switching of plasmonic wave propagation. Our approach makes use of sign-reversal coupling that occurs when switching the coupling configuration between two photonic resonators. Direct experimental measurements of plasmon dispersion curves confirm the forward/backward switching of plasmonic waves propagation in the same frequency range. Chapter 6 studies a mechanically flexible photonic topological insulator that supports robust topological photonic states on a curved surface. Spatial topologies achieved by folding the flexible photonic topological insulator serve as a new freedom to manipulate the propagation of topological photonic states. This work bridges the gap between the emerging field of topological photonics and the technologically promising field of flexible photonics.