Metamaterials on Fibers: Nanofabrication for Optical Applications
Metamaterials (MMs) open a new field of controlling light arbitrarily with artificial deep subwavelength structures. The research of metamaterials brings many unusual optical effects such as negative index of refraction, superlens, hyperlens, cloaking, super absorber and various others. In the past, research of metamaterials has focused on a plane wave or a Gaussian beam incidence in free space. We proposed expanded MMs research with guided light (light propagating in a conventional optical waveguide) and structured light (light containing phase or polarization singularities). In this dissertation, we focus on nanofabrication to realize these expanded studies which may lead to potential applications for MMs in sensing, light-on-a-chip systems and communications. The layout of this dissertation is as following: In chapter 1, we briefly review the development and principles of two major types of metamaterials: negative index MMs and hyperbolic MMs. We introduce the concept of guided light and structured light which we propose to use to expand the study of MMs. We explain the relationship between the three terms. In chapter 2, we first introduce fabrication requirements of optical metamaterials (basic parameters of MMs and feature sizes), and discuss advantages and disadvantages of different fabrication methods. Then, we describe the primary systems that are used (e-beam evaporator and FIB) in our lab. After this introduction, we describe the fishnet structure, fishnet structure with prism, and free standing MMs as examples to describe in detail how to use our systems to fabricate MMs on regular substrates for light in free space. In chapter 3, we discuss methods using both metamaterials and non-metamaterials to generate a typical type of structured light (optical vortices) in free space. We performed a detailed study on the micro spiral phase plate, subwavelength metallic concentric rings, and a metasurface structure. We also demonstrate generating structured light through the use of a nano-waveguide array, a novel metamaterials structure. Such compact and versatile structures are highly compatible with fiber optics and light-on-a-chip systems. These devices enable structured light generation at the micrometer scale, in some cases even down to the nanometer scale. In chapter 4, we propose and demonstrate the integration of optical fibers and metamaterials. Fiber-coupled magnetic MMs in the near-infrared wavelength range are discussed theoretically in detail and experimentally studied. This is the initial study of metamaterials devices working with guided light. Such fiber-MMs integration may provide fundamentally new solutions for light-on-a-chip systems for sensing, biomedical applications, and advanced image processing. In chapter 5, we theoretically study the relation between structured light and modes in a few-mode fiber. We demonstrate the generation of structured light beams using on-fiber devices (non-MMs device: subwavelength concentric rings and MMs device: metasurfaces) and investigated propagation of such structured light with an OAM in a few-mode optical fiber as guided light at visible wavelengths. We demonstrate simultaneous propagation of two orthogonal polarization states in the same conventional step-index optical fiber. In chapter 6, we conclude our works and propose some future directions, including: hyperlens (metamaterials introduced guided light), nano-waveguide array to generate higher order optical vortices, functional metasurfaces on fiber (guided light mode coupler or sorter), and polar shape MMs which can be integrated with fiber mode.