Nonreciprocal Surface Waves on Gyrotropic Interfaces

In this dissertation, the properties of highly nonreciprocal (unidirectional) surface waves guided along the interface between free-space and various 2D and 3D gyrotropic continua are investigated using analytic, numerical, and experimental methods. From a classical electromagnetics perspective, nonreciprocity in the dispersion of surface wave modes supported by the interface is achieved by breaking both time-reversal and space-inversion symmetries in the collective response of the waveguide, which consists of the two interfaced materials. More recently, however, a connection to momentum space topology via the bulk-edge correspondence principle has been made for gyrotropic continua, providing additional insights into the underlying physics that governs the unique propagation characteristics of these unidirectional modes. Building on the previous work done in the areas of nonreciprocal electromagnetics and topological photonics, we (1) develop a new analytic formalism to model excitations of the surface wave modes using a near field current source, (2) investigate a nontraditional way of achieving a gyrotropic response in a 2D continuum, and (3) demonstrate experimentally, for the first time, the unidirectional nature of a recently theorized topologically protected, unidirectional surface wave mode

EM Wave Interaction with a Bounded Plasma Column Supporting an Electron Beam

In this thesis, the disruption of an incoming external transverse electromagnetic wave by an inhomogeneous plasma with energetic electron beam is examined. The plasma-beam characteristics are motivated by theory and experiment. Wave-plasma interactions and wave propagation and reflection in and from a plasma medium is studied. Physical sources such as the plate current density are considered. The inhomogeneity of the plasma slab supporting the energetic electron beam is partially built into the supported fields. The wave-plasma-beam interaction is examined over a wide parameter space. Absorption or reflection of electromagnetic waves can be achieved by changing the plasma number density, collision frequency, beam number density, and the Gaussian nature of the beam and slab thickness. Under appropriate condition in the presence of an energetic electron beam supported by the plasma slab, the externally generated wave incident on and exciting the slab can resonate with the beam. Although insignificant for the parameters studied, this becomes apparent when the operation frequency (both the wave and the beam) approaches the electron plasma frequency. Initial studies conducted have not exhausted all possible parameter space scenarios and physics mechanisms. Based on the results obtained, more involved investigations are warranted

Electrodynamics of Media

Contains research objectives, summary of research and reports on three research projects.Joint Services Electronics Program (Contract DAAB07-75-C-1346)California Institute of Technology (Contract 953524

A new hybrid implicit-explicit FDTD method for local subgridding in multiscale 2-D TE scattering problems

The conventional finite-difference time-domain (FDTD) method with staggered Yee scheme does not easily allow including thin material layers, especially so if these layers are highly conductive. This paper proposes a novel subgridding technique for 2-D problems, based on a hybrid implicit-explicit scheme, which efficiently copes with this problem. In the subgrid, the new method collocates field components such that the thin layer boundaries are defined unambiguously. Moreover, aspect ratios of more than a million do not impair the stability of the method and allow for very accurate predictions of the skin effect. The new method retains the Courant limit of the coarse Yee grid and is easily incorporated into existing FDTD codes. A number of illustrative examples, including scattering by a metal grating, demonstrate the accuracy and stability of the new method

Reflection,Transmission, and Absorption of Vortex Beams Propagation in an Inhomogeneous Magnetized Plasma Slab

Based on the angular spectrum expansion and the 4x4 transfer matrix method, an investigation into the reflection, transmission, and absorption of vortex beams in an inhomogeneous magnetized plasma slab is presented. The reflected and transmitted electric fields are expressed by the inverse Fourier transform of the product of the reflected and transmitted coefficients and the angular spectrum amplitude of the incident beam. The intensity profiles, as well as the distortion of OAM states in both the reflected and transmitted beams are simulated and discussed. Through this investigation it could be concluded that both the incident angle and the plasma parameters have significant impact on the magnitudes of reflected and transmitted intensities, and the distortion of OAM states. The effects of the magnetic field and the incident angle on the reflectance, transmittance, and absorptance of the power have also been reported

Transformation Optics Using Graphene: One-Atom-Thick Optical Devices Based on Graphene

Metamaterials and transformation optics have received considerable attention in the recent years, as they have found an immense role in many areas of optical science and engineering by offering schemes to control electromagnetic fields. Another area of science that has been under the spotlight for the last few years relates to exploration of graphene, which is formed of carbon atoms densely packed into a honey-comb lattice. This material exhibits unconventional electronic and optical properties, intriguing many research groups across the world including us. But our interest is mostly in studying interaction of electromagnetic waves with graphene and applications that might follow. Our group as well as few others pioneered investigating prospect of graphene for plasmonic devices and in particular plasmonic metamaterial structures and transformation optical devices. In this thesis, relying on theoretical models and numerical simulations, we show that by designing and manipulating spatially inhomogeneous, nonuniform conductivity patterns across a flake of graphene, one can have this material as a one-atom-thick platform for infrared metamaterials and transformation optical devices. Varying the graphene chemical potential by using static electric field allows for tuning the graphene conductivity in the terahertz and infrared frequencies. Such design flexibility can be exploited to create patches with differing conductivities within a single flake of graphene. Numerous photonic functions and metamaterial concepts are expected to follow from such platform. This work presents several numerical examples demonstrating these functions. Our findings show that it is possible to design one-atom-thick variant of several optical elements analogous to those in classic optics. Here we theoretically study one-atom-thick metamaterials, one-atom-thick waveguide elements, cavities, mirrors, lenses, Fourier optics and finally a few case studies illustrating transformation optics on a single sheet of graphene in mid-infrared wavelengths

Study of Metallic Nano-Optic Structures

Physical phenomena (optical, electronic and optoelectronic) occurring in metallic nanostructures offer an interesting potential in that they may allow us to overcome the limits of diffractive optics and to develop new functional devices complementing the dielectric-based conventional optics. Optical waves incident to a metallic structure, for example, can excite a collective oscillation of electrons, so-called surface plasmons (SPs). The spatial extension of SP fields is governed by the size of the nanostructure and can be made much smaller than the wavelength of light. These features are potentially useful in developing ultracompact photonic chips, i.e., miniaturizing the optics into subwavelength dimensions.In this thesis, we have investigated the plasmonic phenomena occurring in metallic nanoaperture array structures. A novel fabrication process has been developed to form highly ordered nanoaperture (both slits and holes) arrays on metallic layers. Optical characterization of the fabricated nanostructures revealed many interesting properties (in transmission, reflection, filtering, confinement, etc.) involving plasmonic interactions. The plasmonic phenomena in nanoaperture arrays have been analyzed theoretically: analytical solutions of plasmonic waveguiding inside nanoslits were formulated; funneling of light into nanoslit was simulated; the in-plane surface plasmon band structures at the metal/dielectric interfaces were modeled; the dynamic evolution of polarization in metal islands was analyzed. The finite-difference time-domain (FDTD) analysis of optical field distribution and propagation has been performed, and the simulation results were compared with the analytic results and experimental data. Detailed mechanisms of the plasmonic interactions in nanoaperture arrays have been developed and proposed based on this experimental and theoretical study.We further studied optical transmission properties of bi-layer metallic nanoslit arrays. The structure is found to reveal Fabry-Perot-resonator-like characteristics and the transmittance, passband, and beam polarization properties are determined by structure, dimension, and configuration. Near-field interaction and coupling in the bi-layer slit array structures were also analyzed with FDTD simulation. We also studied surface plasmon effects in reflective metallic grating structures, which show strong reflection quenching under cross-metal SP coupling conditions. We have designed and analyzed metallic nano-optic lenses based on nanoslit array structures. The phase of optical radiation emanating from each aperture is controlled by the metal thickness and aperture size. FDTD simulation of the nano-optic lenses demonstrates refractive transmission of light and beam shaping (focusing and collimation). This study opens up the possibility of developing a new class of optics that can complement the conventional dielectric-based refractive/diffractive optics