Jessie Yao Chin

Broadband ground-plane cloak.

The possibility of cloaking an object from detection by electromagnetic waves has recently become a topic of considerable interest. The design of a cloak uses transformation optics, in which a conformal coordinate transformation is applied to Maxwells equations to obtain a spatially distributed set of constitutive parameters that define the cloak. Here, we present an experimental realization of a cloak design that conceals a perturbation on a flat conducting plane, under which an object can be hidden. To match the complex spatial distribution of the required constitutive parameters, we constructed a metamaterial consisting of thousands of elements, the geometry of each element determined by an automated design process. The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss. Our experimental results indicate that this type of cloak should scale well toward optical wavelengths.

Design of arbitrarily shaped concentrators based on conformally optical transformation of nonuniform rational B-spline surfaces

We study the design of arbitrarily shaped electromagnetic (EM) concentrators and their potential applications. To obtain closed-form formulas of EM parameters for an arbitrarily shaped concentrator, we employ nonuniform rational B-spline (NURBS) to represent the geometrical boundary. Using the conformally optical transformation of NURBS surfaces, we propose the analytical design of arbitrarily shaped concentrators, which are composed of anisotropic and inhomogeneous metamaterials with closed-form constitutive tensors. The designed concentrators are numerically validated by full-wave simulations, which show perfectly directed EM behaviors. As one of the potential applications, we demonstrate a way to amplify plane waves using a rectangular concentrator, which is much more efficient and easier than the existing techniques. Using NURBS expands the generality of the transformation optics and could lead toward making a very general tool that would interface with commercial softwares such as 3D STUDIOMAX and MAYA.

Arbitrarily elliptical–cylindrical invisible cloaking

Based on the idea of coordinate transformation (Pendry, Schurig and Smith 2006 Science 312 1780), arbitrarily elliptical–cylindrical cloaks are proposed and designed. The elliptical cloak, which is composed of inhomogeneous anisotropic metamaterials in an elliptical-shell region, will deflect incoming electromagnetic (EM) waves and guide them to propagate around the inner elliptical region. Such EM waves will return to their original propagation directions without distorting the waves outside the elliptical cloak. General formulations of the inhomogeneous and anisotropic permittivity and permeability tensors are derived for arbitrarily elliptical axis ratio k, which can also be used for the circular cloak when k = 1. Hence the elliptical cloaks can make a large range of objects invisible, from round objects (when k approaches 1) to long and thin objects (when k is either very large or very small). We also show that the material parameters in elliptical cloaking are singular at only two points, instead of on the whole inner circle for circular cloaking, which are much easier to be realized in actual applications. Full-wave simulations are given to validate the arbitrarily elliptical cloaking.

Metamaterial polarizers by electric-field-coupled resonators

The idea of transmission polarizers realized by anisotropic metamaterials is demonstrated by experiments. One of the polarizers converts linear-polarized waves to circular-polarized waves, while the other polarizer converts linear-polarized waves from one polarization to its cross polarization. The excellent agreement between experimental results and theoretical predictions validates the functions and efficiency of the proposed polarizers.

Broadband gradient index microwave quasi-optical elements based on non-resonant metamaterials.

Utilizing non-resonant metamaterial elements, we demonstrate that complex gradient index optics can be constructed exhibiting low material losses and large frequency bandwidth. Although the range of structures is limited to those having only electric response, with an electric permittivity always equal to or greater than unity, there are still numerous metamaterial design possibilities enabled by leveraging the non-resonant elements. For example, a gradient, impedance matching layer can be added that drastically reduces the return loss of the optical elements due to reflection. In microwave experiments, we demonstrate the broadband design concepts with a gradient index lens and a beam-steering element, both of which are confirmed to operate over the entire X-band (roughly 8-12 GHz) frequency spectrum.

An efficient broadband metamaterial wave retarder

Metamaterials with anisotropic electromagnetic properties have the capability to manipulate the polarization states of electromagnetic waves. We describe a method to design a broadband, low-loss wave retarder with graded constitutive parameter distributions based on non-resonant metamaterial elements. A structured metamaterial half-wave retarder that converts one linear polarization to its cross polarization is designed and its performance is characterized experimentally.

Controlling electromagnetic waves using tunable gradient dielectric metamaterial lens

We propose a metamaterial particle which is composed of a dielectric block with a thin metallic rod screwed inside. By adjusting the height of rod inside the dielectric block, we can control the effective medium parameters of a periodic structure composed of the particles. An experiment is presented to retrieve the effective medium parameters, which have good agreements with those from simulation results. Using the unique property of the tunable particles, gradient metamaterial lenses are easily designed to deflect and focus the incident plane waves.

Anisotropic metamaterial devices

In the last few years, a rapid development has been achieved in a subject area, so called optical transformation, which is based on the property of metric invariance in Maxwells equations. Optical transformation, also known as transformation optics, allows metamaterials to be tailor-made according to practical needs. In this paper, we have reviewed the recent progress on the parametric design of transformation devices, such as invisibility cloaks, electromagnetic (EM) concentrator, EM-wave converter, etc. The technique of optical transformation can also be applied when the sources are included in the transformed space.

A SYMMETRICAL CIRCUIT MODEL DESCRIBING ALL KINDS OF CIRCUIT METAMATERIALS

We present a generally symmetrical circuit model to describe all kinds of metamaterials with effective permittivity and permeability. The model is composed of periodic structures whose unit cell is a general T-type circuit. Using the effective medium theory, we derive analytical formulations for the effective permittivity and effective permeability of the circuit model, which are quite different from the published formulas (1, 2). Rigorous study shows that such a generally symmetrical model can represent right-handed materials, left-handed materials, pure electric plasmas, pure magnetic plasmas, electric-type and magnetic-type crystal bandgap materials at different frequency regimes, with corresponding effective medium parameters. Circuit simulations of real periodic structures and theoretical results of effective medium models in this paper and in (1) and (2) are presented. The comparison of such results shows that the proposed medium model is much more accurate than the published medium model (1, 2) in the whole frequency band.

Negative Index Material Composed of Meander Line and Srrs

A compact meander-line resonator is proposed in this paper, which could provide negative permittivity with a small unit-towavelength ratio. The meander-line structure is simple to be designed and is convenient to be controlled. Negative index materials (NIM) are realized using units composed of meander lines and split-ring resonators (SRRs), which have simultaneously negative permittivity and permeability in a specified pass band with relatively low loss. Simulation results show the identified properties of the meander-line resonator and NIM.