Michael Selvanayagam

Discontinuous electromagnetic fields using orthogonal electric and magnetic currents for wavefront manipulation

We introduce the idea of discontinuous electric and magnetic fields at a boundary to design and shape wavefronts in an arbitrary manner. To create this discontinuity in the field we use orthogonal electric and magnetic currents which act like Huygens source to radiate the desired wavefront. These currents can be synthesized either by an array of electric and magnetic dipoles or by a combined impedance and admittance surface. A dipole array is an active implementation to impose discontinuous fields while the impedance/admittance surface acts as a passive one. We then expand on our previous work showing how electric and magnetic dipole arrays can be used to cloak an object demonstrating novel cloaking and anti-cloaking schemes. We also show how to arbitrarily refract a beam using a set of impedance and admittance surfaces. Refraction using the idea of discontinuous fields is shown to be a more general case of refraction than using simple phase discontinuities.

An Active Electromagnetic Cloak Using the Equivalence Principle

Electromagnetic cloaking refers to the ability to prevent an object from scattering an incident electromagnetic field. This has been accomplished in recent works by specially designed materials. Another way of cloaking using sources has been known in the acoustics community. In this letter, we introduce a prescription for canceling the electromagnetic scattering of an object by using an array of sources. By using the equivalence principle, we show that by superimposing magnetic and electric surface current densities at the boundary of an object, the scattered fields from that object can be canceled. These magnetic and electric surface currents can be discretized into electric and magnetic dipoles that are physically implementable by straight and loop wire antennas. Finally, we confirm our results using numerical simulations.

Polarization Control Using Tensor Huygens Surfaces

Controlling the polarization of the electromagnetic field is a crucial aspect of many communication and imaging systems. Building off of previous work on Huygens surfaces, which are surfaces used to manipulate wavefronts using electric and magnetic surface impedances, the concept of a tensor Huygens surface is introduced. A tensor Huygens surface consists of electric and magnetic surface impedances which are tensors as opposed to scalars. In this paper we show how the polarization can be modified from a given initial state to a desired state specifically using a tensor Huygens surface. We examine the fields at the surface to find the required impedance tensors to manipulate the polarization. We also come up with an equivalent circuit model which captures the behavior of the surface as well as its equivalent S-parameter representation. Furthermore, we examine how chiral functionality can be realized using two cascaded Huygens surfaces. We validate these results using a TLM solver as well. Finally, we demonstrate a possible implementation of a tensor Huygens surface made up of skewed dipoles and loops.

Circuit Modeling of Huygens Surfaces

Huygens surfaces are a recently proposed way of manipulating electromagnetic wavefronts using a superposition of subwavelength electric and magnetic dipoles situated on a plane. This allows for interesting phenomena such as refraction, beam manipulation, and focusing using this planar screen. In this letter, a circuit model for the unit cells comprising a Huygens surface is proposed to further understand the functionality of this screen. The equivalent circuit is a lattice network whose constituent series and shunt impedances correspond to the impedances of the magnetic and electric dipoles, respectively. We demonstrate that the corresponding voltage and current terminal relations across the lattice network correspond exactly to the electromagnetic boundary conditions across the Huygens surfaces. We also show how this model can be used to design the required unit cells and, using a two-dimensional circuit solver, how these lattice cells can be used to model an entire Huygens surface.

A Compact Printed Antenna With an Embedded Double-Tuned Metamaterial Matching Network

A compact antenna intended for use in a laptop computer is proposed with the antenna consisting of a simple radiating strip and a matching network based on a metamaterial particle. The matching network is used to increase the -10 dB bandwidth of the antenna. Double-tuned matching network theory is used to increase the bandwidth of the antenna by forming a loop on the Smith Chart inside a given VSWR circle. The matching network is implemented using a complementary-split-ring-resonator (CSRR) microstrip network to act as a shunt LC network. This is confirmed by means of a circuit model to model the CSRR-microstrip network. Finally the antenna is fabricated and tested with the measured and simulated results showing good agreement. The measured antenna has 560 MHz of bandwidth centered at 2.54 GHz with an efficiency of 89%.

Transforming Electromagnetics Using Metamaterials

Electromagnetic metamaterials are artificially structured media with unusual electromagnetic properties that can be engineered from the radio-frequency (RF) and microwave range all the way up to optical frequencies. In its present form, the field of metamaterials is just over ten years old but has already attracted intense interest from many research groups around the globe. Suddenly, classical electromagnetism took on a fresh and exciting perspective, revealing that there are fascinating phenomena still waiting to be discovered and corresponding applications to be invented. In particular, all this excitement is associated with the notion of the macroscopic constitutive parameters, such as the permittivity and permeability. What would be possible if we were able to synthesize electromagnetic materials with arbitrarily valued constitutive parameters? The richness of this possibility becomes more evident when we recall that material parameters can be anisotropic (varying with direction) or spatially inhomogeneous (varying from point to point). Moreover, they can attain values previously not considered (i.e., negative or close to zero), and they can even mix together the electric and magnetic response of a material (chirality).

Polarization Considerations for Scalar Huygens Metasurfaces and Characterization for 2-D Refraction

Transmitarrays and transmissive metasurfaces must efficiently couple incident power to the transmitted beam. Inefficiency is manifested in sidelobe levels, reflections, and insertion loss. The Huygens metasurface embodies the Huygens and equivalence principles, suppressing these sidelobe levels and reflections. This is accomplished with a single thin layer of Huygens sources, which contains both an electric and a magnetic response. In this paper, 2-D interfacial refraction is implemented with a scalar Huygens metasurface. The measured total efficiency is on average 71.06% for a 70° range, the maximum being 80.87% at θi=0°. Moreover, it is on average 68.64% over a fractional bandwidth of 8%, the maximum being 80.87% at 10.0 GHz. This demonstrates that the insertion loss, reflection, and sidelobe powers are low in our design. Furthermore, the questions of polarization purity, and the appropriate polarization definition for a scalar Huygens metasurface, are addressed. Our design contains only printed elements, and consists of two bonded boards, instead of many stacked interspaced layers. This simplifies fabrication, and makes it scalable to millimeter-wave frequencies and beyond. The design is also λ/9.3 thick, in contrast to traditional transmitarrays, which require 3-4 λ/4 spaced layers to obtain the same degree of phase control and matching.

Transmission-Line Metamaterials on a Skewed Lattice for Transformation Electromagnetics

We propose a lattice of skewed transmission lines to implement a full effective material tensor. First we show, using transformation electromagnetics, how a skewed lattice will introduce off-diagonal components in the material tensor. We then show how this can be extended to a transmission-line network placed on a skewed lattice, using periodic analysis of a 2-D transmission-line network. We also show how to design a 2-D transmission-line unit cell to implement a full-material tensor for 2-D propagation. We confirm our results using full-wave simulation of a unit cell, as well as full-wave simulation of refraction in a transmission-line network between an isotropic effective medium and a anisotropic effective medium. Finally, we show how this idea can be extended to 3-D unit cells for transformation electromagnetics applications.

A thin printed metasurface for microwave refraction

This paper presents a novel thin metasurface for microwave refraction. The metasurface is based on the new concept of establishing orthogonal electric and magnetic currents (Huygens sources) on a surface thus physically implementing the equivalence principle. Such thin Huygens Metasurfaces can be used for refraction, focusing and general beam shaping. Here we discuss the design procedure and experimental validation of such a metasurface for 1D refraction. In contrast to previous work, the proposed metasurface does not require stacked layers, but it comprises collocated electric and magnetic dipoles printed on a single panel. This can be represented in one layer of lattice cells, and its behavior described using transmission-line theory. This makes it electrically thin and readily scalable to millimeter-wave frequencies and beyond.

Design And Measurement of Tensor Impedance Transmitarrays For Chiral Polarization Control

Tensor impedance transmitarrays consist of tensor impedance surfaces, separated by dielectric spacers. In this paper, we present a design method for realizing tensor impedance transmitarrays that are capable of controlling the reflection and transmission of circularly polarized waves, referred to as chiral polarization control. To achieve this, we implement a multi-conductor transmission-line (MTL) model to characterize how vertically and horizontally polarized waves are transmitted and reflected through the surface and to synthesize the desired tensor impedances, which comprise the array. Using this MTL model, we show how to synthesize two different chiral transmitarrays. This includes a polarization rotator, which rotates any linear polarization by 90 ° and a circular polarization selective surface (CPSS), which transmits one hand of circular polarization while reflecting the other. This is verified with full-wave simulation showing that our proposed model works. We also fabricate and measure the CPSS at X-band based on our design procedure. To measure the CPSS, we use a novel four-port quasi-optical system to characterize both the reflection and transmission of vertically and horizontally polarized fields off of the surface. We achieve a good agreement with our simulated results though we have -1.7 dB more loss than expected due to the use of FR-4 as our substrate.