arXiv subjects

George V. Eleftheriades

Publications and source records attributed to George V. Eleftheriades.

47 records · Page 3Linked to original sources

arXiv2017

Experimental Verification of Reflectionless Wide-Angle Refraction via a Bianisotropic Huygens' Metasurface

We report the design, fabrication, and characterization of bianisotropic Huygens' metasurfaces (BHMSs) for refraction of normally incident beams towards 71.8 degrees. As previously shown, all three BHMS degrees of freedom, namely, electric polarizability, magnetic polarizability and omega-type magnetoelectric coupling, are required to ensure no reflections occur for such wide-angle impedance mismatch. The unit cells are composed of three metallic layers, yielding a printed-circuit-board (PCB) structure. The fabricated BHMS is characterized in a quasi-optical setup, used to accurately assess specular reflections. Subsequently, the horn-illuminated BHMS' radiation pattern is measured in a far-field chamber, to evaluate the device's refraction characteristics. The measured results verify that the BHMS has negligible reflections, and the majority of the scattered power is coupled to the desirable Floquet-Bloch mode. To the best of our knowledge, this is the first experimental demonstration of such a reflectionless wide-angle refracting metasurface.

physics.optics↗

arXiv2016

Synthesis of passive lossless metasurfaces using auxiliary fields for reflectionless beam splitting and perfect reflection

We introduce a paradigm for accurate design of metasurfaces for intricate beam manipulation, implementing functionalities previously considered impossible to achieve with passive lossless elements. The key concept involves self-generation of auxiliary evanescent fields which facilitate the required local power conservation, without interfering with the device performance in the far field. We demonstrate our scheme by presenting exact reactive solutions to the challenging problems of reflectionless beam splitting and perfect reflection, verified via full wave simulations.

physics.optics↗

arXiv2016

Arbitrary power-conserving field transformations with passive lossless omega-type bianisotropic metasurfaces

We present a general theory for designing realistic omega-type bianisotropic metasurfaces (O-BMSs), unlocking their full potential for molding electromagnetic fields. These metasurfaces, characterized by electric surface impedance, magnetic surface admittance, and magnetoelectric coupling coefficient, were previously considered for wavefront manipulation. However, previous reports mainly considered plane-wave excitations, and implementations included cumbersome metallic features. In this work, we prove that any field transformation which locally conserves real power can be implemented via passive and lossless meta-atoms characterized by closed-form expressions; this allows rigorous incorporation of arbitrary source and scattering configurations. Subsequently, we show that O-BMS meta-atoms can be implemented using an asymmetric stack of three impedance sheets, an appealing structure for printed circuit board fabrication. Our formulation reveals that, as opposed to Huygens' metasurfaces (HMSs), which exhibit negligible magnetoelectric coupling, O-BMSs are not limited to controlling the phase of transmitted fields, but can rather achieve high level of control over the amplitude and phase of reflected fields. This is demonstrated by designing O-BMSs for reflectionless wide-angle refraction, independent surface-wave guiding, and a highly-directive low-profile antenna, verified with full-wave simulations. This straightforward methodology facilitates development of O-BMS-based devices for controlling the near and far fields of arbitrary sources in complex scattering configurations.

physics.optics↗

arXiv2016

Pencil-Beam Single-point-fed Dirac Leaky-Wave Antenna on a Transmission-Line Grid

Leaky-wave antennas (LWAs) are widely used as single-point-fed linear antenna arrays. The extension of LWAs to 2D implies that they can be used as single-point-fed 2D antenna arrays without requiring a complex feeding network. However, generating a pencil beam from 2D LWAs is not straightforward and due care has to be taken for the design of the LWA. On the other hand, transmission-line (TL) grids have demonstrated interesting behaviors, such as an effective negative refractive index and growing of evanescent waves. In this paper, a singlepoint-fed TL-grid 2D Dirac leaky-wave antenna (DLWA) design is proposed that generates a pencil beam at both broadside and slightly tilted angles. The TL-grid unit cell is analytically treated in light of its scattering and impedance matrices. The optimized TL-grid unit cell is shown to exhibit a closed bandgap in the dispersion relation which is also linearly varying with frequency (hence it is a DLWA). The proposed 2D DLWA design is fabricated and the experimental results are presented.

physics.class-ph↗

arXiv2015

Cavity-excited Huygens' metasurface antennas: near-unity aperture efficiency from arbitrarily-large apertures

One of the long-standing problems in antenna engineering is the realization of highly-directive beams using low-profile devices. In this paper we provide a solution to this problem by means of Huygens' metasurfaces (HMSs), based on the equivalence principle. This principle states that a given excitation can be transformed to a desirable aperture field by inducing suitable electric and magnetic surface currents. Building on this concept, we propose and demonstrate cavity-excited HMS antennas, where the single-source cavity excitation is designed to optimize aperture illumination, while the HMS facilitates the current distribution that ensures phase purity of aperture fields. The HMS breaks the coupling between the excitation and radiation spectrum typical to standard partially-reflecting surfaces, allowing tailoring of the aperture properties to produce a desirable radiation pattern. As shown, a single semianalytical formalism can be followed to achieve control of a variety of radiation features, such as the direction of the main beam or the side lobe level, by proper modification of the HMS and the source position. Relying on a cavity excitation, this can be achieved without incurring edge-taper losses and without any degradation of the aperture illumination for arbitrarily-large apertures. With the recent demonstrations of Huygens' metasurfaces at microwave, terahertz, and optical frequencies, the proposed low-profile design may find its use in a myriad of applications across the electromagnetic spectrum, from highly-directive antennas to highly-efficient quantum-dot emitters, reaching near-unity aperture efficiencies.

physics.optics↗

arXiv2014

Passive Lossless Huygens Metasurfaces for Conversion of Arbitrary Source Field to Directive Radiation

We present a semi-analytical formulation of the interaction between a given source field and a scalar Huygens metasurface (HMS), a recently introduced promising concept for wavefront manipulation based on a sheet of orthogonal electric and magnetic dipoles. Utilizing the equivalent surface impedance representation of these metasurfaces, we establish that an arbitrary source field can be converted into directive radiation via a passive lossless HMS if two physical conditions are met: local power conservation and local impedance equalization. Expressing the fields via their plane-wave spectrum and harnessing the slowly-varying envelope approximation we obtain semi-analytical formulae for the scattered fields, and prescribe the surface reactance required for the metasurface implementation. The resultant design procedure indicates that the local impedance equalization induces a Fresnel-like reflection, while local power conservation forms a radiating virtual aperture which follows the total excitation field magnitude. The semi-analytical predictions are verified by finite-element simulations of HMSs designed for different source configurations. Besides serving as a flexible design procedure for HMS radiators, the proposed formulation also provides a robust mechanism to incorporate a variety of source configurations into general HMS models, as well as physical insight on the conditions enabling purely reactive implementation of this novel type of metasurfaces.

physics.optics↗

arXiv2014

Design of Tensor Impedance Transmitarrays For Polarization Control

Tensor impedance transmitarrays consist of layers of tensor impedance surfaces, separated by dielectric spacers. Because the surface impedance of each layer is a tensor, an arbitrarily polarized incident field is scattered into its constituent TE and TM components. This gives rise to transmitarrays capable of altering the polarization state of an incident field. Various tensor impedance transmitarray have been proposed in the literature to alter the polarization, however no comprehensive methodology has been proposed to design these structures. In this work we propose a procedure for designing tensor impedance transmitarrays using multi-conductor transmissionline (MTL) theory. We treat the transmitarray by modelling free-space and the dielectric spacers as an MTL supporting TE and TM modes with each tensor impedance surface as a shunt load along the MTL. By using simple MTL concepts we can design a transmitarray to be reflectionless while controlling the transmission through the layers and thus the transmitted polarization state. We demonstrate this procedure for two classes of tensor impedance transmitarrays while also validating the design using full-wave simulation. The first class are symmetric transmitarrays which can alter a given incident polarization state into a desired polarization state. The second class are asymmetric transmitarrays which can also implement chiral polarization effects such as a linear polarization rotation and a circular polarization selectivity.

physics.optics↗

arXiv2013

Discontinuous Electromagnetic Fields Using Huygens Sources 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 electric and magnetic currents which act like a 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 two novel 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 using phase discontinuities.

physics.optics↗

arXiv2012

Active Cloaking

Electromagnetic cloaking refers to the ability to prevent an object from scattering an incident electromagnetic field. This has been accomplished in recent works by routing the incident field around the object or by changing the scattering properties of the object itself through specially designed materials, surfaces or guiding structures. In this letter, we introduce a new way of cancelling the electromagnetic scattering of an object by using an array of sources. 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 cancelled. These magnetic and electric surface currents can be discretized into electric and magnetic dipoles which are physically implementable by straight and loop wire antennas. Finally we confirm our results using numerical simulations for cloaking a dielectric and a metallic cylinder by means of a thin array of such wire antennas.

physics.class-ph↗

arXiv2008

Two-dimensional subwavelength focusing using a slotted meta-screen

A transmission screen with subwavelength slots is described which can focus electromagnetic radiation to a two-dimensional subwavelength spot. Unlike negative-refractive-index focusing implementations, this "meta-screen" does not suffer from image degradation when losses are introduced and is easily scalable from microwave to Terahertz frequencies and beyond. The slotted geometry is designed using a theory of shifted beams to determine the necessary weighting factors for each slot element, which are then converted to appropriate slot dimensions. The screen was designed using a pair of rotated orthogonal focusing axes to produce a spot on a focal plane located 0.25 λaway. Full-wave simulations are corroborated by experimental measurements at 10 GHz reporting a full-width half-maximum beam width of 0.27 λalong the x-axis and 0.38 λalong the z-axis.

physics.optics↗

arXiv2008

A spatially shifted beam approach to subwavelength focusing

Although negative-refractive-index metamaterials have successfully achieved subwavelength focusing, image resolution is limited by the presence of losses. In this Letter, a metal transmission screen with subwavelength spaced slots is proposed that focuses the near-field beyond the diffraction limit and furthermore, is easily scaled from microwave frequencies to the optical regime. An analytical model based on the superposition of shifted beam patterns is developed that agrees very well with full-wave simulations and is corroborated by experimental results at microwave frequencies.

physics.optics↗