arXiv subjects

George V. Eleftheriades

Publications and source records attributed to George V. Eleftheriades.

At least 37 records · Page 2Linked to original sources

arXiv2021

Multi-functional Metasurface: Visibly and RF Transparent, NIR Control and Low Thermal Emissivity

Rapid advances in metamaterial technology are enabling the engineering of wave-matter interactions heretofore not realized and functionalities with potentially far-reaching implications for major challenges in the fields of energy conservation and radio frequency (RF) communication. We propose a visibly and RF transparent composite metasurface utilizing dielectric-metal spectrally selective coatings with high NIR control and low thermal emissivity, thus achieving a multi-functional metasurface capable of enhancing 5G communication efficiency and exhibiting energy conservation features. The proposed meta-glass yields 92% peak RF transmission at 30 GHz which corresponds to 20% and 90% enhancement when compared to plain glass and low-emissive glass substrates. This meta-glass possesses 86% peak optical transparency at $λ=550 nm$, $>$60% near-IR reflection, and $>$ 80% mid-IR reflection which corresponds to $\approx$ 0.2 thermal emissivity. The proposed metasurface design is highly flexible and can be tuned to operate over different frequency ranges owing to its frequency scalability. This study provides a better alternative using earth-abundant materials compared to low-emissive glass based on indium tin oxide (ITO) while boosting the efficiency of 5G communication amenable to window systems demanding simultaneous functionalities for emergent smart/energy-efficient buildings/cities and autonomous transportation applications.

physics.app-ph↗

arXiv2020

Near-Field Angular Scan Enhancement of Antenna Arrays Using Metasurfaces

In this publication we continue our previous work on extending the angular scan range of phased arrays using metasurfaces. We consider in detail scan enhancement using a single metasurface lens in the near-field of a source and provide analytical expressions for source excitation to obtain a desired beam. We show that such a device suffers from the same directivity degradation as in the case of far-field lens placement. Moreover, we discuss in detail that this approach has limitations, and one cannot place the lens arbitrarily close to the source array. We then propose a two-lens near-field scan enhancer which is not subject to the same limitations, although still subject to the same directivity degradation. Finally we propose a binary metasurface concept which theoretically achieves desired scan enhancement without directivity degradation. Some theoretical claims are then verified via full-field simulations.

physics.app-ph↗

arXiv2020

Approach to the Analysis and Synthesis of Cylindrical Metasurfaces with Non-circular Cross Sections Based on Conformal Transformations

We present methods for analyzing and designing cylindrical electromagnetic metasurfaces with non-circular cross sections based on conformal transformations. It can be difficult to treat surfaces with non-canonical geometries since they generally do not admit straightforward solutions to the Helmholtz wave equation subject to the appropriate boundary conditions. This leads to the reliance on full wave numerical techniques which are only suitable for the analysis, but not the synthesis, of these surfaces. We address this issue by employing conformal transformations to map the physical space into a computational space in which the surface coincides with a circular cylinder. The electromagnetic boundary conditions on the surface remain intact under the transformations due to their angle-preserving nature. However, they are much more easily enforced. As a result, analytical modal solutions for the scattered fields are readily obtainable, which facilitate closed-form analysis and synthesis equations for general non-circular cylindrical metasurfaces. One important utility enabled by the proposed framework is the efficient identification of electromagnetic field distributions that satisfy local power conservation. This leads to passive and lossless surface designs, which are highly desirable in practice as they do not require active and/or lossy components.

physics.optics↗

arXiv2020

A Discrete Fourier Transform-Based Framework for Analysis and Synthesis of Cylindrical Omega-bianisotropic Metasurfaces

This paper presents a framework for analyzing and designing cylindrical omega-bianisotropic metasurfaces, inspired by mode matching and digital signal processing techniques. Using the discrete Fourier transform, we decompose the the electromagnetic field distributions into orthogonal cylindrical modes and convert the azimuthally varying metasurface constituent parameters into their respective spectra. Then, by invoking appropriate boundary conditions, we set up systems of algebraic equations which can be rearranged to either predict the scattered fields of prespecified metasurfaces, or to synthesize metasurfaces which support arbitrarily stipulated field transformations. The proposed framework facilitates the efficient evaluation of field distributions that satisfy local power conservation, which is one of the key difficulties involved with the design of passive and lossless scalar metasurfaces. It represents a promising solution to circumvent the need for active components, controlled power dissipation, or tensorial surface polarizabilities in many state-of-the art conformal metasurface-based devices. To demonstrate the robustness and the versatility of the proposed technique, we design several devices intended for different applications and numerically verify them using finite element simulations.

physics.class-ph↗

arXiv2020

Four-Dimensional Wave Transformations By Space-Time Metasurfaces

Static metasurfaces have shown to be prominent compact structures for reciprocal and frequency-invariant transformation of electromagnetic waves in space. However, incorporating temporal variation to static metasurfaces would result in dynamic apparatuses which are capable of four-dimensional tailoring of both the spatial and temporal characteristics of electromagnetic waves, leading to functionalities that are far beyond the capabilities of conventional static metasurfaces. This includes nonreciprocal full-duplex wave transmission, pure frequency conversion, parametric wave amplification, spatiotemporal decomposition, and space-time wave diffraction. This paper reviews recent progress and opportunities offered by space-time metasurfaces to break reciprocity, revealing their potential for low-energy, compact, integrated non-reciprocal devices and sub-systems, and discusses the future of this exciting research front.

physics.app-ph↗

arXiv2020

Space-Time Medium Functions as a Perfect Antenna-Mixer-Amplifier Transceiver

We show that a space-time-varying medium can function as a front-end transceiver, i.e., an antenna-mixer-amplifier. Such a unique functionality is endowed by space-time surface waves associated with complex space-time wave vectors in a subluminal space-time medium. The proposed structure introduces pure frequency up- and down-conversions and with very weak undesired time harmonics. In contrast to other recently proposed space-time mixers, a large frequency up-/down conversion ratio, associated with gain is achievable. Furthermore, as the structure does not operate based on progressive energy transition between the space-time modulation and the incident wave, it possesses a subwavelength thickness (metasurface). Such a multi-functional, highly efficient and compact medium is expected to find various applications in modern wireless telecommunication systems.

physics.app-ph↗

arXiv2020

Design of Compact Huygens' Metasurface Pairs with Multiple Reflections for Arbitrary Wave Transformations

Huygens' metasurfaces have demonstrated a remarkable potential to perform wave transformations within a subwavelength region. In particular, omega-bianisotropic Huygens' metasurfaces have allowed for the passive implementation of any wave transformation that conserves real power locally. Previous reports have also shown that Huygens' metasurface pairs are capable of realizing transformations that break the local power conservation requirement by redistributing the total power, while the wave propagates between the two metasurfaces. However, the required separation distance overshadows the low-profile characteristics of the individual metasurfaces and leads to bulky designs, especially for lower frequencies. In this work, we develop a method of designing omega-bianisotropic Huygens' metasurface pairs, relying on a point-matching process of the real power at the two metasurfaces. We highlight the versatility of our method by presenting two variations of the configuration, depending on whether the electromagnetic source is located within or outside the metasurface pair. Based on the examples of a cylindrical-wave to plane-wave transformation and a beam expander, we examine the impact of multiple reflections, as a way to overcome the size limitations and design compact structures. Moreover, we explore possible beamforming applications through an example of a Taylor-pattern antenna with a single feed-point between the two metasurfaces.

physics.app-ph↗

arXiv2020

Theory and Simulation of Metasurface Lenses for Extending the Angular Scan Range of Phased Arrays

Recent metasurface developments have led to their consideration for extending the angular scan range of phased arrays. This paper considers the use of Huygens' metasurfaces as lenses to accomplish the task. Ray optics is used to uncover how a single lens can extend the scan range. It is shown that a single scan extending lens leads to a broadside directivity degradation of the original beam. This directivity degradation is quantitatively characterized as a function of the desired angular scan-range expansion. A method of simulating phase boundaries is presented and used to verify the theoretical claims. A Huygens' metasurface lens design is presented and simulated to further validate the theoretical predictions and show that such lenses are physically realizable.

physics.app-ph↗

arXiv2020

Design and experimental demonstration of impedance-matched circular polarization selective surfaces with spin-selective phase modulations

This paper presents the design and experimental demonstration of an impedance-matched circular polarization selective surface which also offers spin-selective phase modulations at microwave frequencies. We achieve this by leveraging the theory of Pancharatnam-Berry phase and cascading four tensor impedance layers, each comprising an array of crossed meander lines. These meander lines are precisely tuned and rotated to implement particular tensor surface impedance values to satisfy the impedance-matching condition for the transmitted right-handed circularly-polarized field while inducing Pancharatnam-Berry phase shift for the reflected left-handed circularly-polarized field. We present a detailed numerical synthesis technique to obtain the required impedance values for satisfying the impedance matching condition, and demonstrate spin-selective phase modulations based on Pancharatnam-Berry phase shifts. To verify the proposed idea, we experimentally demonstrate nearly-reflectionless transmission of right-handed circular polarization at broadside and reflection of left-handed circular polarization at 30 degrees off broadside at 12 GHz. For this purpose, a free-space quasi-optical set up and a near-field measurement system are respectively employed for measuring the transmitted and reflected circularly-polarized fields.

physics.app-ph↗

arXiv2019

Full-Duplex Nonreciprocal-Beam-Steering Metasurfaces Comprising Time-Modulated Twin Meta-Atoms

We present the concept, theoretical model and experimental implementation of a full-duplex nonreciprocal-beam-steering transmissive phase-gradient metasurface. Such a metasurface is realized by exploiting the unique properties of the frequency-phase transition in coupled time-modulated twin meta-atoms. The metasurface may be placed on top of a source antenna to transform the radiation pattern of the source antenna, and introduce different radiation patterns for the transmit and receive states. In contrast to the recently proposed applications of time modulation, here the incident and transmitted waves share the same frequency. The metasurface is endowed with directive, diverse and asymmetric transmission and reception radiation beams, and tunable beam shapes. Furthermore, these beams can be steered by simply changing the modulation phase. The proposed coupled meta-atoms inherently suppress undesired time harmonics, leading to a high conversion efficiency which is of paramount importance for practical applications such as point to point full-duplex communications.

physics.app-ph↗

arXiv2019

Generalized Space-Time Periodic Diffraction Gratings: Theory and Applications

This paper studies the theory and applications of the diffraction of electromagnetic waves by space-time periodic (STP) diffraction gratings. We show that, in contrast with conventional spatially periodic grating, a STP diffraction grating produces spatial diffraction orders, each of which is formed by an infinite set of temporal diffraction orders. Such spatiotemporally periodic gratings are endowed with enhanced functionalities and exotic characteristics, such as asymmetric diffraction pattern, nonreciprocal and asymmetric transmission and reflection, and an enhanced diffraction efficiency. The theory of the wave diffraction by STP gratings is formulated through satisfying the conservation of both momentum and energy, and rigorous Floquet mode analysis. Furthermore, the theoretical analysis of the structure is supported by time and frequency domain FDTD numerical simulations for both transmissive and reflective STP diffraction gratings. Additionally, we provide the conditions for Bragg and Raman-Nath diffraction regimes for STP gratings. Finally, as a particular example of a practical application of the STP diffraction gratings to communication systems, we propose an original multiple access communication system featuring full-duplex operation. STP diffraction gratings are expected to find exotic practical applications in communication systems, especially for the realization of enhanced-efficiency or full-duplex beam coders, nonreciprocal beam splitters, nonreciprocal and enhanced-resolution holograms, and illusion cloaks.

physics.app-ph↗

arXiv2018

Theory, design, and experimental verification of a reflectionless bianisotropic Huygens' metasurface for wide-angle refraction

Huygens' metasurfaces are electrically thin devices which allow arbitrary field transformations. Beam refraction is among the first demonstrations of realized metasurfaces. As previously shown for extreme-angle refraction, control over only the electric impedance and magnetic admittance of the Huygens' metasurface proved insufficient to produce the desired reflectionless field transformation. To maintain zero reflections for wide refraction angles, magnetoelectric coupling between the electric and magnetic response of the metasurface, leading to bianisotropy, can be introduced. In this paper, we report the theory, design, and experimental characterization of a reflectionless bianisotropic metasurface for extreme-angle refraction of a normally incident plane wave towards 71.8$^\circ$ at 20 GHz. The theory and design of three-layer asymmetric bianisotropic unit cells are discussed. The realized printed circuit board structure was tested via full-wave simulations as well as experimental characterization. To experimentally verify the prototype, two setups were used. A quasi-optical experiment was conducted to assess the specular reflections of the metasurface, while a far-field antenna measurement characterized its refraction nature. The measurements verify that the fabricated metasurface has negligible reflections and the majority of the scattered power is refracted to the desired Floquet mode. This provides an experimental demonstration of a reflectionless wide-angle refracting metasurface using a bianisotropic Huygens' metasurface at microwave frequencies.

physics.app-ph↗

arXiv2018

A Leaky-Wave Antenna With Controlled Radiation Using a Bianisotropic Huygens' Metasurface

In this paper, a novel concept of a leaky-wave antenna is proposed, based on the use of Huygens' metasurfaces. It consists of a parallel-plate waveguide in which the top plate is replaced by a bianisotropic metasurface of the Omega type. It is shown that there is an exact solution to transform the guided mode into a leaky-mode with arbitrary control of the constant leakage factor and the pointing direction. Although the solution turns out to be periodic, only one Floquet mode is excited and radiates, even for electrically long periods. Thanks to the intrinsic spurious Floquet mode suppression, broadside radiation can be achieved without any degradation. Simulations with idealized reactance sheets verify the concept. Moreover, physical structures compatible with PCB fabrication have been proposed and designed, considering aspects such as the effect of losses. Finally, experimental results of two prototypes are presented and discussed.

physics.app-ph↗

arXiv2018

Active Huygens' Box: Metasurface-Enabled Arbitrary Electromagnetic Wave Generation Inside a Cavity

This work investigates the generation of arbitrary electromagnetic waveforms inside a geometrical area enclosed by an active metasurface. We introduce the contraption of the Huygens' box, where a region of space is enclosed by an active Huygens' metasurface. We show that, upon generating the necessary electric and magnetic currents, we can create any desired electromagnetic field inside the Huygens' box. Using this method, we demonstrate through simulation and experiment the generation of travelling plane waves, a standing plane wave and a Bessel wave inside a metallic cavity. By linear superposition of these unconventional "modes", we experimentally demonstrate, for the first time, a subwavelength superoscillation focal spot formed without involving evanescent EM waves, and without an accompanying region of exorbitantly high waveform energy. The Huygens' box brings controlled waveform generation to an unprecedented level, with far-reaching implications to imaging, communication and medical therapy.

physics.app-ph↗

arXiv2018

Magneto-electric Uniaxial Metamaterials as Wide-angle, Polarization-insensitive Matching Layers

Antireflection or impedance matching is a topic that has been extensively researched by the optical and microwave communities over the past century and until today. However, due to the diverging wave impedances of TE (s) and TM (p) polarizations with increasing incident angle, it is impossible to achieve perfect matching simultaneously for both polarizations at an oblique incident angle with a single conventional matching layer. In this paper, we show that simultaneous perfecting matching of an arbitrary substrate at an arbitrary incident angle from air is possible with a single uniaxial metamaterial matching layer (UMML) that possesses specific permittivity and permeability tensors. We demonstrate this concept by synthesizing a physical UMML to match a high-index (εr= 10.2) substrate at 45°. The required permittivity and permittivity tensors of the UMML are achieved through the judicious control of the tangential and longitudinal electric and magnetic responses. Around -30 dB reflections are observed for both polarizations at the design angle. Building on the UMML concept, we design a novel radome in microwave regime with a near omni-directional matching performance. For incident angles from 0°-85°, reflections remain below -14 dB for both polarizations. To our best knowledge, this is the widest angular range that has been achieved while maintaining less than 5% power reflection for both polarizations. The proposed UMML concept offers a new paradigm for designing high-performance microwave, terahertz, and optical devices.

physics.app-ph↗

arXiv2018

A Passive Re-Directing Van Atta Type Reflector

The Van Atta retro-reflector can be envisioned as a self-phasing antenna array; a phase gradient, naturally induced by an external incident wave, is inverted so the device re-radiates back towards the incident direction. This Letter demonstrates how to re-direct the re-radiated beam through passive alteration of this phase gradient. For this purpose, crosspropagating isolation is required between the incident and reradiated signal paths. To this end, polarization duplexing can be used to achieve this isolation with a passive and reciprocal structure. To provide further demonstration, a 4-element redirective array is designed and fabricated to offset its re-radiated beam by -15° from incidence. Performance is then verified using fullwave electromagnetic simulation and experimental radiation pattern measurements. It is found that the -15° angular offset is effectively retained over a wide range of incidence angles.

physics.app-ph↗

arXiv2018

Perfect Anomalous Reflection with a Binary Huygens' Metasurface

In this paper we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly non-trivial due to the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces which achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures which allow them to couple to and from leaky and/or evanescent waves. In contrast, we demonstrate that using a Binary Huygens' Metasurface (BHM) --- a passive and lossless metasurface with only two cells per period --- perfect anomalous reflection can be achieved over a wide angular and frequency range. Through simulations and experiments at 24 GHz, we show that a properly designed BHM can anomalously reflect an incident electromagnetic wave from $θ_i = 50^\circ$ to $θ_r = -22.5^\circ$, with perfect power efficiency to within experimental precision.

physics.app-ph↗

arXiv2017

Perfect Anomalous Reflection with an Aggressively Discretized Huygens' Metasurface

This paper investigates the discretization of a periodic metasurface and demonstrates how such a surface can achieve perfect anomalous reflection. Whilst most contemporary theoretical works on metasurfaces deal with continuous current or impedance distributions, we examine how discretization affects a metasurface, and show that in some cases one can discretize a metasurface aggressively --- to the extent of having only two cells per spatial period. Such aggressive discretization can lead to great simplifications in metasurface design, and perhaps more surprisingly, a possible performance improvement from continuous metasurfaces. Using this aggressive discretization technique, we report the design of a binary Huygens' metasurface which reflects an incident plane wave at 50$^\circ$ into a reflected direction of -22.5$^\circ$. Full-wave electromagnetic simulation shows the achievement of anomalous reflection with a power efficiency of 99.1%, which dramatically surpasses the performance of a corresponding passive continuous metasurface, for which the power efficiency is fundamentally limited to 69.6%.

physics.app-ph↗