Journal Publications

39. Younes Ra’di and Andrea Alù, Metagratings for Efficient Wavefront Manipulation, IEEE Photonics Journal 14, 2207513 (2021).

Abstract: Recently, it was revealed that conventional gradient metasurfaces are fundamentally limited on their overall efficiency to reroute the impinging waves towards arbitrary directions in reflection and transmission. Their efficiency is particularly limited for extreme wavefront transformations. In addition, due to the fastly varying impedance profiles that these surfaces require, they usually need high-resolution fabrication processes, limiting their applicability and overall bandwidth of operation. To address these issues, the concept of metagrating was recently introduced, enabling engineered surfaces capable of manipulating light with unitary efficiency even in the limit of extreme wavefront manipulation. Metagratings are periodic or locally periodic arrays operated in the few-diffraction order regime. Their unit cell contains carefully designed scatterers that, in contrast with conventional metasurfaces, do not need to support a continuous gradient of surface impedance, and consequently are far simpler to fabricate and can afford broader bandwidths because of the larger footprint of the constituent elements. Tailoring the coupling towards the few available diffraction channels, metagratings enable wavefront transformations with high efficiency. In this paper, we review the physical mechanisms behind the functionality of metagratings and provide an overview and outlook of the recent progress in this field of science and technology.

38. Shahab Ramezanpour, Andrey Bogdanov, Andrea Alù, and Younes Ra’di, Generalization of Exceptional Point Conditions in Perturbed Coupled Resonators, Phys. Rev. B 104, 205405 (2021).

Abstract: The phase singularity in open systems, known as the exceptional point (EP), has revealed exotic functionalities, especially in optics—as an illustration, ultrasensitive sensors and laser beam unidirectionality. The strong sensitivity to perturbations around the EP has been suggested for sensing applications. Nevertheless, the characteristics of such highly sensitive systems can be affected by unwanted perturbations during the fabrication process. However, if one can control perturbation, it can be considered as an additional degree of freedom to create and tune EPs, enabling fascinating phenomena. In this paper, we propose an analytical method to investigate such systems. We analytically derive the general conditions of EPs in the perturbed pair of coupled ring resonators, where both resonators can be perturbed by different scatterers. Several numerical examples are employed to verify the proposed analytical method. We propose a simple experimental scheme where the predicted effects can be confirmed. It is also shown that by changing the relative position of the scatterers with respect to each other, quite interesting states such as a chiral EP in one resonator or simultaneous chiral EPs in both resonators could be observed, making such a system a highly functional tunable device which can have several applications such as quantized reflection/transmission and q -bits.

37. Diego J. Chachayma-Farfan, Younes Ra’di, and Andrea Alù, Dual-Layer Radio-Transparent Dielectric Core Metasurface Antenna, IEEE Open Journal of Antennas and Propagation 2, 585 (2021).

Abstract: Recent advances in communication systems require densely packed integrated antennas, making the system subject to undesired interferences. Cloaking and scattering cancellation using metamaterials have been proposed as interesting routes towards the suppression of antenna blockage and interference in these dense arrays. Although a few attempts have been made in this direction, finding an optimal design that can suppress the mutual scattering and at the same time preserve radiation features remains a challenge. We explore here an approach that enhances the operational bandwidth and improves angular stability of antenna cloaks, realizing effective radio-transparent antennas for communication systems. Recently, we explored the idea of using a transparent dielectric core metasurface antenna that exploits a low scattering host dielectric rod whose scattering is further reduced by a metasurface cloak with inductive properties immersed into it. At the same time, the metallic traces forming the inductive metasurface can be tailored to efficiently radiate like a conventional dipole antenna. In this work, we significantly improve this design by incorporating two metasurfaces with capacitive nature. With the addition of this second degree of freedom, we are able to optimize our design for more broadband cloaking and at the same time improved radiation characteristics and easier manufacturability.

36. Ahmed Mekawy, Huanan Li, Younes Ra’di, and Andrea Alù, Parametric Enhancement of Radiation from Electrically Small Antennas, Phys. Rev. Applied 15, 054063 (2021).

Abstract: Electrically small antennas are characterized by large quality factors, which yield limited gain-bandwidth products, as a result of the Bode-Fano limit. This bound implies a trade-off between the antenna’s footprint and radiation features, hindering wireless applications that require compact, broadband, and efficient antennas. Here, building on a previous theoretical analysis of parametric matching networks [H. Li, A. Mekawy, and A. Alù, Phys. Rev. Lett. 123, 164102 (2019)], we demonstrate how parametric phenomena can overcome this trade-off, offering a pathway to realize stable non-Foster wideband antennas that go beyond the restrictions of passive systems. We demonstrate our approach in a planar small loop antenna loaded by a time-varying capacitor that oscillates around twice the radiation frequency, showing that it can result in wideband radiation enhancement, exceeding the limitations of passive scenarios.

35. Younes Ra’di and Andrea Alù, Nonreciprocal Wavefront Manipulation in Synthetically Moving Metagratings, Photonics 7, 28 (2020), and featured on the cover* These authors contributed equally to this work.

Abstract: We introduce a metasurface platform for nonreciprocal wave manipulation. We study metagratings composed of nonreciprocal bianisotropic particles supporting synthetic motion, which enable nonreciprocal energy transfer between tailored Floquet channels with unitary efficiency. Based on this framework, we derive the required electromagnetic polarizabilities to realize a metagrating supporting space wave circulation with unitary efficiency for free-space radiation and design a microwave metagrating supporting this functionality. The proposed concept opens new research venues to control free-space radiation with high efficiency beyond the limits dictated by Lorentz reciprocity.

34. Ming Kang*, Younes Ra’di*, Diego Farfan, and Andrea Alù, Efficient Focusing with Large Numerical Aperture Using a Hybrid Metalens, Phys. Rev. Applied 13, 044016 (2020). * These authors contributed equally to this work.

Abstract: Focusing light using electrically thin layers is of paramount importance in several applications, from integrated optics to microwave engineering and sensing. Recently, gradient metasurfaces, which are electrically thin arrays of densely located polarizable particles, have been employed to perform different wave-front transformations, including focusing. In comparison to a bulk lens, these designs provide ultrathin geometries, but they suffer from fundamental limitations on their overall efficiency and achievable numerical aperture. Metagratings offer a solution for efficient beam steering at large angles, but it is challenging to utilize them in the small-angle limit. Here, we introduce a hybrid metalens design, which provides dramatic enhancement in lensing performance compared with that of state-of-the-art metasurfaces, combining metagratings and conventional gradient approaches. Our experimental prototype enables microwave focusing with large efficiency ( η = 0.479 ) and near-unity numerical aperture ( NA = 0.98 ) , yielding a sharp focal point at the diffraction limit in the far field ( FWHM = 0.332 λ ) . We propose a hybrid metalens design with exceptional performance in terms of efficiency and numerical aperture, opening up opportunities for high-throughput optical lithography, high-density data recording, focal plane arrays, radar, and communication systems.

33. Younes Ra’di*, Alex Krasnok*, and Andrea Alù, Virtual Critical CouplingACS Photonics 7, 1468 (2020), and featured on the cover.
* These authors contributed equally to this work.

Abstract: Electromagnetic resonators are a versatile platform to harvest, filter, and trap electromagnetic energy, at the basis of many applications from microwaves to optics. Resonators with a large intrinsic quality factor (Q) are highly desirable, since they can store a large amount of energy, leading to sharp filtering and low loss. However, exciting high-Q cavities with monochromatic signals suffer from poor excitation efficiency, i.e., most of the impinging energy is lost in the form of reflection, since high-Q resonators are weakly coupled to external radiation. Although critical coupling eliminates reflections in steady state by matching the intrinsic and coupling decay rates, this approach requires the introduction of loss in the resonator, causing dissipation and lowering the overall Q-factor. Here, we extend the notion of critical coupling to high-Q lossless resonators, based on tailoring the temporal profile of the excitation wave. Utilizing coupled-mode theory, we demonstrate an effect analogous to critical coupling by mimicking loss with nonmonochromatic excitations at complex frequencies. Remarkably, we show that this approach enables unitary excitation efficiency in open systems, even in the limit of extreme quality factors in the regime of quasi-bound states in the continuum.

32. Zhicheng Xiao, Younes Ra’di, Sergei Tretyakov, and Andrea Alù, Microwave Tunneling and Robust Information Transfer Based on Parity-Time-Symmetric Absorber-Emitter Pairs, Research 2019, 7108494 (2019). 

Abstract: Robust signal transfer in the form of electromagnetic waves is of fundamental importance in modern technology, yet its operation is often challenged by unwanted modifications of the channel connecting transmitter and receiver. Parity-time- (PT-) symmetric systems, combining active and passive elements in a balanced form, provide an interesting route in this context. Here, we demonstrate a PT-symmetric microwave system operating in the extreme case in which the channel is shorted through a small reactance, which acts as a nearly impenetrable obstacle, and it is therefore expected to induce large reflections and poor transmission. After placing a gain element behind the obstacle, and a balanced lossy element in front of it, we observe full restoration of information and overall transparency to an external observer, despite the presence of the obstacle. Our theory, simulations, and experiments unambiguously demonstrate stable and robust wave tunneling and information transfer supported by PT symmetry, opening opportunities for efficient communication through channels with dynamic changes, active filtering, and active metamaterial technology.

31. Robert Duggan, Younes Ra’di, and Andrea Alù, Temporally and Spatially Coherent Emission from Thermal Embedded EigenstatesACS Photonics 6, 2949 (2019).  

Abstract: Thermal emission manipulation is of key importance in numerous applications, yet a number of challenges have prevented its full technical maturation. Efficient mid- and far-infrared sources are valuable for sensing and thermal engineering, yet they are far from matching the sophistication of sources at lower or higher frequencies. Spectrally or spatially narrowband thermal emission has been proposed, partially overcoming the inherent incoherence of thermal processes. However, designing an ideal thermal source with spectral and spatial tailoring of the emission, and enhancing its line width within a small footprint, remains significantly challenging. Here we show how embedded eigenstates in the radiation continuum can be employed to realize compact thermal sources with narrow frequency and spatial spectra of emission, broadening the opportunities for thermal emission manipulation. The proposed concept can be extended to a wide class of thermal emitters, as well as more broadly within the context of thermal management.

30. Giuseppe Trainiti*, Younes Ra’di*, Massimo Ruzzene, and Andrea Alù, Coherent virtual absorption of elastodynamic waves, Science Advances 5, eaaw3255 (2019). * These authors contributed equally to this work.

Abstract: Absorbers suppress reflection and scattering of an incident wave by dissipating its energy into heat. As material absorption goes to zero, the energy impinging on an object is necessarily transmitted or scattered away. Specific forms of temporal modulation of the impinging signal can suppress wave scattering and transmission in the transient regime, mimicking the response of a perfect absorber without relying on material loss. This virtual absorption can store energy with large efficiency in a lossless material and then release it on demand. Here, we extend this concept to elastodynamics and experimentally show that longitudinal motion can be perfectly absorbed using a lossless elastic cavity. This energy is then released symmetrically or asymmetrically by controlling the relative phase of the impinging signals. Our work opens previously unexplored pathways for elastodynamic wave control and energy storage, which may be translated to other phononic and photonic systems of technological relevance.

29. Verena Neder, Younes Ra’di, Andrea Alù, and Albert PolmanCombined Metagratings for Efficient Broad-Angle Scattering Metasurface, ACS Photonics 6, 1010 (2019).

Abstract: Spectrally controlled diffusion and reflection of light are key operations for light management in many optical devices. Integration of this operation in complex nanophotonic devices requires a 2D interface that provides tailored spectrum and directivity control. Here, we present a metagrating superstructure that realizes a resonant light reflector with tailored angular scattering profile. Millimeter-sized metasurfaces are built from arrays of combined supercells of 20–50 μm, composed of 5–7 differently pitched metagratings that tailor at will and with large efficiency the angular response. Each supercell is composed of one or more Si Mie resonators, arranged in a periodic array above an Ag back plane and tailored to resonantly scatter light at 650 nm into only the ±1 diffraction orders with very high efficiency. By varying the pitch and supercell design, we can tailor the overall metasurface reflection profile with large flexibility, realizing a broad-angle Lambertian-type scattering metasurface, as well as a large-angle (35–75°) scattering metasurface, both with resonant optical scattering efficiencies above 70%. These ultrathin structures, fabricated using thin-film deposition, electron beam lithography, and reactive ion etching, can find applications for light trapping and spectrum splitting in solar cells and other devices.

28. Zhanni Wu, Younes Ra’di, and Anthony GrbicTunable Metasurfaces: A Polarization Rotator Design, Physical Review X 9, 011036 (2019).

Abstract: Metasurfaces have enabled unprecedented control over electromagnetic waves, providing new op- portunities in areas such as wireless communications, energy harvesting, imaging, and cloaking. Despite signi cant progress in this area, most metasurface designs provide static functionalities. Here, we theoretically propose and experimentally validate a transparent metasurface with dynamically tunable properties. Notably, we present an electronically tunable metasurface that is capable of rotating the polarization of an arbitrarily-polarized incident wave, without changing its axial ratio. The designed polarization rotator consists of a tunable birefringent structure sandwiched between two 45 degrees rotated metasurface-based quarter-wave plates. The birefringent structure is a bandpass metasurface which relies on integrated varactor diodes for tunability. Through a simple biasing mechanism, different voltages are applied to the diodes to rotate the polarization of a transmitted wave. The proposed idea may find application in various areas where dynamic polarization control is required.

27. Li Quan, Younes Ra’di, Dimitrios L. Sounas, and Andrea AlùMaximum Willis Coupling in Acoustic Scatterers, Physical Review Letters 120, 254301 (2018).

Abstract: Willis coupling in acoustic materials defines the cross-coupling between strain and velocity, analogous to bianisotropic phenomena in electromagnetics. While these phenomena have been garnering significant attention in recent years, to date their effects have been considered mostly perturbative. Here, we derive general bounds on the Willis response of acoustic scatterers, show that these bounds can be reached in suitably designed scatterers, and outline a systematic venue for the realistic implementation of maximally bianisotropic acoustic inclusions. We then employ these inclusions to realize acoustic metasurfaces for bending and steering of sound with unitary efficiency.

26. Younes Ra’diBhakti Chowkwale, Costas Valagiannopoulos, Fu Liu, A Alù, Constantin Simovski, and Sergei TretyakovOn-Site Wireless Power Generation, IEEE Transactions on Antennas and Propagation 66, 4260 (2018).

Abstract: Conventional wireless power transfer systems consist of a microwave power generator and a microwave power receiver separated by some distance. To realize efficient power transfer, the system is typically brought to resonance, and the coupled-antenna mode is optimized to reduce radiation into the surrounding space. In this scheme, any modification of the receiver position or its electromagnetic properties results in the necessity of dynamically tuning the whole system to restore the resonant matching condition. It implies poor robustness to the receiver location and load impedance, as well as additional energy consumption in the control network. In this paper, we introduce a new paradigm for wireless power delivery based on which the whole system, including transmitter and receiver and the space in between, forms a unified microwave power generator. In our proposed scenario, the load itself becomes a part of the generator. Microwave oscillations are created directly at the receiver location, eliminating the need for dynamical tuning of the system within the range of the self-oscillation regime. As a proof-of-concept demonstration, we experimentally show that, such self-oscillating wireless power delivery systems can provide robust operation against changes in the environment or in the location of the load. The proposed concept has relevant connections with the recent interest in parity-time symmetric systems, in which balanced loss and gain distributions enable unusual electromagnetic responses.

25. Younes Ra’di and Andrea Alù, Reconfigurable metagratings, ACS Photonics 5, 1779 (2018).

Abstract: Recently, it was revealed that gradient metasurfaces based on continuous gradients of the local reflection and/or transmission coefficients are fundamentally limited on their overall efficiency in transformation of an impinging wavefront. In addition, due to the fastly varying impedance profile that these surfaces require, they usually need high-resolution fabrication processes. To address these issues, the concept of metagrating was recently put forward, enabling engineered surfaces capable of manipulating light with unitary efficiency. Metagratings are periodic arrays of carefully designed scatterers that, in contrast with conventional metasurfaces, do not need to support a continuous gradient surface impedance, and consequently, are far simpler to fabricate. Building up on this concept, here we explore opportunities to realize reconfigurable multifunctional metagratings. The proposed designs provide a highly efficient reconfigurable platform tuned by electrostatically biasing simply structured graphene sheets, paving the way to a new generation of highly efficient designer surfaces.

24. Hamidreza Chalabi, Younes Ra’di, Dimitrios L. Sounas, and Andrea Alù, Efficient anomalous reflection through near-field interactions in metasurfaces, Phys. Rev. B 96, 075432 (2017).

Abstract: Graded metasurfaces exploit the local momentum imparted by an impedance gradient to mold the impinging wave front. This approach suffers from fundamental limits on the overall conversion efficiency, and it is challenged by fabrication limitations on the spatial resolution. Here, we introduce the concept of metagratings, formed by periodic arrays of carefully tailored bianisotropic inclusions and show that they enable wave front engineering with unitary efficiency and significantly lower fabrication demands. We employ this concept to design reflective metasurfaces for wave front steering without limitations on efficiency. A similar approach can be extended to transmitted beams and arbitrary wave front transformation, opening opportunities for highly efficient metasurfaces for extreme wave manipulation.

23. Younes Ra’di, Dimitrios L. Sounas, and Andrea Alù, Metagratings: Beyond the limits of graded metasurfaces for wave front control, Phys. Rev. Lett. 119, 067404 (2017).

Abstract: Abstract: Graded metasurfaces exploit the local momentum imparted by an impedance gradient to mold the impinging wave front. This approach suffers from fundamental limits on the overall conversion efficiency, and it is challenged by fabrication limitations on the spatial resolution. Here, we introduce the concept of metagratings, formed by periodic arrays of carefully tailored bianisotropic inclusions and show that they enable wave front engineering with unitary efficiency and significantly lower fabrication demands. We employ this concept to design reflective metasurfaces for wave front steering without limitations on efficiency. A similar approach can be extended to transmitted beams and arbitrary wave front transformation, opening opportunities for highly efficient metasurfaces for extreme wave manipulation.

22. Younes Ra’di and Anthony Grbic, Magnet-free nonreciprocal bianisotropic metasurfaces, Phys. Rev. B 94, 195432 (2016).

Abstract: Tellegen and moving metasurfaces are two promising classes of nonreciprocal bianisotropic metasurfaces that offer extended functionalities for electromagnetic wave manipulation. However, in order to realize these metasurfaces, ferrite-based unit cells have been utilized that need to be biased by an external magnetic field. Inspired by recent works on magnet-free non-bianisotropic nonreciprocal building blocks, we introduce designs for magnet-free Tellegen and moving metasurfaces.

21. Viktar S. Asadchy, Mohammad Albooyeh, Svetlana N. Tcvetkova, Ana Díaz-Rubio, Younes Ra’di, and Sergei A. Tretyakov, Perfect control of reflection and refraction using spatially dispersive metasurfaces, Phys. Rev. B 94, 075142 (2016).

Abstract: Nonuniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly nonlocal response to the fields.

20. Younes Ra’di, Dimitrios L. Sounas, Andrea Alù, and Sergei A. Tretyakov, Parity-time-symmetric teleportation, Phys. Rev. B 93, 235427 (2016).

Abstract: We show that electromagnetic plane waves can be fully “teleported” through thin, nearly fully reflective sheets, assisted by a pair of parity-time-symmetric lossy and active sheets in front and behind the screen. The proposed structure is able to almost perfectly absorb incident waves over a wide range of frequency and incidence angles, while waves having a specific frequency and incidence angle are replicated behind the structure in synchronization with the input signal. It is shown that the proposed structure can be designed to teleport waves at any desired frequency and incidence angle. Furthermore, we generalize the proposed concept to the case of teleportation of electromagnetic waves over electrically long distances, enabling full absorption at one surface and the synthesis of the same signal at another point located electrically far away from the first surface. The physical principle behind this selective teleportation is discussed, and similarities and differences with tunneling and cloaking concepts based on PT symmetry are investigated. From the application point of view, the proposed structure works as an extremely selective filter, both in frequency and spatial domains.

19. Younes Ra’di, Viktar S. Asadchy, and Sergei A. Tretyakov, Nihility in non-reciprocal bianisotropic media, EPJ Applied Metamaterials 2, 6 (2015).

Abstract: Here we consider electromagnetic response of non-reciprocal bianisotropic materials in some extreme regimes. The magneto-electric coupling is modeled by symmetric and antisymmetric uniaxial dyadics, which correspond to the so called artificial Tellegen media and moving media, respectively. Extreme electromagnetic properties of uniaxial non-reciprocal bianisotropic materials in the limiting case of nihility, when both permittivity and permeability of the media tend to zero, and only the magneto-electric parameters define the material response, are studied. Among other interesting effects, we show that the moving nihility materials provide the extreme asymmetry in the phase shift of transmitted waves propagating along the opposite directions. Furthermore, we reveal a possibility to create an angular filter with extreme sensitivity to the incidence angle, also using moving nihility slabs.

18. Mikhail M. Omelyanovich, Younes Ra’di, and Constantin R. Simovski, Perfect plasmonic absorbers for photovoltaic applications, Journal of Optics 17, 12 (2015).

Abstract: A novel regime of perfect absorption in a thin plasmonic layer corresponds to a collective mode of an array of plasmonic nanospheres. In our theoretical study we show that the absorption of the incident light occurs mainly in the semiconductor material hosting plasmonic nanospheres, whereas the absorption in the metal is very small. The regime survives when the uniform host layer is replaced by a practical photovoltaic cell. Trapping the light allows the thickness of the doped semiconductor to be reduced to values for which the degradation under light exposure should be insufficient. The light-trapping regime is compatible with both the metal-backed variant of the photovoltaic cell and its semitransparent variant when both electrodes are preformed of a conductive oxide. Negligible parasitic losses, a variety of design solutions and a reasonable operational band make our perfect plasmonic absorbers promising for photovoltaic applications.

17. Younes Ra’di, Viktar S. Asadchy, Sergei U. Kosulnikov, Mikhail M. Omelyanovich, Dmitry Morits, Andrey V. Osipov, Constantin R. Simovski, and Sergei A. Tretyakov, Full light absorption in single arrays of spherical nanoparticles, ACS Photonics 2, 653 (2015).

Abstract: In this paper we show that arrays of core–shell nanoparticles can function as effective thin absorbers of light. In contrast to known metamaterial absorbers, the introduced absorbers are formed by single planar arrays of spherical inclusions and enable full absorption of light incident on either or both sides of the array. We demonstrate possibilities for realizing different kinds of symmetric absorbers, including resonant, ultrabroadband, angularly selective, and all-angle absorbers. The physical principle behind these designs is explained considering balanced electric and magnetic responses of unit cells. Photovoltaic devices and thermal emitters are the two most important potential applications of the proposed designs.

16. Viktar S. Asadchy, Igor A. Faniayeu, Younes Ra’di, Sergei A. Khakhomov, Igor V. Semchenko, and Sergei A. Tretyakov, Broadband reflectionless metasheets: Frequency-selective transmission and perfect absorption, Phys. Rev. X 5, 031005 (2015).

Abstract: Energy of propagating electromagnetic waves can be fully absorbed in a thin lossy layer, but only in a narrow frequency band, as follows from the causality principle. On the other hand, it appears that there are no fundamental limitations on broadband matching of thin resonant absorbing layers. However, known thin absorbers produce significant reflections outside of the resonant absorption band. In this paper, we explore possibilities to realize a thin absorbing layer that produces no reflected waves in a very wide frequency range, while the transmission coefficient has a narrow peak of full absorption. Here we show, both theoretically and experimentally, that a thin resonant absorber, invisible in reflection in a very wide frequency range, can be realized if one and the same resonant mode of the absorbing array unit cells is utilized to create both electric and magnetic responses. We test this concept using chiral particles in each unit cell, arranged in a periodic planar racemic array, utilizing chirality coupling in each unit cell but compensating the field coupling at the macroscopic level. We prove that the concept and the proposed realization approach also can be used to create nonreflecting layers for full control of transmitted fields. Our results can have a broad range of potential applications over the entire electromagnetic spectrum including, for example, perfect ultracompact wave filters and selective multifrequency sensors.

15. Younes Ra’di, Constantin R. Simovski, and Sergei A. Tretyakov, Thin perfect absorbers for electromagnetic waves: Theory, design, and realizations, Phys. Rev. Appl. 3, 037001 (2015).

Abstract: With recent advances in nanophotonics and nanofabrication, considerable progress has been achieved in realizations of thin composite layers designed for full absorption of incident electromagnetic radiation, from microwaves to the visible. If the layer is structured at a subwavelength scale, thin perfect absorbers are usually called “metamaterial absorbers,” because these composite structures are designed to emulate some material responses not reachable with any natural material. On the other hand, many thin absorbing composite layers were designed and used already in the time of the introduction of radar technology, predominantly as a means to reduce radar visibility of targets. In view of a wide variety of classical and new topologies of optically thin metamaterial absorbers and plurality of applications, there is a need for a general, conceptual overview of the fundamental mechanisms of full absorption of light or microwave radiation in thin layers. Here, we present such an overview in the form of a general theory of thin perfectly absorbing layers. Possible topologies of perfect metamaterial absorbers are classified based on their fundamental operational principles. For each of the identified classes, we provide design equations and give examples of particular realizations. The concluding section provides a summary and gives an outlook on future developments in this field.

14. Younes Ra’di, Saeid Nikmehr, and Shahram Hosseinzadeh, A novel method for wave focusing investigation in metamaterial half-space and slab lens structures, International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 29, 63 (2015).

Abstract: Approximate image currents for the Sommerfeld half-space problem with vertical electric and magnetic current sources above double negative half-space is found using the discrete image theory and Laplace transform. This method would found its applications in investigation of dipole radiation above metamaterial half-space and in describing the focusing mechanism of metamaterial lenses. In order to efficiently consider the effects of metamaterial half-space, discrete image dipoles are defined by proposing an exponential series for reflection coefficient. Unknown constants (the loci of images) in the argument of the exponential function are determined by equating the coefficients of the Taylor series for exact reflection coefficient with those of the proposed approximate series. Thus, the calculation of integral with oscillating integrand reduces to the solution of polynomial equation for the zeros of polynomials. As an application for the proposed approach, the field focusing in double negative is investigated. Thanks to the fast and accurate methods for finding the zeros of polynomials, it is shown that in addition to its good accuracy, this method decreases the computational time for field calculations in comparison with the exact methods. It is seen that for the negative refractive index beside −1, the complex images approach to infinity and have negligible effect on the scattering field of dipole. For the other negative indexes the complex image blurs the real image.

13. Viktar S. Asadchy, Younes Ra’di, Joni Vehmas, and Sergei A. Tretyakov, Functional metamirrors using bianisotropic elements, Phys. Rev. Lett. 114, 095503 (2015).

Abstract: Conventional mirrors obey the simple reflection law that a plane wave is reflected as a plane wave, at the same angle. To engineer spatial distributions of fields reflected from a mirror, one can either shape the reflector or position some phase-correcting elements on top of a mirror surface. Here we show, both theoretically and experimentally, that full-power reflection with general control over the reflected wave phase is possible with a single-layer array of deeply subwavelength inclusions. These proposed artificial surfaces, metamirrors, provide various functions of shaped or nonuniform reflectors without utilizing any mirror. This can be achieved only if the forward and backward scattering of the inclusions in the array can be engineered independently, and we prove that it is possible using electrically and magnetically polarizable inclusions. The proposed subwavelength inclusions possess desired reflecting properties at the operational frequency band, while at other frequencies the array is practically transparent. The metamirror concept leads to a variety of applications over the entire electromagnetic spectrum, such as optically transparent focusing antennas for satellites, multifrequency reflector antennas for radio astronomy, low-profile conformal antennas for telecommunications, and nanoreflectarray antennas for integrated optics.

12. Inigo Liberal, Younes Ra’di, Ramon Gonzalo, Inigo Ederra, Sergei A. Tretyakov and Richard W. Ziolkowski, Least upper bounds of the powers extracted and scattered by bi-anisotropic particles, IEEE Transactions on Antenna and Propagation 62, 4726 (2014).

Abstract: The least upper bounds of the powers extracted and scattered by bi-anisotropic particles are investigated analytically. A rigorous derivation for particles having invertible polarizability tensors is presented, and the particles with singular polarizability tensors that have been reported in the literature are treated explicitly. The analysis concludes that previous upper bounds presented for isotropic particles can be extrapolated to bi-anisotropic particles. In particular, it is shown that neither nonreciprocal nor magnetoelectric coupling phenomena can further increase those upper bounds on the extracted and scattered powers. The outcomes are illustrated further with approximate circuit model examples of two dipole antennas connected via a generic lossless network.

11. Mohammad S. Mirmoosa, Younes Ra’di, Viktar S. Asadchy, Constantin R. Simovski, and Sergei A. Tretyakov, Polarizabilities of nonreciprocal bianisotropic particles, Phys. Rev. Applied 1, 034005 (2014).

Abstract: The most general response of a small dipolar particle to electromagnetic fields is determined not by its electric and magnetic polarizabilities only but also by magnetoelectric coupling coefficients. We are interested in creating particles with engineered magnetoelectric coupling of nonreciprocal nature, including artificial “moving particles.” Recently, it has been shown theoretically that using small nonreciprocal bianisotropic particles, one can realize perfect electromagnetic isolators, nonreciprocal twist polarizers, one-way transparent sheets, and other devices. This paper is a necessary step towards these applications. For two electrically small nonreciprocal scatterers, an analytical electromagnetic model of polarizabilities is developed. Both particles are bianisotropic: the so-called Tellegen omega particle and moving chiral particle. Analytical results are compared to the full-wave numerical simulations. Both models satisfy to main physical restrictions and leave no doubts in the possibility to realize these particles experimentally.

10. Younes Ra’di, Viktar S. Asadchy, and Sergei A. Tretyakov, Tailoring reflections from thin composite metamirrors, IEEE Transactions on Antenna and Propagation 62, 3749 (2014).

Abstract: We propose an effective route to fully control the phase of plane waves reflected from electrically (optically) thin composite sheets. This becomes possible using engineered artificial full-reflection layers (metamirrors) formed by arrays of electrically small resonant bi-anisotropic particles. In this scenario, fully reflecting mirrors do not contain any continuous ground plane, but only arrays of small particles. Bi-anisotropic omega coupling is required to get asymmetric response in reflection phase for plane waves incident from the opposite sides of the composite mirror. It is shown that with this concept one can independently tailor the phase of electromagnetic waves reflected from both sides of the mirror array.

9. Viktar S. Asadchy, Igar A. Faniayeu, Younes Ra’di, and Sergei A. Tretyakov, Determining polarizability tensors for an arbitrary small electromagnetic scatterer, Photonics and Nanostructures – Fundamentals and Applications 12, 298 (2014).

Abstract: In this paper, we present a method to retrieve tensor polarizabilities of general bi-anisotropic particles from their far-field responses to plane-wave illuminations. The necessary number of probing excitations and the directions where the scattered fields need to be calculated or measured have been found. When implemented numerically, the method does not require any spherical harmonic expansion nor direct calculation of dipole moments, but only calculations of co- and cross-polarized scattering cross sections for a number of plane-wave excitations. With this simple approach, the polarizabilities can be found also from experimentally measured cross sections. The method is exemplified considering two bi-anisotropic particles, a reciprocal omega particle and a non-reciprocal particle containing a ferrite inclusion coupled to metal strips.

8. Younes Ra’di, Viktar S. Asadchy, and Sergei A. Tretyakov, One-way transparent sheets, Phys. Rev. B 89, 075109 (2014).

Abstract: In this paper we introduce the concept of metasurfaces which are fully transparent when looking from one of the two sides of the sheet and have controllable functionalities for waves hitting the opposite side (one-way transparent sheets). We address the question on what functionalities are allowed, considering limitations due to reciprocity and passivity. In particular, we have found that it is possible to realize one-way transparent sheets which have the properties of a twist polarizer in transmission when illuminated from the other side. Also, active one-way transparent sheets with controllable copolarized reflection and transmission from the opposite side are feasible. We show that particular nonreciprocal magnetoelectric coupling inside the sheet is necessary to realize lossless nonactive transparent sheets. Furthermore, we derive the required polarizabilities of constituent dipole particles such that the layers composed of them form one-way transparent sheets. We conclude with design and simulations of an example of a nonreciprocal one-way transparent sheet functioning as an isolating twist polarizer.

7. Younes Ra’di, Viktar S. Asadchy, and Sergei A. Tretyakov, Total absorption of electromagnetic waves in ultimately thin layers, IEEE Transactions on Antenna and Propagation 61, 4606 (2013).

Abstract: We consider single-layer arrays of electrically small lossy particles that completely absorb electromagnetic waves at normal incidence. Required conditions for electromagnetic properties of bi-anisotropic particles have been identified in the most general case of uniaxial reciprocal and nonreciprocal particles. We consider the design possibilities offered by the particles of all four fundamental classes of bi-anisotropic inclusions: reciprocal chiral and omega particles and nonreciprocal Tellegen and moving particles. We also study the reflection/transmission properties of asymmetric structures with different properties when illuminated from the opposite sides of the sheet. It has been found that it is possible to realize single-layer grids which exhibit the total absorption property when illuminated from one side but are totally transparent when illuminated from the other side (an ultimately thin isolator). Other possible properties are co-polarized or twist polarized reflection from the side opposite to the absorbing one. Finally, we discuss possible approaches to practical realization of particles with the properties required for single-layer perfect absorbers and other proposed devices.

6. Younes Ra’di, and Sergei A. Tretyakov, Balanced and optimal bianisotropic particles: Maximizing power extracted from electromagnetic fields, New Journal of Physics 15, 053008 (2013).

Abstract: Here we introduce the concept of ‘optimal particles’ for strong interactions with electromagnetic fields. We assume that a particle occupies a given electrically small volume in space and study the required optimal relations between the particle polarizabilities. In these optimal particles, the inclusion shape and material are chosen so that the particles extract the maximum possible power from given incident fields. It appears that for different excitation scenarios the optimal particles are bianisotropic chiral, omega, moving and Tellegen particles. The optimal dimensions of resonant canonical chiral and omega particles are found analytically. Such optimal particles have extreme properties in scattering (e.g., zero backscattering or invisibility). Planar arrays of optimal particles possess extreme properties in reflection and transmission (e.g. total absorption or magnetic-wall response), and volumetric composites of optimal particles realize, for example, such extreme materials as the chiral nihility medium.

5. Joni Vehmas, Younes Ra’di, Antti Karilainen, and Sergei A. Tretyakov, Eliminating electromagnetic scattering from small particles, IEEE Transactions on Antenna and Propagation 61, 3747 (2013).

Abstract: This paper presents and discusses the conditions for zero electromagnetic scattering by electrically small particles. We consider the most general bi-anisotropic particles, characterized by four dyadic polarizabilities and study the case of uniaxially symmetric objects. Conditions for zero backward and forward scattering are found for a general uniaxial bi-anisotropic particle and specialized for all fundamental classes of bi-anisotropic particles: omega, “moving”, chiral, and Tellegen particles. Possibility for zero total scattering is also discussed for aforementioned cases. The scattering pattern and polarization of the scattered wave are also determined for each particle class. In particular, we analyze the interplay between different scattering mechanisms and show that in some cases it is possible to compensate scattering from a polarizable particle by appropriate magneto-electric coupling. Examples of particles providing zero backscattering and zero forward scattering are presented and studied numerically.

4. Mohammad Albooyeh, Younes Ra’di, Adil Q. Mohammad, and Constantin R. Simovski, Revised transmission line model for electromagnetic characterization of metasurfaces, Phys. Rev. B 88, 085435 (2013).

Abstract: We have revised the transmission line model with the purpose of simplifying the electromagnetic characterization of metasurfaces with both electric and magnetic responses. The revised model is an efficient tool for the case when a metasurface is located on top of a finite-thickness substrate or inside a finite-thickness slab. We have derived analytical expressions for the grid series impedance and shunt admittance of a substrated metasurface. The presented theory is illustrated by a numerical example of an array consisting of Mie-resonant silicon nanospheres. We have retrieved individual polarizabilities of a single sphere in a uniform host medium and compared them with the predictions of classical Mie theory. Further, we have studied a more practical case when the array of Si spheres is located on top of a finite-thickness glass substrate.

3. Younes Ra’di, Saeid Nikmehr, and Shahram Hosseinzadeh, Input impedance of horizontal dipole located above lossy metamaterial half-space, IEEE Antennas and Wireless Propagation Letters 10, 1255 (2011).

Abstract: Abstract: In this letter, accurate analytical expressions for the impedance of horizontal electric and magnetic dipoles located above half-space materials of arbitrary permittivity and permeability are derived. In this regard, the Fresnel reflection coefficients are approximated by series of exponential functions, and the impedance integrals are analytically solved. The results are compared to the case of natural materials (positive permittivity and permeability), and the key differences are explained. It is shown that for horizontal dipoles above materials of high and/or low conductivities, both results are coincident. For dipoles in the vicinity of low-loss materials, the results are somewhat identical. However, a better agreement could be obtained using higher-order approximations for the integrand.

2. Younes Ra’di, Saeid Nikmehr, and Shahram Hosseinzadeh, A rigorous treatment of vertical dipole impedance located above lossy DPS, MNG, ENG, and DNG half-space, Progress In Electromagnetics Research (PIER) 116, 107 (2011).

Abstract: In this paper, accurate analytical expressions for the impedance of vertical electric and magnetic dipoles which are located over the half-space materials of arbitrary permittivity and permeability are developed. In this regard, the impedance variations are expressed in integral forms. For metamaterial half-space, a proper expression for approximating the Fresnel reflection coefficient is proposed. Using this approximate expression, the impedance integrals are analytically solved, and exact formulas for impedance variations are obtained. The results for the metamaterial half-spaces are compared with the case of natural materials (positive permittivity and permeability), and key differences are explained. The in uences of sign changing in permeability of the half-space material on the impedance of vertical dipole are studied, and the results are validated by comparison with those of numerical solution of integrals. It is shown that for elevated dipoles over materials with high and/or low conductivities, the results of both methods are in complete agreement. For vertical dipoles above low loss materials, the results are somewhat identical. However, a better agreement could be obtained using higher order approximations for the integrand.

1. Younes Ra’di, Saeid Nikmehr, and Ali Pourziad, A novel bandwidth enhancement technique for X-Band RF MEMS actuated reconfigurable reflectarray, Progress in Electromagnetic Research (PIER) 111, 179 (2011).

Abstract: In this paper, a wideband microstrip antenna for X-band (8.2 GHz–12.4 GHz) applications is introduced. First, simple patch antennas are studied. The resultant design demonstrates better performance than the previously published narrowband microstrip reflectarray antennas. The important features of these elements are simple structure, linear operation, and use of RF MEMS switches for programmable pattern control. Next employing our novel method, this narrowband structure is converted to broadband reflectarray antenna that can cover the whole X band. This novel idea is based on introducing several ground plane slots and controlling their electrical lengths by RF MEMS switches. By means of this method, 952 and 587 degree phase swing is achieved for continuous and discrete slot length variation, respectively. Application of this method along with smaller switches results in phase swing improvement of up to 1616 degree. In all structures a RT duroid (5880) substrate is selected to lower the back radiation. The achieved return loss in all cases is less than 0.32 dB. In comparison with the previous publications, our novel method has more generalization capability and results in single layered broadband reconfigurable microstrip reflectarray antennas with linear phase swing, lower cost, and ease of RF MEMS implementation.

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