Bianisotropic Metasurfaces Research Articles

Optimizing an electromagnetic wave absorber for bi-anisotropic metasurfaces based on toroidal modes

The design and optimization of an electromagnetic wave absorber for far-field wireless power transmission (WPT) is the subject of this research study. The goal of the research is to effectively absorb energy from ambient RF electromagnetic waves without the usage of a ground plane by employing metasurfaces with chiral components.By integrating trioidal moments into the design theory, the objective is to create a metasurface that functions in two frequency bands and produces high-quality resonance. The study also explores the dual non-homogeneity property of structures, polarization tensor coefficients, and the electromagnetic response of non-homogeneous metasurfaces. Based on the relative orientation of induced fields and moments, it delves deeper into the two basic possibilities for dual non-homogeneous elements. The development of chiral metasurfaces and the notion of electromagnetic chirality and its implications for polarization properties are introduced.

Simultaneous generation and manipulation of terahertz waves based on nonlinear leaky-waveguide antennas with integrated bianisotropic metasurfaces.

We hereby propose and theoretically investigate a new scheme for simultaneous generation and manipulation of terahertz (THz) waves through difference frequency generation facilitated by a metasurface-assisted nonlinear leaky waveguide antenna. The proposed structure integrates a nonlinear optical waveguide, composed of multiple AlxGa1-xAs layers, with a THz leaky waveguide, wherein a bianisotropic metasurface realizes the radiating aperture. By explicitly utilizing the electric, magnetic, and magnetoelectric coupling responses of the metasurface, we demonstrate that the generated THz wave can be induced as a tightly confined, phase-matched guided mode for efficient generation of the THz wave. Additionally, this approach allows the THz wave to be transformed into a directive beam, radiating at a user-defined leakage rate and direction. Our numerical analyses suggest that THz beams ranging from 2.85 THz to 3.05 THz can be steered from 4 ∘ to 40 ∘, utilizing the inherent beam-steering capabilities of the leaky-waveguide antenna. Within this THz frequency spectrum, the phase matching condition is achieved by adjusting the optical wavelengths between 1.6μm and 1.52μm. In particular, the nonlinear conversion efficiency is 2.9 × 10-5 [1/W] at 3 THz.

Bianisotropic Metasurface for Spatial Separation of Multiple Harmonic Waves

An approach to tuning harmonic responses of surface impedance is introduced to spatially separate fundamental, third-harmonic, and fifth-harmonic waves by focusing on the physical implementation perspective. The concept of the bianisotropic metasurface is adopted to refract the wave toward different angles by assigning identical phase gradients at fundamental and third-harmonic frequencies. On the other hand, high-sheet impedance is realized by maintaining the in-phase condition at the fifth harmonic to bypass the incident wave without refraction. The target properties of each unit cell are implemented using I-shaped patterns based on a three-stacked-layer topology. Different sizes and intervals are applied for the I-shaped patterns, and only their horizontal lengths are tuned to achieve the desired sheet impedances. The feasibility of the proposed approach is validated for the fundamental frequency of 10 GHz and two of odd harmonics through far-and near-field measurements. The results confirm that the fabricated metasurface separates the superimposed harmonic waves toward 44 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> , and 0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> with the measured refraction efficiencies of 65.6%, 67.2%, and 74% at 10, 30, and 50 GHz, respectively.

Metasurface characterization based on eigenmode analysis and averaging of electromagnetic fields

A fully numerical homogenization technique for the retrieval of the effective surface susceptibilities of a periodic composite metasurface is developed in this work. We utilize the so-called dual field-flux finite element formulation scheme to accurately calculate the eigenmodes of a composite periodic metasurface, a scheme that possesses a crucial advantage: The capability of evaluating all field components and their derivatives accurately and with the same order of approximation is a requirement for our proposed technique. Next, we derive the generalized sheet transition condition equations for a general bianisotropic metasurface, which correlate the field components on either sides of the metasurface with its surface susceptibilities. At this point, we establish a new field averaging scheme for the acquisition of the average field components of the modes supported by the metasurface. Combining this computational information, we derive a set of linear algebraic equations based on the GSTCs and the average electromagnetic fields and numerically solve it, so as to obtain the effective surface susceptibilities of a bianisotropic metasurface. Comparison with the results of other techniques in the literature shows very good agreement, relatively to the resonance behavior of the returned values and their position at the frequency spectrum. The advantages that distinguish the proposed technique over other related methods are its foundation on the intrinsic modal information of the eigenmodes supported by the metasurface and its independence of any wave excitation schemes or involvement of analytical polarizability calculations.

Time-Domain Meshless Method Simulation of Metasurfaces With Generalized Sheet Transition Conditions

This paper presents a new time-domain formulation of a meshless method based on the generalized sheet transition conditions (GSTCs). This formulation is applicable for modeling a kind of dispersive metasurface and a time-varying metasurface. This is the first time a meshless method is developed for analysis of the metasurfaces using GSTCs. The significance of time-domain meshless methods originates from their capabilities for multi-scale and conformal modeling in addition to analysis of problems in a broad frequency-range. Here, for simplicity, the meshless formulation is presented for analysis of 1-D problems consisting of monoisotropic metasurfaces. The extension of this method to 2-D and 3-D problems or bianisotropic metasurfaces is straightforward. The scheme is verified by numerical examples.

A Full-Vectorial Spectral Element Method With Generalized Sheet Transition Conditions for High-Efficiency Metasurface/Metafilm Simulation

Recently, metasurfaces as 2-D metamaterials have been widely applied due to their advantages in low loss, ease of fabrication, and tunability in the phase, magnitude, and polarization of incident electromagnetic waves. However, due to the subwavelength-thickness properties of metasurfaces, their direct modeling and simulation are challenging because of the heavy computational burden. In this work, a high-efficiency spectral element method with generalized sheet transition conditions (SEM-GSTCs) is proposed by replacing finite thickness metasurfaces with zero-thickness equivalent boundary conditions to avoid dense elements and the associated enormous number of unknowns. Moreover, both the scalar and the full-vectorial SEM-GSTC are implemented. Some illustrative numerical examples are shown to verify that the proposed SEM-GSTC not only can obtain highly accurate numerical results but also is effective in addressing complex functions ranging from isotropic to fully bianisotropic metasurfaces.

Deep‐Learning Assisted Polarization Holograms

AbstractMultiplexing holography with metasurfaces using different degrees of freedom of light has enabled recent applications in display and information processing. In terms of polarization‐multiplexed holograms, the most general form is an arbitrary Jones matrix profile in storing the maximum amount of information. It requires a relaxation to bianisotropic metasurfaces from a conventional single‐layer implementation of nanostructures, but it will also complicate both the inverse design of the nanostructures and the hologram generation algorithm. Here, an integrated neural network approach, being extended from the recent DeepCGH algorithm, is developed to obtain metasurface structural profiles directly from independent holograms from an arbitrary set of polarizations to another, with maximally four different co‐ and cross‐polarization conversion channels. Such an information‐driven approach enables designing complex polarization holograms directly from an existing metamaterial library without detailed physical knowledge on the constraints, and can be extended to other multiplexing holograms to further facilitate an efficient usage of the information stored on a metasurface.

Emulating bianisotropic coupling through coherent illumination

One of the main advantages of reciprocal bianisotropic metasurfaces is their capability to produce asymmetric scattering depending from which side they are illuminated and on the handedness of circularly polarized illuminations. For most applications, these metasurfaces are designed for illumination by a single source at a time. The resulting bianisotropic metasurface has a specific and usually complex geometrical structure that ensures the expected scattering produced under various illuminations. Here we show that geometrical asymmetry of metasurfaces can be emulated by using non-bianisotropic layers in presence of coherent illumination, which allows us to replicate and optically control the desired asymmetric scattering and chirality effects. In particular, the concept is developed on an example of emulating asymmetric scattering needed to create a 180$^\circ$ hybrid junction for plane waves. We show that this device can be realized either using a bianisotropic metasurface or a set of simple sheets with electric response under simultaneous illumination by two coherent waves.

Transmittable Nonreciprocal Cloaking

Offering a potential pathway to the fantasy of invisibility, cloaking has been a most popular concept since its introduction over 15 years ago. This article furthers the concept by making the cloak transmittable, so as to allow ``transmitting without being seen''. This special cloak consists of a concentric set of bianisotropic metasurfaces with a nonreciprocal innermost element, and allows the entity sitting at its center to transmit information at will in a desired direction while remaining invisible in all other directions. Such cloaks may find application in e.g. stealth technology, blockage avoidance, optical illusion, and cooling.

Classification of Bianisotropic Metasurfaces from Reflectance and Transmittance Measurements

Using fundamental electromagnetic properties of metasurfaces, we build a platform to classify reciprocal bianisotropic metasurfaces from typical experimental measurements and determine isotropic, anisotropic, bi-isotropic (chiral), and bianisotropic (so-called omega) properties. We provide experimental guidelines to identify each class by measuring macroscopic scattering parameters, that is, reflection and transmission coefficients upon plane wave illumination with linear and/or circular polarization. We explicitly provide a recipe of which metasurface properties can and cannot be inferred by means of the chosen polarization, reflection, and transmission properties. We also clarify common confusions in the classification of anisotropic versus chiral metasurfaces based on circular dichroism measurements presented in the recent literature.

Generalizing Dipolar Model for the Characterization of Substrated Periodic Metasurfaces Under Oblique Illumination

Most metasurfaces are fabricated on dielectric substrates and have found various applications in manipulating the characteristics of incident fields, from any direction that is normal or at oblique incidences. As a result, a precise analysis of substrated metasurfaces under oblique illuminations is very attractive and important. In this article, the theory of the analytical modeling of periodic planar metasurfaces, placed on a dielectric substrate for normal incidence, is extended to general oblique cases. This method is based on an independent analysis of the electromagnetic behavior of the constituent particles and the interactions of the particles with each other. Since the interaction coefficients are calculated analytically, this method is time effective. The reflectance and transmittance of different substrated metasurfaces under oblique incidence, which is obtained from our proposed method, are compared to the results of numerical full-wave simulations. Two different cases are considered here—a metasurface consisting of isotropic spherical particles and a more general case of a bianisotropic metasurface composed of SRR inclusions. Good accordance has been observed between the results, confirming the accuracy of the proposed method. Furthermore, the effects of different parameters on the interaction coefficients are also investigated.

Quantum optics of lossy metasurfaces: Propagating the photon-moment matrix by the semiclassical Liouvillian

In this paper we map the quantum optical scattering of a linear dissipative metasurface to an equivalent Liouvillian dynamics with respect to the effective Hamiltonian of the metasurface propagating in fictitious time. By using a lossy bianisotropic metasurface as a generic example, our formulation allows a complete specification of the generalized eigenspace of the Liouvillian superoperator starting from either a diagonalizable or defective classical scattering matrix of the metasurface. With a further factorization of the Liouvillian superoperator, we demonstrate the input-output relationship on the photon density matrix can be obtained by propagating the photon-moment matrix using a complete basis of the semiclassical Liouvillian superoperator. Our investigations will be useful for characterizing quantum scattering of generally bianisotropic metasurfaces, paving the way for investigating and designing passive parity-time symmetric and non-Hermitian metasurfaces for quantum information processing applications.

A Combined Machine-Learning/Optimization-Based Approach for Inverse Design of Nonuniform Bianisotropic Metasurfaces

Electromagnetic metasurface design based on far-field constraints without the complete knowledge of the fields on both sides of the metasurface is typically a time consuming and iterative process, which relies heavily on heuristics and ad hoc methods. This paper proposes an end-to-end systematic and efficient approach where the designer inputs high-level far-field constraints such as nulls, sidelobe levels, and main beam level(s); and a 3-layer non-uniform passive, lossless, omega-type bianisotropic electromagnetic metasurface design to satisfy them is returned. The surface parameters to realize the far-field criteria are found using the alternating direction method of multipliers on a homogenized model derived from the method of moments. This model incorporates edge effects of the finite surface and mutual coupling in the inhomogeneous impedance sheet. Optimization through the physical unit cell space integrated with machine learning-based surrogate models is used to realize the desired surface parameters from physical meta-atom (or unit cell) designs. Two passive lossless examples with different feeding systems and far-field constraints are shown to demonstrate the effectiveness of this method.

Generation of Airy beams in Smith-Purcell radiation.

The metasurface has recently emerged as a powerful platform to engineer wave packets of free electron radiation at the mesoscale. Here, we propose that Airy beams can be generated when moving electrons interact with bianisotropic metasurfaces. By changing the intrinsic coupling strength, full amplitude coverage and 0-to-π phase switching of Smith-Purcell radiation can be realized from the meta-atoms. This unusual property shifts the wave front of the assembled Airy beam toward a parabolic trajectory. Experimental implementation displays that evanescent fields bounded at slotted waveguides can be coupled into Airy beams via Smith-Purcell radiation from a designed bianisotropic metasurface. Our method and design strategy offer an alternative route toward free-electron lasers with diffraction-free, self-accelerating, and self-healing beam properties.

Highly efficient wavefront control through bianisotropic (Willis coupling) metasurfaces

Recent advances in metasurfaces have shown that efficient wavefront transformation can be obtained by carefully controlling the bianisotropic response (Willis coupling) in the metasurfaces. However, applying such a scheme directly for complicated wavefront transformations will lead to nonreciprocal or nonlocal response, which is difficult to realize physically. Here we identify the local power conservation requirements and introduce several strategies to meet such requirements. Our strategies include designing surface waves into the scattered field, designing power-flow conformal metasurfaces, and combining coordinate transformation with bianisotropic metasurfaces. Based on such schemes, we develop a series of bianisotropic metasurfaces design approachs for near-perfect arbitrary wavefront control. These passive and reciprocal design approaches enable experimental demonstration of highly efficient, arbitrary wavefront transformation.

Electromagnetic characteristics of a metasurface composed of chiral spheres under normal incidence

A general methodology for obtaining the reflection and transmission dyads from a general bianisotropic metasurface under oblique incidence have been developed. In this case, it is assumed that the normal electric and magnetic surface polarization currents are negligible. The proposed theory is then used for a metasurface composed of chiral spheres under normal illumination. It is shown that due to an additional parameter of chirality, one can control the reflection, transmission and absorption properties of a metasurface. It is also studied that a metasurface composed of chiral spheres may be used as almost perfect reflector, perfect anti-reflector and good absorber having some specific geometrical and electromagnetic parameters. For completeness, the analytic expressions for electric, magnetic, electromagnetic and magnetoelectric polarizibilities of a chiral sphere have also been derived using Mie theory.

A Beam-Splitting Bianisotropic Metasurface Designed by Optimization and Machine Learning

Electromagnetic metasurfaces have attracted significant interest recently due to their low profile and advantageous applications. Practically, many metasurface designs start with a set of constraints for the radiated far-field, such as main-beam direction(s) and side lobe levels, and end with a non-uniform physical structure for the surface. This problem is quite challenging, since the required tangential field transformations are not completely known when only constraints are placed on the scattered fields. Hence, the required surface properties cannot be solved for analytically. Moreover, the translation of the desired surface properties to the physical unit cells can be time-consuming and difficult, as it is often a one-to-many mapping in a large solution space. Here, we divide the inverse design process into two steps: a macroscopic and microscopic design step. In the former, we use an iterative optimization process to find the surface properties that radiate a far-field pattern that complies with specified constraints. This iterative process exploits non-radiating currents to ensure a passive and lossless design. In the microscopic step, these optimized surface properties are realized with physical unit cells using machine learning surrogate models. The effectiveness of this end-to-end synthesis process is demonstrated through measurement results of a beam-splitting prototype.

One-sided destructive quantum interference from an exceptional-point-based metasurface

We propose the concept of one-sided quantum interference based on non-Hermitian metasurfaces.By designing bianisotropic metasurfaces with a non-Hermitian exceptional point, we show that quantum interference can exist only on only one side but not another. This is the quantum inheritance of unidirectional zero reflection in classical optics.The one-side interference can be further manipulated with tailor-made metasurface. With two photons simultaneously entering the metasurface from different sides, the probability for only outputting one photon on the side with reflection can be modified to zero as a one-sided destructive quantum interference while the output on another side is free of interference. We design the required bianisotropic metasurface and numerically demonstrate the proposed effect. The non-Hermitian bianisotropic metasurfaces provide more degrees of freedom in tuning two-photon quantum interference, in parallel to the celebrated Hong-Ou-Mandel effect.

Design and development of a multi-functional bi-anisotropic metasurface with ultra-wide out of band transmission

This paper presents a multi-functional bi-anisotropic metasurface having ultra-wide out of band transmission characteristics. The proposed metasurface is comprised of 90° rotated T-shaped configuration yielding greater than or equal to 50% out-of-band transmission from above L- to X-band. Moreover, this metasurface achieves a maximum of 99% out-of-band transmission at lower frequency bands (i.e., L-band). The simultaneous absorptive and controlled reflection functionalities are achieved at 15.028 to 15.164 GHz along with polarization-insensitive and angular stable properties. The proposed metasurface yields state-of-the-art features compared to already published papers and has broader scope for Fabry Perot cavity, Radar cross-section (RCS) reduction, electromagnetic compatibility and interference (EMC/I) shielding, selective multi-frequency bolometers, ultrathin wave trapping filters, sensors and beam-splitters in the microwave domain.

Giant Asymmetric Second-Harmonic Generation in Bianisotropic Metasurfaces Based on Bound States in the Continuum

Bianisotropy is a powerful concept enabling asymmetric optical response, including asymmetric reflection, absorption, optical forces, light trapping, and lasing. The physical origin of these asymmetric effects can be understood from magnetoelectric coupling and asymmetrical field enhancement. Here, we theoretically propose highly asymmetric second-harmonic generation in bianisotropic AlGaAs metasurfaces. We show that the ratio of second-harmonic power for the forward and backward illuminations can be increased by around four orders of magnitude. This effect is obtained by altering geometrical parameters that coincide with quasi-bound states in the continuum. Our study paves the way toward a directional generation of higher-order waves and can be potentially useful for nonlinear holograms.