Nonreciprocal Metasurface Research Articles

Magneto-optical chiral metasurfaces for achieving polarization-independent nonreciprocal transmission.

Nonreciprocal transmission, resulting from the breaking of Lorentz reciprocity, plays a pivotal role in nonreciprocal communication systems by enabling asymmetric forward and backward propagations. Metasurfaces endowed with nonreciprocity represent a compact and facile platform for manipulating electromagnetic waves in an unprecedented manner. However, most passive metasurfaces that achieve nonreciprocal transmissions are polarization dependent. While incorporation of active elements or nonlinear materials can achieve polarization-independent nonreciprocal metasurfaces, the complicated configurations limit their practical applications. To address this issue, we propose and demonstrate a passive and linear metasurface that combines magneto-optical and chiral effects, enabling polarization-independent isolation. The designed metasurface achieves a transmittance of up to 80%, with a high contrast between forward and backward propagations. Our work introduces a novel mechanism for nonreciprocal transmission and lays the foundation for the development of compact, polarization-insensitive nonreciprocal devices.

Nonreciprocal Pancharatnam-Berry metasurface for unidirectional wavefront manipulations.

Optical metasurfaces employing the Pancharatnam-Berry (PB) geometric phase, called PB metasurfaces, have been extensively applied to realize spin-dependent light manipulations. However, the properties of conventional PB metasurfaces are intrinsically limited by the Lorentz reciprocity. Breaking reciprocity can give rise to new properties and phenomena unavailable in conventional reciprocal systems. Here, we propose a mechanism to realize nonreciprocal PB metasurfaces of subwavelength thickness by using the Faraday magneto-optical (FMO) effect of yttrium iron garnet (YIG) material in synergy with the PB geometric phase of spatially rotating meta-atoms. Using full-wave numerical simulations and multipole analysis, we show that the metasurface composed of dielectric cylinders and a thin YIG layer can achieve high isolation of circularly polarized lights, attributed to the enhancement of the magneto-optical effect by the resonant Mie modes and Fabry-Pérot (FP) cavity mode. In addition, the metasurface can enable unidirectional wavefront manipulations of circularly polarized lights, including nonreciprocal beam steering and nonreciprocal beam focusing. The results contribute to the understanding of the interplay between nonreciprocity and geometric phase in light manipulations and can find applications in optical communications, optical sensing, and quantum information processing.

Tunable nonreciprocal metasurfaces based on a nonlinear quasi-bound state in the continuum.

Nonreciprocal devices are essential and crucial in optics for source protection and signal separation. A hybrid grating system consisting of a silicon grating, a graphene layer, and a silicon waveguide layer is employed to create a high-Q quasi-BIC (bound state in the continuum). Then, the high-Q properties of the quasi-BIC are harnessed to enhance the third-order nonlinear effect of silicon, thereby improving the nonreciprocal characteristics of the device. The nonreciprocal transmittance ratio of the device can be tunable by adjusting the graphene Fermi energy level, achieving tunability ranging from 0.0865 to 30.57 dB. It also enables the best performance of the device over a wider range of frequency bands. This study provides a new, to the best of our knowledge, method for designing tunable nonreciprocal devices with a wide range of potential applications.

SCATTERING OF ELECTROMAGNETIC WAVES ON FLAT GRID TWO-PERIODIC STRUCTURES

Context. One of the scientific hypotheses for the creation of nonreciprocal optical metasurfaces is based on the use of a wave channel in which rays of the direct and reverse diffraction scenarios are realized on two-periodic flat structures with nonlinear elements. Such processes in the nanometer wavelength range of electronic devices require precise calculations of the interaction of waves and microstructures of devices. It is also important to describe the behavior of antenna devices in mobile communications. Expanding the wavelength range of stable communication is achieved by using prefractal structures in antenna devices in combination with periodic structuring. Similar modeling problems arise when electromagnetic waves penetrate materials with a crystalline structure (radio transparency).
 Objective. To test this hypothesis, it is necessary to carry out mathematical modeling of the process of scattering of electromagnetic waves by metasurfaces under conditions of excitation of several diffraction orders. It is known that among two-periodic flat lattices of different structures there are five types that fill the plane. These are the Bravais grilles. The problem of scattering of an incident monochromatic TE polarized wave on a metal screen with recesses in two-periodic structures filled with silicon was considered.
 Method. The paper builds mathematical models for the study of spatial-amplitude spectra of metasurfaces on Brave lattices and gives some results of their numerical study. The condition for determining the diffraction orders propagating over the grating is proposed. Scattered field amplitudes are from the solution of the boundary value problem for the Helmholtz equation in the COMSOL Multiphysics 5.4 package. Similar problem formulations are possible when studying the penetration of an electromagnetic field into a crystalline substance.
 Results. Obtained relations for diffraction orders of electromagnetic waves scattered by a diffraction grating. The existence of wavelengths incident on a two-periodic lattice for which there is no reflected wave is shown for different shapes (rectangular, square, hexagonal) of periodic elements in the center of which a depression filled with silicon was made. Distributions of reflection coefficients for different geometric sizes of colored elements and recesses are given. The characteristics of the electric field at resonant modes in the form of modulus isolines show the nature of the interaction of the field over the periodic lattice and the scatterersdepressions. At the resonant wavelengths of the incident waves, standing waves appear in the scatterers.
 Conclusions. A mathematical model of the set of diffraction orders propagating from a square and hexagonal lattice into halfspace is proposed z >= 0. It has been shown that flat periodic lattice with square or hexagonal periodicity elements and resonant scatterers in the form of cylindrical recesses filled with silicon can produce a non-mirrored scattered field in metal. The response of the lattices to changes in the wavelength of the incident field by the structure of diffraction orders of the scattered field and high sensitivity to the rotation of the incident plane were revealed. The two-periodic lattices have prospects for creating anti-reflective surfaces of various devices. Two-periodic lattices have prospects for creating anti-reflective surfaces for various devices, laser or sensor electronic devices, antennas in mobile communication elements, and radio transparency elements. They have more advanced manufacturing technologies in relation to spatial crystal structures.

Independent Manipulations of Transmitting and Receiving Channels by Nonreciprocal Programmable Metasurface.

The electromagnetic (EM) beam manipulations such as spatial scanning have always been the focus in information science and technology. Generally, the transmitting and receiving (T/R) beams of the same aperture should be coincident due to the reciprocal theory, and hence, more flexible controls of the spatial information are limited accordingly. Here, we propose a new approach to achieve independent controls of beam scanning in spatial T/R channels based on one aperture made by a nonreciprocal programmable metasurface. The meta-atom is designed to have independent propagation chains for T/R waves by introducing dual-direction power amplifiers (PAs) as the isolators for one-way transparency. A programmable phase shifter with a 360° coverage is loaded with the PA device in the transmitting or receiving chain to realize independent beam scanning in the T/R channels. A prototype of the proposed metasurface is fabricated, and independent beam scanning in the T/R channels is directly acquired with good performance in our measurements. In addition, a proof of concept of integrated sensing and auxiliary communications is accomplished to verify the validity of the presented method.

Wide-angle camouflage detectors by manipulating emissivity using a non-reciprocal metasurface array.

Camouflage detectors that can detect incoming radiation from any angle without being detected are extremely important in stealth, guided missile, and heat-seeking missile industries. In order to accomplish this, the absorption and emission processes must be manipulated simultaneously. However, Kirchhoff's fundamental law suggests that absorption and emission are always in the same direction α(θ) = ε(θ), i.e., absorption and emission are reciprocal. This means that the emission from the detector always points back to the source, for example towards a laser source in a guided missile. Thus, detector emission serves as a complementary measure to hide an object. Here, we present a novel camouflage detector that uses a nonreciprocal metasurface array to independently detect the direction of the incoming radiation as well as manipulate its emissivity response. This is accomplished by using a magneto-optical material called indium arsenide (InAs), which breaks Lorentz reciprocity and Kirchhoff's fundamental law such that α(θ) ≠ ε(θ). This design results in the following absorption and emission: α(θ) = ε(-θ). Nine metasurfaces were designed, optimized, and operated at different incident angles from +50° to -50° at a wavelength of 13 μm. Furthermore, by keeping all metasurfaces in a pixilated array form, one could make a device that works over the full ±50° range. Potentially, this array of nonreciprocal metasurfaces can be used to fabricate thermal emitters or solar-harvesting systems.

Topological Singularity Pair in Nonreciprocal Non-Hermitian Lossy-dielectric Gyromagnetic Metasurface

The transmission properties of a nonreciprocal non-Hermitian metasurface composed of lossy dielectric and gyromagnetic materials are studied with closed-form analytical solutions. It is found that a pair of opposite topological singularities of isolation ratio exist in a two-dimensional (2D) parameter space, and their evolution with parameter adjustment is preliminarily studied. These results provide clues for the design of optical devices. Further study of the mechanism of this singular behavior may reveal the complex systematic effect of non-Hermiticity and non-reciprocity on optical performance.

Passive bias-free non-reciprocal metasurfaces based on thermally nonlinear quasi-bound states in the continuum

Passive bias-free non-reciprocal metasurfaces based on thermally nonlinear quasi-bound states in the continuum

Non-reciprocal electromagnetic metasurface based on the nonlinearity of a liquid metamaterial.

In this Letter, we propose the design of a non-reciprocal electromagnetic metasurface, which uses a highly nonlinear liquid metamaterial as a source of non-reciprocity. We show that the proposed metasurface, whose thickness is comparable to the wavelength, can exhibit a difference between the transmission coefficients in the forward and backward directions of up to 0.95. Moreover, the particularly high nonlinearity of the liquid metamaterial enables the radiation power required to induce the nonlinear effects to be considerably reduced compared with natural materials. The feasibility of the proposed metasurface has been verified by numerical simulations.

MATHEMATICAL MODELING OF SCATTERING OF ELECTROMAGNETIC WAVES ON PLANE TWO-PERIODIC STRUCTURES

One of the scientific hypotheses for the creation of nonreciprocal optical metasurfaces is based on the use of a wave channel in which rays of the direct and reverse diffraction scenarios are realized on two-periodic flat structures with nonlinear elements.
 To test this hypothesis, it is necessary to carry out mathematical modeling of the process of scattering of electromagnetic waves by metasurfaces under conditions of excitation of several diffraction orders. It is known that among two-periodic flat lattices of different structures there are five types that fill the plane. These are the Bravais lattices. The problem of scattering of an incident monochromatic TE polarized wave on a metal screen with recesses in two-periodic structures filled with silicon was considered.
 In this paper, mathematical models are constructed for studying the spatial amplitude spectra of metasurfaces on Bravais lattices and some results of their numerical study are presented. Relationships are obtained for the diffraction orders of scattered electromagnetic waves by a diffraction grating. The existence of wavelengths of incident waves on a two-periodic grating for which there is no reflected wave is shown for different shapes (rectangular, square, hexagonal) of periodic elements in the center of which a recess filled with silicon was made. The distributions of the reflection coefficient are given for various geometric dimensions of periodic elements and recesses.

A self-biased non-reciprocal magnetic metasurface for bidirectional phase modulation

Non-reciprocal metasurfaces can encode optical functions on forward- and backward-propagating waves, and could be used to create non-reciprocal antennas and radomes for full-duplex wireless communication and radar systems. However, such metasurfaces typically require external electric- or magnetic-field biasing or rely on non-linear effects, which makes practical implementation challenging. Here we report a self-biased non-reciprocal metasurface based on magnetic meta-atoms made from lanthanum-doped barium hexaferrite. The metasurface offers a transmittance of up to 77% and an operation angle of ±64°. We show that they can be used for on-demand bidirectional phase modulation, which provides non-reciprocal functionalities including microwave isolation, non-reciprocal beam steering, non-reciprocal focusing and non-reciprocal holography. The approach could also be potentially extended to megahertz and optical frequencies by using different self-biased magnetic materials.

Nonreciprocal transmission of electromagnetic waves with nonlinear active plasmonic metasurfaces

Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as an ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two silver nanostripes with different dimensions placed on both sides of an ultrathin active dielectric spacing layer. The introduced gain leads to an exceptional point formation in the linear operation regime, where unidirectional perfect transmission due to loss compensation is achieved from the presented non-Hermitian parity-time symmetric nanoscale system. The demonstrated plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when its nonlinear response is considered, mainly due to its spatially asymmetric geometry combined with enhanced nonlinearity. Strong and broadband nonreciprocity with unity transmission contrast from opposite directions is realized under relatively low-input laser intensity values. Furthermore, asymmetric nonlinear third-harmonic generation with a pronounced contrast is also illustrated by the same nonreciprocal metasurface design. The findings of this work can lead to the realization of various self-induced nonreciprocal nanophotonic configurations, such as integrated ultrafast switches, ultrathin protective coatings, and asymmetric directional imaging devices.

Tunable optical traps over nonreciprocal surfaces.

We propose engineering optical traps over plasmonic surfaces and precisely controlling the trap position with an external bias by inducing in-plane nonreciprocity on the surface. The platform employs an incident Gaussian beam to polarize targeted nanoparticles, and exploits the interplay between nonreciprocal and spin-orbit lateral recoil forces to construct stable optical traps and manipulate their position within the surface. To model this process, we develop a theoretical framework based on the Lorentz force combined with nonreciprocal Green's functions and apply it to calculate the trapping potential. Rooted on this formalism, we explore the exciting possibilities offered by graphene to engineer stable optical traps using low-power laser beams in the mid-IR and to manipulate the trap position in a continuous manner by applying a longitudinal drift bias. Nonreciprocal metasurfaces may open new possibilities to trap, assemble and manipulate nanoparticles and overcome many challenges faced by conventional optical tweezers while dealing with nanoscale objects.

Retroreflector With Polarization Isolation Based on Nonreciprocal Metasurface

This letter proposes a nonreciprocal metasurface retroreflector, which introduces isolation between two orthogonal polarizations. The proposed metasurface is based on the transistor-loaded nonreciprocity mechanism and receiving-reradiating structure. Benefitting from this design, the nonreciprocal polarization response introduced by the backloaded transistor amplifier can provide isolation between the two orthogonal polarizations. Besides, the unit cross-polarized reflection phase can be designed arbitrarily to realize desired phase gradient. As a result, the metasurface converts vertically polarized waves to horizontally polarized waves with amplification and retroreflects at a specific incident angle. Meanwhile, absorption occurs when horizontally polarized waves are incident at the same angle. A prototype is fabricated and measured. The measured results prove that the proposed metasurface can realize retroreflection at a 30° incident angle with cross-polarization isolation of 29.42 dB at 5.44 GHz. The proposed metasurface is expected to expand applications of planar retroreflectors.

Breaking Transmission Symmetry Without Breaking Reciprocity in Linear All-Dielectric Polarization-Preserving Metagratings

Transmission asymmetry in reciprocal systems offers an appealing alternative to bulkier nonreciprocal implementations for certain applications. Common reciprocal routes to transmission asymmetry of linearly polarized light involve a rotation of its polarization. Here, we explore a different route with a linear all-dielectric metagrating that preserves polarization, while lacking inversion symmetry along the surface-normal direction. Our all-angle transmission calculations reveal an abrupt transition from a symmetric to an asymmetric transmission response that traces the Bragg critical wavelength of higher-order beam emergence as a function of the incident angle. By adopting an analogy between scattering from a multiport network and the metagrating paradigm we establish why the only necessary condition for transmission symmetry breaking in this class of systems is the emergence of any higher-order Bragg diffracted beam. We further show how such a transmission symmetry breaking is consistent with reciprocity and also demonstrate the underpinning symmetry-breaking mechanism with a first-principles numerical experiment. Finally, we shed light on some previous misconceptions regarding transmission symmetry breaking related to the role of the substrate or need for change of diffraction order number at each interface. Our proposed metagrating can exhibit a strong transmission asymmetry, with contrast that can be as high as approximately 75%, thus underlining its potential as a blueprint for passive asymmetric or nonlinear self-biasing nonreciprocal metasurfaces relevant to integrated and active photonics.

Nanophotonic devices based on magneto-optical materials: recent developments and applications

Abstract Interaction between light and magnetism in magneto-optical (MO) nanophotonic devices has been actively studied in the past few years. The recent development of MO all-dielectric resonators and metasurfaces has led to the emergence of various novel MO phenomena that were not observed in their bulk counterparts. For example, a large s-polarized transverse MO Kerr effect can be observed at magnetic resonance wavelength, which cannot exist in the bare MO films. We review recent developments in nanophotonic devices based on MO materials and focus on different modes and related MO effects in nanophotonic structures with emphasis on recently discovered new MO phenomena in magnetoplasmonics and all-dielectric nanostructures, such as dark mode, all-dielectric Mie resonance and waveguide mode. Further, we discuss the potential applications of these nanostructures for biological/chemical sensing, magnetic field sensing, and magnetic field-controlled active and nonreciprocal metasurfaces.

Nonreciprocal Control of Electromagnetic Polarizations Applying Active Metasurfaces

AbstractThe electromagnetic (EM) controls of spatial nonreciprocal performance have gained much attention due to the extreme attraction of the unusual physical phenomenon. Especially, the emergence of metasurfaces, has enormously promoted the design of manipulating the spatial wave and field to achieve various EM functions. Herein is proposed a method to realize the spatial nonreciprocal manipulation for the EM polarizations using an active metasurface which elaborately integrates the elements of power amplifiers. The proposed nonreciprocal metasurface can have the distinct transmission appearance of EM polarizations. When the spatial wave impinges on metasurface with vertical polarization from one side, the transmitted wave has the same polarization; however, when the spatial wave with vertical polarization is incident from the opposite side, the polarization of the transmitted wave will be converted to horizontal. This novel performance is realized by using the meta‐atoms composed of three cascaded components: receiving component, nonreciprocal guiding system, and transmitting component. A prototype of the active metasurface is fabricated and measured, and good agreement is observed between the simulation and measurement results.

Dual-Polarized Nonreciprocal Spatial Amplification Active Metasurface

This letter presents a dual-polarized nonreciprocal active metasurface (MTS) for spatial electromagnetic wave amplification. The MTS is designed based on receiving, amplifying, and reradiating scheme. Two square patch antennas printed on the opposite sides of two-layer substrates are used as receiving and reradiating antennas, respectively. A T-shaped microstrip line coupling structure is adopted to achieve impedance matching and dc-block function. Two bipolar transistors, which are place on a pair of orthogonal microstrip feed lines, are used as amplifiers to achieve dual polarization spatial amplification of the electromagnetic wave. A prototype MTS is fabricated and tested to verify the design. The measured effective spatial amplification coefficients higher than 0 dB has a band from 4.88 to 5.19 GHz with the peak value of 10.4 dB. Besides, the proposed MTS is insensitive to polarization. The spatial amplification coefficients variation for different polarization is below 0.6 dB. The proposed MTS could be used in the radio frequency transceiver systems to reduce the pressure of the amplifier on the circuit board.

Transistor-Loaded Nonmagnetic Nonreciprocal Metasurfaces

Reciprocal metasurfaces represent prominent compact structures for bidirectional and frequency-invariant transformation of electromagnetic waves. However, their application to modern wireless communication are limited due to their reciprocal response and lack of a controllable, versatile and programmable mechanism. On the other hand, nonreciprocal metasurfaces are capable of steering electromagnetic waves unidirectionally, leading to functionalities that are far beyond the capabilities of conventional static metasurfaces. In addition, they are controllable and programmable through their active constituents. This paper provides an overview on recent progress and opportunities offered by transistor-loaded metasurfaces to break reciprocity, and discusses their potential for low-energy, compact, and integrated nonreciprocal devices and systems.

Magnetless reflective gyrotropic spatial isolator metasurface

We present the concept of a magnetless reflective gyrotropic spatial isolator (RGSI) metasurface. This is a birefringent metasurface that reflects vertically polarized incident waves into a horizontally polarized waves, and absorbs horizontally polarized incident waves, hence providing isolation between the two orthogonal polarization. We first synthesize the metasurface using surface susceptibility-based generalized sheet transition conditions. We then propose a mirror-backed metaparticle implementation of this metasurface, where transistor-loaded resonators provide the desired magnetless nonreciprocal response. Finally, we demonstrate the metasurface by full-wave simulation results. The proposed RGSI metasurface may be used in various electromagnetic applications, and may also serve as a step towards more sophisticated magnetless nonreciprocal metasurface systems.