Nonreciprocal Metasurface Research Articles

Active switching control of field-localized waveguide using time-modulated non-reciprocal reconfigurable metasurface

Active metamaterials with embedded control allow reconfiguring of the physical properties needed for wave propagation control. This flexibility provides useful features important for various waveguiding applications. Previous approaches to active metamaterials could change the material properties; however, their overall physical properties remained static. Time-modulated metamaterials, which can change physical properties dynamically, have recently attracted great interest. In this work, we present a time-varying, non-reciprocal, reconfigurable metasurface, which achieves active switching control of the field-localized waveguide. An experimental demonstration of the non-reciprocal metasurface is presented using two examples of sub-wavelength waveguides operating at 13 MHz and 13.5 MHz. The dynamic reconfiguring capability provides a wide range of versatile, practical applications, overcoming static passive metamaterials’ limitations. Because the waveguide’s location and shape can be controlled readily, the proposed method can be used for a broad range of applications that require dynamic wave propagation control.

Free-Space Nonreciprocal Transmission Based on Nonlinear Coupled Fano Metasurfaces

Optical nonlinearities can enable unusual light–matter interactions, with functionalities that would be otherwise inaccessible relying only on linear phenomena. Recently, several studies have harnessed the role of optical nonlinearities to implement nonreciprocal optical devices that do not require an external bias breaking time-reversal symmetry. In this work, we explore the design of a metasurface embedding Kerr nonlinearities to break reciprocity for free-space propagation, requiring limited power levels. After deriving the general design principles, we demonstrate an all-dielectric flat metasurface made of coupled nonlinear Fano silicon resonant layers realizing large asymmetry in optical transmission at telecommunication frequencies. We show that the metrics of our design can go beyond the fundamental limitations on nonreciprocity for nonlinear optical devices based on a single resonance, as dictated by time-reversal symmetry considerations. Our work may shed light on the design of flat subwavelength free-space nonreciprocal metasurface switches for pulsed operation which are easy to fabricate, fully passive, and require low operation power. Our simulated devices demonstrate a transmission ratio >50 dB for oppositely propagating waves, an operational bandwidth exceeding 600 GHz, and an insertion loss of <0.04 dB.

Возбуждение анизотропной импедансной метаповерхности в виде эллиптического цилиндра.

The problem of arbitrary excitation of waves by a system of external sources near an anisotropic metasurface in the form of an elliptical cylinder with a surface homogenized impedance tensor of general form is solved. The solution to the problem is written as a superposition of E- and H-waves in elliptical coordinates. The partial reflection coefficients of waves were found from the boundary conditions using the orthogonality of the Mathieu angular functions. For these coefficients, four coupled infinite systems of linear algebraic equations of the second kind are obtained. The conditions under which the solution of the excitation problem by the method of eigenfunctions is obtained in an explicit form are found and analyzed. It is shown that for this, the surface impedance tensor of a uniform metasurface must belong to a class of deviators (have zero diagonal elements). In the particular case of a mutual (most easily realized) metasurface, its impedance tensor should only be reactance. In another special case, the impedance tensor of a set of deviators describes a class of anisotropic nonreciprocal metasurfaces with the so-called perfect electromagnetic conductivity (PEMC).

Electromagnetic characteristics of a metasurface composed of uncoated and coated spherical particles

A general theory for the reflection, transmission and equivalent surface impedance of a symmetric and non-reciprocal metasurface composed of dipole scatterers has been developed under oblique incidence. The proposed theory is then used for metasurfaces composed of uncoated and coated spheres under normal illumination. The materials considered for uncoated and coated spheres are realistic, i.e., magnetodielectric and Gallium Arsenide. Some specific designs of anti-reflector, reflector and absorber metasurfaces with the free space background have been discussed. It is also observed that isorefractive background media can be used to enhance the reflection magnitude from a metasurface composed of Gallium Arsenide coated magnetodielectric spheres as compared to the free space background under some specified conditions.

Topological theory for perfect metasurface isolators

We introduce topological theory of perfect isolation: perfect transmission from one side and total reflection from another side simultaneously. The theory provides an efficient approach for determining whether such a perfect isolation point exists within a finite parameter space. Herein, we demonstrate the theory using an example of a Lorentz non-reciprocal metasurface composed of dimer unit cells. Our theory also suggests that perfect isolation points can annihilate each other through the coalescence of opposite topological charges. Our findings could lead to novel designs for high-performance optical isolators.

Magnetic field driven light control by hybrid magneto-optical metasurfaces

Demonstration of magneto-optical effects enhancement in hybrid Ni/Si metasurfaces is reported. Our results show that the polarization plane rotation angle reaches the considerable value with a change of a sign in a very narrow spectral range in the vicinity of the optical magnetic dipole Mie - type resonance. The specific Faraday rotation is 160 degrees/μm, which is an outstanding effect for an ultra-thin magnetic material with a thickness of only 5 nm as used in this work. These results could be used for the creation of active nonreciprocal photonic nanostructures and metasurfaces.

Time-moduated nonreciprocal metasurface absorber for surface waves.

We have investigated a magnet-free, nonreciprocal surface wave absorber based on high impedance surfaces (HISs) using a spatial-temporal modulation approach. By controlling embedded switches with a travelling wave, the HIS metasurface is modulated to break the time and spatial symmetry, which enables surface waves to propagate in one direction but be absorbed when propagating in the reverse direction. The nonreciprocity has been demonstrated by an EM-circuit co-simulation. We envision that this could be possibly applied in future communication systems that preferably transmit unidirectionally but absorb interference from the reverse direction caused by reflections or other devices.

Nonreciprocal metasurface with space\u2013time phase modulation

Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations on wave propagation. However, it is challenging to realize efficient and ultrafast temporal modulation in a photonic system. Here, leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface, we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The metasurface, with travelling-wave modulation upon nonlinear Kerr building blocks, creates spatial phase gradient and multi-terahertz temporal phase wobbling, which leads to unidirectional photonic transitions in both the momentum and energy spaces. We observed completely asymmetric reflections in forward and backward light propagations over a large bandwidth around 5.77 THz within a sub-wavelength interaction length of 150 nm. Our approach highlights a potential means for creating miniaturized and integratable nonreciprocal optical components.

Digital Nonlinear Metasurface with Customizable Nonreciprocity

AbstractRecently, investigation of metasurface has been extended to nonreciprocity through breaking time‐reversal symmetry. Among a number of magnetless strategies, nonlinearity is an important nonreciprocal principle amenable to the metasurface. As passive analog‐based devices, most of the existing nonlinear nonreciprocal metasurfaces are inherently characterized by the relatively unchangeable performance, sharp frequency response, and hysteresis loops. Here, an analog–digital–analog mechanism is proposed to realize the nonlinear nonreciprocity, which provides a digitally reconfigurable solution for a family of nonreciprocal performance within a shared hardware architecture. This concept is validated by a metasurface prototype with the integration of a digital module at microwave frequencies. Based on the proposed mechanism, the properties can be customized as demanded, ranging from a normal reciprocal response to a variety of nonreciprocal functions, including electromagnetic (EM) diode and EM unidirectional limiting functions, which have been experimentally demonstrated with direction reversibility and threshold tunability. The proposed metasurface is also underpinned by the nonhysteretic performance and wide operating bandwidth, thereby potentially making it an inexpensive and stable candidate for advanced manipulations of EM waves.

Numerical demonstration of non‐reciprocal waveform‐selective metasurfaces

The authors demonstrate two types of non-reciprocal waveform-selective metasurfaces that control the level of reflectance, depending not only on the direction of the incident wave but also on the waveform, namely, on the pulse width. A capacitor-based waveform-selective metasurface exhibits strong reflectance for a short pulse compared to that for a continuous wave (CW) at the same frequency. In contrast, an inductor-based waveform-selective metasurface more strongly reflects a CW than a pulse. The difference between them is demonstrated to be more than 50% but is reduced for a wave propagating in the other direction. The concept of the authors' structures is expected to offer an additional degree of freedom with which to design non-reciprocal devices and applications.

Optical Circulation and Isolation Based on Indirect Photonic Transitions of Guided Resonance Modes

While there has been enormous progress in meta-surface designs, most meta-surfaces are constrained by Lorentz reciprocity. Breaking reciprocity, however, enables additional functionalities and greatly expands the applications of meta-surfaces. Here, we introduce a realistic nonreciprocal meta-surface that can achieve optical circulation and isolation. This device consists of a photonic crystal slab that supports two bands of guided resonances, and upon a temporal modulation in each unit cell with a spatially varying phase, an indirect photonic transition can be induced between the guided resonances, which breaks Lorentz reciprocity without the use of magneto-optic materials. We provide direct first-principle numerical simulations, using the multifrequency finite-difference frequency-domain method, to demonstrate that this device can achieve optical circulation and isolation with no back reflection under realistic modulation frequency and modulation strength.

Magnet-free nonreciprocal bianisotropic metasurfaces

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.

Dynamic non-reciprocal meta-surfaces with arbitrary phase reconfigurability based on photonic transition in meta-atoms

We introduce a distinct class of dynamic non-reciprocal meta-surfaces with arbitrary phase-reconfigurability. This meta-surface consists of an array of meta-atoms, each of which is subject to temporal refractive index modulation, which induces photonic transitions between the states of the meta-atom. We show that arbitrary phase profile for the outgoing wave can be achieved by controlling the phase of the modulation at each meta-atom. Moreover, such dynamic meta-surfaces exhibit non-reciprocal response without the need for magneto-optical effects. The use of photonic transition significantly enhances the tunability and the possible functionalities of meta-surfaces.

Synthesis of electromagnetic metasurfaces: principles and illustrations

The paper presents partial overview of the mathematical synthesis and the physical realization of metasurfaces, and related illustrative examples. The synthesis consists in determining the exact tensorial surface susceptibility functions of the metasurface, based on generalized sheet transition conditions, while the realization deals with both metallic and dielectric scattering particle structures. The examples demonstrate the capabilities of the synthesis and realization techniques, thereby showing the plethora of possible metasurface field transmission and subsequent applications. The first example is the design of two diffraction engineering birefringent metasurfaces performing polarization beam splitting and orbital angular momentum multiplexing, respectively. Next, we discuss the concept of the electromagnetic remotely controlled metasurface spatial processor, which is an electromagnetic linear switch based on destructive interferences. Then, we introduce a non-reciprocal non-gyrotropic metasurface using a pick-up circuit radiator (PCR) architecture. Finally, the implementation of all-dielectric metasurfaces for frequency dispersion engineering is discussed.