We show that the hybrid resonances of a DMR backed by a cavity are meta-resonances, in that they can be made as perfect as possible by fine tuning the structural parameters but without the requirements of extreme materials properties, such as zero dissipation. Instead, dissipation in the DMR is essential for the realization of perfect meta-resonances. We experimentally demonstrate such perfection by tuning the structure of a HMR till its reflection is as low as 0.426 % . Besides the primary meta-resonances that are originated from the strong resonances of the DMR, weak hitchhiker resonances can also produce meta-resonances as perfect as the primary ones. The depth of the reflection dips is insensitive to the strength of the resonances involved, but critically depends on the degree of impedance match to air brought mostly by fine tuning the structure parameters, such as the cavity volume, the mass of the platelet, or the pre-tension in the membrane. Using the eccentricity of the platelet position in the DMR, a number of resonances and anti-resonances are generated, resulting in up to five meta-resonances within the range of 200 Hz to 1000 Hz, with the highest reflection being 7 % and the lowest being 1.2 % . Other means of introducing hitchhiker meta-resonances are also reported.
Here we make use of vanadium dioxide (VO2) to design a bifunctional metasurface working at the same targeted frequency. With the increase of temperature, the functionality of the designed metasurface can switch from a multi-channel retroreflector to a perfect absorber, caused by the phase transition of VO2 from insulator to conductor. Different from traditional bifunctional metasurfaces designed by simple composition of two functionalities, our proposed bifunctional metasurface is based on the interaction between two functionalities. The device shows good potential for the combination of wavef…
Perfect all-angle asymmetric transmission via normal susceptibilities: exact spatial derivative by local meta-atoms and nonlocal metasurfaces
Andrew and Erna Viterbi Faculty of Electrical and Computer Engineering, Technion—Israel Institute of Technology（安德鲁和埃尔纳·维特比电气与计算机工程学院，以色列理工学院）；Photonics Initiative of the Advanced Science Research Center, City University of New York（纽约市立大学高级科学研究中心光子学计划）
We introduce chiral gradient metasurfaces that allow perfect transmission of all the incident wave into a desired direction and simultaneous perfect rotation of the polarization of the refracted wave with respect to the incident one. Besides using gradient polarization densities which provide bending of the refracted wave with respect to the incident one, using metasurface inclusions that are chiral allows the polarization of the refracted wave to be rotated. We suggest a possible realization of the proposed device by discretizing the required equivalent surface polarization densities realized…
We present an asymmetric bifacial graphene metasurface to realize tunable and broadband coherent perfect absorption (CPA) at terahertz (THz) frequencies. The proposed design is composed of two dissimilar graphene square patch metasurfaces stacked over an ultrathin dielectric substrate. Nonlinear CPA performance without resorting to optical nonlinearities and amplification are demonstrated based on wave interference phenomena taking place along and inside this subwavelength structure. Broadband asymmetric reflection from opposite directions is obtained in the case of single wave illumination. T…
Recent advances in metasurfaces have shown the importance of controlling the bianisotropic response of the constituent meta-atoms for maximum efficiency wavefront transformation. Under the paradigm of a bianisotropic metasurface, full control of the local scattering properties is allowed opening new design avenues for creating reciprocal metasurfaces. Despite recent advances in the perfect transformation of both electromagnetic and acoustic plane waves, the importance of bianisotropic metasurfaces for transforming cylindrical waves is still unexplored. Motivated by the possibility of arbitrari…
In this paper we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly non-trivial due to the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces which achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures which allow them to couple to and from leaky and/or evanescent…
This paper investigates the discretization of a periodic metasurface and demonstrates how such a surface can achieve perfect anomalous reflection. Whilst most contemporary theoretical works on metasurfaces deal with continuous current or impedance distributions, we examine how discretization affects a metasurface, and show that in some cases one can discretize a metasurface aggressively --- to the extent of having only two cells per spatial period. Such aggressive discretization can lead to great simplifications in metasurface design, and perhaps more surprisingly, a possible performance impro…
Motivated by the idea of smart metasurfaces, we will demonstrate a graphene-based tunable absorber in which perfect absorption can be achieved for all angles of incidence, only by tuning the Fermi level of graphene. We place an unpatterned graphene sheet on a mushroom-type high impedance surface whose resonant frequency is stable for all incident angles. For TM-polarization, perfect absorption can be realized from normal to grazing incidence at the same frequency when modulating the Fermi level of graphene from 0.18 eV to 1 eV.
Actively tunable and reconfigurable wavefront shaping by optical metasurfaces poses a significant technical challenge often requiring unconventional materials engineering and nanofabrication. Most wavefront-shaping metasurfaces can be considered 'local' in that their operation depends on the responses of individual meta-units. In contrast, 'nonlocal' metasurfaces function based on the modes supported by many adjacent meta-units, resulting in sharp spectral features but typically no spatial control of the outgoing wavefront. Recently, nonlocal metasurfaces based on quasi-bound states in the con…
We demonstrate tunable perfect anomalous reflection with metasurfaces incorporating lumped elements. The tunable capacitance of each element provides continuous control over the local surface reactance, allowing for controlling the evanescent field distribution and efficiently tilting the reflected wavefront away from the specular direction. The performance of the metasurface is evaluated for both TE and TM polarization and for reflection to the first and second diffraction order.
Graded metasurfaces exploit the local momentum imparted by an impedance gradient to transform the impinging wavefront. This approach suffers from fundamental limits on the overall conversion efficiency and it is challenged by fabrication limitations on the maximum spatial resolution. Here, we introduce the concept of meta-gratings, which enables arbitrary wavefront engineering with unitary efficiency and significantly lower demands on the fabrication precision. We employ this concept to design reflective metasurfaces for wavefront steering without limitations on efficiency. A similar approach …
We demonstrate that rotationally symmetric chiral metasurfaces can support arbitrarily sharp resonances with the maximum optical chirality determined by precise shaping of bound states in the continuum (BICs). Being uncoupled from one circular polarisation of light and resonantly coupled to its counterpart, a metasurface hosting the chiral BIC resonance exhibits a narrow peak in the circular dichroism spectrum. We propose a realization of such chiral BIC metasurfaces based on pairs of dielectric bars and validate the concept of maximum chirality by numerical simulations
The unusual viscoelastic properties of silica aerogel plates are efficiently used to design subwavelength perfect sound absorbers. We theoretically, numerically and experimentally report a perfect absorbing metamaterial panel made of periodically arranged resonant building blocks consisting of a slit loaded by a clamped aerogel plate backed by a closed cavity. The impedance matching condition is analyzed using the Argand diagram of the reflection coefficient, i.e., the trajectory of the reflection coefficient as a function of frequency in the complex plane. The lack or excess of losses in the …
Coherent Perfect Absorber (CPA) and Laser is a pair of inversely worked wave structure satisfying the time-reversal relation. We point out that, if the wave energy absorbed by CPA can be recovered by somewhat way and then be reemitted by its corresponded Laser, their combination can be a wave-controlling device that can "move" the wave energy from one place to another. By extending the concepts of CPA and Laser from the previously studied one-dimensional non-Hermitian systems to the two-dimensional ones, this understanding is used to simplify the designing of the metasurface in this letter. As…
Recently, the investigation of metasurface has been extended to wave control through exploiting nonlinearity. Among all of the ways to achieve tunable metasurfaces with multiplexed performances, nonlinearity is one of the promising choices. Although several proposals have been reported to obtain nonlinear architectures at visible frequencies, the area of incorporating nonlinearity in form of passive-designing at microwave metasurfaces is open for investigation. In this paper, a passive wideband nonlinear metasurface is manifested, which is composed of embedded L-shape and Γ-shape meta-atoms wi…
Modern electronic systems operate in complex electromagnetic environments and must handle noise and unwanted coupling. The capability to isolate or reject unwanted signals for mitigating vulnerabilities is critical in any practical application. In this work, we describe the use of a binary programmable metasurface to (i) control the spatial degrees of freedom for waves propagating inside an electromagnetic cavity and demonstrate the ability to create nulls in the transmission coefficient between selected ports; and (ii) create the conditions for coherent perfect absorption. Both objectives are…
Previous research has attempted to minimize the influence of loss in reflection- and transmission-type acoustic metasurfaces. This letter shows that, by treating the acoustic metasurface as a non-Hermitian system and by harnessing loss, unconventional wave behaviors that do not exist in lossless metasurfaces can be uncovered. Specifically, we theoretically and experimentally demonstrate a non-Hermitian acoustic metasurface mirror featuring extremely asymmetrical reflection at the exception point. As an example, the metasurface mirror is designed to have high-efficiency retro-reflection when th…
Huygens' metasurfaces have demonstrated a remarkable potential to perform wave transformations within a subwavelength region. In particular, omega-bianisotropic Huygens' metasurfaces have allowed for the passive implementation of any wave transformation that conserves real power locally. Previous reports have also shown that Huygens' metasurface pairs are capable of realizing transformations that break the local power conservation requirement by redistributing the total power, while the wave propagates between the two metasurfaces. However, the required separation distance overshadows the low-…
In recent years, metasurfaces have shown extremely powerful abilities for manipulation of electromagnetic waves. However, the local electromagnetic response of conventional metasurfaces yields to an intrinsic performance limitation in terms of efficiency, minimizing their implementation in real-life applications. The efficiency of reconfigurable metasurfaces further decreases because of the high density of meta-atoms, reaching 74 meta-atoms per $λ^2$ area, incorporating lossy tunable elements. To address these problems, we implement strong electromagnetic non-local features in a \textit{sparse…
As wireless power transfer (WPT) repeaters, metasurfaces can enhance field coupling, improving the WPT operation. In this paper, we show that metasurfaces can also be used as transmitters of capacitive WPT systems. Such a metasurface-based WPT system can feed multiple receivers and provide robust operation against load or position variations. We formulate an analytical model of such WPT systems. We also discuss the exact solution of a particular example with $N$ identical receivers.
Huygens' metasurfaces are electrically thin devices which allow arbitrary field transformations. Beam refraction is among the first demonstrations of realized metasurfaces. As previously shown for extreme-angle refraction, control over only the electric impedance and magnetic admittance of the Huygens' metasurface proved insufficient to produce the desired reflectionless field transformation. To maintain zero reflections for wide refraction angles, magnetoelectric coupling between the electric and magnetic response of the metasurface, leading to bianisotropy, can be introduced. In this paper, …
Electromagnetic metasurfaces can be characterized as intelligent if they are able to perform multiple tunable functions, with the desired response being controlled by a computer influencing the individual electromagnetic properties of each metasurface inclusion. In this paper, we present an example of an intelligent metasurface which operates in the reflection mode in the microwave frequency range. We numerically show that without changing the main body of the metasurface we can achieve tunable perfect absorption and tunable anomalous reflection. The tunability features can be implemented usin…
A metasurface particle with independent transmission and reflection full phase coverage for circularly polarized waves is introduced. This particle is constituted of two parts, one controlling the power splitting and the reflection phase, and the other one controlling the transmission phase, both leveraging the Pancharatnam-Berry phase principle. Given its unique flexibility, this particle may find various applications in metasurface technology.
Metasurfaces formed of arrays of subwavelength resonators are often tuned to 'critically couple' with incident radiation, so that at resonance dissipative and radiative damping are balanced and absorption is maximised. Such design criteria are typically derived assuming an infinite metasurface, whereas the absorption characteristics of finite metasurfaces, even very large ones, can be markedly different in certain frequency intervals. This is due to the excitation of surface waves, intrinsic to resonant metasurfaces, and especially meta-resonances, namely collective resonances where the surfac…
In this work, we investigated a simple structured graphene terahertz (THz) metasurface (MSF) with perfect absorption, wideband, polarization insensitive, oblique incidence insensitive and frequency reconfiguration. The graphene MSF structure is composed of a two-dimensional periodic array of graphene meta-atoms deposited on the silicon substrate terminated by a metal ground plane. The performance of the proposed MSF is numerically analyzed. An equivalent circuit model of the structure and its closed-form solution is introduced. The graphene MSF thin structure at 2.5 THz provides 100% of absorp…
Metasurface with gradient phase response offers new alternative for steering the propagation of waves. Conventional Snell's law has been revised by taking the contribution of local phase gradient into account. However, the requirement of momentum matching along the metasurface sets its nontrivial beam manipulation functionality within a limited-angle incidence. In this work, we theoretically and experimentally demonstrate that the acoustic gradient metasurface supports the negative reflection for all-angle incidence. The mode expansion theory is developed to help understand how the gradient me…
