Thin Metasurface Research Articles

Single-Layer Dual-Band Microwave Metasurface Cloak of Conducting Cylinder

A single-layer dual-band metasurface cloak for a conducting cylinder illuminated by TM waves is presented. The metasurface is designed with dual-resonance unit cells to fulfil the sheet impedances necessary for scattering cancelation at two frequency bands. The electrically thin reciprocal metasurface cloaking layer is wrapped around the conducting cylinder separated by a dielectric spacer. As an example, the dual-band cloaking of a conducting cylinder with radius of $0.1\lambda _{\mathrm {L}}$ ( $\lambda _{\mathrm {L}}$ is the wavelength in free space at 2 GHz), height of $2.1\lambda _{\mathrm {L}}$ and a spacer distance of $0.03\lambda _{\mathrm {L}}$ is verified. The simulation results show more than 10 and 7 dB reduction in total scattering width, with 5.2% and 4.2% of 5 dB reduction bandwidth, centered at 2 and 3 GHz, respectively. The design is validated experimentally at four bistatic angles.

Shell-type acoustic metasurface and arc-shape carpet cloak

We systematically propose a thin shell-type acoustic metasurface, which could be used to design a carpet cloak that closely covers an arc-shaped object, therefore providing the necessary support for hiding an object with any arbitrary shape. To facilitate the experimental measurement, however, the work here starts with some rotary spherical shell-type and ellipsoidal shell-type cell structures. The measured and calculated sound transmission loss (STL) results of these structures suggest that the sound insulation performances of the shell-type structure are quite different from those of the plate-type structure, indicating a possible break in the shape of the classical sound insulation curve. Considering also that cylindrical shell structures are more widely used in practice than the rotary shell structures, a number of two-dimensional bilayer cylindrical and elliptic cylindrical shell structures were, therefore, designed in this assay. Due to the asymmetry of the structure, the shell-type cells could exhibit bianisotropic sound absorption, reflection and effective parameters. Furthermore, the stiffness of the thin shell structure changed nonlinearly with the changing of the radius of curvature, with a wing shape tendency. In addition, a bilayer cylindrical shell-type acoustic metasurface and an arc-shaped carpet acoustic cloak were successively designed, wherein the phased compensation of differently shaped cell structures could be adjusted by means of a new engineering iso-phase design method. This work could provide the necessary guidance to extend existing results in the field of membrane- and plate-type acoustic metamaterials for shell-type structures, and the realization of the arc-shaped cloak could provide support for the design of a carpet acoustical cloak for use with arbitrary shapes.

Review: recent progress in metal-less metasurfaces and metamaterials

Plasmonic metamaterials (MMs) are based on light coupling to metal free electrons, which inherently accompanied with severe dissipation losses that usually limit device performance. Furthermore, the complexity in designing 3D plasmonic MMs have left them far beyond from being realized in the real market for on-chip nanophotonic applications. Dielectric MMs have alternatively served as an ultimate solution to tackle the low efficiency problem of metal-based MMs. In this review paper, we present optically thin dielectric-based metasurfaces (DMs) that allow for phase and polarization manipulation of light. DMs are essentially based on excitation of double Mie resonance effect, which inherits DMs the ability to modulate light with 2π phase modulation, thus, serving in variety of applications including metalens, beam steering, holograms and optical chirality. Two main classes of DMs are mainly covered which are: Huygens’s metaurfaces and Geometric phase metasurfaces. We first introduce the underlying physics of DMs and propose applications that have been realized with high efficiencies. In the second part, we demonstrate another class of MMs which is semiconductor based hyperbolic metamaterial (HMM). Semiconductor based HMM has two main advantages which are: their ease of fabrication in analogy with DMs and their ability to operate in the mid IR range. We outline HMMs’ applications including thermal harvesting, focusing and mid IR absorption. Finally, we overview the current challenges and future goals for the field of dielectric MMs in order to be realized for real market application.

Atomically thin planar metasurfaces

Enhancing light absorption in two-dimensional (2-D) materials using simple planar structures is important for the development of large-area atomic-scale photonic devices but also has been a major challenge. We theoretically present a general strategy to create an atomically thin planar metasurface consisting of a semiconductor monolayer (replacing the array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector, to achieve near-perfect, polarization-sensitive absorption in 2-D materials. This strategy is validated by experimentally demonstrating planar metasurfaces based on atomically thin Ge films. Higher than 90% exclusive absorption, 2.5 times greater than prior demonstration, is achieved in the MoS2 monolayer for TE polarization. Our strategy can be extended to the infrared spectrum and thus has potential for thermal energy applications.

EM lens design using thin planar metasurfaces for high antenna gain and low SLL applications

A novel electromagnetic (EM) lens with high gain, suppressed side-lobe level (SLL) and low profile is designed using thin planar metasurfaces. The proposed design consists of a source patch antenna and an EM lens based on two thin planar metasurfaces. The two thin metasurfaces with an air gap are placed at the focal distance H of about λ /2.25 above the source patch antenna. This configuration forms a thin planar lens in which the source patch antenna emitted quasi-spherical wave in order to transform into plane wave. The total size of source antenna and an EM lens is same. The miniaturised EM lens antenna faces high SLL, which can be reduced by maintaining the uniform transmission phase coefficient and tunable transmission amplitude coefficient of metasurface unit cells. By implementing this topology on metasurfaces, the SLL of the proposed EM lens can be reduced without affecting the boresight gain of source patch antenna. The measured results of the proposed EM lens achieve 7 dB gain enhancement at designed frequency (5.8 GHz) with the SLL suppression of -24 dB. The total volume of planar lens antenna is 1.17 λ × 1.17 λ × λ /2.25 mm 3 , which is very compact compared with other reported planar lens designs.

Terahertz Absorber With Reconfigurable Bandwidth Based on Isotropic Vanadium Dioxide Metasurfaces

The design of a broadband tunable absorber is proposed based on a thin vanadium dioxide metasurface, which is composed of a simple array of vanadium dioxide and a bottom gold film. When the conductivity of vanadium dioxide is equal to 2000 Ω -1 cm -1 , simulated absorptance exceeds 90% with 71% bandwidth from 0.47 to 0.99 THz and full width at half maximum is 98% from 0.354 to 1.036 THz with center frequency of 0.695 THz. Simulated results show that absorptance peak can be tuned from 5% to 100% when the conductivity changes continually from 10 Ω -1 cm -1 to 2000 Ω -1 cm -1 . The designed absorber may have useful applications in terahertz spectrum such as energy harvesting, thermal emitter, and sensing.

A thin low-frequency broadband metasurface with multi-order sound absorption

We present a theoretical and experimental realization of a thin multi-unit metasurface with multi-order sound absorption that exhibits a continuous near-perfect absorption spectrum in the broadband range of 450 Hz–1360 Hz. The metasurface unit is a perforated composite Helmholtz-resonator (PCHR) that is constructed by inserting one or more separating plates with a small hole into the interior of a Helmholtz resonator (HR). The multi-order sound absorption mechanism can be achieved so that with the original absorption peak and the structural size unchanged, multiple near-perfect peaks are obtained in higher frequencies by a PCHR unit. This extraordinary multi-peak performance is the result of the upgraded multi-degree-of-freedom system with the separating plates, which is explained well by the equivalent acoustic circuit. The specific absorption properties of the PCHR unit are investigated thoroughly with a theoretical approach similar to the plane wave expansion method, and verified via the finite element simulations. On this basis, by precisely balancing the parameters of each unit, the absorption bandwidth of the subwavelength 8-unit metasurface is dramatically broadened about 65% by the proposed mechanism. This work would offer a new guidance for the achievement of the wider absorption band and has great potential in engineering applications.

Metasurface of deflection prism phases for generating non-diffracting optical vortex lattices.

A functional metasurface of both transparent medium slices and multiple deflection prisms is proposed, where phase retardations for generating non-diffracting vortex lattices are integrated and encoded as rotation angles of nano-apertures. Under plane-wave illumination, the transmitted waves from the thin flat metasurface act analogously as multiple beams, each with a designed propagating direction and pre-scribed phase shift, that generate an optical lattice within their overlapping region of space. By altering the design parameters of the metasurface, lattice type and size can be controlled. Both numerical simulations and experiments were conducted, verifying the possibility of the proposed method and the non-diffracting properties of the generated vortex lattices.

Strong Plasmon–Exciton Coupling with Directional Absorption Features in Optically Thin Hybrid Nanohole Metasurfaces

Plasmons and excitons can interact to form new hybridized light–matter states, with a multitude of potential applications including optical logic circuits and single-photon switches. Here, we report the first observation of strong coupling based on optically thin plasmonic nanohole films. The absorptive plasmon resonances of these nanohole films lead to suppressed transmission and Fano-shaped extinction peaks. We prepared silver nanohole films by colloidal lithography, which enables large-scale fabrication of nanoholes distributed in a short-range order. When coated with J-aggregate molecules, both extinction and absorption spectra show clear formation of two separated polariton resonances, with vacuum Rabi splitting on the order of 300 meV determined from anticrossing experiments. In accordance with strong coupling theory, the splitting magnitude increases linearly with the square root of molecular concentration. The extinction peak positions are blue-shifted from the absorption polariton positions, as e...

Wide band radar cross section reduction by thin AMC structure

A new thin and wideband RCS reduction metasurface is designed and fabricated by using two different AMC unit cell tiles arranged in chessboard like configuration. One of these tiles is formed by saltire arrow unit cells, whereas the other one is formed by Jerusalem unit cells which are designed to achieve out phase difference at broad frequency range. This metasurface reduces the RCS more than 10-dB from 15.75 GHz to 41.3 GHz (90% bandwidth). The designed metasurface works properly at both TE and TM propagation modes, also. Moreover, it has stable 10-dB RCS reduction bandwidth for TM propagation mode up to 50° oblique incident angle and 30° for TE ones. These good specifications prove the designed metasurface capability compared with the state of the art references.

Low cost and thin metasurface for ultra wide band and wide angle polarization insensitive radar cross section reduction

A planar low cost and thin metasurface is proposed to achieve ultra-wideband radar cross section (RCS) reduction with stable performance with respect to polarization and incident angles. This metasurface is composed of two different artificial magnetic conductor unit cells arranged in a chessboard like configuration. These unit cells have a Jerusalem cross pattern with different thicknesses, which results in wideband out-phase reflection and RCS reduction, consequently. The designed metasurface reduces RCS more than 10-dB from 13.6 GHz to 45.5 GHz (108% bandwidth) and more than 20-dB RCS from 15.2 GHz to 43.6 GHz (96.6%). Moreover, the 10-dB RCS reduction bandwidth is very stable (more than 107%) for both TE and TM polarizations. The good agreement between simulations and measurement results proves the design, properly. The ultra-wide bandwidth, low cost, low profile, and stable performance of this metasurface prove its high capability compared with the state-of-the-art references.

Design of a randomised arrangement AMC surfaces for RCS reduction based on equivalent circuit modelling

This study aims to present a thin and broadband meta-surface for both mono-static and bi-static radar cross-section (RCS) reduction being able to operate in large range of incident angles and different polarisations. Meta-surfaces are formed through the combination of two artificial magnetic conductor (AMC) cells arranged in periodic and randomised fashions in order to suppress the scattered fields in low levels in all directions as well as minimising the maximum RCS of the metallic surface. By appropriate design of each AMC, and using equivalent circuit method, the reflection coefficients of these surfaces are designed in a way to reach 180 (±37°) phase difference over a wide range of frequencies and incident angles. The bandwidth for 10 dB mono-static RCS reduction is about 53% for both periodic and randomised structures at the normal incidence. It is illustrated that the randomised arrangement structure has 3 dB lower maximum bi-static RCS compared with the periodic structure. Finally, the periodic and randomised arrangement prototypes are fabricated and measurements of their mono-static RCS are conducted. The validity of the study design is fulfilled by achieving an acceptable agreement between the measured and simulated results.

Systematic design and experimental demonstration of bianisotropic metasurfaces for scattering-free manipulation of acoustic wavefronts

Recent advances in gradient metasurfaces have shown that by locally controlling the bianisotropic response of the cells one can ensure full control of refraction, that is, arbitrarily redirect the waves without scattering into unwanted directions. In this work, we propose and experimentally verify the use of an acoustic cell architecture that provides enough degrees of freedom to fully control the bianisotropic response and minimizes the losses. The versatility of the approach is shown through the design of three refractive metasurfaces capable of redirecting a normally incident plane wave to 60°, 70°, and 80° on transmission. The efficiency of the bianisotropic designs is over 90%, much higher than the corresponding generalized Snell’s law based designs (81%, 58%, and 35%). The proposed strategy opens a new way of designing practical and highly efficient bianisotropic metasurfaces for different functionalities, enabling nearly ideal control over the energy flow through thin metasurfaces.

An ultralight and thin metasurface for radar-infrared bi-stealth applications

We present a thin metasurface with large microwave absorptivity and low infrared emissivity simultaneously. By properly tuning the resonance peaks and impedance of the meta-atom, broadband microwave absorptivity greater than 90% from 8.2 to 16.0 GHz is achieved. In the meantime, owing to large coverage of periodic metal patches on the top surface, low infrared emissivity is exhibited in the infrared region (IR) of 8 µm–14 µm. The excellent agreement between numerical simulation and experimental result demonstrates the good performance of the proposed metasurface. Due to the usage of polymethacrylimide (PMI) and polyethylene terephthalate (PET) as the substrate, the metasurface is especially advantageous for the light weight, making it a favorite in real engineering applications.

Single-Layer Plasmonic Metasurface Half-Wave Plates with Wavelength-Independent Polarization Conversion Angle

Manipulation of polarization state is of great fundamental importance and plays a crucial role in modern photonic applications such as optical communication, imaging, and sensing. Metamaterials and metasurfaces have attracted increasing interest in this area because they facilitate designer optical response through engineering the composite subwavelength structures. Here we propose a general methods of designing half-wave plate and demonstrate in the near-infrared wavelength range an optically thin plasmonic metasurface half-wave plates that rotate the polarization direction of the linearly polarized incident light with a high degree of linear polarization. The half-wave plate functionality is realized through arranging the orientation of the nanoantennas to form an appropriate spatial distribution profile, which behave exactly as in classical half-wave plates but over in a wavelength-independent way.

A thin and conformal metasurface for illusion acoustics of rapidly changing profiles

Recently developed metasurfaces have been used for surface engineering applications. However, the ability to cloak or mimic reflective surfaces with a large in-plane phase gradient remains unexplored. One major challenge is that even with a small incidence angle, the strong acoustic impedance variation induced by the random height profile creates additional scattering and increases the complexity of the analysis and design of the metasurface. Here, we introduce an acoustic metasurface with 1/12 wavelength thickness to realize an acoustic carpet cloak for a randomly rapid-change surface and a virtual acoustic diffuser from a flat surface using a set of Helmholtz resonators. The limitation of the metasurface for large phase gradient application is explored based on a nonlocal model that considers the contributions from neighboring surface profiles. This study extends the integration of smart acoustic surface and may find applications of surface engineering such as in architectural acoustics.

Cooperative Resonances in Light Scattering from Two-Dimensional Atomic Arrays.

We consider light scattering off a two-dimensional (2D) dipolar array and show how it can be tailored by properly choosing the lattice constant of the order of the incident wavelength. In particular, we demonstrate that such arrays can operate as a nearly perfect mirror for a wide range of incident angles and frequencies, and shape the emission pattern from an individual quantum emitter into a well-defined, collimated beam. These results can be understood in terms of the cooperative resonances of the surface modes supported by the 2D array. Experimental realizations are discussed, using ultracold arrays of trapped atoms and excitons in 2D semiconductor materials, as well as potential applications ranging from atomically thin metasurfaces to single photon nonlinear optics and nanomechanics.

Experimental consideration of RCS reduction using thin metasurface

Experimental consideration of RCS reduction using thin metasurface

Enhancing the Radiation Performance of a Pyramidal Horn Antenna by Loading a Subwavelength Metasurface

We present the detailed design and experimental demonstration of a low-sidelobe horn antenna based on controlling the amplitude and the phase distribution of the electromagnetic fields over the aperture of the horn antenna using a thin single-layer metasurface lens. The lens is composed of two identical subwavelength metallic square-ring arrays printed on the two sides of a single dielectric layer. By placing the metasurface lens inside a standard pyramidal horn, the electromagnetic fields over the horn aperture can be manipulated, resulting in low sidelobe radiation in both $E$ - and $H$ -planes. Both simulated and measured results demonstrate the tapered field distribution over the horn aperture and the significant reduction of sidelobes in the $E$ -plane as well as the reduction of the backlobes.

Broadband RCS Reduction Based on Spiral-Coded Metasurface

In this letter, an efficient approach for broadband radar cross-section (RCS) reduction is proposed. By coding eight types of linear-phase gradients in a spiral pattern, the reflected wave was uniformly diffused, and thus, the backscatter RCS reduction was achieved within an ultrabroadband. For verification, a spiral-coded metasurface composed of $8\times 8$ subarrays is designed, fabricated, and measured. Numerical and experimental results both show that the thin metasurface enables a 10 dB RCS reduction over an ultrabroadband ranging from 12.2 to 23.4 GHz. Moreover, the good merit of our approach is further demonstrated by comparing the scattering behavior to the steering beam of a regular sequential layout pattern. Our approach features broad band, polarization insensitivity, wide incident angles, and easy design without any optimization, promising great potential in real-world low-scattering stealth applications.