Focusing Metasurface Research Articles

Vortex beam generated by circular-polarized metasurface reflector antenna

Vortex beams with orbital angular momentum (OAM) can remarkably increase the channel capacity without signal process, thus attract more and more attention of researchers, and various novel methods of generating vortex beams blow out. In this work, a circular-polarized vortex beam reflector antenna was proposed via integrating vortex beam generated metasurface and beam focusing metasurface. The antenna consists of the metasurface reflector and a feed helical antenna. The phase distribution of the metasurface combines the OAM phase distribution and the beam focusing required phase distribution which is used to generate vortex beam from a point source. The measured results agree well with the simulated results, and both demonstrate that the reflector antenna can generate a vortex beam with OAM from 12 GHz to 18 GHz, and the realized gain is 22.4 dB at the center frequency 15 GHz.

Terahertz high-resolution wideband focusing metasurface based on catenary structure

Due to several excellent properties, electromagnetic terahertz waves may be very useful for imaging applications, provided sufficiently high-resolution can be achieved. A tightly focused beam can be produced using a two-dimensional metasurface and catenary structure that enables phase-tuning of electromagnetic waves. We modified the catenary structure to produce a metasurface and a non-diffracting 0th-order Bessel beam. According to our calculation, the formed wideband focal spots can be smaller than Abbe’s diffraction limit, between 350 GHz and 450 GHz, with a maximum gain of 15.1 dB. Such a metasurface could open new doors for future high-resolution medical imaging research and technology.

Nonplanar acoustic metasurface for focusing

Acoustic metasurfaces have attracted considerable attention in recent years because of their unprecedented manipulation of acoustic waves within subwavelength planar structures. However, planar rigid structures are not compatible with human skin, which possesses dynamically varying and highly unconventional geometries. This limits the applicability of acoustic metasurfaces in a promising industry. In this paper, we describe a nonplanar acoustic metasurface for focusing based on the ability of metasurfaces to impart nontrivial phase shifts. We propose a common mapping method for the pattern design that enables focusing behind the free surface. By cutting hollowed-out patterns on a nonplanar metasurface membrane, forming a specialized transmitted phase distribution, we obtain the approximate constructive interference on the focal spot, thus implementing high-efficiency focusing behind an arbitrary nonplanar metasurface. Specifically, we demonstrate focusing on spherical, conical, and chaotic surfaces and discuss the scope of application of our design rule. Moreover, by applying the pattern to conical surfaces, we design an “umbrella structure” to implement high-efficiency focusing with a dynamic, tunable focal length. The proposed nonplanar metasurface not only expands the application range of metamaterials but also demonstrates the real-world applicability of cutting-edge metamaterial research.

A circular-polarized metasurface planar reflector antenna based on Pancharatnam–Berry phase

Due to the significance of circular-polarized antennas for satellite communications applications, a planar circular-polarized reflector antenna is proposed in this paper, which consists of a circular polarization beam focusing metasurface and a feed helical antenna. The metasurface is realized using circular polarization conversion resonators with deep sub-wavelength size which is based on Pancharatnam–Berry phase. As an example, a Ka-band 150 × 150 mm2 planar circular-polarized antenna is designed, fabricated and measured with focal length of 150 mm. Both the simulated and measured results agree well and verify that the designed antenna works over 30–34 GHz with gain larger than 20 dB, half-power band width (HPBW) about 4° and axis ratio lower than 3 dB.

Ultrathin Microwave Devices for Polarization-Dependent Wavefront Shaping Based on an Anisotropic Metasurface

Metasurfaces have recently become a promising material, offering new degrees of freedom in molding electromagnetic wave properties. In this work, we propose and numerically investigate ultrathin microwave devices for polarization-dependent wavefront shaping based on an anisotropic metasurface, which consists of a square metal ring with a cross, a dielectric substrate, and a metal ground plane. It is demonstrated the proposed metasurface can independently manipulate reflective x- and y-polarized wavefronts at frequency of 15 GHz via engineering of the metal cross. Furthermore, the reflective efficient is extremely high, reaching a near-unity value of 0.98. Based on this anisotropic metasurface, a polarization beam splitter is achieved by artificially arranging the spatial distribution of metasurfaces with prescribed geometries. In addition, we successfully design a focusing metasurface to separate the x- and y-polarized beams via focusing them at different positions. The proposed approach paves a way toward the applications of the metasurface in a microwave band.

Continuously Tunable Acoustic Metasurface for Transmitted Wavefront Modulation

Previously reported acoustic metasurfaces that consist of fixed channels, are untunable to meet the broadband requirement and alterable functionalities. To overcome this limitation, we propose screw-and-nut mechanism of tunability and design a type of continuously tunable acoustic metasurface with unit components of helical cylinders which are screwed into a plate. The spiral channel length can be tuned continuously by the screwed depth; and then a metasurface with continuously tunable acoustic phase at independent pixels is attained. Different distributions of the unit components can shape an arbitrary metasurface profile. We also developed an approximate equivalent medium model to predict the tunability of the unit component. As an example, we present the design details of a circular tunable metasurface for three-dimensional acoustic focusing of different airborne sound sources in a wide frequency region. A sample of the metasurface is manufactured by poly lactic acid (PLA) plastic with the helical cylinders being 3D-printed. Experiments of sound focusing are performed. It is shown that the results of the equivalent medium model, the finite element simulation and the experiments are in a good agreement.

Optimal design of SPP-based focusing metasurface optical elements in terahertz frequency

A structure with new meta-atom is proposed. The meta-atom with C3 rotational symmetry consists of three slits. The slits are drilled in the metal. Based on this meta-atom, metasurface lenses with different functions are designed. It is based on the phase and amplitude modulation by tuning the rotational angle of the meta-atom. The rotational angle of each meta-atom is achieved by simulated annealing algorithm. Three kinds of focal lenses, which can realize one or two focus points on the left and right sides, have been designed in terahertz frequency. The finite-difference time-domain method is employed to verify and appraise the properties of the designed lenses. Simulation results show that the designed lenses can perform the preset functions well. It demonstrates that such meta-atom is more suitable for focusing metasurface in different directions. It will promote the lightness, miniaturization and integration of optical components. And it is more suitable for microwave and terahertz wavefront modulation.

Broadband Tunable Acoustic Asymmetric Focusing Lens from Dual-Layer Metasurfaces

The acoustic asymmetric focusing effect attracts more and more attention, with its extensive applications in high-intensity focused ultrasound (HIFU) therapy and medical ultrasound imaging. Typical focusing devices have symmetric structures, though, which do not yield asymmetric transmission simultaneously. This work realizes a broadband acoustic lens using dual-layer metasurfaces for both subwavelength focusing and asymmetric transmission. Without changing the metasurface's structures, horizontal and vertical focusing are manipulated by mechanically changing the distance between metasurfaces, and the angle of incidence.

A Novel Metasurface for Dual-Mode and Dual-Band Flat High-Gain Antenna Application

A dual-mode and dual-band flat high-gain antenna based on anisotropic focusing metasurface (MS) is proposed in this communication. The elements of the MS can steer the reflected phases of ${y}$ -polarized waves around 10 GHz and the transmitted phases of ${x}$ -polarized waves around 14 GHz independently. The function of the anisotropic MS is to focus the incident plane waves around 10 and 14 GHz on spots with different focal lengths and spaces. To reduce the block effect of the reflection mode, an extra phase gradient is introduced to the initial phase distribution of the ${y}$ -polarization to deflect the reflection mode main-beam to an angle of 30°. Finally, the new high-gain antenna obtains a deflecting reflection beam with peak gain of 17.2 dB and a normal transmission beam with peak gain of 19.5 dB. Besides single-band working, the antenna can also operate at 10 and 14 GHz simultaneously with gains of 14.7 and 16.7 dB, respectively. Measured results make a good agreement with the simulated results.

Wide-Band Circularly Polarized ReflectarrayUsing Graphene-Based Pancharatnam-Berry Phase Unit-Cells for Terahertz Communication.

A wide-band and high gain circularly polarized (CP) graphene-based reflectarray operating in the THz regime is proposed and theoretically investigated in this paper. The proposed reflectarray consists of a THz CP source and several graphene-based unit-cells. Taking advantages of the Pancharatnam Berry (PB) phase principle, the graphene-based unit-cell is capable of realizing a tunable phase range of 360° in a wide-band (1.4–1.7 THz) by unit-cell rotating, overcoming the restriction of intrinsic narrow-band resonance in graphene. Therefore, this graphene-based unit-cell exhibits superior bandwidth and phase tunability to its previous counterparts. To demonstrate this, a wide-band (1.4–1.7 THz) focusing metasurface based on the proposed unit-cell that exhibits excellent focusing effect was designed. Then, according to the reversibility of the optical path, a CP reflectarray was realized by placing a wide-band CP THz source at the focal point of the metasurface. Numerical simulation demonstrates that this reflectarray can achieve a stable high gain up to 15 dBic and an axial ratio around 2.1 dB over the 1.4–1.7 THz band. The good radiation performance of the proposed CP reflectarray, as demonstrated, underlines its suitability for the THz communication applications. Moreover, the design principle of this graphene-based reflectarray with a full 360° phase range tunable unit-cells provides a new pathway to design high-performance CP reflectarray in the THz regime.

A thin wideband high-spatial-resolution focusing metasurface for near-field passive millimeter-wave imaging

In this paper, we present a flat transmission-type focusing metasurface for the near-field passive millimeter-wave (PMMW) imaging systems. Considering the non-uniform wavefront of the actual feeding horn, the metasurface is configured by unit cells consisting of coaxial annular apertures and is optimized to achieve broadband, high spatial resolution, and polarization insensitive properties important for PMMW imaging applications in the frequency range from 33 GHz to 37 GHz, with the focal spot as small as 0.43λ0 (@35 GHz). A prototype of the proposed metasurface is fabricated, and the measurement results fairly agree with the simulation ones. Furthermore, an experimental single-sensor PMMW imaging system is constructed based on the metasurface and a Ka-band direct detection radiometer. The experimental results show that the azimuth resolution of the system can reach approximately 4 mm (≈0.47λ0). It is shown that the proposed metasurface can potentially replace the bulky dielectric-lens or reflector antenna to achieve possibly more compact PMMW imaging systems with high spatial resolution approaching the diffraction-limit.

The novel hybrid metal-graphene metasurfaces for broadband focusing and beam-steering in farfield at the terahertz frequencies

A hybrid metal-graphene metasurface was proposed and researched in terahertz (THz) frequencies. It is found that these hybrid structure metasurfaces give rise to a larger phase variation in comparison to bare graphene metasurface. Based on the analysis of electric field of the hybrid structures, it implies that due to the enhanced localized electric field and the Fabry-Perot resonance cause the phase increasing quadratically with the improvement of the graphene Fermi energy (EF) in hybrid metal-graphene structures. By designing different metal sizes and modulating EF, the structure shows up to 295° of smooth phase modulation at the frequency of 4.5 THz. Based on the phase profile design, a reflection-type focusing metasurface with near 1000 μm focal length in a broadband frequency range from 4 to 4.8 THz and 1-bit dynamically tunable coding metasurface to steer the reflected THz waves into multiple beams in farfield were both demonstrated successively. Moreover, the degree of freedom for beam-steering of 2-bit coding metasurfaces was enhanced and the process of manipulation was also qualitatively interpreted by the convolution principle. This work may supply a potentially effective method to instruct the design of desirable metasurfaces with multi-function in the modulation of electromagnetic waves.

Focusing Microwaves in the Fresnel Zone With a Cavity-Backed Holographic Metasurface

We present the design and experimental demonstration of a cavity-backed, holographic metasurface capable of focusing microwaves in the Fresnel zone. The circular cavity consists of two stacked plates: microwaves are injected into the bottom plate via a coaxial connector, which forms the feed layer, and are coupled to the top holographic metasurface layer via an annular ring on the periphery of the cavity. This coupling results in an inward traveling cylindrical wave in the top layer, which serves as the reference wave for the hologram. A sparse array of slots, patterned into the upper plate, constitutes the hologram that produces the focal spot. To mitigate high sidelobe levels and improve performance, a tapered design, facilitated by varying the slot size, is also introduced. The proposed designs—which have a 10-cm diameter, operate at 20 GHz, and form a focal spot at a distance of 10 cm—are validated using full-wave simulations as well as measurements of fabricated samples. The proposed focusing metasurface may find application as a compact source for Fresnel zone wireless power transfer and remote sensing schemes.

Polarization-switchable and wavelength-controllable multi-functional metasurface for focusing and surface-plasmon-polariton wave excitation.

Realizing versatile functionalities in a single photonic device is crucial for photonic integration. We here propose a polarization-switchable and wavelength-controllable multi-functional metasurface. By changing the polarization state of incident light, its functionality can be switched between the flat focusing lens and exciting surface-plasmon-polariton (SPP) wave. Interestingly, by tuning the wavelength of incident light, the generated SPP waves can also be controlled at desired interfaces, traveling along the upper or lower interface of the metasurface, or along both of them, depending on whether the incident light satisfies the first or second Kerker condition. This polarization-switchable and wavelength-controllable multifunctional metasurface may provide flexibility in designing tunable or multifunctional metasurfaces and may find potential applications in highly integrated photonic systems.

Efficient Analysis of Metasurfaces in Terms of Spectral-Domain GSTC Integral Equations

We present a spectral-domain (SD) technique for the efficient analysis of metasurfaces. The metasurface is modeled by generalized sheet transition conditions (GSTCs) as a zero-thickness sheet creating a discontinuity in the electromagnetic field. The SD expression of these GSTCs for a specified incident field leads to a system of four surface integral equations for the reflected and transmitted fields, which are solved using the method of moments in the spectral domain. Compared to the finite-difference and finite-element techniques that require meshing the entire computational domain, the proposed technique reduces the problem to the surface of the metasurface, hence eliminating one dimension and providing substantial benefits in terms of memory and speed. A monochromatic generalized-refractive metasurface and a polychromatic focusing metasurface are presented as illustrative examples.

Transmissive focusing meta-surface with nearly 100% efficiency

Planar lens is one of the most important components of both microwave and optical frequency regimes. However, current available transmissive lenses are limited by their low efficiency, complex configurations and large sizes. Here, we propose a general strategy to design focusing meta-surface by using carefully designed Huygens elements. Each element on the meta-surface can realize very high transmission due to the electric and magnetic currents deduced by electric and magnetic structures, respectively. As proof of the concept, a meta-lens, working at frequency of 10 GHz is designed, fabricated and measured. Our meta-lens consists of 31 × 25 Huygens elements, occupying a total area of 155 × 164.3 mm2. Numerical and experimental results agree well with each other. The measured absolute efficiency of our meta-lens is higher than 86.7% (90.6% for simulation) at the working frequency. Moreover, the bandwidth is about 30.7% (32.5% for simulation). Our finding opens up a new avenue to design high-performance microwave meta-devices working in transmission geometry.

High performance terahertz metasurface quantum-cascade VECSEL with an intra-cryostat cavity

A terahertz quantum-cascade (QC) vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated with over 5 mW power in continuous-wave and single-mode operation above 77 K, in combination with a near-Gaussian beam pattern with a full-width half-max divergence as narrow as ∼5° × 5°, with no evidence of thermal lensing. This is realized by creating an intra-cryostat VECSEL cavity to reduce the cavity loss and designing an active focusing metasurface reflector with low power dissipation for efficient heat removal. Also, the intra-cryostat configuration allows the evaluation of QC-VECSEL operation vs. temperature, showing a maximum pulsed mode operating temperature of 129 K. While the threshold current density in the QC-VECSEL is higher compared to that in a conventional edge-emitting metal-metal waveguide QC-laser, the beam quality, slope efficiency, maximum power, and thermal resistance are all significantly improved.

Anomalous reflection focusing metasurface based on a dendritic structure

In this study, we designed a metasurface based on dendritic model structures. This metasurface consists of three branches, which rotate symmetrically around the z-axis. In accordance with the principle of optical resonator, the dendritic metasurface achieved distinct anomalous reflection and refraction in the infrared spectrum at an incidence angle range of 0–90°. On the basis of this phenomenon, the structure of an asymmetric dendritic metasurface unit was designed, with each unit consisting of two branches. This metasurface unit could achieve abnormal reflection and refraction for a normal incident plane wave. An opposite phenomenon would occur when the metasurface was rotated 180° around the z-axis. We used this feature to design a metasurface that could achieve a prominent reflective plane focusing effect at the mid-wave infrared band as a result of anomalous reflection. The dendritic metasurface had a uniform structure that could be adjusted flexibly. The anomalous phenomenon could be realized over a wide range of wavebands and angles at the mid-wave infrared band. In addition, the focusing effect was evident, and the reflection efficiency was higher than those of other structures.

Electrically Tunable Metamaterials Based on Multimaterial Nanowires Incorporating Transparent Conductive Oxides

We present novel design approaches for metasurfaces and metamaterials with electrical tunability offering real-time manipulation of light and serving as multifunctional devices in near-infrared frequency regime (at the specific wavelength of 1.55 μm). For this purpose, we integrate indium-tin-oxide (ITO) as a tunable electro-optical material into multimaterial nanowires with metal-oxide-semiconductor and metal-insulator-metal configurations. In particular, an active metasurface operating in the transmission mode is designed which allows for modulation of the transmitted light phase over 280 degrees. This large phase modulation is afforded in the cost of low transmission efficiency. We demonstrate the use of such active metasurfaces for tunable bending and focusing in free-space. Moreover, we investigate the implementation of this material in deeply subwavelength multimaterial nanowires, which can yield strong variations in the effective refractive index by the virtue of internal homogenization enabling tunability of the performance in gradient refractive index metamaterials. In the theoretical modeling of these structures, we adopt a hierarchical multiscale approach by linking drift-diffusion transport model with the electromagnetic model which rigorously characterizes the electro-optical effects.

Single-layer broadband planar antenna using ultrathin high-efficiency focusing metasurfaces**Project supported by the National Natural Science Foundation of China (Grant No. 61372034).

Phase gradient metasurfaces (PGMS) offer a fascinating ability to control the amplitude and phase of the electromagnetic (EM) waves on a subwavelength scale, resulting in new applications of designing novel microwave devices with improved performances. In this paper, a reflective symmetrical element, consisting of orthogonally I-shaped structures, has been demonstrated with an approximately parallel phase response from 15 GHz to 22 GHz, which results in an interesting wideband property. For practical design, a planar antenna is implemented by a well-optimized focusing metasurface and excited by a self-designed Vivaldi antenna at the focus. Numerical and experimental results coincide well. The planar antenna has a series of merits such as a wide 3-dB gain bandwidth of 15–22 GHz, an average gain enhancement of 16 dB, a comparable aperture efficiency of better than 45% at 18 GHz, and also a simple fabrication process. The proposed reflective metasurface opens up a new avenue to design wideband microwave devices.