Transmissive Metasurface Research Articles

Planar microwave retroreflector based on transmissive gradient index metasurface

In this paper, a novel planar microwave retroreflector based on a transmissive gradient metasurface combined with a curved metal mirror is proposed and demonstrated. The transmissive metasurface can efficiently converge a wide-angle incident wave to a pre-designed curved metal mirror behind it with a proper distance, which acts as an effective reflective surface that can greatly enhance the backscattering of the incident wave with a wide-angle view. According to the full-wave simulations, the proposed metasurface retroreflector can perform an excellent retroreflective effect for incident microwaves of angle view between −30° and 30° range. A prototype was fabricated and the experimental results verify that the metasurface retroreflector can realize the monostatic radar cross section (RCS) enhancement with a continuous wide incident angle view from −30° to 30° at 10 GHz within a stable 3 dB RCS level. It is further demonstrated that the excellent wide-angle backscattering performance (absolute RCS enhancement value, operational bandwidth and/or incident angle view) of the proposed microwave metasurface retroreflector is competitive against the traditional trihedral corner reflector with comparable dimensions, thus opening up new possibilities to substitute the traditional bulky radar retroreflector by using a planar compact metasurface structure for microwave engineering. The presented microwave metasurface retroreflector is promising to develop into a low-profile, light weight and planar radar retroreflector which possesses tremendous RCS backscattering enhancement and wide-angle view operation range.

Independent Energy Allocation of Dual‐Helical Multi‐Beams with Spin‐Selective Transmissive Metasurface

AbstractAlthough metasurfaces have provided unprecedented control of electromagnetic (EM) waves, it is still challenging for the arbitrary manipulation of spatial radiation modes, energy distribution, and spin state of EM wave especially in microwave frequencies. Here, a cascaded dual‐helicity‐decoupled transmissive metasurface to realize independent modulation of two orthogonal circularly polarized waves is proposed. Leveraging this appealing feature, distinct multiple beams with freely designed energy intensities for two spin states generated from a metasurface are demonstrated experimentally with high efficiency, providing full control of sophisticated wavefronts, spin states, and their energy allocations. Two proof‐of‐concept designs are implemented in microwave region, which may offer a new way to achieve spatially energy‐editable multifunctional devices that can be used in practical applications, such as beam forming, multiple target tracking, and wireless communications.

A Tunable Semiconductor‐Based Transmissive Metasurface: Dynamic Phase Control with High Transmission Level

AbstractHerein, a tunable semiconductor‐based metasurface, based on electro‐optical modulation of an array of geometrically‐fixed silicon (Si) nanobars, sandwiched between two distributed Bragg reflectors (DBRs) is proposed. A flat‐top transmission spectrum with a steep phase profile, which turns out to be essential to realize a highly‐efficient spatial light modulator in transmission mode, is formed by excitation of two spectrally close Fabry‐Pérot‐type and guided‐mode resonances. The refractive‐index of Si nanobars is electrically modulated by the injection of electrons and holes using a P–I–N junction configuration, considered along each nanobar. It is theoretically demonstrated that wide phase agility of 215° and transmission amplitude higher than 0.6 (with an average of ≈0.83) can be accomplished at the operating wavelength of ≈1.55 µm. This wide‐range tunability is realized by introducing free electron and hole carrier densities of ∆N = ∆P = 5 × 1018 cm−3 accompanied with Si refractive‐index‐change of ∆nSi = 0.01. The transmission phase of each unit cell can be separately controlled, which in turn allows to design a tunable meta‐array with real‐time beam control. As a proof of concept, a dynamic focusing metalens with an adjustable focal length is designed and numerically investigated.

Planar Spoof Surface Plasmon Polariton Antenna by Using Transmissive Phase Gradient Metasurface

AbstractSpoof surface plasmon polariton (SSPP) antennas are of particular importance in communication and radar systems. Currently available SSPP radiation devices are limited to low performance with high side‐lobes because it is extremely challenging to accurately control the wave vector of SSPP and the inherent momentum mismatch between the SSPP and spatial waves. Inspired by the optical principle of reversibility, high‐performance radiation control of SSPP is proposed to be achieved with transmissive phase gradient metasurface (TPGM). The TPGM, placed a meticulously optimized distance above the SSPP propagation structure, can provide an additional opposite wave vector to match the momentum between the SSPP and spatial waves. When the propagating SSPP transmits on the TPGM, it can be decoupled into the free space accurately and flexibly. Numerical results coincide well with the measurements, indicating that the radiation control of SSPP achieves high‐performance and low side‐lobe within 9 to 10.5 GHz with the measured radiation efficiency higher than 50%. The measured maximum efficiency appears at 9.8 GHz for 69%. Thanks to the flexible and accurate manipulation of the dispersion relation of SSPP provided by TPGM, the findings may open an avenue in achieving larger angle scanning antenna.

Independent Control of Copolarized Amplitude and Phase Responses via Anisotropic Metasurfaces

AbstractA class of anisotropic, transmissive electromagnetic metasurfaces is presented, which enable independent and simultaneous control of copolarized phase and amplitude responses to two linear, orthogonal polarizations. By varying the geometrical parameters, the transmission response of the proposed structure can yield a full phase coverage, accompanied by broadly adjustable amplitude and negligible cross‐polarized components. The full amplitude‐phase control together with the novel anisotropic character allows efficient implementation of complicated field manipulations. As representative application examples, which cannot be realized via conventional (phase‐only) metasurfaces, it is presented here: (1) the radiation of multiple equal‐power vortex beams (along arbitrarily predesigned directions, with designable orbital angular momentum modes under different polarizations), and (2) the realization of polarization‐reconfigurable multifocal metalenses. Full‐wave numerical simulations and experimental results demonstrate good agreement and confirm the versatility and effectiveness of the proposed approach to design advanced field‐manipulation systems.

A novel composite transmission metasurface with dual functions and its application in microstrip antenna

Generally, a double corner-cut square structure transmission linear to circular polarization conversion metasurface (DCS-PCM) is difficult to realize circularly polarized radiation when it is applied to a linearly polarized microstrip antenna as electromagnetic surfaces superstrate. A novel composite transmission metasurface (NCTM) with dual functions of linear to circular polarization conversion and polarization selection is proposed and verified. For a y-polarized incident wave, transmission linear to right-hand circular polarization conversion can be realized from 8.43 GHz to 9.50 GHz, and for an x-polarized incident wave, co-polarized total reflection can be realized. Different from the DCS-PCM, the unit cell of the NCTM is made up of a corner-cut square, a substrate, and a polarization gate. Thanks to the novel design of polarization gates, the proposed NCTM is easier to realize circularly polarized radiation when it is applied to a linearly polarized source antenna. The realized gain of the antenna with NCTM can be improved because of the Fabry–Pérot resonant cavity, and the maximum amplitude of the gain improvement is 6.8 dBi. The results of simulation and experiment show that the linearly polarized microstrip antenna with NCTM can realize circularly polarized radiation with an axial ratio less than 3 dB and a significant gain improvement simultaneously.

High-Efficiency Refracting Millimeter-Wave Metasurfaces

Printed circuit metasurfaces have attracted significant attention in the microwave community for their versatile wavefront manipulation capability. Despite their promising potential in telecommunications and radar applications, few transmissive metasurfaces have been reported operating at millimeter-wave frequencies. Several secondary effects including fabrication tolerances, interlayer near-field coupling and the roughness of conductors are more severe at such high frequencies and can cause significant performance degradation. Additionally, very accurate experimental techniques are required in order to characterise these effects. In this work, we present highly efficient refracting metasurfaces operating at 83 GHz. We use a synthesis technique that minimizes performance degradation due to effects such as interlayer near-field coupling and conductor roughness. Our experimental characterization includes an accurate determination of the intensity of all forward propagating Floquet harmonics in a broad frequency range. The experimental data shows very good agreement with full-wave simulation and verifies our synthesis method.

High-efficiency ultra-thin polarization converter based on planar anisotropic transmissive metasurface

We present the design, simulation, and characterization of an ultrathin polarization converter based on planar anisotropic transmissive metasurface. The metasurface consists of two identical metallic structures printed on both sides of a dielectric substrate and possesses a nearly uniform transmittance with a π phase difference over the whole bandwidth for waves of orthogonal linear polarizations. It is demonstrated numerically and experimentally that the designed metasurface can high-efficiently change the polarization plane of a linearly polarized plane wave or convert the circularly polarized plane wave into the wave of opposite handedness in a wide frequency bandwidth. The polarization conversion ratios in both cases could reach almost 100% from 15.8 GHz to 16.7 GHz. The proposed metasurface converter may find uses in various applications combining highly efficient control of polarization with a low insertion loss.

Huygens Metasurfaces Based on Congener Dipole Excitations

Metasurfaces are powerful platforms for controlling light, with bandwidth and efficiency being the two key parameters. Here conventional transmissive metasurfaces suffer, because they rely on multipoles of different resonant nature to construct Huygens sources. The authors explore the possibility of utilizing congener dipoles (solely electric, without a magnetic component) with similar resonant properties to release these constraints. Beam deflection and focusing, over a broad wavelength band and with high efficiency, are demonstrated. This approach extends the design methodology and improves the performance of metasurfaces.

A Review of Orbital Angular Momentum Vortex Beams Generation: From Traditional Methods to Metasurfaces

In this paper, we review the generation of vortex beams carrying orbital angular momentum in the microwave domain. We firstly present the theory of Laguerre–Gaussian beams where it is demonstrated that they carry such type of momentum. We further provide an overview of the classical methods used to generate orbital angular momentum vortex beams, which rely on two main methods; plane wave to vortex wave conversion and direct generation using radiating antennas. Then, we present recent progress in the physics of metasurfaces devoted to the generation of vortex beams with a discussion about reflective and transmissive metasurfaces for plane wave to vortex wave conversion as well as methods to reduce the intrinsic divergence characteristics of vortex beams. Finally, we conclude on this rapidly developing research field.

A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption

AbstractA thin screen exhibiting dynamically switchable transmission/absorption functionalities is highly desired in practice. However, a trilayer transmissive metasurface with active elements controlled in a uniform manner cannot exhibit independently controlled transmission and absorption properties due to intriguing interplays between the scattering and absorbing properties in systems exhibiting inversion symmetries. This motivates to employ the coupled‐mode theory to establish a generic phase diagram for such transmissive metasurfaces with active elements loaded in different layers tuned independently and to guide researchers design tunable metadevices with completely decoupled transmission/absorption responses. Based on such a phase diagram, a microwave metasurface is designed/fabricated with PIN diodes incorporated, and it is experimentally demonstrated that its functionality can switch from perfect transparency to perfect absorption, controlled by external voltages applied across the diodes. In addition to finding immediate applications in practice (e.g., smart radomes), the results of this study also provide a new type of tunable meta‐atom for building metasurfaces with flexible wave‐front control abilities.

Manipulation of Terahertz Wave Based on Three-Layer Transmissive Pancharatnam-Berry Phase Metasurface

In order to improve the transmitted efficiency of metasurfaces, the three-layer unit cell structure with coupling characteristics between layers was proposed. Using these Pancharatnam-Berry phase element particles with three-layer structure, we constructed encoded metasurfaces with different sequences to control the transmitted scattered waves. However, the manipulation of continuous transmission angle requires the continuous change of encoding metasurface period. Since the size of encoding particles in the coded metasurfaces cannot be designed to be infinitesimally small, it is impossible to obtain the continuously changing period of coded metasurfaces. In order to obtain the continuous manipulation of transmission scattering angles, we introduced the principle of Fourier convolution operation in digital signal processing on encoding metasurface sequences. By performing the addition and subtraction operation between two different encoding metasurface sequences, a new encoding metasurface sequence can be obtained with different scattering angle. The transmission scattering angles can be obtained continuously, and the terahertz wave can be manipulated freely. Moreover, by using the proposed three-layer highly efficient Pancharatnam-Berry phase encoding meta-atoms, these coded particles with different rotation angles can be precisely arranged to build the generators of the orbital angular momentum beam with different topological charges.

Wide-Angle Frequency Scanning Metasurface Antenna Fed by Spoof Plasmonic Waveguide

We propose novel wide-angle frequency scanning metasurface antenna and antenna array fed by spoof plasmonic waveguide. The spoof plasmonic waveguide adopts the ultra-thin corrugated metallic strip which can support and propagate spoof surface plasmon polariton (SSPP) with high efficiency. By compensating the momentum difference between SSPP and free-space propagation modes via constant phase gradient, metasurface antenna can exhibit wide-angle and continuous radiation property according to the Generalized Snell's law. A prototype is designed, fabricated and measured, which consists of a wideband polarization rotating transmissive phase gradient metasurface (TPGM) placed a certain distance above spoof plasmonic waveguide. Such antenna works in a wideband range from 6.8 to 14.0 GHz, and the main beam can continuously scan from the forward direction to the backward direction with high efficiency. The measured results show that the scanning angle can reach 55°, which agree well with the simulated and theoretically calculated results. Furthermore, the proposed metasurface antenna can be combined easily as an antenna array with a simple feed network. Compared to single metasurface antenna, the antenna array exhibits the pencil-shaped directional radiation property, and the average gain of antenna array is obviously enhanced. The proposed frequency scanning metasurface antenna is of great value in the design of conformal antenna and the large aperture antenna array, especially on curved faces.

Composite polarization conversion metasurface and its application in integrated regulation radiation and scattering of antenna

The transmission polarization conversion metasurface has been widely concerned, because it has the advantage of being easy to be conformal with the antenna. Based on the reasonable arrangement of transmission polarization conversion units, various and complex electromagnetic functions can be realized. As the electromagnetic open window on the flight platform, the antenna is the bottleneck that restricts the decrease of radar cross section (RCS) of the whole flight platform. It is difficult to simultaneously realize the normal and efficient radiation of the antenna and the decrease of the RCS of the antenna. When the designed transmission metasurface is used in the antenna design, the radiation and scattering of the antenna can be regulated comprehensively. In this paper, a composite polarization conversion metasurface is proposed and verified. The unit cell of composite polarization conversion metasurface consists of two mirror symmetrical anisotropic metal patches in the upper layer, a dielectric layer and a polarization gate in the lower layer. When the polarization direction of the incident electromagnetic wave is perpendicular to the extension direction of the polarization gate and arrives at the composite polarization conversion surface, the conversion surface can realize the conversion from transmission linear polarization to right-hand circular polarization in a frequency range from 9.3 GHz to 10.9 GHz. When the polarization direction of the incident electromagnetic wave is parallel to the extension direction of the polarization gate, co-polarized total reflection can be realized. The chessboard arrangement metasurface is composed of composite polarization conversion unit and its mirror unit. A novel linearly polarized chessboard arrangement metasurface antenna is composed of the linearly polarized source microstrip antenna with a bandwidth of 9.4–10.7 GHz and the chessboard arrangement metasurface. By using the counter rotating cancellation characteristic of circular polarization, the chessboard arrangement metasurface antenna maintains linearly polarized radiation. Comparing with the source microstrip antenna, the linear polarization purity of chessboard arrangement metasurface antenna is improved from 9.5 GHz to 10.5 GHz. At the same time, the forward gain of the chessboard arrangement antenna increases and the radar cross section decreases. The maximum reduction is 39.2 dB. To further verify the practicability of the design and analysis, the chessboard arrangement metasurface antenna sample is fabricated and measured in microwave anechoic chamber with an Agilent 5230C network analyzer. The experimental results are in good agreement with the simulation results. This study has important reference value in the design of high gain, low RCS antenna and integrated regulation radiation and scattering of antenna.

Theoretical study on 2.52 terahertz beam shaping and polarization conversion based on the transmissive all-dielectric metasurface

Based on the physical optics vector diffraction integral formula and the commercial electromagnetic software EastWave, a transmissive all-dielectric square metasurface with a side length of 12.4 mm is designed in this paper. It can simultaneously modulate the phase and polarization of the terahertz wave. The 2.52 terahertz x-polarized normal incident Gaussian beam can be shaped into a radially polarized narrow-width annular Bessel–Gaussian beam (radii of the inner and outer circles are 4.125 mm and 5.5 mm, respectively) at a propagation distance of 23.8 mm away from the metasurface. The metasurface is composed of the poly (4-methyl-1-pentene) substrate and the rectangular silicon resonators with different dimensions and rotation angles (counterclockwise rotation angle of the rectangle long axis as the reference of positive x axis). The relative square error between the amplitude distribution of the shaped beam and the target beam is 7.36%, and the fitting coefficient of them is 96.6%. The diffraction efficiency (defined as the ratio of the total light field energy in the target region to that in the rear plane of the metasurface along the incident beam propagation direction) of the shaped annular region is 75.5%. By focusing the narrow-width annular Bessel–Gaussian beam obtained in this paper, we can get smaller focal spot and longer focal depth, which is of great significance for using terahertz wave in particle acceleration and material processing. In addition, the emergent polarization can be flexibly switched between radially and azimuthally polarized beams by changing the polarization direction of the linearly polarized incident beam.

All-dielectric three-element transmissive Huygens’ metasurface performing anomalous refraction

Metasurfaces have pioneered a new avenue for advanced wave-front engineering. Among the various types of metasurfaces, Huygens’ metasurfaces are thought to be a novel paradigm for flat optical devices. Enabled by spectrally overlapped electric resonance and magnetic resonance, Huygens’ metasurfaces are imparted with high transmission and full phase coverage of 2π, which makes them capable of realizing high-efficiency wave-front control. However, a defect of Huygens’ metasurfaces is that their phase profiles and transmissive responses are often sensitive to the interaction of neighboring Huygens’ elements. Consequently, the original assigned phase distribution can be distorted. In this work, we present our design strategy of transmissive Huygens’ metasurfaces performing anomalous refraction. We illustrate the investigation of Huygens’ elements, realizing the overlapping between an electric dipole and magnetic dipole resonance based on cross-shaped structures. We find that the traditional discrete equidistant-phase design method is not enough to realize a transmissive Huygens’ surface due to the interaction between neighboring Huygens’ elements. Therefore, we introduce an extra optimization process on the element spacing to palliate the phase distortion resulting from the element interaction. Based on this method, we successfully design unequally spaced three-element transmissive metasurfaces exhibiting anomalous refraction effect. The anomalous refractive angle of the designed Huygens’ metasurface is 30°, which exceeds the angles of most present transmissive Huygens’ metasurfaces. A transmissive efficiency of 83.5% is numerically derived at the operating wavelength. The far-field electric distribution shows that about 93% of transmissive light is directed along the 30° refractive direction. The deflection angle can be tuned by adjusting the number of Huygens’ elements in one metasurface unit cell. The design strategies used in this paper can be inspiring for other functional Huygens’ metasurface schemes.

Generalized Design Methodology of Mode Conversion Based on Transmissive Metasurfaces

A novel and generalized methodology of mode conversion based on the transmissive metasurfaces is studied in this article. The mathematic formulations, the design criterions, and procedures have been presented in detail. To further demonstrate the proposed methodology, four typical examples are designed. Subsequently, a prototype with the input mode of circular waveguide TM0,1 mode is designed, fabricated, and measured at ${X}$ -band. The simulated and measured results show that the input mode has been converted into the Guassian-like mode, which could verify the correctness of the proposed design methodology. In addition, due to the rich applications of the transmissive metasurfaces, the proposed methodology will start a new page to realize the mode conversion, especially for the high-frequency case.

A wideband transmission metasurface for generating Bessel beam carrying orbital angular momentum

Orbital angular momentum (OAM) microwave waves have received increasing research interest because of their potential to improve communication capacity and radar resolution. However, the traditional orbital angular momentum wave has the problem of diffraction divergence. The ideal Bessel beam with orbital angular momentum has the property of nondiffraction. Hence, the Bessel beam carrying orbital angular momentum has great research potential. In this work, a wideband transmission metasurface for generating Bessel beam carrying OAM is designed, fabricated, and measured. And it is the first experimental to show that the Bessel beam carrying OAM has higher gain and lower crosstalk than the traditional OAM wave in far field transmission.

Dual-Wide-Band Dual Polarization Terahertz Linear to Circular Polarization Converters based on Bi-Layered Transmissive Metasurfaces

Transmissive metasurface-based dual-wide-band dual circular polarized operation is needed to facilitate volume and size reduction along with polarization diversity for future THz wireless communication. In this paper, a novel dual-wide-band THz linear polarization to circular polarization (LP-to-CP) converter is proposed using transmissive metasurfaces. It converts incident X polarized waves into transmitted left-hand circular polarized (LHCP) and right-hand circular polarized (RHCP) waves at two frequency bands. The structure consists of bi-layered metasurfaces having an outer conductor square ring and three inner conductor squares diagonally intersecting each other. The proposed converter works equally well with incident Y polarizations. Operational bandwidths for the dual-band LP-to-CP are 1.16 THz to 1.634 THz (34% fractional bandwidth) and 3.935 THz to 5.29 THz (29% fractional bandwidth). The electromagnetic simulation was carried out in two industry-standard software packages, High Frequency Structure Simulator (HFSS) and Computer Simulation Technology (CST), using frequency and time domain solvers respectively. Close agreement between results depicts the validity and reliability of the proposed design. The idea is supported by equivalent circuits and physical mechanisms involved in the dual-wide-band dual polarization operation. The impact of different geometrical parameters of the unit cell on the performance of LP-to-CP operation is also investigated.

3D plasmonic design approach for efficient transmissive Huygens metasurfaces.

In this paper we present a design concept for 3D plasmonic scatterers as high- efficiency transmissive metasurface (MS) building blocks. A genetic algorithm (GA) routine partitions the faces of the walls inside an open cavity into a M x N grid of voxels which can be either covered with metal or left bare, and optimizes the distribution of metal coverage needed to generate electric and magnetic modes of equal strength with a targeted phase delay (Φt) at the design wavelength. Even though the electric and magnetic modes can be more complicated than typical low order modes, with their spectral overlap and equal strengths, they act as a Huygens source, with the accompanying low reflection magnitude. Square/hexagonal voxels inside square/rectangular cavities are thoroughly analyzed for operation at 8 µm, although the technique can be applied to different cavity geometries for operation across the electromagnetic spectrum. Results from full-wave simulations show the GA routine can repeatedly pinpoint scatterer geometries emitting at any Φt value across 2π phase space with transmittances of at least 60%, making these MS building blocks an attractive plasmonic alternative for practical optical applications. Full-scale metasurface devices are calculated from near-fields of the individual elements to validate the optical functionality.