Transmissive Metasurface Research Articles

Spectral imaging of flexible terahertz coding metasurface

Coding metasurfaces have emerged as a promising venue for terahertz (THz) beam steering and beamforming. In this study, we designed a transmission metasurface with a complementary structure based on Babinet's principle. The beam-steering capability of the coding metasurface is implemented by encoding “0” and “1” elements with different phase responses and by controlling the coding sequences. The deflection angle can be controlled by changing the period of these 0 and 1 elements. Despite the development of beam-steering technology, measurements of the direction pattern of steered THz beams remain challenging. Systems for THz time-domain spectroscopy and spectral imaging are used to characterize the flexible coding metasurfaces. The THz imaging system can provide information on the directional pattern of the beam. To verify the performance of the proposed metasurface, the experimental measurements of beam deflection were found to be consistent with the values yielded by a simulation. Our study provides an effective platform for the design and measurement of THz beam steering.

Generating Bessel Beams Efficiently in Microwave With High Transmission Metasurfaces

In this article, two transmissive metasurfaces with different phase control mechanisms and working polarization condition are proposed to efficiently generate zero order Bessel beams (BBs), respectively. For the linear polarization condition, a dual-mode meta-atom constructed by three layers of dielectric substrate and four layers of metal patch is designed. By changing the size of metal patches of meta-atom along x-axis and y-axis, respectively, the phase shift for x- and y-polarized incident wave can be tuned relative independently of 0-2 π while the transmission coefficient keeps high value at 10 GHz. For the circular polarization condition, a sandwich type meta-atom applying geometric phase to realize full phase control while keep high efficiency near 10 GHz is designed. According to the designed phase profile of BBs, select meta-atoms with certain phase and efficiency, then arrange them to construct metasurfaces. Full wave simulation results indicate that the dual-mode metasurface working in linear polarization condition generates BBs with the maximum transmission efficiency 0.79 and 0.85 for x and y polarization wave, respectively, and the geometric phase metasurface working in circular polarization condition generates BBs with the maximum transmission efficiency 0.92. Besides, the propagation characteristics of the excited BBs are studied.

Highly efficient transmissive metasurface for polarization control

Metasurface-based polarization control is a promising application where significant miniaturization and broadband operation bandwidth can be achieved. Several highly efficient reflective polarization control devices are presented in the literature. However, most transmittive devices suffer from several problems such as having low efficiency (50% or less), limited operation bandwidth and incomplete polarization conversion. From a practical perspective, transmittive polarization controllers are of paramount importance as the feasible applications of the reflective ones are very limited. Thus far, most transmittive structures have a limited control of the polarization. Additionally, CMOS compatibility is not discussed. In this paper, a novel design is presented to generally control the polarization state of the transmitted light, based on triangular-wave shaped connected meta-strips. In this context, a one-layered polarization controller is designed to perform a general control of the phase difference between the two orthogonal electric field components over a tremendous operation bandwidth. Moreover, two flipped resonators are suggested to design a highly efficient broadband transmittive polarization controller. Such a structure provides a high power conversion efficiency up to a peak of 95% over a broadband bandwidth. CMOS compatibility is also achieved for mid infrared operation. Further, the reported structure is subwavelength, where the total thickness of the two-layered quarter-wave plate is equal to 0.2 $$\lambda _{center}$$ . Full-wave simulations are performed using Lumerical finite difference time domain (FDTD) software package.

A Low-Profile Transmissive Metasurface for Transformation of Plane Wave to Contour Beam Pattern Using 4-Arm Spiral Element

A low-profile transmissive metasurface is proposed to synthesize contour beam pattern for a European region. A slot array feed is used to illuminate same size metasurface aperture with a plane wave at a near field distance of 24 mm. A two step optimization process is proposed to solve asymmetric and complicated pattern synthesis problem for transformation of plane wave to contour beam pattern using modified Alternating Projection Method (APM). A new subwavelength 4-arm spiral element is proposed to achieve desired performance for contour beam pattern synthesis in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Ku$ </tex-math></inline-formula> band centered at 12.5 GHz. A prototype of low profile transmissive metasurface is also fabricated and measured to verify its gain and bandwidth performance for contour beam pattern synthesis for the European region.

A dual band spin-selective transmission metasurface and its wavefront manipulation.

Chiral metasurfaces which can achieve different optical responses for left-handed and right-handed circularly polarized (CP) light have been proposed. Most of the research studies on chiral metasurfaces focus on improving circular dichroism (CD) and realizing dynamic manipulation of the chiro-optical response. However, there have only been a few reports on the multi-band chiro-optical response. Here, we propose an all-silicon chiral meta-atom which can realize spin-selective transmission in a dual band. In addition, a terahertz metasurface with spin-selective transmission through phase arrangement is designed by using chiral meta-atoms satisfying a gradient geometric phase. Under left-hand circularly polarized (LCP) incidence, the metasurface generates a focused right-hand circularly polarized (RCP) beam which is focused at a distance of 4.8 mm from the exit surface of the metasurface. Our work broadens the concept of metasurface design and may attract more researchers' attention on the applications of chiral metasurfaces.

A new method of implementing simultaneous multiplexing holographic display of wavelength and polarization state with simple structure metasurface

In this paper, we propose a new method to realize both polarization-multiplexing and wavelength-multiplexing using a simple structure, which can realize hologram by the multiplexing of double wavelengths and double polarization in the visible band. Our design can reduce color cross-talk and have a higher diffraction efficiency. We design a transmission metasurface composed of simple rectangular cells. Firstly, we establish the relationship of structural parameters with the transmission phase under various incident conditions of light beams. Then we propose a fitness function that can optimize the structural parameters of the unit cell at each pixel point, which can display different images by 532 nm &lt;i&gt;x&lt;/i&gt;-polarization and 633 nm &lt;i&gt;y&lt;/i&gt;-polarization incident light beams respectively. Finally, finite difference time domain method is used to analyze the structure, and the holographic result fits the theoretical design very well. This work proposes using single metasurface structure to solve the problems of wavelength cross-talk appearing when using simple structures, and will have great importance in coding and anti-counterfeiting.

Theoretical study on generation of radially polarized beam from linearly polarized beam with all-silicon metasurface in the terahertz regime

A transmissive all-silicon dielectric square metasurface with a side length of 12.4 mm is designed in this paper. At 2.52 terahertz, the normally incident x-polarized Gaussian beam can be shaped into the radially polarized narrow-width annular beam with Bessel-Gaussian amplitude distribution at a propagation distance of 45 mm away from the metasurface. Anisotropic rectangular silicon pillars with different dimensions and rotation angles are arranged on the silicon substrate to manipulate the beam. To suppress the reflection loss, a polarization-independent square silicon pillar array is also proposed, which is patterned at the other side of the substrate. In addition, the errors analysis is also discussed. It is expected that our study may have potential for applying terahertz wave in high-resolution microscopy and capturing particles in three dimensions.

Dual-band transmissive metasurface with linear to dual-circular polarization conversion simultaneously

Metasurfaces, as two dimensional periodic structures, are an important candidate to manipulate the polarization of electromagnetic (EM) waves. However, there are few reports on the simultaneous realization of wideband dual-circular polarization conversion using transmissive metasurfaces with one periodic structure in measurement. In this paper, we propose a dual-band dual-circular transmissive metasurface (DCT-MS) for polarization conversion. A DCT-MS simultaneously converts linearly polarized (LP) EM waves into left-hand circularly polarized (LHCP) EM waves in a lower band and right-hand circularly polarized (RHCP) EM waves in a higher band. The unit cell of a DCT-MS consists of a thin substrate and two metal patches in the shape of symmetrical arrows above and below the substrate. The simulated results show that LP waves can be converted into LHCP waves from 7.31 GHz to 10.58 GHz with the axial ratio less than 3 dB. Moreover, the RHCP waves can be realized from 14.26 GHz to 17.36 GHz with the same standard. A DCT-MS sample is measured, and the experimental results are in good agreement with the simulation results.

Planar multi-angle retro-reflectors based on the wave-vector-reversion of spoof surface plasmon polaritons.

Spoof surface plasmon polariton (SSPP) is kind of sub-wavelength electromagnetic (EM) mode, which is favorable for miniaturization and thinning of EM devices. In this paper, we propose a method of designing thin planar retro-reflector that can operate under multiple incidence angles at the same frequency. The retro-reflector is composed of a transmissive phase gradient metasurface (TPGM) placed above a metallic patch array (MPA), where the former couples and decouples SSPPs while the latter supports eigen-mode propagation of SSPPs. Under oblique incident angles, the TGPM can impart 0 and π Pancharatnam-Berry (P-B) phases alternatively, producing P-B phase gradients along its surface. Incident waves can be coupled as SSPPs propagating on the MPA which will be reflected at the borders of the MPA, thus the wave-vector of SSPPs is reversed. In this way, retro-reflection can be realized under the two incidence angles θ=±45.0°. Moreover, due to mode mismatch between the TPGM and MPA under normal incidence, the retro-reflector acts like a planar metallic plate under θ=0°. To verify this method, a prototype was designed, fabricated and measured. Both the simulation and measurement results verify significant backscattering enhancement under θ=±45.0° and 0° at 10.0 GHz. This work provides an alternative method of designing planar retro-reflectors and may find applications in wireless communication, target tracking, etc.

Construction of a Cost-Effective Phased Array Through High-Efficiency Transmissive Programable Metasurface

Programmable metasurfaces have shown great potential in the areas of low-complexity phase array systems in comparison with the conventional phased array antennas. In this document, a 1-bit transmissive programable metasurface with high efficiency is proposed for the cost-effective beam-steering phased array. The designed transmissive metasurface is made up of reconfigurable cells with perfect 1-bit phase tuning and less transmission losses. Through dynamically programming the 1-bit code distributions of the metasurface, real-time scanning pencil beams in desired directions can be created.

Manipulation of the arbitrary scattering angle based on all-dielectric transmissive Pancharatnam Berry phase coding metasurfaces in the visible range.

In order to improve the transmitted efficiency of the metasurface in the visible range, an all-dielectric Pancharatnam-Berry phase unit structure was proposed. Using these Pancharatnam-Berry phase element particles with different rotation angles, all-dielectric encoding metasurfaces can be constructed. The encoding metasurface connects the physical coding particles with digital coding in digital signal processing. The manipulation of the continuous transmission angle requires the continuous change of the encoding metasurface period. Since the size of encoding particles on the coded metasurfaces cannot be designed to be infinitesimally small, it is impossible to obtain the continuously changing period of the coded metasurfaces. To manipulate effectively and freely the angle of scattering in the visible range, Fourier convolution principle in digital signal processing was introduced on all-dielectric encoding metasurfaces with Pancharatnam-Berry phase meta-atoms. The addition and subtraction operations on two initial encoding sequences can be implemented to obtain a new encoding sequence. The manipulation of the arbitrary scattering pattern after Fourier convolution operations on different encoding sequences can be realized, especially for larger abnormal deflection angles. The checkerboard encoding metasurface was also designed to further prove the applicability of the Fourier convolution principle. Moreover, by using the proposed all-dielectric 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.

Broadband terahertz transmissive quarter-wave metasurface

Polarization conversion devices are key components in spectroscopy and wireless communications systems. Conventional terahertz waveplates made of natural birefringent materials typically suffer from low efficiency, narrow bandwidth, and substantial thickness. To overcome the limitations associated with conventional waveplates, a terahertz quarter-wave metasurface with enhanced efficiency and wide bandwidth is proposed. The transmissive quarter-wave metasurface is rigorously designed based on an extended semi-analytical approach employing network analysis and genetic algorithm. Simulation results suggest that the design can achieve linear-to-circular polarization conversion with a 3-dB axial ratio relative bandwidth of 53.3%, spanning 205 GHz–354 GHz. The measurement results confirm that the proposed design enables a 3-dB axial ratio from 205 GHz to at least 340 GHz with a total efficiency beyond 70.2%, where the upper frequency bound is limited by the available experimental facility. This quarter-wave metasurface can cover an entire terahertz electronics band and can be scaled to cover other nearby bands under the same convention, which are technologically significant for future portable systems.

Airy Beam Generation: Approaching Ideal Efficiency and Ultra Wideband with Reflective and Transmissive Metasurfaces

AbstractAiry beams are essential for photonic applications, owing to their intriguing diffraction‐free, self‐accelerating, and self‐healing properties. Recently, 2D metasurfaces have offered great opportunities for generating Airy beams within subwavelength thickness. However, they suffer from the bottlenecks of low efficiency and narrow bandwidth, which may limit their applications in practice. In this study, a strategy is proposed to design amplitude‐ and phase‐controllable metasurfaces that can generate desirable high‐efficiency and ultra wideband Airy beams, based on the Jones matrix analysis. Two exemplary prototypes of Airy beam generators operating in reflection and transmission geometries are demonstrated in the microwave region, achieving high efficiency approaching the theoretical limitation in an ultrawide bandwidth (fractal bandwidth of 84% for reflective prototype and 118% for transmissive). The results show a significant advantage of the bandwidth and total efficiency over low‐thickness metasurfaces. The proposed strategy and metasurface generators may trigger great interest in ultra wideband and high‐efficiency wave manipulations, with small thickness and flat profile compatible with integrated systems, which hold promise for structured light, metaoptics, beam shaping, and advanced holography.

High-efficiency transmissive metasurface for dual-polarized dual-mode OAM generation

In this paper, a high-efficiency dual-polarized transmissive metasurface is presented for the generation of dual-mode convergent orbital angular momentum (OAM) beams. The proposed transmissive metasurface is composed of dual-polarized metasurface units, which are constructed from four stacked patches coupled through a Jerusalem-cross aperture on the interlayer ground, to complete the phase modulation for the orthogonal polarizations. The proposed transmissive metasurface units can achieve full 2π phase modulation along with low transmission loss. Converged OAM beams with modes numbers ℓx = +1 and ℓy = −2 are thus stimulated through the proposed transmissive metasurface for both orthogonal polarizations, which have been demonstratednumerically andexperimentally.

튜토리얼: 메타표면을 이용한 빔포밍/빔조향 투과 안테나

In this paper, we describe the operation principle of a reconfigurable transmitarray antenna(TA) using a transmissive metasurface (MS) and introduce state-of-the-art technology trends. The TA comprises a source antenna; further, the design of a planar MS allows for the control of the wave front from the feed source when the MS exhibits a good transmittance and a full transmission phase variation of 360°. When the MS varies the phase of the transmitted wave passing through the MS such that it is in-phase at the normal plane of the desired direction, a highly directive beam can be obtained in this direction. Two types of methods can be employed to achieve beam steering in the desired direction. One is to control the shape of the transmission phase using an active MS with a changeable transmission phase variation. The other is to control the shape of the transmission phase by varying the wave front of the feed source. The reconfigurable TA has been intensively researched to achieve high efficiency, low-profile, and low-cost performance; further, the implementation and characteristics of various reconfigurable high-gain TAs are analyzed in this study.

Anisotropic Metasurface Holography in 3-D Space With High Resolution and Efficiency

Electromagnetic metasurfaces are structured surfaces consisting of a thin array of subwavelength elements with engineered scattering properties, thus providing a promising means of holographic display by controlling the field distributions. However, the existing metasurface holography has limited capabilities in achieving 3-D fields with simultaneous features of high spatial resolution and energy efficiency due to the design methods. In this work, we present a transmissive metasurface to achieve polarization-dependent field distributions in 3-D space with both high resolution and efficiency. In the design, a new method that utilizes dyadic Green's function (DGF) as the rigorous propagator is used. Using the method, the sources and fields are linked directly and can be located on arbitrary 3-D points with the intermediate spaces being equal or unequal. The design is successfully validated by full-wave simulations and experimental measurements. The broadband simulation results indicate that the design has a bandwidth of 0.8 GHz. This work demonstrates a feasible and simple route of field synthesis for real-world applications that require high resolution, high efficiency, and 3-D scenarios.

Full-Duplex Nonreciprocal Beam Steering by Time-Modulated Phase-Gradient Metasurfaces

We present the concept, theoretical model and experimental implementation of a full-duplex nonreciprocal-beam-steering transmissive phase-gradient metasurface. Such a metasurface is realized by exploiting the unique properties of the frequency-phase transition in coupled time-modulated twin meta-atoms. The metasurface may be placed on top of a source antenna to transform the radiation pattern of the source antenna, and introduce different radiation patterns for the transmit and receive states. In contrast to the recently proposed applications of time modulation, here the incident and transmitted waves share the same frequency. The metasurface is endowed with directive, diverse and asymmetric transmission and reception radiation beams, and tunable beam shapes. Furthermore, these beams can be steered by simply changing the modulation phase. The proposed coupled meta-atoms inherently suppress undesired time harmonics, leading to a high conversion efficiency which is of paramount importance for practical applications such as point to point full-duplex communications.

Highly transmissive bilayer Huygens’ metasurface with over 315° phase coverage

We develop a generic approach to design highly transmissive metasurfaces with a full control over the phase of transmitted waves. Our approach allows one to obtain very thin metasurfaces based on a single printed circuit board with specially designed copper patterns on the top and bottom surfaces. The phase control of the transmitted waves is achieved by exploiting the independent tunability of the electric and magnetic resonances of the subwavelength unit cells. In contrast to the previously reported metasurfaces that had multiple layers to maintain high transmissivity over the 2π phase range, here we exploit multiple resonators in the same plane to overcome the efficiency limit of the bilayer design. The proposed unit cells are then optimized for a nearly 2π phase range at 9 GHz and used to design a focusing lens. The performance of the fabricated lens is characterized by numerical simulations and experiments.

An efficient method of generating vortex electromagnetic wave based on double‐layer transmissive metasurface

An efficient method of generating vortex electromagnetic wave based on double‐layer transmissive metasurface

High‐Efficiency Transmissive Programmable Metasurface for Multimode OAM Generation

AbstractMetasurfaces have been extensively studied for generating electromagnetic waves carrying orbital angular momentum (OAM). In particular, programmable metasurfaces enable real‐time switching between multiple OAM modes in a digital manner. However, the current programmable metasurfaces are mostly based on reflective mode, which suffer from low efficiency as well as serious feed blockage. In this paper, a transmissive programmable metasurface is presented for the highly efficient generation of multimode convergent OAM beams. The proposed transmissive metasurface is composed of electronically reconfigurable units with 1‐bit phase resolution (0/π), which are obtained by integrating two PIN diodes in the radiating layer for current direction modulation. Through the antisymmetry configuration of the two PIN diodes, nearly uniform transmission magnitudes but inversed phase states in a wide band can be obtained. The simulation results show that the proposed reconfigurable unit can achieve good 1‐bit phase tuning, with minimum insertion loss of 0.2 dB and 2 dB transmission bandwidth of more than 10%. Through the dynamic modulation of the quantized code distributions on the metasurface, programmable multimode OAM beams can thus be constructed. Both simulated and measured results verify the effectiveness of the proposed design.