Metasurface Structure Research Articles

A multifunctional metasurface with asymmetric transmission and high-efficiency cross-polarization converter

A three-layer multifunctional metasurface structure is proposed to achieve polarization rotation, perfect polarization conversion, and asymmetric transmission. The design consists of mutually perpendicular rectangular patches, metal pillars, and open-slit metal sheets. By propagating the current through the metal pillars and changing the surface current direction, the dipole can be orthogonally steered to accomplish polarization conversion. The metal in the middle layer can be used to both improve the polarization conversion ratio and ensure high transmittance. The operating band of asymmetric transmission is 8.3–14.7 GHz, where the conversion ratio is above 90% in all of 9–14.2 GHz. In order to verify the proposed concept, the related parameters are designed and measured, the final experimental results match with the simulation results, and the design can be used in the radome, electromagnetic stealth.

Observation of full-parameter Jones matrix in bilayer metasurface

Metasurfaces, artificial 2D structures, have been widely used for the design of various functionalities in optics. Jones matrix, a 2×2 matrix with eight parameters, provides the most complete characterization of the metasurface structures in linear optics, and the number of free parameters (i.e., degrees of freedom, DOFs) in the Jones matrix determines the limit to what functionalities we can realize. Great efforts have been made to continuously expand the number of DOFs, and a maximal number of six has been achieved recently. However, the realization of the ultimate goal with eight DOFs (full free parameters) has been proven as a great challenge so far. Here, we show that by cascading two layer metasurfaces and utilizing the gradient descent optimization algorithm, a spatially varying Jones matrix with eight DOFs is constructed and verified numerically and experimentally in optical frequencies. Such ultimate control unlocks opportunities to design optical functionalities that are unattainable with previously known methodologies and may find wide potential applications in optical fields.

High-Sensitivity Angle Modulation Biosensor Based on Surface Plasmon Resonance of Metasurface

Metasurface is a popular artificial layered material. In this paper, a sensor based on surface plasmon resonance with a new type of symmetrical triangular structure is designed. This paper designs a metasurface structure, the highest sensitivity of 364 °/RIU is achieved by adopting angle modulation. We used COMSOL simulation software, the metal layer structure and size parameters are optimized to obtain the best reflectivity curve, and the accuracy of the simulation is improved through the linear interpolation method. Compared with the traditional gold layer structure, the sensitivity of 135 °/RIU is increased by 1.7 times. We believe that this design can have potential application value in the field of biosensing.

Dynamic Tunable Meta-Lens Based on a Single-Layer Metal Microstructure

Ultra-thin focusing meta-lenses based on the metasurface structure with adjustable focal length show important applicant value in compact systems, especially in on-chip terahertz spectroscopy, imaging systems, and communication systems. A stretchable substrate, dynamic focusing meta-lens based on the cross-polarized metal C-shaped split ring resonators (SRRs) is designed and investigated. At the operation frequency of 0.1 THz, the operation characteristics of the unit cell structure and the formed meta-lens are investigated. The phase of the unit cell structures can be modulated by changing the rotation angle, width, and symmetry axis of the C-shaped metal SRRs. When the terahertz wave is incident vertically, the focusing performance can be achieved based on the specific arrangement of the metasurface unit cells. By stretching the flexible substrate of the meta-lens, the dynamic focusing effect can be realized. When the substrate stretches from 100% to 120%, the focal length changes from 59.8 mm to 125.2 mm, the dynamic focusing range is 109.4% of the minimum focal length, and the focusing efficiency changes between 5.5% and 10.5%.

Anti-reflecting metasurface for broadband polarization independent absorption at Ku band frequencies

An impedance matched metasurface can efficiently channel the electromagnetic fields for maximum power transfer. The thin film based impedance matching techniques often utilize highly dissipative materials and destructive interference of reflection components from multiple subwavelength layers. Here, we propose a novel method to achieve anti reflection characteristics through destructive interference of antiparallel electromagnetic scattering emerging from chiral metasurface. The supercell structure of metasurface consists of four adjacent multi split-rings on FR-4 substrate. The split-rings are arranged to induce anti-parallel surface currents leading to destructive interference for scattered fields. The antireflection characteristics results in near perfect broadband absorption at dual frequency bands. A broadband absorption of 983 MHz is achieved between 12.687 and 13.669 GHz. Similarly, a narrow band absorption of 108 MHz is achieved in frequency range of 15.307–15.415 GHz. The impedance matched with unique symmetric design of supercell results in identical absorption for both x- and y-polarized incident fields. The numerical and experimental results verify broadband absorption characteristics at Ku band frequencies. The proposed metasurface absorber can be used for microwave energy harvesting applications.

High-Performance Quality Factor Based Sensor With Diagonal Cylinder Metasurface of the Bound State in the Continuum

High-quality-factor (high-Q-factor) electromagnetic resonance plays an important role in sensor applications. Previously proposed gas refractive index sensors are often limited by the large cavity length or microscale fabrication process in practical applications. Recently, ultra-high Q factor resonance based on the bound state in the continuum (BIC) has provided a feasible approach to solve these problems. In this paper, we propose a metasurface structure consisting of a single size tetramer cylinder. It supports dual band toroidal dipole (TD) resonances driven by BIC. The physical mechanism of double TD resonances is clarified by the multipole decomposition of the metasurface band structure and far-field scattering power. The sensor structure based on this achieves a sensitivity of 518.3 MHz/RIU, and the maximum line width does not exceed 680 kHz. The high-Q-factor electromagnetic resonance has the advantages of polarization independence and simplicity to manufacture. These findings will open up an avenue to develop the ultrasensitive sensor in the gigahertz regime.

Spatial domain communication technique for future chipless ID sensors based on vortex terahertz beams generated by metasurfaces

As future communication systems move toward the terahertz (THz) spectrum with a much higher speed and more densification, enhanced security utilizing the novel concepts will be inevitable, particularly in automated identification systems. This paper proposes a novel spatial domain technique based on vortex beams generated by metasurface structures for efficient chipless identification (ID) sensors where the information is saved in the vortex modes. This approach using the vortex concept offers the high security strength in the automated systems compared to existing solutions, including time-based and frequency-based sensors. Furthermore, combining these vortex-based sensors and conventional ones results in substantially increasing the stored information capacity. To verify the idea, here, we present an uneven dielectric metasurface (UDM) to generate distinct vortex modes to identify different information. The proposed sensor is designed by using an equivalent transmission line (TL) model, and the extracted results exhibited a good agreement between simulation and theoretical approaches. Furthermore, it is demonstrated that the transmitted information capacity can be notably enhanced by using a sensor tag with simultaneous multiple modes and also using more than one tag simultaneously.

Development of a Metasilencer Considering Flow in HVAC Systems

Although the driving noise of electric vehicles has been reduced compared with that of internal combustion engine vehicles, a new interior noise problem is emerging. It is crucial to reduce the noise of the heating, ventilation, and air conditioning (HVAC) system, which is one of the main causes of interior noise. Therefore, in this study, a metasilencer with an acoustic metasurface structure is presented. The metasilencer was designed to restrain the travel direction of the sound wave of the target frequency into a U-shaped configuration using an acoustic metasurface while considering the flow noise effect of the HVAC system. Acoustic analysis confirmed the noise reduction frequency range and refraction effect of the metasurface. The speaker test confirmed the noise reduction effect of the silencer. The same was also confirmed via HVAC tests, even in the presence of a flow.

Monopole Antenna with Enhanced Bandwidth and Stable Radiation Patterns Using Metasurface and Cross-Ground Structure.

In this paper, a printed monopole antenna with stable omnidirectional radiation patterns is presented for applications in ocean buoy and the marine Internet of Things (IoT). The antenna is composed of a rectangular patch, a cross-ground structure, and two frequency-selective surface (FSS) unit cells. The cross-ground structure is incorporated into the antenna design to maintain consistent monopole-like radiation patterns over the antenna's operating band, and the FSS unit cells are placed at the backside of the antenna to improve the antenna gain aiming at the L-band. In addition, the FSS unit cells exhibit resonance characteristics that, when incorporated with the cross-ground structure, result in a broader impedance bandwidth compared to the conventional monopole antenna. To validate the structure, a prototype is fabricated and measured. Good agreement between the simulated and measured results show that the proposed antenna exhibits an impedance bandwidth of 83.2% from 1.65 to 4 GHz, compared to the conventional printed monopole antenna. The proposed antenna realizes a peak gain of 4.57 dBi and a total efficiency of 97% at 1.8 GHz.

Terahertz polarization conversion from optical dichroism in a topological Dirac semimetal

Topological Dirac semimetals (TDSMs), such as Cd3As2 and Na3Bi, exhibit strong optical dichroism with contrasting dielectric permittivity along different crystal axes. However, such optical dichroism is often overlooked in the study of TDSM-based optoelectronic devices and whether such optical dichroism can lead to unique functionalities not found under the isotropic approximation remains an open question, thus far. Here, we show that the optical dichroism in TDSM lead to starkly different terahertz (THz) responses and device performance as compared to the isotropic case. Using finite-difference time-domain simulations of a Cd3As2-based metasurface, we demonstrate that such optical dichroism can lead to an unexpected THz wave polarization conversion even if the metasurface structure remains fourfold rotationally symmetric, a useful feature not achievable under the isotropic model of TDSM. Our findings concretely reveal the contrasting spectral response between isotropic and anisotropic media and shed important light on the capability of anisotropic TDSM in THz applications, leading not just to the more accurate device modeling but also a new route in realizing THz wave polarization conversion without the need for complex device morphology commonly employed in conventional polarization converters.

Reconfigurable broadband metasurfaces with nearly perfect absorption and high efficiency polarization conversion in THz range

Reconfigurable metasurfaces (RMSs) that enable the switching function of absorption and polarization conversion have attracted increasing attention. However, the design of RMSs to achieve wideband and high efficiency for both absorption and polarization conversion functions simultaneously remains a great challenge. Here, we propose the design of a RMS structure with a high-efficiency cross-polarization conversion and nearly perfect absorption. The reconfiguration between different functions of polarization conversion and absorption is obtained based on the reversible insulator-to-metal phase transition of Vanadium dioxide (VO_{2}). When the VO_{2} is in insulator state, the RMS realizes the cross-polarization conversion function in the wideband of 1.04–3.75 THz with a relative bandwidth up to 113 % due to the multi-resonant modes of electric and magnetic resonances. Meanwhile, the nearly-perfect absorption is achieved in the range of 1.36–3.38 THz with the corresponding relative bandwidth up to 85 % for the VO_{2} in metallic state. Specially, the wideband and high-efficiency performance of these functionalities is maintained for a wide angle incidence. The capability of bi-functional switch and integration with polarization conversion and absorption in a single metasurface structure endowed with both wideband and high-efficiency characteristics for a wide incident angle is very promising for emerging RMS devices in the terahertz region.

Parallel configuration terahertz metasurface

Coding metasurfaces have offered considerable abilities of freedom manipulation electromagnetic waves. Most of reported works exhibit that the coding metasurfaces are composed of coding particles in terms of a certain coding sequences or random digital sequence generated by MATLAB. Generally, these methods of designing metasurfaces are a time-consuming and considerably challenging process for researchers. Herein, we propose a novel terahertz metasurface structure which consists of coding supercells in parallel. The coding element is a sandwich structure made of two identical metal pattern layers and silicon dioxide layer. Under the incidence of right-handed circularly polarized (RCP) terahertz wave, the proposed parallel metasurface realizes vortex beam with topological charge of l=1 and symmetrical four beam splitting. The structure provides a new simple and easy method for the design of terahertz metasurface devices and has a wide application prospect in terahertz system.

Design of a metasurface filtering antenna with tunable radiation nulls

In this article, a low profile wideband metasurface filtering antenna with tunable radiation nulls is proposed. Metasurface structure is used to excite adjacent resonant modes to obtain broadband responses. A U-shaped slot is introduced into the microstrip line to generate a radiation null in the lower stopband. Complementary split-ring resonators (CSRRs) are etched on the metasurface unit to produce another radiation null in the upper stopband edge to improve the filtering effect. The radiation nulls position can be flexibly controlled by changing the size of U-shaped slot and CSRRs. For verification, the corresponding antenna is fabricated and tested. The antenna size is 0.556 λ 0 × 0.556 λ 0 × 0.044 λ 0 (30 × 30 × 2.4 mm3). The measured results agree well with the simulated. The antenna operates from 4.9 to 6 GHz with a flat gain (5.5 dBi), a sharp passband skirt and stable radiation patterns.

E-shaped split ring resonator used for metasurface-based structure with an ultra-high Q-factor for terahertz application

Plasmonic devices based on split ring resonator (SRR) of ultra-low-loss performance (high Q-factor) are significant for a broad range of applications and studies. However, due to the inevitable ohmic and radiation losses, it has always been a challenge to design various types of plasmonic devices with high Q-factor. Here, we theoretically present a unique E-shaped SRR that can destructively suppress dipole radiation by adjusting the grating-arm position to produce asymmetry. Meanwhile, the proposed terahertz metasurface structure formed by periodically arranged E-shaped SRRs can give rise to high Q-factor resonances. The results show that an ultra-high Q-factor beyond 104 can be obtained in the terahertz frequency region by optimizing the structural parameters. The sensing characteristics of such a metasurface-based structure served as a refractive-index sensor are investigated as well, demonstrating its advantage of high sensitivity. Unique radiation properties of the metasurface-based structure enable it to use in the design of novel low-loss, ultra-sensitive sensors, narrowband filters, and other plasmonic devices.

Dynamically tunable multimodal vortex beam generator based on reflective metasurface

How efficiently generating a multi-mode vortex beam is the key to the new system of wireless communication technology based on orbital angular momentum. Due to the advantages of simple structure and flexible tunability of the metasurface, in recent years, people have been devoted to generating dynamic orbital angular momentum more simply and efficiently using programmable metasurfaces. To address the current problem of a single number of modes for generating orbital angular momentum vortex beams from metasurface, the programmable reflective metasurface structure has been designed to realize the function of dynamically tunable multi-mode of vortex beams. By loading two varactor diodes inside each metal patch to construct eight digital coding units, and using the programmable bias circuit to modulate the coding distribution and resonance characteristics on the metasurface in real-time, the metasurface obtains low reflection loss and stable reflection phase, and finally obtains vortex beams with mode numbers of ℓ = ±1, ℓ = ±2, and ℓ = ±3, respectively. The designed metasurface also has the advantages of a simple structure, small electrical size, and low profile, which has a large potential value in the communication of vortex beams and a perfect application in wireless transmission.

Millimeter‐wave frequency reconfigurable antenna using simple VO 2 ‐based paired metasurface

In this article, a millimeter-wave frequency-reconfigurable metasurface antenna by electrically controlling vanadium dioxide (VO2) is proposed and studied. As a phase change material, VO2 is an insulator at room temperature but can turn into high conductivity metallic state when activated by continuous direct current (DC) voltage. By integrating the VO2 film between two simple rectangle metasurface cells, a frequency-reconfigurable paired metasurface structure (FRPMS) is achieved. The FRPMS can be considered as one unit when VO2 film is activated to metallic state while as two independent cells when VO2 film is in insulating state. In this way, the FRPMS can operate, respectively at 28 and 38 GHz with independent tunability. Moreover, a coplanar and staggered DC bias circuit with little effect on radiation is designed to switch the operating modes of VO2 easily. The antenna is fed by a stepped waveguide with ridges through a loop slot, which is suitable for dual-band feeding with large frequency ratio. One prototype is fabricated and measured for demonstration. When the VO2 film is in metallic state, the measured working band covers from 28.5 to 29.6 GHz with the maximum gain of about 7.1 dBi. When cooled down to insulating state, the antenna can work in a higher band from 36.7 to 38.3 GHz with the maximum gain of about 7.9 dBi. Due to the advantages of simple structure, high gain and easy tuning, the antenna can be expected to use in mmW wireless communication.

Ultracompact Polarization Splitter–Rotator Based on Shallowly Etched Subwavelength Gratings and Anisotropic Metasurfaces

Polarization splitter–rotators (PSRs) are an essential component in on-chip polarization-sensitive and polarization–division multiplexing systems. In this work, we propose an ultracompact and high-performance silicon-based polarization splitter–rotator utilizing anisotropic metasurfaces, which is the first to combine the two, to our knowledge. The tilted periodic metasurface structure has different modulation effects on different polarized light fields, such as the transverse–electric (TE) mode and the transverse–magnetic (TM) mode, which are beneficial for designing polarization management devices. According to the results, the entire length of the silicon PSR was ~13.5 μm. The TE-to-TM conversion loss and polarization conversion ratio ere −0.154 dB and 96.5% at 1.55 μm, respectively. In the meanwhile, the cross talk and reflection loss were −27.0 dB and −37.3 dB, when the fundamental TE mode was input. The insertion loss and cross talk were −0.19 dB and −25.01 dB at the central wavelength when the fundamental TM mode was input. In addition, the bandwidth reached up to ~112 nm with polarization conversion loss and insertion loss higher than −0.46 dB and −0.36 dB. The simulations also show that the designed devices had good fabrication tolerance.

Biobased and Programmable Electroadhesive Metasurfaces.

Electroadhesion has shown the potential to deliver versatile handling devices because of its simplicity of actuation and rapid response. Current electroadhesion systems have, however, significant difficulties in adapting to external objects with complex shapes. Here, a novel concept of metasurface is proposed by combining the use of natural fibers (flax) and shape memory epoxy polymers in a hygromorphic and thermally actuated composite (HyTemC). The biobased material composite can be used to manipulate adhesive surfaces with high precision and controlled environmental actuation. The HyTemC concept is preprogrammed to store controllable moisture and autonomous desorption when exposed to the operational environment, and can reach predesigned bending curvatures up to 31.9 m–1 for concave and 29.6 m–1 for convex shapes. The actuated adhesive surface shapes are generated via the architected metasurface structure, incorporating an electroadhesive component integrated with the programmable biobased materials. This biobased metasurface stimulated by the external environment provides a large taxonomy of shapes—from flat, circular, single/double concave, and wavy, to piecewise, polynomial, trigonometric, and airfoil configurations. The objects handled by the biobased metasurface can be fragile because of the high conformal matching between contacting surfaces and the absence of compressive adhesion. These natural fiber-based and environmentally friendly electroadhesive metasurfaces can significantly improve the design of programmable object handling technologies, and also provide a sustainable route to lower the carbon and emission footprint of smart structures and robotics.

Double Fano Resonance and Independent Regulation Characteristics in a Rectangular-like Nanotetramer Metasurface Structure.

Fano resonance, which is based on a plasmonic metasurface, has many potential applications in various fields, such as biochemical sensors, slow light effect, and integrated optical circuits. In this study, a rectangular-like nanotetramer metasurface structure composed of four round-head nanorods was designed. The transmission spectrum, surface charge, and electrical field distributions of the proposed structure were simulated using the finite element method. A double Fano resonance profile was observed in the transmission spectrum. One of the Fano resonances was caused by the symmetry breaking and plasmon hybridization between the horizontal double rods, whereas the other resonance was due to the plasmonic modes’ hybridization among four nanorods. These resonances could be independently tuned because of different formation mechanisms. The number of Fano resonances could be adjusted by changing the coupling distance between the horizontal and vertical rods. The results contributed to designing the highly sensitive sensors based on the plasmonic metasurface.

On-demand design based on deep learning and phase manipulation of all-silicon terahertz chiral metasurfaces

Chiral metasurfaces have been widely used in sensing, imaging and other fields because they can manipulate light through the efficient circular dichroism (CD). However, its on-demand design is still a very challenging task. In this work, we propose an on-demand multiple reverse design based on deep learning, named target-driven conditional generative network (TCGN). It can reverse design the metasurface structure that meets the required CD, and its mean square error (MAE) is 0.0089. We use this method to inversely design multiple sets of metasurfaces with different structures, and all their CD values can exceed 0.36. Both simulations and experiments prove the feasibility and effectiveness of using deep learning to reverse design metasurfaces. In addition, the designed metasurface can realize chiral wavefront control under dual frequency. This design method based on deep learning can rapidly and efficiently design the chiral metasurfaces, which provides a new way for the design of metasurfaces.