Metasurface Patterns Research Articles

Broadband optically transparent microwave absorber made of interdigital metasurfaces in rotational symmetry with a single air spacer

A broadband optically transparent metasurface microwave absorber (MMA) is designed and experimentally studied. The MMA is made of two indium tin oxide (ITO) resistive films deposited on two transparent polyethylene terephthalate substrates respectively, between which is sandwiched a single air spacer. The top ITO resistive film is etched with periodic interdigital metasurface patterns in rotational symmetry, while the bottom ITO resistive film is an integrated sheet with a low resistance working as the backplane. By carefully optimizing the functional interdigital metasurface structures in a numerical solver, a desirable 4-octave broadband MMA is achieved. The absorbing bandwidth is 4.53–18.71 GHz (122.03%) in the numerical predictions for the perpendicular incidence, in which the absorptivity is greater than 90%. Its total thickness is only 5.8 mm or 0.088λ L, where λ L is the wavelength (66.23 mm) at the lowest 4.53 GHz. The absorber is validated in experiments. Results are observed in good agreement with the simulated ones. The interdigital MMA is polarization-insensitive and able to operate for wide-angle incidences up to 45°. These properties are demonstrated in both simulations and experiments.

Terahertz structured deep-subwavelength plasmonic grating metasurface for manipulating polarization-dependent coupling response

The complex electromagnetic anisotropy of terahertz (THz) metasurfaces with geometric symmetry breaking has attracted extensive attention. Typical effects arise from the coupling of polarization responses in orthogonal directions of various components of the metasurface structure, such as the electromagnetically induced transparency (EIT) effect. However, it is a challenge to precisely control or perfectly avoid the polarization-dependent coupling responses. In this work, deep-subwavelength plasmonic gratings (PGs) with a fine wire width of 1 μm at the order of deep subwavelengths of 1/100 THz wave are fabricated by electron beam lithography, and these wire gratings are graphically designed as a C-shaped metasurface pattern with a period of 100 μm in sub-wavelength scale. The complete anisotropic response in the single-oriented PG metasurface is demonstrated by both simulation and experiments, where the polarization-dependent coupling effect is eliminated. More interestingly, the hybrid-oriented PG metasurface exhibits narrowband and wideband EIT effects in the x and y polarization directions with the maximum polarization extinction ratio of 20 dB, respectively, indicating this mechanism can realize more flexible manipulation of polarization-dependent coupling. This patterned deep-subwavelength PG provides a new structure and mechanism for excitation, regulation, and restriction of polarization-dependent mode coupling, and has important applications in THz spectroscopic detection, polarization imaging, and wireless communication.

Multiaxis Manufacture of Conformal Metasurface Antennas

A conformal metasurface antenna exhibiting a pencil beam radiation pattern at 10.0 GHz has been designed using the Voronoi partition approach, and fabricated on the Kahu Uninhabited Aerial System (UAS) fuselage. Two manufacturing methods are presented and compared. The first approach utilized a 3-axis Trotec fiber laser to etch the flattened metasurface geometry in copper foil. The etched pattern was then ‘stretched’ over the UAS geometry. The second approach utilized a 6-axis nScrypt (retrofitted with an IDS aerosol jetting tool) to conformally print the metasurface pattern directly on the UAS fuselage. An electroless copper plating step was then utilized to improve the radiofrequency (RF) conductivity of the printed silver. Both manufacturing methods yielded functional metasurface antennas with equivalent performance at the operating frequency. However, the first method is limited to geometries that can be ‘flattened’ with acceptable tolerances, whereas the second approach is amenable to all practical geometries. This demonstration of two manufacturing techniques is a critical step forward in the cost-effective deployment of truly conformal metasurface antennas on realistic geometries.

High-temperature flexible metasurface with broadband microwave polarization rotation and low infrared emissivity

Advanced electromagnetic functional materials are more and more inclined to integrated design, for instance the compatibility in different spectra as well as wide temperature ranges. Metasurface provides an effective solution for multi-spectrum compatible design owning to its extraordinary performance. In this work, we propose a high-temperature flexible metasurface framework, which can simultaneously realize microwave polarization rotation and low infrared emissivity. As verification, a high-temperature resistant multi-spectrum compatible flexible metasurface is fabricated and measured, which can availably work up to 300 °C. For microwaves, the metasurface achieves the orthogonal polarization rotation efficiency of more than 90% in 5.2–7.5 GHz. For infrared, the metallic filling ratio of 84.42% is constructed by designing the metasurface pattern, thus reducing the infrared emissivity to 0.22 in 3–14 µm. This work provides a flexible metasurface framework for implementing multi-spectrum compatibility at high temperatures, which can be easily attached to the surface of high-temperature components and extended to more functional integration.

Research on a demand design method of a cross polarization converter metasurface based on a depth generation model

As a two-dimensional electromagnetic metamaterial, the cross-polarization conversion (CPC) metasurface is thin, easy to develop, and has attracted wide attention. However, existing CPC cell surface designs still rely on inefficient full-wave numerical simulation. Although some researchers have explored deep learning CPC metasurface structure design methods, the generated metasurface patterns are of poor quality. In this paper, an on-demand design method for cross-polarization conversion metasurface based on depth-generation model is proposed. Firstly, Wasserstein generative adversarial network (WGAN) is used to reverse design CPC metasurface, and Wasserstein distance is introduced to replace JS divergence and KL divergence to optimize the target. The problem of training difficulty caused by gradient elimination of original generative adversarial network (GAN) is fundamentally solved. Secondly, in the WGAN model, U-Net architecture generator is used to generate images, which greatly improves the surface image quality of CPC. In addition, a simulator composed of convolutional neural network (CNN) is also added in this paper to carry out forward prediction of S-parameter spectrum diagram. By inputting the patterns generated by WGAN into the simulator, the corresponding S-parameter spectrum diagram is generated and compared with the real S-parameter spectrum diagram, so as to verify whether the surface patterns of generated elements meet the requirements. The depth generation model proposed in this paper organically combines the forward spectrum prediction model and the reverse CPC metasurface structure design model, so that the CPC metasurface structure satisfying the expected electromagnetic response can be designed quickly on demand. This on-demand design method is expected to promote the rapid design, fabrication and application of electromagnetic devices.

Efficient design method for terahertz broadband metasurface patterns via deep learning

The traditional design method of metasurfaces is the trial-and-error process with full-wave electromagnetic simulation. Recently, as an effective method, deep learning has been widely used in a variety of fields to solve complex problems. Here, forward prediction and inverse design methods for terahertz (THz) random metasurfaces are proposed based on deep Convolutional Neural Networks (CNN) and Genetic Algorithms (GA). According to the metasurface pattern, the forward prediction model accurately obtains the reflection amplitude and phase response. Compared with the Full-wave solver, the calculation speed is increased by 40,000 times. Furthermore, the THz random structure can be accurately and quickly derived from the target response, operating in a broadband range of 0.2–2 THz. Then, we discuss the advantages of single-objective and multi-objective optimization in the inverse design of metasurface patterns. By combining with the GA, the design efficiency is greatly improved. This can serve as an efficient method for global optimization in complex designs. Finally, we obtain two meta-atoms used to encode metasurface in only 10 min and built a three-beam splitter. The model proposed in this paper provides a new approach to metasurface design at THz frequencies.

Etching metasurfaces on bluff bodies for vortex-induced vibration energy harvesting

Bluff bodies play crucial roles in affecting the functionality of wind energy harvesting systems. Inspired by the versatility and wide application of metasurfaces in many fields, we propose to etch several metasurface patterns on a cylinder bluff body and investigate their effects on vortex-induced vibration (VIV). A general lumped parameter model is developed for VIV piezoelectric energy harvesters (VIVPEHs). Three-dimensional CFD simulation models are built to obtain the lift and drag coefficients. Wind tunnel tests are conducted to experimentally study the dynamics and evaluate the voltage output. The theoretical results matched well with the experimental ones. It is unveiled that etching the concave metasurface patterns on the bluff bodies has a significant influence on the dynamic behavior of the bluff body, thus enhancing or suppressing VIV. Beside, we demonstrate the influences of the etched metasurface patterns on the generated vortex street behind the bluff bodies through CFD simulations. It is shown that different concave metasurface patterns could significantly affect the wake pressure, thus influencing the aerodynamics. Furthermore, the effects of the electrical parameters, including the load resistance and electromechanical coupling strength, on energy harvesting performance are investigated based on the theoretical model.

Neural Optimizer for Inverse Design of Complex‐Modulated Hologram Implemented by Plasmonic Metasurfaces

Inverse design of a metasurface involves searching parameters in a high‐dimensional space, which needs huge computational power. To ease the computational burden, neural network, a well‐researched computer science stream, has demonstrated its potential usage in the inverse design of a photonic device. Many studies primarily focused on the nanostructure's configuration. However, the near field of a metasurface requires further optimization to achieve a desired holographic image at relieved computational power. Here, a convolutional neural network is developed to optimize the complex field of a computer‐generated hologram, which can be fabricated into a metasurface to generate the desired holographic image upon lensless image projection. The neural optimizer can accelerate the design speed 400 times faster than theoretical computation and reduce the time complexity from O(n 4) to O(n 2). The neural optimizer has been compared against three other methods, e.g., gradient‐based optimization, global genetic algorithm, and coupled‐mode theory, to demonstrate a lowered error rate from more than 10% to 1.38% for the benchmark testing and a reduced running time from hours to near 1 s. The neural optimizer is envisioned to play a key role in lensless image projection and real‐time metasurface pattern design.

Physical insight into the terahertz polarization modulator by using Smith chart

Polarization modulation (PoM) provides an additional degree of freedom for the modulation of carrier waves and may be an alternative to conventional wireless communication techniques that use amplitude, frequency, and phase modulations. In this study, we proposed a metasurface-based modulator that transforms a linearly polarized wave into a circularly polarized wave in the terahertz range of 1.53 THz–1.57 THz. The polarization manipulation of the metasurface is analyzed using the Smith chart method, which builds a bridge between the metasurface pattern and its equivalent circuit impedance. We show the reflection/transmission coefficient and impedance of the metasurface in the Smith chart, providing clear physical images for the designers. According to the Smith chart analysis, we obtained a two-layer metasurface modulator with 90% efficiency for the transmitted field. All the results are validated from the simulation. The proposed Smith chart method opens a new design methodology of metasurface-based devices.

Design of metasurface-based polarization modulators by using Smith chart method

Polarization modulation (PoM) provides an additional degree of freedom for the modulation of carrier waves and may be an alternative to conventional wireless communication techniques that use amplitude, frequency, and phase modulations. Metasurfaces have provided unprecedented freedom for modulating the polarization state of electromagnetic waves. In this study, we introduce the Smith chart method to guide the design of metasurface-based modulator that transforms a linearly polarized wave into a circularly polarized wave. The proposed method bridges the metasurface pattern and its equivalent circuit impedance. We show that the parameter changes in the pattern can be directly displayed in the Smith chart, which provides clear physical image guidance to designers. According to the Smith chart analysis, we obtained a single-layer modulator with 50% efficiency and a three-layer modulator with 100% efficiency, all of which are verified using the full-wave simulation method. The Smith chart method will also be helpful in the design of other metasurface-based devices, such as metasurface absorbers.

A Novel Ultrathin Checkerboard Inspired Ultrawideband Metasurface Absorber

A cost-effective, conformal, polarization-insensitive, light-weight, ultrathin, and ultrawideband electromagnetic wave absorber covering the entire Ku, K, and Ka bands and the lower V band is presented in this article. The absorber consists of a single layer of inkjet printed resistive metasurface laid over a flexible paper supported by a grounded foam. A novel metasurface pattern is designed by combining a checkerboard inspired symmetrical structure around a central square patch. The notable features of the proposed absorber are its ultrathin thickness of 2.17 mm (0.09<inline-formula><tex-math notation="LaTeX">$\boldsymbol{\lambda _l}$</tex-math></inline-formula> at the lowest operating frequency) for achieving a fractional bandwidth of 117&#x0025; with a small radar cross section and its suitability for conformal applications. It exhibits more than 90&#x0025; absorptivity over a frequency range from 11.87 to 45.28 GHz for all the angles of polarization and is also angularly stable. The theoretical, simulation, circuital, and experimental analysis validates the various features exhibited by the absorber.

A Differential Modulation Scheme for Metasurface-Based Terahertz Communications

Metasurface-based terahertz (THz) modulators, which carry information by changing the spatial pattern of the metasurface at every symbol time, are an important component to establish low-cost low-energy–consuming THz communication systems. This paper proposes a differential spatial THz modulation (DSTM) scheme for metasurface-assisted THz communications. In DSTM, the metasurface pattern activation orders are employed to carry information, instead of the metasurface patterns themselves. In this way, the DSTM receiver can perform differential detection without knowing the metasurface response to incident THz signals and the channel state information. The proposed DSTM scheme is applicable to all kinds of metasurfaces enabling THz communications, including transmissive metasurfaces and reflective metasurfaces. For high-rate DSTM systems, we propose an efficient bits-to-activation-order mapping method and a low-complexity detection method. Simulations are conducted to demonstrate the performance of the proposed scheme. It is shown that the proposed scheme pays acceptable penalty compared to the non-differential modulation scheme with coherent detection, which needs to know the metasurface response to the incident signals as well as all the channel state information.

Phase-to-pattern inverse design paradigm for fast realization of functional metasurfaces via transfer learning

Metasurfaces have provided unprecedented freedom for manipulating electromagnetic waves. In metasurface design, massive meta-atoms have to be optimized to produce the desired phase profiles, which is time-consuming and sometimes prohibitive. In this paper, we propose a fast accurate inverse method of designing functional metasurfaces based on transfer learning, which can generate metasurface patterns monolithically from input phase profiles for specific functions. A transfer learning network based on GoogLeNet-Inception-V3 can predict the phases of 28×8 meta-atoms with an accuracy of around 90%. This method is validated via functional metasurface design using the trained network. Metasurface patterns are generated monolithically for achieving two typical functionals, 2D focusing and abnormal reflection. Both simulation and experiment verify the high design accuracy. This method provides an inverse design paradigm for fast functional metasurface design, and can be readily used to establish a meta-atom library with full phase span.

Large-area optical metasurface fabrication using nanostencil lithography.

We demonstrate a large-area fabrication process for optical metasurfaces utilizing reusable SiN on Si nanostencils. To improve the yield of the nanostencil fabrication, we partially etch the front-side SiN layer to transfer the metasurface pattern from the resist to the nanostencil membrane, preserving the integrity of the membrane during the subsequent potassium hydroxide etch. To enhance the reliability and resolution of metasurface fabrication using the nanostencil, we spin coat a sacrificial layer of resist to precisely determine the gap between the nanostencil and the metasurface substrate for the subsequent liftoff. 1.5 mm diameter PbTe meta-lenses on ${\rm{Ca}}{{\rm{F}}_2}$ fabricated using nanostencils show diffraction-limited focusing and focusing efficiencies of 42% for a 2 mm focal length lens and 53% for a 4 mm focal length lens. The nanostencils can also be cleaned using chemical cleaning methods for reuse.

Silicon-Based TM0-to-TM3 Mode-Order Converter Using On-Chip Shallowly Etched Slot Metasurface

Mode-order converters drive the on-chip applications of multimode silicon photonics. Here, we propose a TM0-to-TM3 mode-order converter by leveraging a shallowly etched slot metasurface pattern atop the silicon waveguide, rather than as some previously reported TE-polarized ones. With a shallowly etched pattern on the silicon waveguide, the whole waveguide refractive index distribution and the corresponding field evolution will be changed. Through further analyses, we have found the required slot metasurface pattern for generating the TM3 mode with high conversion efficiency of 92.9% and low modal crosstalk &lt;−19 dB in a length of 17.73 μm. Moreover, the device’s working bandwidth and the fabrication tolerance of the key structural parameters are analyzed in detail. With these features, such devices would be beneficial for the on-chip multimode applications such as mode-division multiplexing transmission.

On the use of metasurface for Vortex-Induced vibration suppression or energy harvesting

This paper explores the use of metasurface in designing an aerodynamic system for potential vortex-induced vibration (VIV) suppression or energy harvesting. Four kinds of metasurface patterns are designed to decorate an ordinary cylinder bluff body for modifying its aerodynamic characteristics. A theoretical model is developed for the VIV system by describing the wake as a van der Pol oscillator. The aerodynamic parameters, i.e., the lift and drag coefficients, used in the theoretical model, are determined from three-dimensional CFD simulations. A wind tunnel experiment is conducted to validate the theoretical model and investigate the aerodynamic behaviors of the VIV systems with different bluff bodies. It is found that the metasurface pattern has a significant influence on the aerodynamic characteristics of the bluff body. Using different metasurface patterns to decorate the bluff body, the vortex-induced vibration of the system could be either enhanced or suppressed, compared with the baseline reference using an ordinary bluff body with a smooth surface. Furthermore, the vortex shedding processes are simulated to give further insights into the wake oscillating motions. The roles of the metasurface in suppressing or enhancing the vortex-induced vibration are reasonably explained: the existence of metasurfaces could alter the flow field around the bluff bodies and consequently the aerodynamic force. In addition, based on the validated theoretical model, it is learned from a parametric study that the power output reaches saturation when the electromechanical coupling strength is increased to a certain level. Therefore, piezoelectric transducers with moderate coupling coefficients are recommended for practical applications from the cost-effective perspective.

Active Switching of Toroidal Resonances by Using a Dirac Semimetal for Terahertz Communication

The dynamical switching of a toroidal dipole resonance channel is demonstrated by tuning the Fermi level of a Dirac semimetal film sandwiched between the back substrates. As the Fermi level is increased from 30 to 150 meV, the resonance frequency is switched from 0.283 to 0.201 THz because of the transition from toroidal mode to hybrid mode. The hybrid mode is formed by the interaction between the toroidal mode and the plasmonic mode (induced by a Dirac semimetal film with metallic properties). The influence of the sandwiched dielectric layer (between the toroidal metasurface pattern and the Dirac semimetal film) on the switching effect was also investigated. This active dual-channel terahertz switching may have potential applications in advanced terahertz communication.

Terahertz Polarizers Based on 2D Ti3C2Tz MXene: Spin Cast from Aqueous Suspensions

Herein, the fabrication of terahertz (THz) polarizers is reported on by simply spin casting two‐dimensional (2D) MXene Ti3C2Tz nanosheets on a photolithographically patterned THz‐transparent substrate and subsequent immersion in acetone. Lines 30 nm‐thick and 10–20 μm wide result in electric field (E) extinction ratios (ER) of up to 3 dB, or power ER of up to 6 dB. Simulations show the possibility of achieving ER beyond 16 dB, or power ER higher than 32 dB by increasing the thickness of the MXene lines to 1.5–2 μm and optimizing the metasurface patterns. The Ti3C2Tz nanosheets are solution‐processed and can be deposited on a variety of substrates, including flexible ones. Once encapsulated, chemically stable THz polarizers, that combine high performance and low production costs, can be readily manufactured, with characteristics that compare favorably with the much more involved metallic wire grid polarizers, including gold and tungsten. Moreover, recent demonstration of dynamic tunability of Ti3C2Tz THz conductivity by ultrafast optical pulses opens the possibility of using MXene wire‐grids in high‐speed THz modulators.

Deep Learning Designs of Anisotropic Metasurfaces in Ultrawideband Based on Generative Adversarial Networks

Metasurfaces have been developed as a promising approach for manipulating electromagnetic waves. Recently, deep learning algorithms have been introduced to design metasurfaces, but the network can only output one solution for each desired input and suffers from nonunique issue. To overcome the aforementioned challenges, a deep neural network model for inverse designs of anisotropic metasurfaces with full phase properties in ultrawideband is proposed. Given the target reflection spectra as inputs, the candidate metasurface patterns are generated through a generative adversarial network (GAN), and the corresponding predictions are simply achieved by the accurate forward neural network model to match the target spectra in the whole band with high fidelity. By training the generator and discriminator in GAN in an alternating order combined with setting a threshold of discriminator loss to trigger the phase prediction, the proposed method is much more efficient and consumes less time in the training process. Numerical simulations and experimental results demonstrate that the reflection phases of the generated meta‐atoms have excellent agreements with the given targets, providing an efficient way in automatically designing metasurfaces. The most important advantage of this approach over the previous schemes is to improve the design speed significantly with very good accuracy.

Information theory of metasurfaces.

We propose a theory to characterize the information and information processing abilities of metasurfaces, and demonstrate the relation between the information of the metasurface and its radiation pattern in the far-field region. By incorporating a general aperture model with uncertainty relation in L2-space, we propose a theory to predict the upper bound of information contained in the radiation pattern of a metasurface, and reveal the theoretical upper limit of orthogonal radiation states. The proposed theory also provides guidance for inverse design of the metasurface with respect to given functionalities. Through investigation of the information of disordered-phase modulated metasurfaces, we find the information invariance (1−γ, where γ is Euler's constant) of chaotic radiation patterns. That is to say, the information of the disordered-phase modulated radiation patterns is always equal to 1−γ, regardless of variations in size, the number of elements and the phase pattern of metasurface. This value might be the lower bound of radiation-pattern information of the metasurface, which can provide a theoretical limit for information modulation applications, including computational imaging, stealth technologies and wireless communications.