Narrow Results

An additional concave incident surface comprised of two-dimensional (2D) sonic crystals (SCs) is employed to tune the acoustic image in the far-field region. The tunability is realized through changing the curvature of the concave surface. To explain the tuning mechanism, a simple ray-trace analysis is demonstrated based on the wave-beam negative refractive law. Then, a numerical confirmation is carried out. Results show that both the position and the intensity of the image can be tuned by the introduced concave surface.

In this paper, a new method is proposed to manipulate seismic Rayleigh waves using phase-gradient metasurfaces. This highly compact artificial structure enables the anomalous refraction of Rayleigh waves according to the generalized Snell’s law (GSL). The soil-embedded metasurface is composed of only one column of commercial rubber blocks, which can provide an accurate phase shift to the Rayleigh wave. To verify the flexibility of this method, several metasurfaces are designed. Numerical results demonstrate that the Rayleigh waves can be focused, split, or converted into evanescent waves by using specific phase gradient configurations. The investigation also suggests the strong potential of metasurface as a smart device for shielding of seismic surface waves.

Terahertz (THz) focusing metasurface plays a key role in micro-focusing and micro-imaging. Most of the existing focus sets only show the number and deflection of focuses. However, few researches are concerned about the adjustment of focus intensity. Herein we designed a complementary metasurface based on a C-ring and cross-shaped composite slot structures, and the effect of structural thickness on focusing intensity and focal length were studied. We find that the angle of anomalously transmitted beam can be scanned from $17.5^{\circ }$ to $26.5^{\circ }$ when the frequency changes from 1.2 THz to 1.8 THz, while the focal length of the focusing metasurface can be linearly changed from 1000 $\mu \mathrm{\text{m}}$ to 1650 $\mu \mathrm{\text{m}}$. With increase in the thickness of the C-ring area, more interestingly, the focus intensity decreased linearly from 8.0 dB (V/m) to 4.5 dB (V/m). However, the focus intensity showed no obvious change when the thickness of the cross-shaped area was increased from 0.2 $\mu \mathrm{\text{m}}$ to 5 $\mu \mathrm{\text{m}}$. Therefore, our work would enhance the application of THz in high-resolution imaging, new THz device, and flat lens with adjustable focus intensity.

A novel polarization insensitive metasurface with over 25 dB monostatic radar cross-section (RCS) reduction is introduced. The proposed metasurface is comprised of carefully arranged unit cells with spatially varied dimension, which enables approximate uniform diffusion of incoming electromagnetic (EM) energy and reduces the threat from bistatic radar system. An iterative fast Fourier transform (FFT) method for conventional antenna array pattern synthesis is innovatively applied to find the best unit cell geometry parameter arrangement. Finally, a metasurface sample is fabricated and tested to validate RCS reduction behavior predicted by full wave simulation software Ansys HFSS${}^{\mathrm{\text{TM}}}$ and marvelous agreement is observed.

In this paper, using a cross-shaped complementary Indium Tin Oxide (ITO)-based metasurface design, the transmission of THz radiation is shown to be filtered within the 3 dB level from maximum in the frequency range of interest ($\sim$333 GHz). Various metasurface structures primarily composed of cross-shaped openings with a 400 micron unit cell size are patterned on top of 1750 micron thick fused silica substrates. They are patterned using UV lithography methods after the films were grown using DC sputtering. The fabricated structures were characterized using Terahertz Time Domain Spectroscopy (THz-TDS) measurement technique. The measured transmission agrees well with the simulation of the structure for four different samples with different geometries. These results suggest that metasurface and/or metamaterial patterns based on ITO in visibly transparent media can be utilized for filtering of frequencies in the long wavelength spectrum. These types of filters can be very useful in the near future for THz communication and security applications.

A wideband metasurface filter based on complementary split-ring resonators (CSRR) has been prepared. The frequency and transmission bandwidth of the metasurface filters with different split widths are discussed. After analyzing the mechanism of the metasurface, the proposed metasurface filters are fabricated. The electromagnetic properties of the metasurface are measured by a designed test system. The measured results are in good agreement with the simulated ones, which shows that the metasurface filter has a wideband property. As the split width of the CSRR increases, the frequency of the passband shifts to higher frequency regions and the transmission bandwidth decreases.

In this paper, we present a wide band metasurface (MS) polarization converter which converts a linearly polarized signal to a right-handed or left-handed circularly polarized signal both numerically and experimentally. The unit cell of MS has three nested rectangular resonators which have two metallic patches at its crossed corners. The simulated and measured results are achieved by a commercial full wave EM simulator and a vector network analyzer with two horn antennas in microwave frequency regime. The S-parameters are obtained for co-polarized and cross-polarized responses and axial ratio is evaluated by the division of these two responses. The axial ratio is kept below 3 dB for efficient polarization converting activity. Correspondingly, axial ratio bandwidth of 800 MHz is obtained. The proposed MS can easily be fabricated and integrated into many desired applications by proper configurations depending on the application area and frequencies. The proposed MS has potential such as polarization converter with 0.75 efficiency in WiMAX frequency band, PMC-like treatment with a phase reflection around 0${}^{\circ }$ and reflection coefficient nearly unity at some frequency points. Beside this, the three nested rectangle MSs also provide opportunities to design low profile antennas with conversion characteristics.

A terahertz metasurface perfect absorber with multi-band performance is demonstrated. The absorber is composed of a ground plane and four split-ring resonators (SRRs) with different dimensions, separated by a dielectric spacer. The numerical simulation results illustrate that the proposed absorber has four distinct absorption peaks at resonance frequencies of 4.24, 5.66, 7.22, and 8.97 THz, with absorption rates of 96.8%, 99.3%, 97.3%, and 99.9%, respectively. Moreover, the corresponding full width at half-maximum (FWHM) values are about 0.27, 0.35, 0.32, and 0.42 THz, respectively, which are much broader than those of previously reported absorbers. Besides, the calculated magnetic field distributions allow us to understand the absorption mechanism in detail. The effects of incident angle and azimuthal angle on the absorber are also investigated. The results show that the proposed absorber is partially sensitive to the incident angle, which makes this design promising for practical applications in terahertz imagers and detectors.

An ultrathin metasurface-based absorber consisting of titanium nitride (TiN) nano-disk arrays–dielectric layer-TiN substrate is proposed in this paper. The absorber exhibits near-unity absorption in the whole visible range of 380–780 nm. Our results demonstrate that the proposed metasurface-based absorber is not only independent of light polarization, but also exhibits angle-independent absorption behavior for oblique incidence up to 70${}^{\circ }$. The high absorption performance of the TiN nano-disk arrays-based absorber can attribute to two different loss mechanisms associated with the intrinsic loss and plasmonic resonance.

Polarization is a property of transverse waves that specify the geometrical orientation of the oscillations, and it is of significant importance to control the polarization of waves at the information storage and processing. In this paper, we propose a new type of polarization conversion metasurface based on the $\mathrm{\Phi }$-shaped structure in reflection mode. An ultrawideband cross-polarization conversion is obtained. The simulated results show that this metasurface can rotate the polarized direction of the linearly polarized electromagnetic wave by 90${}^{\circ }$ and the polarization conversion efficiency reaches up to 90% in the ultrawideband from 6.9 GHz to 22.2 GHz. The relative bandwidth is 105%. In addition, the experimental results of polarization conversion efficiency are in good agreement with the simulations. With the advantages of high conversion efficiency, ultrawide operating band and the simple geometric structure, the application of this polarization conversion metasurface structure can be extended to the frequency range of terahertz or even the visible light.

Structural colors engineering as a promising research area provides a high-resolution and environmental-friendly sustainable colors implementation. Here, metasurface-based concept was used to design waveguides and gratings of Si₃N₄ material on silicon dioxide substrates. The shift of reflection peaks can be controlled by structural parameters and efficiencies exceeded 90% with half height and width less than 10 nm. This induced structural color with superior saturation. Moreover, such nanostructures showed good sensitivity to polarization angle and incident angle of incident light. The proposed coloring devices look promising for applications in active color displays, imaging devices, and anti-counterfeiting and so on.

A novel metasurface is proposed that aims to generate underwater acoustic waves with various functions by only one actuator. Each metasurface unit consists of an air cavity sandwiched on one side by a vibration plate and connecting rubber supports. By properly selecting the ratio of the plate to unit lengths, a phase shift of $\pi$ can be attained to constitute a binary coding metasurface. Three demonstrations, including focusing, branching and self-bending waves, are chosen to validate the functionality of the design. The design is also shown to work over a wide frequency range by changing the ratio. In addition, the design is extremely compact, with the thickness only about 1/100 of the target wavelength. Compared with commonly used phased array transducers that are utilized to generate underwater acoustic waves, this design offers has the advantage of needing only a single actuator as opposed to needing a lumped electrical control system.

Fluorescence microscopy possesses the advantages of high resolution, high sensitivity, molecular specificity and noninvasiveness, providing an important tool in life science research. The multifocal array and 3D structured light are two kinds of important light fields that are often used in scanning fluorescence microscopy systems and wide-field fluorescence microscopy systems. However, traditional methods for generating multifocal arrays and 3D structured light illumination rely on various bulk optical components, making it challenging to achieve compact optical systems. Besides, generating these two types of illumination typically requires two separate and independent optical systems, hindering the integration of different types of fluorescence microscopy systems. Here, a dielectric metasurface is proposed that can achieve the switching between multifocal arrays and 3D structured light through polarization state modulation, greatly simplifying the illumination optics of fluorescence microscopy systems and facilitating the integration of different types of fluorescence microscopy systems.

Tunable three-dimensional (3D) electromagnetic (EM) metasurfaces are critical for dynamic modulation of EM responses but their construction and tuning mechanism are still complex. Here, we report a simple yet effective 3D reconfigurable EM metasurface, which was obtained from a planar kirigami polyimide substrate printed with periodically arranged copper split-ring resonator. Under mechanical stretch, the two-dimensional (2D) planar metasurface can be uniformly deformed into a 3D state, which is effective for tuning its EM transmission characteristic. By combining mechanics and EM simulations as well as experimental measurements, we revealed the deformation mode and active EM transmission modulation capability of the metasurface. It is shown that at the initial state, the planar kirigami metasurface exhibits ideal frequency selective transmission to transverse electric (TE) wave but allows for complete transmission for transverse magnetic (TM) wave. As the applied strain increases from 0% to 20%, the transmission was adjusted from −17.74dB to −9.74dB for TE wave but merely from 0dB to −3.25dB for TM wave. Meanwhile, the resonant frequency experienced a visible shift for both TE and TM waves. Finally, the equivalent circuit analysis and simulated surface current density were conducted to reveal the tuning mechanism of the proposed metasurface.

The much thicker intrinsic absorption layer (IAL) in normal-incidence Ge-on-Si photodetectors (NIPD) usually causes a contradiction between responsivity and bandwidth. In response to this issue, here, we simulate the design of an NIPD with geranium (Ge) layers based on a “fishnet” metasurface, leading to a reduced device thickness as thin as 380nm. The optical simulation results show that the light field can be perfectly localized in the 210nm IAL, and the absorptivity is as high as 99.45% at 1550nm, which is even better than bulk materials. Moreover, the electrical simulation results suggest that the horizontal size of the photosensitive region can be reduced to 11.2 $\mu$m, while the responsivity of the photodetector is close to 1 A/W at −1V bias voltage, which is nearly 23 times that of a bulk device with the same thickness, and the 3dB bandwidth is up to 40GHz, which can be compared with waveguide photodetectors. Besides, this device also demonstrates a high signal-to-noise ratio with a low dark current of 28.68 nA, making it an excellent PD for opto-electrical communication.

An ultra-thin metasurface is proposed to realize wideband polarization-independent anomalous reflection. The sub-wavelength resonator can produce different resonance modes, which are the result of the combined effect of dielectric and the metallic structure. The gradient metasurface is done by six discrete orientation of the local sub-wavelength resonator which provides a phase gradient. The simulation and measured results show that 9GHz bandwidth of the anomalous reflection is achieved.

In this paper, two optimized autofocusing metasurfaces (AFMs) with different desired focal distances are designed by using particle swarm optimization (PSO) algorithm. Based on the finite element simulation software COMSOL Multiphysics, the performance of ultrasound transducer (UT) with AFM at different design parameters in Airy distributions $(r_0,\omega )$ and the bottom thickness (d) of AFM are simulated and analyzed. Based on the simulation data, the artificial neural network model is trained to describe the complex relationship between the design parameters of AFM and the performance parameters of UT. Then, the multiobjective optimization function for AFM is determined according to the desired performance parameters of UT, including focal position, lateral resolution, longitudinal resolution and absolute sound pressure. In order to obtain AFMs with the desired performance, PSO algorithm is adopted to optimize the design parameters of AFM according to the multiobjective optimization function, and two AFMs are optimized and fabricated. The experimental results well agree with the simulation and optimization results, and the optimized AFMs can achieve the desired performance. The fabricated AFM can be easily integrated with UT, which has great potential applications in wave field modulation underwater, acoustic tweezers, biomedical imaging, industrial nondestructive testing and neural regulation.

Metasurfaces refer to the sub-wavelength nanostructures that are capable of manipulating the amplitude, phase, polarization, and other characteristics of light, to enable diverse applications across the ultraviolet, visible, infrared, and terahertz spectra. In this review paper, we aim to provide an introductory note on metasurfaces, from fundamentals and design methods to applications, such as biosensing, environmental monitoring, metalenses, optical cloaking, electromagnetic scattering, structural color, miniaturized devices, and others. Moreover, we also identify the key challenges and limitations of metasurfaces, such as fabrication, integration, optimization, tuning, signal processing, and analysis, and suggest possible directions and solutions for future research. At last, we envision several emerging and promising trends for metasurfaces, such as new materials and structures, new phenomena and mechanisms, machine learning, and artificial intelligence techniques. The review is expected to inspire future development in this exciting and rapidly evolving field of metasurface devices.

Metasurfaces refer to the sub-wavelength nanostructures that are capable of manipulating the amplitude, phase, polarization, and other characteristics of light, to enable diverse applications across the ultraviolet, visible, infrared, and terahertz spectra. In this review paper, we aim to provide an introductory note on metasurfaces, from fundamentals and design methods to applications, such as biosensing, environmental monitoring, metalenses, optical cloaking, electromagnetic scattering, structural color, miniaturized devices, and others. Moreover, we also identify the key challenges and limitations of metasurfaces, such as fabrication, integration, optimization, tuning, signal processing, and analysis, and suggest possible directions and solutions for future research. At last, we envision several emerging and promising trends for metasurfaces, such as new materials and structures, new phenomena and mechanisms, machine learning, and artificial intelligence techniques. The review is expected to inspire future development in this exciting and rapidly evolving field of metasurface devices.