Metasurface Absorber Research Articles

Large Optical Modulation of Dielectric Huygens’ Metasurface Absorber (Advanced Optical Materials 10/2023)

Mie-type optical resonators are applicable to all-optical modulation. In article number 2300102, Hao-Yu Cheng, Kung-Hsuan Lin, Kuo-Ping Chen, and co-workers demonstrate both theoretically and experimentally that modulation depth is enhanced by two orders of magnitude in Huygens' metasurface absorber where electric dipole lattice resonance matches the magnetic dipole resonance. Within picosecond scale delay after ultrafast photoexcitation and a few square micrometer spatial confinement, over 100% photothermal modulation depth is experimentally achieved in Huygens' metasurface absorber based on amorphous silicon.

Design of Metasurface Absorber Based on Improved Deep Learning Network

Metasurfaces have received extensive attention for their unique electromagnetic properties. However, traditional metasurface design is hugely labor-intensive and computationally resource-intensive, especially when using complex structures to obtain suitable targets. In this paper, a design method based on deep learning (DL) is proposed, which can efficiently reduce design time and resource consumption. The DL model is composed of two parts, an autoencoder (AE) and a deep learning network (DLN). It can quickly fit the relationship between the electromagnetic response and the metasurface structure. For demonstration, two different absorbers are designed based on the proposed DL method, and the target spectrum are in good agreement with the simulation results. The proposed DL method achieves an average accuracy of 95% and 85% on two different absorbers, respectively, verifying its powerful predictive ability. In addition, the high performance of DL on two different structures shows its transferability.

Triple‐band metasurface absorber for RF energy harvesting applications

AbstractEnergy harvesting is expected to be ubiquitous and essential to modern‐day life in the future. The limitations of ambient sources, such as thermal, mechanical, and solar, for energy harvesting can be overcome by RF energy harvesting for low‐power devices. With the help of perfect metasurface absorbers, an electromagnetic wave can be used as an ambient source for energy harvesting. In this paper, a simple polarization‐insensitive triple‐band perfect metasurface absorber is proposed for RF energy harvesting applications. The three‐layer (metal–dielectric–metal) meta‐atom of the designed metasurface consists of a cross‐shaped metal patch surrounded by slotted square‐shaped metallic patches and of meta‐atoms. The absorber is typically developed in S‐ and C‐bands with resonant frequencies of 3.06, 3.9, and 5.97 GHz. The absorption is near unity which persists over a wide angle of incidence. The experimental results are in close agreement, within 2%, with the simulation results. The reliability of the proposed absorber has been determined for the RF energy harvesting applications, and a high harvesting efficiency of 94.4% and 84% has been observed at 3.07 and 5.97 GHz, respectively.

Ultra-broadband tunable terahertz absorber based on graphene metasurface with multi-square rings

In recent years, thinness, lightness, broadband, high absorption, and tunability have become five important elements for electromagnetic (EM) absorbers. This article proposes a novel single-layered graphene metasurface absorber (GMSA) that perfectly achieves all of the above elements. The GMSA differs from common hybrid patterns in that it adopts a graphene layer with a simple multi-square ring structure. The simulation results show that Fabry-Perot resonance (FPR) and graphene-based surface plasmon resonance (GSPR) can be combined to produce the broadband absorption of 3.04 THz in the terahertz (THz) frequency range. The proposed GMSA is completely polarization-insensitive because of its symmetrical configuration, and it exhibits wide-angle oblique incidence properties for both TE and TM-polarized waves, allowing it to achieve broadband absorption with at least 90% absorption in the range of incident angles from 0° to 45°. In addition, the chemical potential of graphene can be altered to change the effective absorption bandwidth, which can be easily tuned by the applied external voltage. The GMSA has broad application prospects in THz detection, imaging, and object cloaking due to its flexible and straightforward construction, polarization insensitive, wide-angle incident, broadband, and high absorption qualities.

Elimination of interference and cross talk from a face‐to‐face‐antennas system by a metasurface absorber

AbstractIn this letter, a design and an experimental verification of a face‐to‐face‐antennas system combined with a metasurface absorber are presented, from which interference and cross talk are eliminated. First, the metasurface absorber unit cell composed of square copper pixels is optimally designed with the far‐field‐incidence condition utilizing the genetic algorithm to satisfy the −10 dB reflectance from 5.75 to 5.83 GHz with the minimum reflectance −13.90 dB at 5.8 GHz. Then, the metasurface absorber is integrated into a pair of 2 × 1 array antennas, of which the resonance frequency is 5.8 GHz. The two array antennas are set to face each other to enable short‐distance wireless‐data transmission between two antennas aligned in a line of sight. By absorbing undesired reflected waves using the metasurface absorber, interference induced on the transmitted spectrum along the vertical direction and cross talk along the horizontal and diagonal directions can be eliminated at the same time. They are confirmed by the simulation results calculated for various distances between the pair of array antennas. The measurements are well matched to the simulations, verifying the accuracy of the proposed design.

Inverse design of a broadband tunable terahertz metasurface absorber

A tunable terahertz vanadium dioxide (VO2) metasurface absorber based on inverse design is proposed and demonstrated numerically. The inverse design is a novel design method by which the optical structure found is based on the desired functional properties, significantly improving the design efficiency of optical structures. The metasurface absorber accomplishes more than 90% absorption between 2.88 and 7.15 THz. Peak absorption intensity may be dynamically changed between 4% and 96% as VO2 conductivity is changed between 200 S/m and 2 × 105 S/m. The physical mechanism is discussed by interference phase extinction, the theory of impedance matching and field distributions. The suggested absorber can be used for energy harvesting and terahertz filtering.

Design of a broadband metasurface sound absorber based on Hilbert fractal

In this paper, a new fractal-based broadband metasurface absorber is developed, which is based on the excellent space-filling ability of the Hilbert fractal. Each unit cell in absorber consists of a micro perforated panel (MPP) and a coplanar coiled cavity. In order to gain a deep insight into the sound absorption mechanism and perform a rapid design, a theoretical model for analyzing the sound absorption characteristics of the unit cell is established, in which the Fok function is used to account for the coupling effect between holes. Afterward, the absorption mechanism as well as the effects of parameter variations on absorption characteristics are investigated. To increase the space utilization, each unit cell is arranged in space according to the Hilbert fractal curve. Consequently, a metasurface absorber with 6 detuned unit cells is constructed. The multiple resonant cavities with dissimilar lengths can provide peak absorptions at multiple frequencies, thereby broadening the attenuation frequency range. Finally, the absorption performance of the designed absorber is obtained by theoretical calculations, finite element (FEM) simulations and experimental measurements, respectively. The experimental results show that a continuous absorption spectrum is achieved in the range of 855–1359 Hz with absorption coefficient above 0.8 under a deeply sub-wavelength thickness (22.2 mm). This study provides an effective way for the design of a space-limited broadband absorber. With the advantages of ultrathin thickness, broadband, and compactness, the developed fractal-based metasurface absorber has great potential in the field of noise reduction.

Large Optical Modulation of Dielectric Huygens’ Metasurface Absorber

AbstractHigh‐index nanostructures are found to exhibit large nonlinearity of spectral responses under photoexcitation, which is applicable to all‐optical modulation. The nonlinearity stems from Mie‐type resonance shift due to photoinduced variation of refractive index. Here, amorphous silicon‐based Huygens’ metasurface absorber and ultrafast excitation are used to demonstrate two orders of magnitude enhancement of modulation depth, both theoretically and experimentally, when electric dipole lattice resonance matches the magnetic dipole resonance. Within a few square micrometer spatial confinement and picosecond scale delay, over 100% photothermal modulation depth is experimentally achieved at matched resonance condition. This concept is not limited to photothermal modulation in the amorphous silicon but is applicable to dielectric materials with various mechanisms to achieve modulation of refractive index. This work opens the avenue toward all‐optical information processing via optimized modulation by careful spatiotemporal and spectral control over a metasurface.

Broadband Metasurface Absorber Based on an Optimal Combination of Copper Tiles and Chip Resistors

In this study, a broadband metasurface absorber composed of an optimal combination of copper tiles connected with four chip resistors is designed and experimentally verified. After fixing the locations of the chip resistors and setting their resistances to 100 Ω, the genetic algorithm (GA) is utilized to design the optimal copper tile pattern for broadband absorption. The optimal combination of the copper tiles is identified by determining the states of the square tile pairs between copper or air, depending on the one or zero states of the bit sequence created by GA, respectively. The full-wave simulation results of the optimized metasurface absorber confirmed a -10 dB reflectance bandwidth within the frequency range of 6.57 to 12.73 GHz for the normal incidence condition, with the fractional bandwidth being 63.83%. The accuracy of the metasurface absorber was verified through an experimental result that matched well with the full-wave simulated one.

Research on multifunctional integrated multispectral stealthy metasurface absorbers for infrared and radar

Owing to the widespread application of multispectral combination detection technology in the military, the research of multispectral combination stealth technology has become imminent. To address that problem, an integrated ultra-broadband radar stealth and high-efficiency infrared stealth multispectral flexible stealth metasurface absorber (MFSA) is proposed, fabricated, and measured. The MFSA consists of an infrared shielding layer (IRSL) and a radar-absorbing layer (RAL). The simulation results of MFSA show that the absorptivity is higher than 90% in the radar stealth band 3.6 ∼ 29 GHz, the infrared emissivity is lower than 0.322 in the infrared stealth band 3–14 μm, and the MFSA achieves radar scattering cross-section (RCS) reduction in 3.6 ∼ 29 GHz effectively. In addition, the multiphysics simulation results at the peak frequency point show that the slight deformation of the structure caused by the heat loss effect of the MFSA on the electromagnetic (EM) wave has almost no effect on its absorptivity. After measuring, radar stealth properties and infrared stealth properties are verified. Due to flexibility, thin thickness (only 5.1 mm), excellent stability, wide incidence angle (0°∼60°), polarization-insensitive, ultra-broadband radar stealth, and high-efficiency infrared stealth indicate the designed MFSA has promising applications on military platforms and sites with multispectral combination stealth technology.

Polarization-Independent Dual-Band Metasurface Absorber for Wireless Power Transmission

A new design of a polarization-independent dual-band metasurface absorber based on an improved Minkowski fractal structure is proposed in this paper. It is designed for wireless power transmission. The proposed metasurface absorber is composed of many sub-wavelength elements. By adjusting the structure of the elements, it has rotational symmetry and multi-frequency resonance characteristics. The proposed metasurface absorber array can keep more than 87% harvested efficiency at 2.45GHz and 93% harvested efficiency at 5.8GHz for arbitrary polarization of incident electromagnetic waves. The working frequency of the proposed absorber can cover the main working frequency bands of the current microwave wireless energy transmission system. The proposed metasurface absorber, which has the great advantage of subwavelength, polarization-independent, and high harvested efficiency, is suitable for wireless power transmission, especially in ambient power harvesting and UAV remote power supply.

Fractal metasurface for THz applications with polarization and incidence angle insensitivity

A Fractal metasurface-based absorber is designed to obtain a wideband absorption in THz frequency band. The developed absorber has a broadband absorption over more than 600 GHz bandwidth, from 0.6 THz to 1.2 THz, with a minimum absorption percentage of 70% and total absorption from 1 to 1.1 THz. The proposed fractal structure is investigated using a Modified Split Ring Resonator (MSRR). The absorption coefficient of the proposed fractal metasurface is compared with the modified initially SRR metasurface. The results indicate that by using fractal design, the metasurface operating band is summed up, resulting in a wide bandwidth. The absorption coefficient of the proposed fractal metasurface is studied using two different simulation software programs to ensure the obtained results, with a high agreement between the results of them. The designed metasurface has a similar absorption response for the different polarization of the incident wave. The impedance of the designed metasurface indicates that, for the operating band, the fractal metasurface has a surface impedance of around 377 Ω, indicating a good matching between the designed metasurface and air. Different incidence and polarization angles are studied with results showing its insensitivity to both of them. The designed metasurface is compared with previously published works, showing wider bandwidth, higher absorption, and incidence and polarization insensitivity. The proposed metasurface absorber can be used for biomedical applications.

Bicontrollable all dielectric metasurface absorber for chemical and biosensing applications

A bi-controllable all dielectric metasurface structure is designed at terahertz, which consists of an InAs micro-cylindrical array on the gold ground plane. The maximum absorptance frequency is tunable with the help of an external magnetic field and also by geometrical rotation of the metasurface as InAs is a gyrotropic material whose property depends on the magnitude and direction of an external magnetic field. Simulation results indicate that the proposed metasurface can achieve an absorptance of more than 99 % at 9.756 THz as well as it can provide a high tunability rate of around 2.55 THz T−1 in lower band and 3.33 THz T−1 in the upper band frequency spectrum with the application of the magnetic field. Additionally, the metasurface also acts as polarization insensitive when no external magnetic field is applied. Furthermore, the maximum absorptance frequency shows sensitivity towards the change in the refractive index value of the surrounding medium, thus the metasurface can be used for chemical and biosensing applications. It shows high chemical sensitivity of around 2727 GHz/RIU for the detection of different chemicals and bio sensitivity of 787 GHz/RIU for the detection of various viruses.

An optically transparent flexible metasurface absorber with broadband radar absorption and low infrared emissivity

The rapid development of surveillance technology has driven the research of multispectral stealth. Demand for infrared and microwave radar compatible stealth is becoming increasingly urgent in military applications. Herein, a versatile metamaterial absorber is designed and fabricated to simultaneously achieve ultra-broadband radar scattering reduction, low infrared emission, and high optical transparency. The designed structure consists of an infrared stealth layer, radar absorption layers, and backing ground. The infrared stealth layer employs specifically indium tin oxide (ITO) square patches, while the radar absorption layers can be obtained by stacking different size ITO patterned films of the same structure with high surface resistances, realizing broadband microwave stealth performance in the 1.98–18.6 GHz frequency range with an incident angle of 45°. The broad radar stealth and low infrared emissivity of 0.283 are consistent with the simulations and calculations. Furthermore, the designed structure exhibits characteristics such as polarization insensitivity, wide incident angles, optical transparency, and flexibility, allowing for a wide range of applications in various environments.

Highly conformable terahertz metasurface absorbers via two-photon polymerization on polymeric ultra-thin films

Highly conformable terahertz metasurface absorbers via two-photon polymerization on polymeric ultra-thin films

Highly conformable terahertz metasurface absorbers via two-photon polymerization on polymeric ultra-thin films

Abstract The continuously increasing interest in flexible and integrated photonics requires new strategies for device manufacturing on arbitrary complex surfaces and with smallest possible size, respectively. Terahertz (THz) technology can particularly benefit from this achievement to make compact systems for emission, detection and on-demand manipulation of THz radiation. Here, we present a novel fabrication method to realize conformable terahertz metasurfaces. The flexible and versatile character of polymeric nanomembranes is combined with direct laser writing via two-photon polymerization to develop free-standing ultra-thin quasi-perfect plasmonic absorbers with an unprecedentedly high level of conformability. Moreover, revealing new flexible dielectric materials presenting low absorption and permittivity in the THz range, this work paves the way for the realization of ultra-thin, conformable hybrid or all-dielectric devices to enhance and enlarge the application of THz technologies, and flexible photonics in general.

RCS Reduction on Patterned Graphene-Based Transparent Flexible Metasurface Absorber

A metasurface absorber consisting of patterned graphene sandwich structure, transparent flexible polyvinyl chloride (PVC) dielectric layer, and indium tin oxide (ITO) bottom plate is proposed in microwave band. The graphene layer is patterned as periodic strip bands with the same rectangular holes. The radar cross section (RCS) characteristics are studied by simulation and experiment. The results indicate that the proposed transparent flexible metasurface shows more than 10 dB RCS reduction in dual band, wideband, and single band when the sheet resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{s}$ </tex-math></inline-formula> ) of graphene is 70, 240, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$350~\Omega $ </tex-math></inline-formula> /sq, respectively. For <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{s} = 240\,\,\Omega $ </tex-math></inline-formula> /sq, RCS reduction in the conformal case is better than 10 dB in 8.52–16.98 GHz, and corresponding relative bandwidth is 66.35%. The regulation characteristics are attributed to the strong interference at the resonant frequency or in the broadband. It effectively captures the incident electromagnetic (EM) waves in the metasurface, and the incident wave energies are dissipated with high ohmic loss. Moreover, the RCS suppression ability of the conformal metasurface becomes better with the increase of curvature radius when the metasurface size is fixed. The proposed metasurface initiates a new way for research and development of multifunctional metasurface devices, which has important application in EM stealth in microwave band.

High-quality factor mid-infrared absorber based on all-dielectric metasurfaces.

The absorption spectrum of metasurface absorbers can be manipulated by changing structures. However, narrowband performance absorbers with high quality factors (Q-factor) are hard to achieve, mainly for the ohmic loss of metal resonators. Here, we propose an all-dielectric metasurface absorber with narrow absorption linewidth in the mid-infrared range. Magnetic quadrupole resonance is excited in the stacked Ge-Si3N4 nanoarrays with an absorption of 89.6% and a Q-factor of 6120 at 6.612 µm. The separate lossless Ge resonator and lossy Si3N4 layer realize high electromagnetic field gain and absorption, respectively. And the proposed method successfully reduced the intrinsic loss of the absorber, which reduced the absorption beyond the resonant wavelength and improved the absorption efficiency of Si3N4 in the low loss range. Furthermore, the absorption intensity and wavelength can be modulated by adjusting the geometric parameters of the structure. We believe this research has good application prospects in mid-infrared lasers, thermal emitters, gas feature sensing, and spectral detection.

Optimal design of broadband acoustic metasurface absorbers

The absorption of low-frequency noise has always been limited by structural thickness, but the novel physical properties of sound-absorbing metamaterials provide a solution to this problem. Based on genetic algorithm, an acoustic metasurface absorber (AMA) composed of micro-perforated plates (MPPs) and impedance matching coiled-up cavities (IMCCs) is proposed. Different from previously reported metamaterials, this structure can easily provide flexible and accurate broadband sound absorption in different target frequency bands. The theoretical model behind the algorithm is established, and two optimal structures (AMA I/AMA II) are obtained for low and mid-high frequency bands. Broadband sound absorption is realized with a thickness of only 71 mm (about 1/13 of the relevant wavelength at 369 Hz), and an average sound absorption coefficient of 0.931 is achieved in the low-frequency band of 350–1000 Hz. Furthermore, by changing the frequency band of the quasi-perfect absorber to 500–2000 Hz, the average sound absorption coefficient exceeds 0.945 with a thickness of only 55 mm (about 1/11 of the relevant wavelength at 563 Hz). The reflection coefficient in the complex plane and theoretical impedance analysis are utilized to reveal the underlying mechanism of the absorption and the acoustic characteristics of the two structures, which show excellent broadband absorption performance in the low and mid-high frequency bands. This work provides a method of arbitrarily modulating surface acoustic impedance in broadband and a reference for broadband noise control.

Colloidal self-assembly based ultrathin metasurface for perfect absorption across the entire visible spectrum

Abstract Perfect absorption over the entire visible spectrum can create a dark background for acquiring images with high contrast and improved resolution, which is crucial for various applications such as medical imaging, biological detection, and industrial non-destructive testing. The broadband absorption is desired to be achieved in an ultrathin structure for low noise as well as high integration. Here, we experimentally demonstrate a metasurface broadband perfect absorber with an ultrathin thickness of 148 nm and a large area of ∼10 cm2. Such a metasurface, with more than 97% absorption in the wavelength range from 400 to 800 nm, is composed of chromium nanodisk hexagonal array deposited on a chromium substrate with a silica spacer. A self-assembly based colloidal lithography nanofabrication method is developed for the scalable fabrication of the proposed nanostructure. We attribute the broadband absorption to the spectrally overlapped Fabry–Perot resonance, surface plasmon polariton, and localized surface plasmon resonances. Our results offer a novel approach to wafer-scale and low-cost manufacturing of absorption-based devices for applications such as high-contrast imaging and optical modulation.