Multi-band Absorber Research Articles

Multi-band perfect absorber based on an elliptical cavity coupled with an elliptical metal nanorod.

We proposed a triple-band narrowband device based on a metal-insulator-metal (MIM) structure in visible and near-infrared regions. The finite difference time domain (FDTD) simulated results illustrated that the absorber possessed three perfect absorption peaks under TM polarization, and the absorption efficiencies were about 99.76%, 99.99%, and 99.92% at 785 nm, 975 nm, and 1132 nm, respectively. Simulation results matched well with the results of coupled-mode theory (CMT). Analyses of the distributions of the electric field indicated the "perfect" absorption was due to localized surface plasmon polaritons resonance (LSPPR) and Fabry-Perot resonance. We developed a multi-band absorber with more ellipsoid pillars. The four band-absorbing device presented perfect absorption at 767 nm, 1046 nm, 1122 nm, and 1303 nm, and the absorption rates were 99.45%, 99.41%, 99.99%, and 99.94%, respectively. By changing the refractive index of the surrounding medium, the resonant wavelengths could be tuned linearly. The maximum sensitivity and Figure of Merit were 230 nm RIU-1 and 10.84 RIU-1, respectively. The elliptical structural design provides more tuning degrees of freedom. The absorber possessed several satisfactory performances: excellent absorption behavior, multiple bands, tunability, incident insensitivity, and simple structure. Therefore, the designed absorbing device has enormous potential in optoelectronic detection, optical switching, and imaging.

Actively tunable and switchable terahertz metamaterials with multi-band perfect absorption and polarization conversion.

In this paper, we theoretically present and numerically demonstrate an actively tunable and switchable multi-functional metamaterial based on vanadium dioxide (VO2) and graphene in the terahertz region. When VO2 is in the metallic phase, the proposed metamaterial serves as a multi-band perfect absorber, which exhibits the characteristics of insensitive polarization and robust tolerance for variations of the incidence angle. When VO2 is in the insulator phase, the proposed metamaterial acts as a polarization converter, which can simultaneously achieve perfect linear-to-linear and linear-to-circular polarization conversions. The simulation results show the cross-polarization conversion rate can reach ∼100% at the frequency region from 6.09 to 6.43 THz as well as 8.15 THz. Moreover, the ellipticity of linear-to-circular polarization conversion reaches ±1 at frequencies of 5.75 and 8.34 THz, respectively, which means the linear polarization waves can be completely converted into circular polarization waves. The proposed metamaterial provides new insight for the design of optoelectronic devices with multi-functionality in the terahertz region.

An Ultrathin Multiband Chiral Metasurface for Transmission and Asymmetric Absorption of Electromagnetic Waves

This article presents an ultrathin chiral metasurface which can exhibit multiband asymmetric absorption as well as symmetric transmission in a specific frequency band outside the absorption regions. Unlike most electromagnetic metasurface absorbers, the proposed structure does not have a continuous conducting sheet at the bottom which also allows it to act as a bandpass spatial filter. The metasurface has a substrate thickness of only λhigh/62.5 and λlow/34 at the highest and lowest operational free‐space wavelengths, respectively. The transmission band is centered at 5.5 GHz, and the asymmetric absorption bands are centered at 3, 3.33, and 4.5 GHz, respectively. The operational bands can be tuned as per user requirements. The metasurface has an angular stability of 45° for both TE and TM incidence. It can be used for radar cross‐section (RCS) reduction, electromagnetic shielding, and as a spatial bandpass filter.

Phytic acid-assisted hybrid engineering of MOF-derived composites for tunable electromagnetic wave absorption

Hybrid engineering is gradually deemed as a powerful approach to solving the bottleneck problem of metal-organic framework (MOF) derived absorbers for practical application. Herein, a new type of semiconductor/carbon-based hybrid material was successfully prepared by phytic acid (PA) modification and carbonization of MOF/bacterial cellulose (BC) precursors, which remedied the drawbacks of structural instability, lethal byproducts and complicated steps reported previously. Specifically, the obtained Fe(PO3)2@C/phosphorus-doped carbon foam (Fe(PO3)2@C/PCF) had a 3D hybrid micro-nanostructure that integrated spatial microcurrent network, multi-level pores, heterogeneous interfaces and lattice defects, showing its unique advantages of low filler content (15 wt.%), moderate surface reflectivity, multi-band microwave absorption and radar stealth. The experimental analysis and CST simulation further revealed that PA dosage can precisely adjust the hybrid phase content, pore texture and electromagnetic parameters of the final product to achieve synergistic enhancement of multiple dielectric response, impedance matching and attenuation capacity. As a result, an effective bandwidth (EAB) of 6 GHz and a minimum reflection loss (RLmin) of −57.0 dB were obtained in the Ku- and C-bands, respectively. These encouraging results may advance the development of novel MOF-derived absorbents based on the hybridization principle.

Tri-band and high FOM THz metamaterial absorber for food/agricultural safety sensing applications

Multi-band based terahertz metamaterial absorber is of great importance in current research and applications because of its ability to achieve multi-band absorption of electromagnetic waves and multi-point matching of detection material information. In this paper, a three-band terahertz metamaterial absorber based on metal split-opening ring is proposed, which produces harp peaks with high Q values due to the resonant response of the metal resonator matched with the surface lattice resonance. By optimizing the structure parameters, the Q-factor is as high as 528 and the FOM value is 153.2, which is better than the FOM of the currently reported absorbers. The sensing performance of absorber as refractive index sensor and thickness sensor is studied, and the results show that it can be used for refractive index sensing and thickness sensing detection of analytes. Finally, five pesticides were simulated and tested. The maximum frequency shift of the absorber was 16ghz when the refractive index changed by 0.02 steps, which proved its ability to recognize different pesticides. The detection of proline solutions of 2–10 g/100 ml proves that the absorber could detect analytes with a refractive index difference of 0.003. These results show that the absorber has high sensitivity, high Q factor, ultra-high FOM, and polarization-insensitive characteristics, which can be used for detecting food/agricultural safety testing and other fields.

Ultra-efficient machine learning design of nonreciprocal thermal absorber for arbitrary directional and spectral radiation

Development of nanophotonics has made it possible to control the wavelength and direction of thermal radiation emission, but it is still limited by Kirchhoff's law. Magneto-optical materials or Weyl semimetals have been used in recent studies to break the time-reversal symmetry, resulting in a violation of Kirchhoff's law. Currently, most of the work relies on the traditional optical design basis and can only realize the nonreciprocal thermal radiation at a specific angle or wavelength. In this work, on the basis of material informatics, a design framework of a multilayer nonreciprocal thermal absorber with high absorptivity and low emissivity at any arbitrary wavelength and angle is proposed. Through a comprehensive investigation of the underlying mechanism, it has been discovered that the nonreciprocal thermal radiation effect is primarily attributed to excitation of the cavity mode at the interface between the metal and the multilayer structure. Moreover, the impact of factors, such as layer count, incidence angle, extinction coefficient, and applied magnetic field on nonreciprocal thermal radiation, is thoroughly explored, offering valuable insights to instruct the design process. Additionally, by expanding the optimization objective, it becomes feasible to design fixed dual-band or even multi-band nonreciprocal thermal absorbers. Consequently, this study offers essential guidelines for advancing the control of nonreciprocal thermal radiation.

Direct ink writing printed flexible double-layer staggered woodpile structure for multi-band compatible absorption of gigahertz and terahertz waves

Soft multi-band compatible electromagnetic wave absorbers have wide applications in 6G communication, radar stealth, detection instruments, and radio astronomy. In this context, a kind of double-layer staggered woodpile absorber is designed and manufactured by the direct ink writing (DIW) technology with a composite ink system composed of reduced graphene oxide (RGO)/spherical carbonyl iron (SCI) and polydimethylsiloxane (PDMS). The as-prepared absorber shows compatible absorption in both microwave and terahertz frequency bands. The effective absorption bandwidth (EAB) and minimum reflection loss (RLmin) reach 8.24 GHz and −52.2 dB at a thickness of 3.2 mm, respectively. In the terahertz band (0.5–3.5 THz), it shows effective absorption (absorption rate greater than 99 %) and achieves RL < -20 dB throughout the entire tested band. In addition, the double-layer staggered woodpile absorber has good flexibility and can be conveniently used for curved substrates. This work enables the potential for rapid manufacturing of flexible and compatible absorbing devices through DIW 3D printing for applications in 5/6G communication, military stealth and flexible wearable intelligent devices.

Polarization-independent and all-optically modulated multiband metamaterial coherent perfect absorber

A polarization-independent, all-optically modulated multiband metamaterial coherent perfect absorber (CPA) is proposed. Numerical simulation results reveal that the absorber can be controlled over a wide range of amplitudes by changing the phase difference of two incident light beams. The all-optical modulation can be extended to telecom wavelengths, allowing for simultaneous modulation of absorption at 1310 nm and 1550 nm. In addition, switchable logical gate states can be achieved at the dip between absorption peaks. The proposed absorber utilizes bound states in the continuum (BIC) mode to realize another absorption band, expanding the application range and enhancing its flexibility. The results suggest that the proposed absorber has a promising modulated optical response, with potential applications in developing optical switches, smart multi-band absorbers, telecom devices and so on.

Tailored heterogeneous interface based on porous hollow In-Co-C nanorods to construct adjustable multi-band microwave absorber

By rationally controlling the growth of zeolite-imidazolium salt skeleton (ZIF-67) nanoparticles on the hollow indium oxide (In2O3) and the subsequent pyrolysis process, In-Co-C hollow rod-like composites with multiple components were successfully prepared in this work. The synergistic impact of diverse components including In2O3, C, Co3InC0.75, and In not only optimizes impedance matching and improves conductive loss, but also creates significant interfacial polarization. Furthermore, the addition of Co source and pyrolysis temperature was found to have a significant affected on impedance matching and microwave absorption. The optimized In-Co-C sample displayed ultra-broad absorption bandwidth (EAB) of up to 7.12 GHz (10.88–18.0 GHz) at 2.26 mm thickness, which could cover the whole Ku band and part of X-band, outperforming the previously reported MOFs-derived composites. Furthermore, by adjusting the thickness to 2.91 mm and 4.01 mm, the EAB could cover the entire X band and most of the C band. The attenuation mechanisms were systematically investigated through the delta function method and ANSYS high-frequency structure simulator. These findings suggest that the MOFs-derived In-Co-C hollow nanorods could serve as high-performance microwave absorbers with ultra-broad absorption.

Sustainable synthesis of tunable 2D porous carbon nanosheets toward remarkable electromagnetic wave absorption performance

The realization of microwave absorbers that demonstrate wide frequency bands and strong absorption properties, especially for realizing multi-band absorption characteristics, presents an ongoing challenge in the field. Herein, we reported on a simple and sustainable method for the fabrication of gram-scale B/N co-doped porous carbon nanosheets (PCNs). These PCNs exhibited excellent microwave absorption performance, and their intrinsic electromagnetic wave absorption performance can be tuned by adjusting the size of nanosheets and heteroatoms. The PCNs achieved dual-band absorption in the C- and Ku- bands. Furthermore, the GC20 achieved an RLmin value of −29.5 dB at a thickness of 1.7 mm, with an effective adsorption band (EAB) of 7.2 GHz. The porous nanosheets, which provide the ideal synergistic effect of polarization loss and conductivity loss, were the factors that contributed to these desirable features. These absorbers with multi-band absorption are not only expected to address the prevalent issue of narrow EAB but also fulfill the requirements for applications in the different frequency bands, inspiring innovative designs of multi-band electromagnetic absorbers.

Multiband absorber design and optimization with Lorenz chaotic attractor

Metamaterials are artificial materials that don’t come from nature and have unusual electromagnetic properties. It has applications in many fields such as medical, image processing, signal absorption, invisibility cloak, sensor, and antenna. In this study, signal absorption of metamaterials was discussed and a simulation study was carried out. The most important difference of the proposed structure is that the pattern obtained from the Lorenz chaotic attractor was used in the resonator’s design, unlike the geometric shaped works. The chaos pattern consists of units that have both random shapes and sizes. Chaos can be defined as the order of disorder, which is extremely sensitive to initial conditions and has a broad power spectrum like noise. In addition, this study aims to contribute to the science of chaos. The resonator structure used in the proposed structure showed similar results with the studies in the literature. Rogers RT5870 material was used as substrate in the design. The proposed design is an eight-band structure with excellent signal absorption, partially insensitive to polarization and angle of incidence. The results show that the proposed design absorbs signals in the infrared region of the spectrum at 14.24, 15.44, 19.72, 21.04, 33.92, 35.16, 36.44 and 38.79 THz. Near-perfect signal absorption at the indicated frequencies is 97.98%, 98.73%, 99.85%, 92.03%, 95.62%, 92.56%, 96.21% and 97.09%, respectively. The proposed design includes heat sensor and thermal imaging etc can be used in application areas.

Dual strategy of A-site ion substitution and self-assembled MoS2 wrapping to boost permittivity for reinforced microwave absorption performance

With the rapid development of electronic technology, how to effectively eliminate electromagnetic pollution has become a serious problem. Perovskite oxides have shown great potential in the field of electromagnetic wave absorption due to their unique structure and excellent physicochemical properties. Herein, by rationally manipulating the A-site ion substitution strategy, the theoretically directed doping of Sr ions into La ionic sites was utilized and the layered MoS2 was loaded by the hydrothermal process to modify its surface. Consequently, the introduced polarization phenomenon improved the dielectric performance of the perovskite oxides, achieving a collaborative dielectric/magnetic loss mechanism. Accordingly, the prepared La0.7Sr0.3FeO3(LSFO)/MoS2 as coating filler in the epoxy resin coating system can obtain the minimum reflection loss of –67.09 dB at 1.9 mm and the maximum effective absorption bandwidth of 7.28 GHz at 2.3 mm. More importantly, it also exhibits excellent absorption performance for multi-band electromagnetic waves, covering a wide range of specified frequency bands. It provides inspiration for exploring novel perovskite oxide-based electromagnetic wave absorbing coatings and broadens the choice of ideal candidate materials for designing highly efficient, multi-band absorbers to cope with sophisticated electromagnetic environments.

Multi-band tunable strongly nonreciprocal thermal radiation in topological edge state coupled mode

Violating Kirchhoff's law of thermal radiation can break the reciprocity constraints on spectral emissivity and absorptivity, which provides new capabilities for managing photonic heat flow and fundamentally improves energy harvesting systems' efficiency. Currently, most micro-nano structures that violate Kirchhoff's law are mainly confined to single- or dual-band models, significantly limiting applications such as photonic energy conversion and thermal management. Here, a scheme is proposed for a topological one-dimensional photonic crystal heterostructure consisting of multiple photonic crystals and Weyl semimetals, and its absorptivity and emissivity are theoretically calculated by the transfer matrix method. The results show that the multi-band nonreciprocity that completely violates Kirchhoff's law can persist over broad angular and frequency ranges without requiring any external magnetic field via tuning the relevant parameters. Furthermore, the electric field distribution reveals that the formation of such multiple bands originates from the coupling between topological edge states, and the significant enhancement of the local field near the interface of the heterostructure provides favorable conditions for the realization of strong nonreciprocity. Our work may contribute to the design and development of multiband nonreciprocal devices for energy conversion and thermal management and may also provide design ideas for multiband reciprocal absorbers, filters, optical switching, etc.

Switchable polarization-independent THz reflector/absorber

This paper introduces a novel metamaterial switchable terahertz (THz) reflector/absorber, which incorporates a composite graphene and vanadium dioxide (VO2) structure. The structure comprises gold and graphene layers separated by VO2 dielectric layers. By leveraging the electrical adjustability of graphene and the temperature adjustability of VO2, the operating frequency and absorption amplitude of the absorption spectrum can be controlled by adjusting the Fermi levels of graphene and the conductivity of VO2. The designed dual-function switch can be utilized as a reflector when VO2 exhibits a metallic state and as a perfect multiband absorber when VO2 exhibits dielectric properties. Numerical simulation results demonstrate that the proposed device exhibits exceptional performance for TE and TM waves, achieving a more than 99% absorption rate at 2.2, 6.5, and 10.5 THz. This structure holds considerable potential for a wide range of applications, including temperature monitoring, stealth technology, wireless communication, and biomedicine.

Dynamically switchable tri-functional THz-integrated metamaterial absorber based on VO2-graphene

Based on the easily accessible phase transition temperature of VO2 and the electrically tunable properties of graphene, a tri-functional THz-integrated metamaterial absorber with dynamically switchable low-frequency broadband, high-frequency broadband, and multi-band absorption are proposed. This absorber is composed of a patterned VO2 layer, a SiO2 dielectric layer with a layer embedded patterned graphene, a thick unpatterned VO2 layer, both sides of which are coated with ML graphene, a SiO2 dielectric layer, and an Au substrate. When the patterned and unpatterned VO2 layers are both metallic, the proposed absorber behaves as a low-frequency broadband absorber from 2.7 to 6.2 THz with average absorptivity exceeding 90 %, with the value of absorption sensitive to the structure of the patterned VO2. When the patterned VO2 is insulating and the unpatterned VO2 is metallic, the absorber exhibits high-frequency broadband absorptivity, above 90 % from 5.8 to 7.6 THz. When the patterned and unpatterned VO2 are both insulating, the device acts as a multi-band absorber with 12 tunable resonance peaks. In addition, the proposed absorber is robust against the incidence angle and polarization. Furthermore, the electric field strength distributions and multiple interference theory are presented to explain the physical mechanisms of tri-functional near-100 % absorptivity. Our work may provide a promising path for the development of multifunctional THz-integrated metamaterial absorbers.

Near-perfect multi-band graphene absorber with a compound grating-based resonant structure

We proposed a multi-band graphene absorber composed of a resonant compound grating (RCG), monolayer graphene, a thin silver layer, and a distributed Bragg reflector (DBR). A multi-band near-perfect absorption spectrum can be determined by three different physical mechanisms, including guided mode resonance (GMR), quasi-bound states in the continuum (quasi-BIC), and Tamm plasmon polaritons states (TPPs). Interestingly, the quasi-BIC mode can be achieved by changing grating layer spacing. In addition, the BIC mode can also be realized by varying grating periods. The variation of the absorption peak can be observed by changing the spacing layer thickness of the DBR structure, and the waveguide layer and grating layer thickness of the compound grating. Simulation results show that the absorption peaks of the three modes excited by our proposed absorbers are high as 80% at an incident light angle of 25°.

Terahertz multi-band absorber and dual-bandwidth polarization converter based on VO2 and graphene

A terahertz metamaterial with multiple functions is proposed based on patterned graphene, concentric gold rings and gold columns, and vanadium dioxide (VO2). By adjusting the temperature of VO2, it can be in metal state or insulator state, and correspondingly the metamaterial can work as a dual-bandwidth polarization converter or a multi-bandwidth absorber. When the metamaterial acts as a polarization converter, by adjusting the Fermi level of graphene, the converter can realize simultaneously x-polarized wave to y-polarized wave polarization conversion (CPC) and line-to-circular (LTC) polarization conversion (PC). In addition, by adjusting the Fermi level of graphene, the operating frequency and operating efficiency of the absorber and polarization converter can be dynamically tuned. By studying the electric field distribution and the surface current of the metamaterial, the physical mechanism of efficient absorption and dual-bandwidth polarization is explored. Moreover, the proposed multifunctional metamaterial is highly robust to incidence angle. It has potential applications in communication, spectroscopy, sensors, and physical detection.

Tunable multiband THz perfect absorber due to the strong coupling of distributed Bragg reflector cavity mode and Tamm plasma polaritons modes based on MoS2 and graphene

In this work, we propose a multiband terahertz perfect absorber by combining a distributed Bragg reflector (DBR) cavity with molybdenum disulfide (MoS2) and graphene. The absorption peak comes from the strong coupling of the DBR cavity mode and the two Tamm plasma polaritons (TPPs) modes. Our numerical simulations demonstrate that the degree of coupling in this hybrid system is tunable by adjusting the structural parameters, thereby allowing for control over the position and intensity of the absorption peaks. Furthermore, utilizing the tunable Fermi energy level of graphene and controlled variation of the molybdenum disulfide carrier concentration enables active switching between absorption-free, dual-band, and triple-band absorption. Additionally, the high absorption efficiency in a wide angular range and sensitivity make this device highly versatile. The coupled harmonic oscillator model is used to explain the strong coupling phenomena of the system. This simple layered structure provides a hybrid coupling system with a controlled number of bands suitable for potential active terahertz optoelectronic devices.

Dual-band absorption due to simultaneous excitation of TE and TM modes in ITO-ES metamaterial for IR stealth technology

We proposed a plasmonic based Indium Tin Oxide Embedded Silver (ITO-ES) infrared dual-band perfect absorber for the utilization in IR stealth technology. The proposed structure comprised of nano size Ag ring and desk embedded in ITO. The optical losses in optical regime of nanoscale plasmonic Ag were incorporated in terms of collision frequency using the modified Drude model. The optical response of the proposed structure was predicted by exciting it with a planwave using full wave vector frequency domain solver by employing Finite Element Method (FEM) in Computer Simulation Technology (CST) Microwave Studio. The simultaneous excitation of higher order transverse electromagnetic (TE and TM) modes in the proposed structure lead to high values of both absorption and Front to Back ratio (FTBR). The controlled and selective wavelength multiband absorption is crucial for IR stealth technology. The proposed metamaterial structures exhibit dual-bands IR absorption at wavelengths λ = 1.40 μm and 6.4 μm. First absorption peak suppresses the back scattering of the laser used by laser guided devices for detection, while the second peak suppresses the IR signature of the structure due thermal radiation which can be used by tracking devices to detect warm/hot objects.

Miniaturized Near All-Angle Stable 2.5-D FSS for Dual-Band Shielding in Transmission Mode and Multiband Absorption in Reflection Mode

Miniaturized Near All-Angle Stable 2.5-D FSS for Dual-Band Shielding in Transmission Mode and Multiband Absorption in Reflection Mode