Multi-band Absorber Research Articles

Broadband microwave absorption enabled by a novel carbon nanotube gratings/cement composite metastructure

Microwave absorption performance is increasingly needed in cement composites, while multiband or broadband absorption properties are challenging to realize. This study provides a simple and reproducible metastructure for cement composites based on carbon nanotube periodic gratings, exhibiting averaged −15.5 dB reflectivity and 14.2 GHz effective bandwidth in 1–18 GHz. Theoretical requirements for the perfect absorption are analyzed, and the absorption performance and mechanism of the metastructure are studied. Furthermore, three factors affecting absorption performance are investigated, including the gratings periodicity, filling fraction, and the thickness of the cement dielectric. The metastructure with 24 mm periodicity gratings demonstrates multiband absorption performance, showing five peaks lower than −20 dB. The metastructure with 14/24 filling fraction gratings exhibits the reflectivity peak of −38.7 dB and bandwidth for −15 dB of 13.0 GHz, demonstrating broadband nearly perfect absorption performance. In addition, the metastructure with a thinner cement dielectric layer performs better absorbing properties. This study not only proposes an effective method for cement composites to achieve broadband absorption, but also provides cement composites and fireproof materials as novel candidates for the dielectric of metamaterial absorbers, which promotes the application of microwave absorbers to practical construction engineering.

Quad-Band Polarization-Insensitive Square Split-Ring Resonator (SSRR) with an Inner Jerusalem Cross Metamaterial Absorber for Ku- and K-Band Sensing Applications

The development of metamaterial absorbers has become attractive for various fields of application, such as sensing, detectors, wireless communication, antenna design, emitters, spatial light modulators, etc. Multiband absorbers with polarization insensitivity have drawn significant attention in microwave absorption and sensing research. In this paper, we propose a quad-band polarization-insensitive metamaterial absorber (MMA) for Ku- and K-band applications. The proposed patch comprises two square split-ring resonators (SSRR), four microstrip lines, and an inner Jerusalem cross to generate four corresponding resonances at 12.62 GHz,14.12 GHz, 17.53 GHz, and 19.91 GHz with 97%, 99.51%, 99%, and 99.5% absorption, respectively. The complex values of permittivity, permeability, refractive index, and impedance of MMA were extracted and discussed. The absorption mechanism of the designed MMA was explored by impedance matching, equivalent circuit model, as well as magnetic field and electric field analysis. The overall patch has a rotational-symmetrical structure, which plays a crucial role in acquiring the polarization-insensitive property. The design also shows stable absorption for both transverse electric (TE) and transverse magnetic (TM) modes. Its near-unity absorption and excellent sensing performance make it a potential candidate for sensing applications.

Multi-band perfect absorber based on graphene monolayer coupled to photonic nanostructure

An active plasmonic device is designed to support multiple perfect absorption peaks using the highly confined graphene surface plasmons excited by silicon-based diffractive gratings. The physical origination corresponding to different absorption peaks is analyzed and the principle of impedance matching is used to explain perfect absorption. We show that the absorption spectrum is effectively controllable over a wide wavelength range by changing the Fermi levels, relaxation time of the graphene and geometric parameters of the device. This study could provide some possibilities facilitating the design of next-generation optical and photoelectronic structures by combining graphene and dielectric grating.

Bulky Thiolate-Protected Silver Nanocluster Ag 213 (Adm-S) 44 Cl 33 with Excellent Electrocatalytic Performance toward Oxygen Reduction

Bulky Thiolate-Protected Silver Nanocluster Ag ₂₁₃ (Adm-S) ₄₄ Cl ₃₃ with Excellent Electrocatalytic Performance toward Oxygen Reduction

Multiband polarization insensitive and tunable terahertz metamaterial perfect absorber based on the heterogeneous structure of graphene

In this paper, we present and investigate a multi-band metamaterial perfect absorber based on the heterogeneous structure of graphene with Cu and SiO2 substrates. The top layer of structure consists of one graphene disk at the center and four graphene solid triangle with semicircular cuts on them that surround the central disk. This heterogeneous structure causes us to achieve 99.5%, 99.7%, 94.71% and 97.06% perfect absorptions peaks at 4.24 THz, 5.89 THz, 9.66 THz and 10.62 THz, respectively. The absorption mechanism based on electric fields has been investigated. We can shift the wavelength of absorption peaks to our required wavelength by changing the Fermi level (µc) of graphene. Two absorption peaks of this absorber remain unchanged in different light incident angles. In addition, very important point about this structure is that it is not sensitive to polarization and this feature makes the proposed absorber very suitable for applications such as imaging, filtering, sensing and detecting applications.

Design of quad-band polarization insensitive ultra-thin compact metamaterial based absorber for terahertz applications

The paper attempts to design and characterize the compact multiband absorber in the terahertz regime. The applications in the terahertz regime demand applicability of metamaterial-based absorbers. The proposed structure incorporates simple geometry that provides high absorption of 99.53%, 96.88%, 95.27%, and 96.37% at 5.53THz, 5.87THz, 6.34THz and 6.59THz respectively. The absorption characteristics attained is by incremental development in one consolidated patch. The proposed geometrical design acquired through gradual development provides polarization insensitive behavior, making the structure practically realizable. The work focuses on achieving multi absorption peaks in the terahertz regime, which is authenticated by field and current distribution mechanism prevailing in the proposed absorber. The simple design geometry with the attainment of multiband absorption provides ample scope for applicability in the terahertz regime.

Broadband terahertz metamaterial absorber and modulator based on hybrid graphene-gold pattern

We have numerically demonstrated a tunable broadband metamaterial absorber (MA) by introducing a graphene pattern and a gold disk, of which the bandwidth is up to 3.4 THz. When the chemical potential of graphene changes from 0 eV to 0.5 eV, the proposed MA structure can also be used as a modulator with the minimum and maximum modulate depth being about 69.5% and 92.5%, respectively. By tuning the relaxation time of the graphene from 0.1 ps to 0.5 ps, several obvious absorption peaks will occur in the broad and flat absorption band , which will deteriorate the broadband absorption characteristics and make it show a trend of multi-band absorption characteristics. To further increase the absorption bandwidth, a new graphene pattern and an additional gold disk are added into the proposed MA structure to form a new type of MA structure. Simulation result shows that the newly designed MA structure can achieve a wider absorption bandwidth of more than 4.0 THz. Meanwhile, it still has the performance of excellent modulation characteristic, polarization insensitive and wide tolerance of incident angles. All these optimal characteristics of the proposed MA structures will undoubtedly benefit the use of metamaterial absorber, high speed communication, terahertz detection devices, optoelectronic devices and so on. • Both graphene and gold patterns are used in the design of proposed metamaterial absorber. • The proposed structure can achieve broadband absorption with a bandwidth of 3.4 THz. • It has the characteristics of both polarization insensitivity and wide incident angles. • By adjusting the chemical potential of the graphene, dynamic tunability of the absorption performances can be achieved.

Reconfigurable multi-band water-graphene cascade metamaterial perfect absorbers loaded with vanadium dioxide.

We demonstrate the perfect trapping of electromagnetic fields over multi-band frequencies through all-dielectric terahertz absorbers using water graphene cascade metamaterials. More specifically, the coating water layer greatly enhances the higher-order Fabry-Pérot resonant absorbing modes and can achieve more than 8 absorbing peaks with the absorptions exceeding 99% in the spectrum below 3 THz. Especially such multiple perfect absorbing bands can readily be reset when the proposed water-graphene metamaterial absorbers integrate with thermal controlled vanadium dioxide. Such a perfect absorbing capacity would also be valid for the wide angular illuminations with different polarizations, and the reconfigurable characteristics of graphene can also enable the dynamically tuning of the absorbing frequencies, offering great freedom of extensive applications in energy harvesting and wave manipulation.

Approaching multi-band and broadband high absorption based on one-dimensional layered structures containing monolayer MoS2

MoS2 has attracted considerable attention owing to its unusual and intriguing potential applications in optoelectronic devices. In this study, the absorption properties of a simple one-dimensional (1D) layered structure composed of monolayer MoS2 are analyzed by transfer matrix method. The dielectric permittivity of monolayer MoS2 is employed using the Lorentz model. The influences contributed to the period of the structure and the incident angle are numerically investigated. Our results indicate that a tunable multi-band (broadband) absorber can be achieved by using such a 1D layered structure. A multi-band (broadband) absorption phenomenon can be obtained by increasing the period of the structure. Furthermore, the absorption peaks and broadband absorptance spectra have blue-shifted as the incident angle increases.

Transferring A4 Paper to FeNi3/NiCx Coated Carbon Skeleton for Efficient Absorption of Multiband Microwave

Herein, A4 typing paper was used as a novel source to manufacture FeNi3 and NiCx coated carbon skeleton via facile routes. The product was examined for its ability to absorb electromagnetic emission which can be a health hazard. The impact of precursor concentration on the final electromagnetic wave absorption of samples was evaluated; the composite prepared under suitable concentration possesses outstanding multiband absorption ability of −34.64 dB and −26.7 dB at 2.32 GHz and 17.2 GHz, respectively, together with an ultra-wide effective absorption bandwidth of 9.58 GHz at only 3.9 mm. The strong dipole polarization and broad frequency range of preferable impedance matching, along with the coupling of other auxiliary mechanisms, are responsible for this excellent property. The as-prepared absorber has great potency for multiband absorption of electromagnetic waves.

High-performance dual-control tunable absorber with switching function and high sensitivity based on Dirac semi-metallic film and vanadium oxide

This study combines two tunable metamaterials, vanadium oxide (VO2) and Dirac semimetal thin films (DSF), to design a tunable multi-band narrowband absorber with on-off switching function. When VO2 is in metallic state, the absorber has three perfect absorption peaks with absorptivity greater than 97%. By lowering the temperature, VO2 can be transformed into an insulating state, which greatly reduces the absorption rate of the absorber and realizes the change of the state of the absorber. We have conducted more in-depth studies on the absorption mechanism and structural parameters of the absorber in the on state. By changing the Fermi level (EF) of the DSF, the resonant frequency of the absorber is tunable up to 0.444 THz while maintaining the absorption rate above 90%. The absorber can maintain a stable working state in various complex electromagnetic environments, and has a refractive index sensitivity (S) of up to 462 GHz/RIU. Due to the advantages of various control methods, good performance and diverse functions, our designed absorber has good practical development prospects and market value in many fields such as switching and detection.

Independently tunable triple-band infrared perfect absorber based on the square loops-shaped nano-silver structure

Recently, the keen demand for multi-band perfect absorbers in spectroscopy, infrared detection, and other fields has intensified. To meet the energy demand, here we propose an Ag-Dielectric-Ag multilayer triple-band perfect absorber exhibiting perfectly resonance absorption in the near-infrared region. And the numerical calculations indicate that we obtain three resonance absorption peaks (908 nm, 1085 nm, and 1216 nm) with the maximal absorption of 99.09%, 99.84%, and 99.21%, respectively. Based on this, we propose that the physical formation mechanism of three resonance peaks are the Fabry-Pérot resonance effect and Localized surface plasmon polaritons resonance. Theoretically combined with the electromagnetic coupling theory to explain how the resonance-enhanced absorption. At the same time, by the comparison with the complementary structure, it is theoretically proposed that the MIM structure has a Fabry-Pérot resonance effect in the near-infrared region and then computationally verified the Fabry-Pérot formula. Furthermore, we can adjust the amplitude and wavelength of resonance peaks by modifying the geometry unit cell. And our proposed triple-band perfect absorber has some excellent properties, such as high absorptivity, multi-band, tunable, suitability for wide incidence angles, excellent angle tolerance, polarization tolerance, etc. In addition, our TBPA has a simple structure, which simplifies the processing technology and economically saves processing costs. Furthermore, the following simulations investigate the absorption spectrum of our TBPA exhibit property changes significantly as the environment changes. So we can calculate the sensitivity of three modes (sorted by resonance wavelength from short to long) is S1 = 70.09 nm/RIU, S2 = 208.26 nm/RIU, and S3 = 50.06 nm/RIU, respectively. Therefore, we believe that this triple-band perfect absorber can capture maximize optical energy and apply it in devices and materials for light conversions, like plasmon-enhanced photovoltaics, optical absorption switching, and modulator-optical communications.

Tunable ultra-high quality factor graphene absorber based on semicylindrical silica array and distributed Bragg reflector structure

We propose a tunable narrowband absorber by utilizing a graphene monolayer placed between a dielectric semicylindrical array and a multilayer silica/silicon distributed Bragg reflector (DBR) structure. The multi-band perfect absorption can be achieved due to the excitation of multiple resonant modes in the absorber, including the guided mode resonance of the dielectric silica array and BR-based guided mode resonance in the DBR structure. The ultra-high quality factor (Q) is mainly attributed to the low external leakage loss of the resonator and the low intrinsic loss of the graphene monolayer. Moreover, the Q-factor of absorption peaks can be tuned by electrically controlling the Fermi energy of graphene. The sensitivity of a spectral wavelength shift for the refractive index change of the resonator is up to 730 nm/RIU, and the figure of merit is 1043. The proposed graphene-based metamaterial offers potential applications for photodetectors, optical modulators, and sensors in the near infrared frequency regime.

A Review on Metamaterial Absorbers: Microwave to Optical

Metamaterials (MM) are artificially designed materials that possess unique properties due to their geometrical design. They also display some peculiar properties, such as negative refractive index, Snell’s law reversal, Doppler effect reverse, and left-handed behavior. MMs are used in a myriad of applications, including invisibility cloaking, perfect lensing, perfect absorption, and sensing. In this review article, the property of electromagnetic absorption by structures known as metamaterial absorbers (MMAs) is discussed. An MMA is a composite made up of many layers of metallic patterns separated by dielectric. This novel device helps in achieving near-unity absorption by various mechanisms, which are investigated in this article. The MMAs are classified based on their absorption characteristics, such as polarization tunability, broadband operation, and multiband absorption, in different frequency regimes.

A carbon nanotube coated metawall with ultra-wideband perfect wave absorption utilizing cement dielectric

Metamaterials (MMs) have attracted intense attention owing to their unprecedented control over the propagation of electromagnetic waves. Researches on MMs usually concentrate on the metallic resonators layer, while the carbon materials-based resonators and the dielectric layer of MMs are relatively overlooked. Here, a novel and simple “metawall” perfect MM absorber based on carbon nanotube (CNT) is conducted, which enables ultra-wideband perfect absorption, showing three peaks with 99% absorption and yielding over 97% absorption in 5.6–11.0 GHz. A reproducible and cost-efficient fabrication method is proposed, using CNT paste as the resonator and the combination of lossy cement board and lossless ceramic fiberboard as the dielectric. Moreover, only one kind of one-dimensional gratings CNT pattern is required to achieve multi-band perfect absorption. Through the control experiments to investigate the roles of the components of the metawall structure, it is found that the wideband perfect absorption of the metawall can be attributed to its unique structure and the absorption performance of the cement board. The proposed metawall provides a new perspective for achieving wideband perfect absorption and may serve as a promising platform to combine carbon nanomaterials with practical load-bearing engineering components.

Ultra-Narrowband Anisotropic Perfect Absorber Based on α-MoO3 Metamaterials in the Visible Light Region.

Optically anisotropic materials show important advantages in constructing polarization-dependent optical devices. Very recently, a new type of two-dimensional van der Waals (vdW) material, known as α-phase molybdenum trioxide (α-MoO3), has sparked considerable interest owing to its highly anisotropic characteristics. In this work, we theoretically present an anisotropic metamaterial absorber composed of α-MoO3 rings and dielectric layer stacking on a metallic mirror. The designed absorber can exhibit ultra-narrowband perfect absorption for polarizations along [100] and [001] crystalline directions in the visible light region. Plus, the influences of some geometric parameters on the optical absorption spectra are discussed. Meanwhile, the proposed ultra-narrowband anisotropic perfect absorber has an excellent angular tolerance for the case of oblique incidence. Interestingly, the single-band perfect absorption in our proposed metamaterials can be arbitrarily extended to multi-band perfect absorption by adjusting the thickness of dielectric layer. The physical mechanism can be explained by the interference theory in Fabry–Pérot cavity, which is consistent with the numerical simulation. Our research results have some potential applications in designs of anisotropic optical devices with tunable spectrum and selective polarization in the visible light region.

Multi-Band Polarization-Insensitive Metamaterial Absorber for Microwave Based on Slotted Structure and Magnetic Rubber.

A design method of five-band polarization-insensitive metamaterial absorber (MMA) based on the slotted structures and the magnetic rubber is proposed for L-, S-, C-, X-, and Ku-band applications. The slotted structures of the top layer, which evolved from two square rings, are used to excite multi-resonance. The range of the electromagnetic (EM) parameters of a magnetic rubber substrate, which is used to adjust the equivalent impedance of the absorber to match the free space impedance in different bands, is estimated using the impedance matching principle. A series of magnetic rubber substrates based on the estimated EM parameters are prepared and measured, whose thickness is only 0.7 mm, meeting the thin design requirements. The absorption of the proposed absorber greater than 90% at 1.7 GHz, 3.87 GHz, 5.96 GHz, 9.4–10.4 GHz, and 14 GHz is achieved when the doping amount of the carbonyl iron powders is 200%. The absorbing performance of the absorber with measured EM parameter agrees well with the theoretical estimates, which validates the accuracy of the proposed design method.

Graphene-Integrated Plasmonic Metamaterial for Manipulation of Multi-Band Absorption, Based on Near-Field Coupled Resonators

We demonstrated a multi-band plasmonic metamaterial absorber (MA), based on the near-field coupled resonators. In addition to the individual resonances of resonators in the proposed structure, which were split-ring resonator (SRR) and cross-shape structures, another resonance was also excited owing to the coupling of resonators, revealing a triple-band absorption. Furthermore, to control the absorption behavior, on the top of the SRRs, the identical SRRs made of graphene ink were pasted. By increasing the resistance of graphene ink, the coupling strength was weakened, changing the triple-band absorption to a dual-band one. Our work might be useful as the controllable devices, based on graphene-integrated plasmonic MA, such as filters, detectors and energy harvesters.

Cost-Effective Bull's Eye Aperture-Style Multi-Band Metamaterial Absorber at Sub-THz Band: Design, Numerical Analysis, and Physical Interpretation.

Theoretical and numerical studies were conducted on plasmonic interactions at a polarization-independent semiconductor–dielectric–semiconductor (SDS) sandwiched layer design and a brief review of the basic theory model was presented. The potential of bull’s eye aperture (BEA) structures as device elements has been well recognized in multi-band structures. In addition, the sub-terahertz (THz) band (below 1 THz frequency regime) is utilized in communications and sensing applications, which are in high demand in modern technology. Therefore, we produced theoretical and numerical studies for a THz-absorbing-metasurface BEA-style design, with N-beam absorption peaks at a sub-THz band, using economical and commercially accessible materials, which have a low cost and an easy fabrication process. Furthermore, we applied the Drude model for the dielectric function of semiconductors due to its ability to describe both free-electron and bound systems simultaneously. Associated with metasurface research and applications, it is essential to facilitate metasurface designs to be of the utmost flexible properties with low cost. Through the aid of electromagnetic (EM) coupling using multiple semiconductor ring resonators (RRs), we could tune the number of absorption peaks between the 0.1 and 1.0 THz frequency regime. By increasing the number of semiconductor rings without altering all other parameters, we found a translation trend of the absorption frequencies. In addition, we validated our spectral response results using EM field distributions and surface currents. Here, we mainly discuss the source of the N-band THz absorber and the underlying physics of the multi-beam absorber designed structures. The proposed microstructure has ultra-high potentials to utilize in high-power THz sources and optical biomedical sensing and detection applications based on opto-electronics technology based on having multi-band absorption responses.

Multi-order resonators for acoustic multiband asymmetric absorption and reflection

We propose a multiband asymmetric acoustic absorption and reflection system constructed from a waveguide and multi-order resonators with different radiant modes. Theoretical and experimental results confirm that the coupling of the dark and bright modes at each resonant frequency enables multiband high-efficiency absorption requiring only two functional units. More interestingly, we demonstrate a hybrid multiband asymmetric system based on customized multi-mode pairs. One side of the hybrid multiband system almost completely absorbs the lower frequency incident sound waves while reflecting the higher frequency ones; conversely, the other side effectively reflects the lower frequency ones and absorbs the higher frequency ones. Our design showcases the flexibility of customized multiband asymmetric absorption and also provides an approach for the design of bidirectional wave-manipulation devices.