Dual-band Absorber Research Articles

Comment on Lu et al. Ultrathin Terahertz Dual-Band Perfect Metamaterial Absorber Using Asymmetric Double-Split Rings Resonator. Symmetry 2018, 10, 293

In this study, the design of a dual-band terahertz absorber, previously published by Lu et al. (Symmetry 2018, 10, 293), was re-simulated. Our findings showed significantly different absorption results from those published in the article. A detailed analysis was conducted to explain this discrepancy, which was attributed to the reflection of an unaccounted orthogonal component of the waves from the design, rather than absorption. The metasurface design has two resonances at 4.48 THz and 4.76 THz, respectively. It was reported that at these frequencies, the structure achieved absorption of 98.6% and 98.5%, respectively. However, in our results, it was found that at the second resonance (4.76 THz), the structure acted as a strong cross-polarization converter, reflecting a significant amount of incident energy in the cross-polarization component of the reflected wave. When this component is considered in the reflection coefficient calculations, the absorption reduces to 41% (from 98.5%), which is not an acceptable level for an absorber. In addition, the structure was simulated for both lossy and lossless (FR4) substrate cases to understand the effect of substrate losses. The results showed that the absorption response significantly deteriorates at the first resonance (4.48 THz) in the case of a lossy FR4 substrate.

Metasurface-based polarization-insensitive absorber with dual-band tunable absorption intensity

In this paper, a polarization-insensitive absorber with dual-band tunable absorption intensity is proposed. Two absorption bands are generated by introducing external branches into a ring structure. Since the resistance of the loaded PIN diode changes with bias current, the reflectivity of two absorption bands can be dynamically adjusted from almost 0 dB to less than −10 dB at normal incidence. Inductors are loaded on both sides of the metal structure, which guarantees not only that the units are connected during DC feeding, but also the symmetry of the metal structure in RF band to ensure polarization insensitivity. The mechanisms of dual absorption bands and tunable absorption intensity are revealed through the detailed analysis of resonant modes, equivalent circuit model, surface current, and loss density distribution. A sample prototype has also been fabricated, and the measured results are well consistent with the simulated ones. The proposed absorber has the unique advantage of dynamic tuning of dual-band absorption intensity, making it a versatile choice in modern communication, radar, and stealth systems.

Wide-angle ultra-wideband metamaterial absorber based on complex dielectric layer in long and very long-wave infrared

Long and very long-wave infrared are the most important bands in infrared detection technology because of their high atmospheric window radiation energy. At present, long and very long-wave infrared detectors are widely used in atmospheric monitoring, night reconnaissance, deep space exploration and other fields. In this paper, we first analyze the coupling resonance in the dual-band absorber (Ti–Si–Ti), and the absorption rates are 97.05% and 98.95% at 6.1 μm and 19.2 μm, respectively. Then, a four-layer (Ti–Si–SiO2–Ti) absorber with a complex dielectric structure is obtained by using the surface plasmon resonance and the inherent absorption of the lossy material SiO2. In addition to the traditional electromagnetic field analysis, we also used the layer absorption energy loss theory to study the inherent absorption mechanism of SiO2, and the average absorption of the absorber from 19 to 24.7 μm reached 92.87%. Finally, based on the composite dielectric layer, a thin ultra-wideband absorber with four nanorods of the same size surface structure was designed, and the average absorption was 92.03% from 13.3 to 24.6 μm. The polarization-insensitive ultra-wideband absorber proposed by us is light, simple in structure and easy to manufacture, and has potential application value in atmospheric monitoring, night reconnaissance, deep space exploration and other fields.

Two bits dual-band switchable terahertz absorber enabled by composite graphene and vanadium dioxide metamaterials

This article presents the design of a 2-bit dual-band switchable terahertz absorber using a stacked combination of graphene and vanadium dioxide (VO2) metamaterials. For the first time, the proposed absorber design offers four switchable states by controlling the conductivity of graphene and VO2 metamaterial layers. The lower absorption band is produced by the graphene metamaterial, whereas the upper band is implemented by the VO2 metamaterial pattern. The structure shows two absorption bands (State 11) at 0.745–0.775 THz and 2.3–5.63 THz, when the Fermi graphene level of graphene is 0.2 eV and the VO2 is in the metallic phase. The lower absorption band is turned off, while keeping the upper band (State 01), when the graphene Fermi level is 0 eV and the VO2 layer is in the metallic phase. The upper absorption band is turned off, while preserving the lower absorption band (State 10) by switching the VO2 into the insulator phase and keeping the graphene Fermi level at 0.2 eV. Finally, both of the absorption bands are turned off by setting the graphene Fermi level to 0 eV and switching the VO2 into the insulating phase. Equivalent circuit modelling analysis and full-wave electromagnetic simulations are used to explain the operation principle of the proposed absorber. Very good agreement is obtained between the theoretical analysis and the simulations confirming the presented design principle for the 2-bit switchable absorber.

Optically Transparent Dual-Band Metamaterial Absorber Using Ag Nanowire Screen-Printed Second-Order Cross-Fractal Structures

In this paper, we propose an optically transparent dual-band metamaterial absorber (MMA) that uses Ag nanowire screen-printed fractal structures. The proposed MMA exhibits near-perfect absorption in the C- and K-bands. This dual-band absorption property is achieved through two inductive–capacitive (L-C) resonances located at 6.45 and 21.14 GHz, which are generated by the second-order fractal structures. We analyzed the microwave absorbing mechanisms through the distributions of the surface current and electromagnetic field on the top and bottom layers. The MMA demonstrates an optical transmittance of 63.1% at a wavelength of 550 nm. This high optical transmittance is attained by screen printing transparent Ag nanowire ink onto a transparent PET substrate. Since screen printing is a simple and low-cost fabrication method, the proposed MMA offers the advantages of being low cost while having the properties of optical transparency and effective dual-band absorption. Consequently, it holds great potential for the radar stealth application of C- and K-bands in that it can be attached to the windows of stealth aircraft due to its optical transparency and dual-band near-perfect absorption property.

Flat-band Friedrich-Wintgen bound states in the continuum based on borophene metamaterials.

Many applications involve the phenomenon of a material absorbing electromagnetic radiation. By exploiting wave interference, the efficiency of absorption can be significantly enhanced. Here, we propose Friedrich-Wintgen bound states in the continuum (F-W BICs) based on borophene metamaterials to realize coherent perfect absorption with a dual-band absorption peak in commercially important communication bands. Metamaterials consist of borophene gratings and a borophene sheet that can simultaneously support a Fabry-Perot plasmon resonance and a guided plasmon mode. The formation and dynamic modulation of the F-W BIC can be achieved by adjusting the width or carrier density of the borophene grating, while the strong coupling leads to the anti-crossover behavior of the absorption spectrum. Due to the weak angular dispersion originating from the intrinsic flat-band characteristic of the deep sub-wavelength periodic structure, the proposed plasmonic system exhibits almost no change in wavelength and absorption at large incident angles (within 70 degrees). In addition, we employ the temporal coupled-mode theory including near- and far-field coupling to obtain strong critical coupling, successfully achieve coherent perfect absorption, and can realize the absorption switch by changing the phase difference between the two coherent beams. Our findings can offer theoretical support for absorber design and all-optical tuning.

Multifunctional metamaterial device based on VO2 and the equivalent diode.

This paper proposes a switchable multifunctional metamaterial device operating in the terahertz (THz) band. The device is loaded with an equivalent diode and utilizes vanadium dioxide (V O 2). The middle layer of the whole device, a metal layer, divides the device into the I side and the II side. When the diode is ON, the I side can achieve dual-band absorption at 1.975 and 4.345THz. When the diode is OFF, the I side can achieve single-band absorption at 4.28THz. In the case of V O 2 being insulating, the II side can achieve linear-to-linear (LTL) polarization conversion at 2.342-4.18THz. In the case of V O 2 being conductive, the II side can realize linear-to-circular (LTC) polarization conversion at 2.105-3.283THz. The device provides a new strategy for the subsequent combination of multiple functions. The device can be used in electromagnetic stealth, intelligent applications, radiometers, and sensors and has relatively large application potential in miniaturized multifunctional metamaterials and THz band research.

Dual-band polarization insensitive graphene-based reconfigurable absorber in a terahertz regime.

In this paper, a dual-band graphene-based absorber is structured and investigated in a terahertz frequency domain. The designed absorber contains a plus-shaped graphene sheet, as well as four dual concentric graphene-based rings. It is designed according to the SiO2-based substrate. A dual concentric graphene ring provides more than 90% absorption from 2.36-3.24THz, while a plus-shaped graphene sheet is accountable for absorption from 5.0-5.32THz. The maximum absorption is found at 2.56, 3.08, and 5.16THz with 99.81%, 97.34%, and 99.74%, respectively. The variation in the chemical potential of the graphene material offers re-configurability in the dual absorption bands. Due to the four-fold symmetry of the designed structure, the absorptivity value is approximately constant for various polarizations with a wide incident angle (more than 60°). The aforementioned features of the proposed graphene-based absorber make it appropriate in a terahertz frequency regime.

Adjustable THz Absorber Exploiting Nested Graphene Disk-Ring

This work introduces a THz dual-band absorber with the capability of being tuned via external stimulation. A nested disk-ring graphene pattern is proposed on top of the lossless substrate while the other side of the substrate is coated by a thick layer of gold. The proposed absorber is modeled using an equivalent circuit model (ECM) and also simulated by the full wave method. A perfect match between the circuit model and numerical full wave simulation highlights the developed circuit modeling. Since the circuit modeling can reduce CPU time and need memory considerably. The impedance-matching concept interpretations are discussed in detail. Ample simulation results show the dual-band operation of the proposed absorber at 2THz and 10THz, while an interesting tuning capability is provided by changing chemical potentials. Such a simple and adjustable wave absorber is in demand for numerous applications ranging from medical imaging to indoor communication.

Independent dual-band absorptions with large frequency ratio and large bandwidth ratio from composite metamaterials of water pyramids over Helmholtz resonators

We demonstrate the perfect trapping of electromagnetic fields at dual frequencies through composite metamaterials of a water pyramid cascaded meta-surface loaded with a Helmholtz resonator array. More specifically, the water pyramid cascaded meta-surface functions as an ultra-wideband absorber at the frequency range of 6.6–100 GHz with more than a 175 % relative bandwidth, while operating as a transparent screen at lower frequencies for the Helmholtz resonators. On the other hand, the Helmholtz resonator array will absorb electromagnetic fields at 100 MHz with the Q factor of 63 and the total thickness of 1/160 wavelengths, while performing as the ground for the water pyramids. The proposed design, integrating the water pyramid cascaded meta-surface with a Helmholtz resonator array for the independent dual-band absorptions with a large frequency ratio and large bandwidth ratio, should pave the way for building up more advanced energy-harvesting meta-devices.

Graphene‐Based Flexible Optically Transparent Dual‐Tunable Metasurface

AbstractDynamically tunable flexible metasurfaces with optical transparency are essentially useful in stealth, electromagnetic sensing, and camouflage while keeping continuous optical observation and communication. In this paper, a dual‐tunable graphene‐based metasurface is designed based on patterned graphene sandwich structures, showing independent dynamic control of reflection amplitude and resonance frequency, excellent optical transparency, and outstanding flexibility. Both the simulated and measured results demonstrate that the transition from dual band absorption at 12 and 24.8 GHz to broad band absorption from 10 to 25.7 GHz, and the absorption amplitude can be tuned between −5 and −15 dB on average in each case dynamically and independently by applying bias voltage to graphene layers. Lastly, the distributions of the induced electric field and surface currents, as well as an equivalent circuit model, are provided to illustrate the tunable electromagnetic absorption mechanism. The designed metasurface has potential applications that are speculated in the fields of electromagnetic tune and electromagnetic stealth.

Dynamically tunable multifunctional terahertz absorber based on hybrid vanadium dioxide and graphene metamaterials.

In this work, in pursuit of a multifunctional device with a simple structure, high absorption rate, and excellent bandwidth, a tunable broadband terahertz (THz) absorber based on vanadium dioxide (V O 2) and graphene is proposed. Due to the phase transition of V O 2 and the electrically tunable properties of graphene, the structure realizes single broadband and dual-band absorption characteristics. When graphene is in the insulating state (E f=0e V) and V O 2 is in the metallic state, the developed system has more than 90% absorption and a wide absorption band from 1.36 to 5.48THz. By adjusting the V O 2 conductivity, the bandwidth absorption can be dynamically varied from 23% to more than 90%, which makes it a perfect broadband absorber. When graphene is in the metallic state (E f=1e V), V O 2 is in the insulating state, and the designed device behaves as a tunable and perfect dual-band absorber, where the absorptivity of the dual-band spectrum can be continuously adjusted by varying the Fermi energy level of graphene. In addition, both the broad absorption spectrum and the dual-band absorption spectrum maintain strong polarization-independent properties and operate well over a wide incidence angle, and the designed system may provide new avenues for the development of terahertz and other frequency-domain tunable devices.

Fabrication of graphene-based magnetic carbon composites derived from iron cobalt metal-organic framework as dual-band electromagnetic wave absorbers

The fabrication of advanced magnetic carbon electromagnetic wave (EMW) absorbers derived from metal-organic framework (MOF) with thin thickness, wide bandwidth, strong absorption strength and low filling ratio remains a huge challenge. In this paper, CoFe2O4/CoFe/C/Graphene composites were prepared by a simple two-step way of solvothermal reaction and carbonization treatment. The results of micromorphology analysis revealed that as the carbonization temperature increased, the obtained carbon frameworks changed from a spindle shape to dodecahedron-like shape and finally to a tile shape, and the carbonization temperature had a notable influence on the electromagnetic parameters and EMW absorption properties of the magnetic carbon composites. It should be noted that the obtained CoFe2O4/CoFe/C/Graphene composites exhibited comprehensive excellent EMW absorption performance when the carbonization temperature was 750.0 ℃ and the filling radio was only 15.0 wt%, namely the effective absorption bandwidth reached up to 4.8 GHz and the optimal minimum reflection loss was −40.2 dB at a thickness of 2.06 mm. A distinct dual-band (C band and Ku band) absorption feature was observed when the thickness exceeded or equaled 4.5 mm. Furthermore, the underlying electromagnetic dissipation mechanisms were also elucidated. Therefore, our results could provide a valuable reference for preparing excellent magnetic carbon EMW absorbers derived from MOF.

Optimized design for absorption metasurface based on autoencoder (AE) and BiLSTM-Attention-FCN-Net

In order to speed up the process of optimizing design of metasurface absorbers, an improved design model for metasurface absorbers based on autoencoder (AE) and BiLSTM-Attention-FCN-Net (including bidirectional long-short-term memory network, attention mechanism, and fully-connection layer network) is proposed. The metasurface structural parameters can be input into the forward prediction network to predict the corresponding absorption spectra. Meantime, the metasurface structural parameters can be obtained by inputting the absorption spectra into the inverse prediction network. Specially, in the inverse prediction network, the bidirectional long-short-term memory (BiLSTM) network can effectively capture the context relationship between absorption spectral sequence data, and the attention mechanism can enhance the BiLSTM output sequence features, which highlight the critical feature information. After the training, the mean square error (MSE) value on the validation set of the reverse prediction network converges to 0.0046, R2 reaches 0.975, and our network can accurately predict the metasurface structure parameters within 1.5 s with a maximum error of 0.03 mm. Moreover, this model can achieve the optimal design of multi-band metasurface absorbers, including the single-band, dual-band, and three-band absorptions. The proposed method can also be extended to other types of metasurface optimization design.

Photo-excited separated tuning for the frequency and magnitude of dual-band absorption in the terahertz regime

Introducing tuning materials into metasurfaces is a crucial development trend for dynamic nanodevices. In this paper, we achieve the separate tuning for the frequency and magnitude of dual-band absorption in a photo-excited metasurface in the terahertz regime. By tuning the conductivity of the embedded photo-excited silicon, the two absorption peaks shift by 0.33 THz and switch to zero from near unity, respectively. In contrast, the absorption spectrum of x-polarized waves shows insensitivity to the variation of silicon conductivity. The LC equivalent circuit model is used to demonstrate the resonance frequencies of absorption peaks, which agree well with simulation results. This photo-excited Si based metasurface has wide potential applications in dynamic terahertz devices, such as modulators and frequency selectors.

An ultrathin and dual-band bidirectional metasurface absorber based on ring-disk resonators

An ultrathin and dual-band bidirectional metasurface absorber based on ring-disk resonators

Retracted: The Study of Midinfrared Extremely Narrow Dual-Band Absorber Based on a Novel Metamaterial Structure

Retracted: The Study of Midinfrared Extremely Narrow Dual-Band Absorber Based on a Novel Metamaterial Structure

Design nano cluster high Q-factor perfect plasmonic absorber for bacteria detection in the optical range

Plasmonic nanostructures enable strong light absorption at the nanometer scale, making them highly attractive for a variety of applications including sensing, imaging, and spectroscopy. In this work, we propose and simulate a dual-band plasmonic absorber based on a Pentamer and split ring resonator (SRR) hybrid structure. The absorber operates at terahertz frequencies of 209 and 229 THz, corresponding to free-space wavelengths of 1310 nm and 1430 nm. At the lower frequency band, the absorber provides perfect absorption up to 100 %, while at the higher band, the absorption is 95 %. The high absorption selectivity at the two target bands is enabled by the hybrid heptamer-SRR design. The central Pentamer consists of a central nanodisk surrounded by four peripheral nanodisks with precise dimensions and spacing. This Pentamer provides a dual-band response tuned to the desired frequencies. To further enhance the quality factor and absorption, the Pentamer is encircled by an SRR designed to boost the Q-factor. At 229 THz, the SRR pushes the quality factor to around 635, significantly higher than typical plasmonic absorbers. The final optimized design enables strong absorption together with high Q-factor and frequency selectivity. It exhibits sensitivity around 450 nm/RIU and a Figure of Merit of 227 RIU−1. The full-wave simulation is used for designing the absorber in CST Microwave Studio to optimize the dimensions for maximum dual-band absorption for biosensing applications. This absorber can be realistically fabricated for terahertz sensing applications such as detecting bacteria as studied.

Graphene-enhanced polarization-independent high-Q terahertz metamaterial absorber with tunability for sensing applications

Terahertz metamaterial absorber (TMMA) has broad prospects for sensing applications. To achieve high sensitivities and low detection limits in practice, a TMMA should exhibit a high Q-factor preferably independent of polarization, tunability, and consists of simple structures that are easy to realize. However, simultaneously achieving these is difficult. Here, we propose a graphene-enhanced TMMA that exhibits a high Q-factor independent of polarization with tunability, using couplings between a continuous monolayer graphene and the underneath metallic fractal structures with rotational symmetry. Simulation results show that the TMMA exhibits dual-band absorption with broad tunability from ∼1.5 to 3.0 THz. A maximum polarization-independent Q-factor of 133.12 with a maximum frequency sensitivity (S) of 0.42 THz/RIU and a figure of merit of 18.28 RIU-1 are achieved theoretically. The presented graphene-enhanced TMMA has many potential applications, particularly in the field of chemical and biological sensing.

Dual-band perfect absorption graphene metasurface with high modulation in near-infrared

Graphene has garnered significant attention in tunable metasurfaces due to its unique optical properties. The absorption of the graphene metassurface can be dynamically adjusted by manipulating the chemical potential. However, the working wavelengths of most graphene metasurfaces are limited in terahertz and mid-infrared, or have a low reflection range. In this paper, we design a hybrid graphene metasurface with a slot structure. The working wavelengths are optimized to 1550 nm and 1910 nm. Additionally, the slot structure greatly enhances the field intensity near the monolayer graphene to improve its interaction with light. As a result, perfect absorption is achieved at both working wavelengths, along with a maximum reflection range of 0.9 and a modulation depth of 99%.