Multi-band Absorber Research Articles

Multi-band terahertz superabsorbers based on perforated square-patch metamaterials.

This paper presents a multi-band terahertz superabsorber with a surface structure that consists of a square metallic patch with a very small rectangular hole whose area is only 3.94% of the square patch. The introduction of a rectangular hole in the square patch plays an important role in achieving multi-band absorption. Three resonant bands with very high absorption (>95%) were observed in the terahertz range. Different from the near-field distributions of the traditional square patch with no modification, the introduction of a rectangular hole in the square patch can break the near-field distributions of the traditional square patch with no modification or can rearrange them to form some new or extra resonance modes, thereby generating multi-band absorption. Considering the fact that the introduced rectangular hole plays the key role in the rearrangement of the near-field and the introduction of some new resonance modes, the parameter changes of the rectangular hole introduced in the square patch provide considerable freedom in controlling the number of absorption peaks, and the resonant bands can be tuned to quad- or dual-band absorption. The multi-band superabsorbers given here should have potential applications in numerous areas.

Design of A Perfect Metamaterial Absorber for Microwave Applications

In this manuscript, a multi-band and low-profile metamaterial absorber with polarisation independence from 00 to 450 is presented. The proposed metamaterial structure is composed of a single ring with a rectangular patch, consisting of periodic unit cells with a size of 150mm × 250mm × 1.5mm. The structure exhibits three absorption peaks under normal incidence, which cover the X-band. According to the results, the desired material can excellently absorb the electromagnetic wave signal, with an outstanding absorption rate of about 95% at the microwave x-band frequency. The proposed structure shows three absorption bands where two of them exceed 90% absorption level. The results displayed a high Q-factor of 103.5 at a resonance frequency of 8.58 GHz and the figure of merit (FOM) is 98.4, which can be used to enhance the sensor sensing, narrowband band filter and image sensing. The proposed structure is fabricated, and experiments are carried out to validate the design principle. Strong agreements are observed between the measured and the corresponding simulated results.

Ultra-thin dual-band perfect metamaterials absorber for microwave applications

Abstract This article studies an ultra-thin dual-band perfect electromagnetic metamaterials absorber. It presents a typical artificial microwave structure that can realize multi-band perfect metamaterial absorption. The structure design was implemented by using a Four Quarters of Circle Resonator (FQCR), an ultra-thin FR-4 dielectric substrate and an entire metal plate. The simulated results show two perfect absorption peaks with absorption rates, where the first peak hits 99.35%, and the second peak hits 97.81%. In addition, the ratios of the structure thickness to the wavelengths of the proposed design are 1/82 and are 1/66 at resonance frequency 9.12 GHz and 11.34 GHz respectively, which are ultra-thin comparing with wavelengths. By using the distributions of electric surface current, the mechanism of the ultra-thin dual band is described. This absorber is non-sensitive to the TE polarization wave ranging from 0° to 90° and non-sensitive to the TM polarization wave ranging from 0° to 60° for the first peak and to 45° for the second peak. Furthermore, such multi-band absorber promises many x-band applications, such as harvesting, radar cross section RCS, sensing, antenna enhancement, thermal image and detection.

Multiband terahertz metasurface absorber using Hilbert fractal

Most of the natural materials have very weak interaction with the terahertz (THz) electromagnetic waves. Designing an absorber in the THz range is of considerable interest. We present multiband THz metasurface absorbers formed by the metallic Hilbert fractals separated with a dielectric layer from the metallic ground-plane. Numerical simulations reveal that the absorption spectra of the second-order Hilbert fractal absorber have four absorption bands located at 1.61, 3.41, 4.61, and 5.61 THz with absorption efficiency of 1, 0.97, 0.99, and 0.93, respectively. We also compare three orders of Hilbert fractals as resonant elements and examine the role of different geometrical parameters on the absorption spectra. The proposed absorbers can be readily tuned to desired frequencies for applications, such as imaging, filtering, sensing, and selective thermal emission.

Equivalent circuit model of graphene chiral multi-band metadevice absorber composed of U-shaped resonator array.

In this study, we have designed an equivalent circuit model (ECM) by use of a simple MATLAB code to analyze a single-layered graphene chiral multi-band metadevice absorber which is composed of U-shaped graphene resonator array in terahertz (THz) region. In addition, the proposed metadevice absorber is analyzed numerically by the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed device which is the first tunable graphene-based chiral metadevice absorber can be used in polarization sensitive devices in THz region. It is single-layered, tunable, and it has strong linear dichroism (LD) response of 94% and absorption of 99% for both transverse electric (TE) and transverse magnetic (TM) electromagnetic waves. It has four absorption bands with absorption >50% in 0.5-4.5 THz : three absorption bands for TE mode and one absorption band for TM mode. Proposed ECM has good agreement with the FEM simulation results. ECM analysis provides a simple, fast, and effective way to understand the resonance modes of the metadevice absorber and gives guidance for the analysis and design of the graphene chiral metadevices in the THz region.

Multi-band and high-sensitivity perfect absorber based on monolayer graphene metamaterial

The Finite Difference Time Domain (FDTD) method is used for the simulation, and a new method based on critical coupling and guided resonance is proposed theoretically and numerically to realize a multi-band ideal absorber (PA) of monolayer graphene. Its physical mechanism can be more perfectly analyzed through impedance matching and coupled mode theory (CMT). Due to the guided resonance (100% transmission or reflection efficiency is obtained through the coupling of the leakage mode and the guided mode under the phase matching condition), the perfect absorber can obtain four perfect absorption peaks. The resonance wavelengths are located at λ1 = 1085.03 nm, λ2 = 1131.48 nm, λ3 = 1187 nm and λ4 = 1365.35 nm, respectively. Their absorption rates are 95.88%, 99.81%, 97.44% and 95.30%. At the same time, we can also see a phenomenon in which the spectral position and value of the absorption peak can be adjusted by changing the relevant geometric parameters in the system (the geometric size, period, and incident angle of the hexagonal air hole absorber). Meanwhile, the structure we designed has certain advantages in the field of similar absorber research by briefly calculating related values of sensing performance. The sensitivity of its four resonance peaks are 46.45, 94.35, 151 and 598.9 nm/RIU, and FOM are 5.445, 11.192, 19.895 and 85.680. So we believe that the research has huge application prospects in terms of sensors, tunable spectrum detection, environmental monitoring and medical diagnosis, modulators and optoelectronic device sensors.

Low-voltage controlling high order plasmonic modes based on graphene/metal electrodes

To improve the compatibility of the nanophotonic device with the electronic device, a kind of hybrid system based on graphene/metal electrode is demonstrated and characterized by both coupled mode theory and numerical simulation. Our results show that, assisted by metallic slits, multiple high order plasmonic resonant modes, including even and odd ones, are efficiently excited in graphene ribbons. The slits perforated in a metal electrode act as magnetic dipole sources and break the structural symmetry in individual graphene ribbons. Due to nano-sized space between graphene and metal gate electrodes, low voltages are required to dynamically control plasmonic modes of such hybrid system. At last, based on Salisbury effect, a low-voltage controlling multi-band absorber are successfully designed. The proposed structure would find potential applications in the field of integrated photonics which are compatible with electronic technology, specifically in low-voltage controlled plasmonic devices such as tunable multiband mid-infrared absorbers, modulators, biosensors, photonic memory cells etc.

Phase change material-based nano-cavity as an efficient optical modulator

Structural phase transition induced by temperature or voltage in phase change materials has been used for many tunable photonic applications. Exploiting reversible and sub-ns fast switching in antimony trisulfide (Sb2S3) from amorphous (Amp) to crystalline (Cry), we introduced a reflection modulator based on metal–dielectric–metal structure. The proposed design exhibits tunable, perfect, and multi-band absorption from visible to the near-infrared region. The reflection response of the system shows >99% absorption of light at normal incidence. The maximum achievable modulation efficiency with a narrow line width is ∼98%. Interestingly, the designed cavity supports critical resonance in an ultrathin (∼λ/15) Sb2S3 film with perfect, broadband, and tunable absorption. Finally, we proposed a novel hybrid cavity design formed of Cry and Amp Sb2S3 thin films side-by-side to realize an optical modulator via relative motion between the incident light beam and cavity. The proposed lithographic free structure can be also used for filtering, optical switching, ultrathin photo-detection, solar energy harvesting, and other energy applications.

Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption.

In this paper, we propose and demonstrate a switchable terahertz metamaterial absorber with broadband and multi-band absorption based on a simple configuration of graphene and vanadium dioxide (VO2). The switchable functional characteristics of the absorber can be achieved by changing the phase transition property of VO2. When VO2 is insulating, the device acts as a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz. The broadband absorber exhibits excellent absorption performance under a wide range of incident and polarization angles for TE and TM polarizations. Moreover, the absorption bandwidth and intensity of the absorber can be dynamically adjusted by changing the Fermi energy level of graphene. When VO2 is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies at 1 THz, 2.45 THz, and 2.82 THz. The multi-band absorption is achieved owing to the fundamental resonant modes of the graphene ring sheet, VO2 hollow ring patch, and coupling interaction between them. Moreover, the multi-band absorber is insensitive to polarization and incident angles for TE and TM polarizations, and the three resonance frequencies can be reconfigured by changing the Fermi energy level of graphene. Our designed device exhibits the merits of bi-functionality and a simple configuration, which is very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Investigation of acoustic plasmons in vertically stacked metal/dielectric/graphene heterostructures for multiband coherent perfect absorption.

Coherent absorption, as the time-reversed counterpart to laser, has been widely proposed recently to flexibly modulate light-matter interactions in two-dimensional materials. However, the multiband coherent perfect absorption (CPA) in atomically thin materials still has been elusive. We exploit the multiband CPA in vertically stacked metal/dielectric/graphene heterostructures via ultraconfined acoustic plasmons which can reduce the photon wavelength by a factor of about 70 and thus enable multiple-order resonances on a graphene ribbon of finite width. Under the illumination of two counter-propagating coherent beams, the two-stage coupling scheme is used for exciting multispectral acoustic plasmon resonances on the heterostructure simultaneously, thereby contributing to the ultimate multiband CPA in the mid-infrared region. The strong dependence of the nearly linear dispersion of acoustic plasmons on the chemical potential in graphene and the separation between the metal and the graphene allows the tunability in spectral positions of absorption peaks. Intriguingly, the absorption of each resonant peak is continuously tuned by varying the relative amplitude of two counter-propagating beams, and even their phase difference, respectively. The maximum modulation depth of 4.46*105 is observed. The scattering matrix is employed to demonstrate the principle of CPA and the finite-difference time-domain (FDTD) simulations are used for elucidating the flexible tunability. More importantly, the multiband coherent absorber is robust to the incident angle, and thus undoubtedly benefits extensive applications on optoelectronic and engineering technology areas for modulators and optical switches.

Fabrication of Infrared-Compatible Nanofluidic Devices for Plasmon-Enhanced Infrared Absorption Spectroscopy

Nanofluidic devices have offered us fascinating analytical platforms for chemical and bioanalysis by exploiting unique properties of liquids and molecules confined in nanospaces. The increasing interests in nanofluidic analytical devices have triggered the development of new robust and sensitive detection techniques, especially label-free ones. IR absorption spectroscopy is one of the most powerful biochemical analysis methods for identification and quantitative measurement of chemical species in the label-free and non-invasive fashion. However, the low sensitivity and the difficulties in fabrication of IR-compatible nanofluidic devices are major obstacles that restrict the applications of IR spectroscopy in nanofluidics. Here, we realized the bonding of CaF2 and SiO2 at room temperature and demonstrated an IR-compatible nanofluidic device that allowed the IR spectroscopy in a wide range of mid-IR regime. We also performed the integration of metal-insulator-metal perfect absorber metamaterials into nanofluidic devices for plasmon-enhanced infrared absorption spectroscopy with ultrahigh sensitivity. This study also shows a proof-of-concept of the multi-band absorber by combining different types of nanostructures. The results indicate the potential of implementing metamaterials in tracking several characteristic molecular vibrational modes simultaneously, making it possible to identify molecular species in mixture or complex biological entities.

Stereo Perfect Metamaterial Absorber Based on Standing Gear-Shaped Resonant Structure With Wide-Incident-Angle Stability

In this work, a single-band metamaterial absorber (MA) based on a three dimensional (3D) resonant structure is presented. The unit cell is composed of a standing gear-shaped resonator, which is embedded in the dielectric substrate. The proposed 3D MA is ultrathin with a total thickness of 2.3 mm, corresponding 0.077λ0 at its center frequency. The simulation results demonstrate a high absorption peak at 10.1 GHz with absorptivity of 99.9%. The proposed 3D MA is insensitive to the polarization of the incident wave due to its rotationally symmetric structure. Moreover, the proposed 3D MA exhibits a wide-incident-angle stability, as absorptivity of more than 85% can be achieved for both TE and TM incidences with incident angle up to 60°. Most importantly, multiband electromagnetic wave absorption of the stereo MA can be enabled by adjusting the structural parameters of the standing gear. The proposed structure is compatible with 3D printing technology and has potential applications in electromagnetic shielding.

Rapid prototyping of microwave metasurfaces by ink-jet printing on polyester (PET) transparencies

Microwave metasurfaces have been developing rapidly for various applications, typically fabricated on rigid substrates such as silicon and printed circuit board using conventional microfabrication and related techniques. Rapid prototyping and production is desirable for quick design changes, design flexibility for different applications, manufacturing and low cost. Here, we report on the production of microwave metasurfaces using inkjet-printed Ag conductive patterns on polyester sheets as transparent and flexible substrates. The design is shown to be easily reconfigurable between single-band absorption achieved with a single layer of metal array pattern and multi-band absorption achieved either by stacking various single-band sheets or by printing a complex pattern on a single sheet. The dielectric thickness is varied by the simple addition of blank polyester sheets between the sheet with the printed pattern and the ground plane. Optimal dielectric thickness for each combination of materials and geometries defining the metasurface has been achieved by varying the number of blank sheets to maximize the absorption to near perfect levels in each case. This method is amenable for rapidly producing narrowband and broadband metasurfaces for various applications.

Experimental Verification of a TH\(\text{z}\) Multi-band Metamaterial Absorber

Multi-band metamaterial absorbers have been of great interest owing to their potentials for a wide range of communicating, sensing, imaging, and energy harvesting applications. In this work, we experimentally investigate a four-band metamaterial absorber operating at THz frequencies. The metamaterials are fabricated using the maskless UV photolithography and e-beam evaporation techniques. The absorption spectra of the proposed absorber are measured using the micro-Fourier transformed infrared spectroscopy. It was demonstrated that multi-band absorption behavior originates from different individual metamaterial resonators. The thickness of the dielectric spacer plays a key role in optimizing the absorption performance, in line with the predicted results on single-band THz absorbers.

Toward an Ultra-Wideband Hybrid Metamaterial Based Microwave Absorber.

In this paper, we propose a novel design of an ultra-wideband hybrid microwave absorber operating in the frequency range between 2 GHz and 18 GHz. This proposed hybrid absorber is composed of two different layers that integrate a multiband metamaterial absorber and a lossy dielectric layer. The metamaterial absorber consists of a periodic pattern that is composed of an arrangement of different scales of coupled resonators and a metallic ground plane, and the dielectric layer is made of epoxy foam composite loaded with low weight percentage (0.075 wt.%) of 12 mm length carbon fibers. The numerical results show a largely expanded absorption bandwidth that ranges from 2.6 GHz to 18 GHz with incident angles between 0° and 45° and for both transverse electric and transverse magnetic waves. The measurements confirm that absorption of this hybrid based metamaterial absorber exceeds 90% within the above-mentioned frequency range and it may reach an absorption rate of 99% for certain frequency ranges. The proposed idea offers a further step in developing new electromagnetic absorbers, which will impact a broad range of applications.

A bi-tunable switchable polarization-independent dual-band metamaterial terahertz absorber using VO2 and Dirac semimetal

A bi-tunable switchable dual-band THz absorber using vanadium dioxide (VO2) and Dirac semimetal films (DSFs), which is independent on polarization, is proposed. The frequency and intensity of absorptances of the two absorption peaks can be dynamically changed by tuning the Fermi energy of the DSF or the VO2 conductivity. Moreover, the absorber can alternate between an absorption function (with an absorptance of 99.4%) and a reflection function (with a reflectance of 77.4%) at the frequencies of absorption peak B in case of VO2 transitions from metallic state to insulator state. In addition, the proposed absorber exhibits advantages such as polarization-independent nature and wide incident angle tolerance. This study provides references to future studies of THz bi-tunable dual-band or multi-band polarization-independent absorbers.

Computational analysis of octagonal torus-shaped nano meta-atom through fractional absorption bandwidth for applications in the terahertz regime

In this paper, we present a computational analysis method for the study of silicon dioxide (SiO2) graphene-based and amorphous silicon (a-Si)-graphene based nano meta-atoms in the terahertz (THz) regime. The proposed meta-atoms were modeled using a SiO2 substrate, optimized using a-Si substrate, and “sandwiched” between graphene layers. The octagonal-shaped torus was geometrically designed as a 3D structure to achieve optimal electromagnetic wave absorption from separate atoms. The upper and lower tori had identical diagonal arms to capture the maximum number of photons with minimal absorption losses. Graphene was chosen as the upper and ground layer material to optimize the electrical and optical properties of our proposed nano metamaterial. Finally, the following four fractional bandwidths with absorption percentages were optimized from the simulation: 4 bands (225.7–254.74 THz, 99.8%; 356.64–401.1 THz, 97.28%; 525.4 THz, 99.19%; 656.34 THz, 94.94%). The multi-band fractional absorption characteristic in the THz regime highlighted our proposed nano meta-atom as a potential candidate for absorption in the infrared and visible spectrum.

Quad band metamaterial absorber based on asymmetric circular split ring resonator for multiband microwave applications

This article presents a metamaterial based on a modified asymmetric circular split ring resonator for absorber applications in the microwave regime. This metamaterial absorber (MMA) exhibits four absorption peaks to cover C, X, and Ku band applications. The MMA unit cell is formed on a low-cost FR4 substrate with electrical dimensions of 0.106 λ0 × 0.106 λ0, for which wavelength, λ0 is calculated at 4.1 GHz. The resonator of the unit cell is structured with three concentric intercoupled circular split rings with dimensions modified to obtain maximum absorption peaks of 97.9%, 99.1%, 99.5%, and 99.95% at 4.1, 6.86, 11.3, and 13.45 GHz, respectively. Metamaterial and absorber properties are investigated in the analysis of the surface current, electric, and magnetic fields. The unit cell exhibits single negative metamaterial properties with an effective medium ratio (EMR) of 9.15 and quality factor (Q factor) greater than 20. Beyond that, the equivalent circuit of the MMA unit cell is modeled and validated by comparing S11 obtained from Advanced Design System (ADS) and CST software. The MMA array’s absorption properties are also examined in the simulations. Measured results of the unit cell and array correspond well with the result of simulation in terms of absorption at intended frequencies. Given its good EMR, superior quality factor, and high maximum absorption, the MMA can be a good candidate for a multiband absorber in microwave applications like sensing, detecting, notch filtering and to decrease the indirect reverberation and reflections caused by the metallic part of the radar and satellite antennas.

Broadband/multiband absorption through surface plasmon engineering in graphene-wrapped nanospheres

In this paper, a thin film constructed by a periodic assembly of graphene-wrapped particles with spherical geometry has been proposed as a polarization-insensitive reconfigurable perfect absorber. The performance of the proposed structure is based on the cooperative excitation of the quadrupole localized surface plasmons on graphene shells. By sweeping the quality of graphene shells, it is recognized that the low-quality graphene material is the best choice for the absorber design. Moreover, the effect of graphene chemical potential and periodicity of the particles on the absorptivity of the structure is investigated. The physical mechanism of performance is clarified by investigating the excited localized surface plasmon resonances. In addition, the angle-independent behavior up to around 60 degrees for both transverse electric (TE) and transverse magnetic (TM) waves is proved. Interestingly, by engineering the substrate height, our proposed absorber exhibits dynamic broadband performance due to the impedance matching and multiband absorption by enhancing the Fabry-Perot resonances of a micrometer-sized substrate. The possibility of attaining a similar static broadband response by stacking multiple layers is also proved. Our proposed sub-wavelength absorber can be suitable for novel optoelectronic devices due to its simple geometry.

Multiband metamaterial selective absorber for infrared stealth.

Nanostructured selective absorbers have widespread applications ranging from artificial color to thermophotovoltaics and radiative cooling. In this paper, we propose a metamaterial selective absorber with a metal-insulator-metal structure for infrared stealth. It can realize multiband absorption, and one sharp peak is at 1.54 µm, which can be used to reduce the scattering signals in laser-guided missiles. The other two relatively broad absorption peaks are at 2.83 µm and 6.11 µm, which can match the atmospheric absorption band. It can reduce up to 90 % of the detected infrared signals while maintaining a relatively high level of thermal emission capability. The dependence of the spectral characteristics on the incident angle is studied. The infrared signatures of the structure could be suppressed across a wide temperature range.