Graphene-based Absorber Research Articles

Graphene-Based Tunable Plasmonic Perfect Absorber in FIR-Band Range

In this paper, a structure based on metal-graphene-insulator–metal (MGIM) is designed as a tunable plasmonic perfect absorber (PPA) in the FIR-band range. Simulation results performed with the 3D finite difference time domain (FDTD) method show that in the perpendicular incidence of a plane wave light in the range of 35 to 105 µm, the absorption spectrum of the proposed device has a resonance peak with absorption above 95%, which can show excellent tunability by applying a gate bias voltage to the graphene nanolayers in the structure. In our proposed structure unit cell, there is a C1 graphene nanolayer in the center and a C2 graphene nanolayer around it that bias voltage is applied only to C2, then changing the chemical potential of the two graphene nanolayers relative to each other, the absorption spectrum of the device shifts in the desired wavelength range.

Multi-band absorption characteristics of a metal-loaded graphene-based photonic crystal

In this paper, the absorption characteristics of a metal-photonic crystal-metal structure in the visible light region have been investigated by the transfer matrix method. The metal-photonic crystal-metal composite structure is formed by loading a metal layer on both sides of a one-dimensional graphene photonic crystal composed of dielectric material and graphene. The calculation results show that the multi-band absorption can be achieved for this structure. The influence of structural parameters on the multi-band absorption characteristics of the structure was investigated detailly. It is found that the center wavelength of the absorption band basically keeps invariant with the change of the chemical potential of graphene. It is also found that the number of absorption bands is tunable by changing the periodic number of graphene photonic crystals or the thickness of the dielectric material. These results provide a useful reference for the development of graphene-based multi-band light absorbers. • A M-GPC-M structure which can realize multi-band absorption in the visible wavelength region is proposed. • The number of absorption bands is adjustable. • The influence of structural parameters on the multi-band absorption characteristics is investigated.

A four-bias three-layer graphene-based THz absorber

Wave absorbers are considered to be fundamental building blocks for the manipulation of light. Almost all optical systems exploit absorbers to realize some functions. A highly tunable wide-band THz absorber is presented herein. Utilizing a dual-bias scheme with a single graphene layer leads to greater freedom to control the absorption response, while a conventional periodic array of graphene ribbons and a layer of graphene sheet are also exploited. Also, a circuit model representation for all the constituent parts of the proposed absorber is developed with an evolved design methodology. According to the simulation results, wide-band absorption from 3.5 to 6 THz is achieved.

Angularly tunable perfect absorption in graphene-mushroom hybrid structure for all angles

In this work, the authors propose a graphene-based tunable perfect metamaterial absorber at microwaves that can achieve perfect absorption at all angles. This is realized by stacking an unpatterned graphene sandwich structure and a dielectric layer on top of a mushroom-type high impedance surface whose resonant frequency is stable for all angles of incidence. A perfect-absorption-angle tunable absorber working at TM polarization is designed and realized, achieving perfect absorption covering a wide range of angles from normal to grazing incidence at the same frequency when the impedance of graphene is modulated by different bias voltages. The design concept of integrating different tunable materials and meta-structures opens up promising possibilities for wave manipulation with applications in smart and multifunctional electromagnetic devices.

A multilayer graphene-based absorber for the sub-THz regime

This work presents a four-layer absorber based on graphene patterns. Periodic arrays of biased graphene nanodisks and ribbons are exploited, while a graphene sheet top layer enhances the overall absorption. The presented structure is modeled using the circuit model method and simulated to verify its validity and accuracy. A sufficient number of simulations are performed to investigate the robustness of the proposed structure versus the variation of physical parameters such as the thicknesses of the layers, the chemical potentials, and the electron relaxation time. Wideband absorption in the frequency range of 8–9.5 THz is obtained for the proposed device, which can thus be considered for use in several optical applications including optical sensing and detection.

High-sensitivity and independently tunable perfect absorber using a nanohole and a cross-shaped graphene

In this paper, a highly sensitive and tunable graphene-based quad-band perfect or nearly perfect absorber is theoretically investigated. The proposed design is composed of a stacked nanohole and cross-shaped graphene on top of a gold layer separated by an insulator. Based on the numerical simulations, four absorption peaks are observed in the proposed structure, which can reach to 100% at 9.47 µm, 93% at 10.03 µm, 97% at 13.93 µm, and 96% at 18.81 µm in the infrared range. It has been shown that the absorption peaks can be independently tuned by varying the Fermi energy level of the graphene layers as well as the geometrical parameters of the structure, which is highly desired for wavelength selective sensing. In addition, the proposed absorber can be used as a plasmonic sensor for refractive index sensing with maximum sensitivity of 12.86 µ m R I U and figure of merit of 49.90, which is very remarkable compared to the previous reported values. Due to the symmetry of the proposed device, a wide angle-insensitive high absorption is maintained for all of the bands under transverse-electric and transverse-magnetic polarizations. With its excellent performance, our graphene-based tunable multiband absorber can be used in biosensors, chemical sensors, and many other nanophotonics devices.

A Tunable Perfect THz Metamaterial Absorber with Three Absorption Peaks Based on Nonstructured Graphene

In this paper, a graphene-based tunable multi-band terahertz absorber is proposed and numerically investigated. The proposed absorber can achieve perfect absorption within both sharp and ultra-broadband absorption spectra. This wide range of absorption is gathered through a unique combination of periodically cross- and square-shaped dielectrics sandwiched between two graphene sheets; the latter enables it to offer more absorption in comparison with the traditional single-layer graphene structures. The aforementioned top layer is mounted on a gold plate separated by a Topas layer with zero volume loss. Furthermore, in our proposed approach, we investigated the possibility of changing the shapes and sizes of the dielectric layers instead of the geometry of the graphene layers to alleviate the edge effects and manufacturing complications. In numerical simulations, parameters, such as graphene Fermi energy and the dimensions of the proposed dielectric layout, have been optimally tuned to reach perfect absorption. We have verified that the performance of our dielectric layout called fishnet, with two widely investigated dielectric layouts in the literature (namely, cross-shaped and frame-and-square). Our results demonstrate two absorption bands with near-unity absorbance at frequencies of 1.6–2.3 and 4.2–4.9 THz, with absorption efficiency of 98% in 1.96 and 4.62 THz, respectively. Moreover, a broadband absorption in the 7.77–9.78 THz is observed with an absorption efficiency of 99.6% that was attained in 8.44–9.11 THz. Finally, the versatility provided by the tunability of three operation bands of the absorber makes it a great candidate for integration into terahertz optoelectronic devices.

Tunable plasmonic absorber in THz-band range based on graphene arrow-shaped metamaterial

The tunable selective absorber consisting of periodic arrow-shaped graphene arrays that operates in the far infrared and terahertz range is proposed. It is achieved by depositing a set of arrow-shaped graphene ribbons on a SiO2 dielectric spacer. The absorption characteristics of the structure are investigated by the Finite Difference Time Domain (FDTD) method. The results show that when the Fermi level and relaxation time increase at the same time, the maximum pure absorption increases from 0.0109 to 0.1276, which is improved by nearly 12 times. Moreover, when the relaxation time is increased from 0.1 ps to 1.0 ps, the maximum value of the absorption peak increases from 0.0162 to 0.1155. In addition, this paper also conducts a comparative study on the bisymmetric arrow-shaped graphene structure based on this structure. The absorption spectrum of this structure shows a multi-peak phenomenon, which can achieve the purpose of selective absorption and individual modulation. The research results have certain guiding significance for the design of next–generation graphene-based perfect terahertz absorbers, and can be applied to the fields of label-free biomedical sensing, photodetectors and photonic devices.

Tunable and polarization-sensitive graphene-based terahertz absorber with eight absorption bands

A metamaterial absorber consisting of stacked and orthogonal elliptical graphene layers is proposed to achieve eight absorption bands in the terahertz (THz) frequency range. By adjusting the length of the short or long axis for each graphene ellipse, the corresponding absorption peak can be tuned independently. Electric field distributions are present to reveal the physical mechanism. The absorption spectrum can also be tuned by varying the chemical potential of each graphene ellipse and the manipulation for the polarization angle of the incidence. Moreover, the absorber can work properly under oblique incidences. Our work paves a way for designing tunable THz absorbers with numerous absorption bands.

A perfect electrically tunable graphene-based metamaterial absorber

In this paper, a metamaterial absorber based on graphene is proposed, designed and simulated numerically using multilayer structures. Before describing our work, the performance response of a three-layer structure is studied concerning one cylinder as a unit cell. Then, by varying the chemical potential of graphene (Fermi level of graphene) by applying external potential, the center frequency of this absorber can be adjusted. We will prove that by increasing the number of cylinders as fundamental elements in the unit cell, the bandwidth is also adjustable. Also, the center frequency can be affected by changing the dimensions of fundamental elements. Considering the above-mentioned items, the structure is investigated through increasing the graphene layers in which the absorber’s frequency response is wider respected to previous structures and the center frequency is adjustable as a result of variations in the chemical potential of graphene layers. It is shown that the proposed perfect absorber’s central frequency shift through graphene’s voltage variation is about 0.15 THz which can be increased to 0.3 THz by changing radius. In some of the introduced absorbers, the maximum value of the absorption has reached over 95%. The most important advantage of the proposed structure, which is the main purpose of designing terahertz metamaterial absorbers, is its adjustable bandwidth and center frequency, and simple fabrication.

Wideband Graphene-Based Fractal Absorber and its Applications as Switch and Inverter

In this paper, the idea of square fractal geometry has been utilized to introduce a tunable wideband graphene-based perfect plasmonic absorber in the near-infrared region. It consists of a MgF2 layer and an array of gold squares fractal loaded on a graphene layer. In the designed absorber a single layer of graphene has been used instead of multilayered graphene structures. The structure is polarization-insensitive under normal incidence due to the geometric symmetry. The absorption and bandwidth of the structure are almost insensitive to the incident angle up to 15° and 45° for TE and TM polarizations, respectively. Moreover, by choosing appropriate structural parameters, the resonance wavelength of the desired plasmonic absorber can be controlled. The absorption of the introduced structure can be tuned by changing the chemical potential of the graphene. Therefore, the proposed fractal absorber can act as switch and inverter at λ = 1995 nm. Furthermore, the equivalent circuit model of the absorber has been derived to confirm the validity of the simulation results. The superiorities of our fractal absorber are wide full-width at half-maximum of 406 nm, multi-applicant, perfect absorption, and fabrication feasibility due to the simple structure with the maximum absorption tolerance error of 5.12%.

A Review on Heavy Metal Ions and Containing Dyes Removal Through Graphene Oxide-Based Adsorption Strategies for Textile Wastewater Treatment.

Textile wastewater heavy metal pollution has become a severe environmental problem worldwide. Metal ion inclusion in a dye molecule exhibits a bathochromic shift producing deeper but duller shades, which provides excellent colouration. The ejection of a massive volume of wastewater containing heavy metal ions such as Cr (VI), Pb (II), Cd (II) and Zn (II) and metal-containing dyes are an unavoidable consequence because the textile industry consumes large quantities of water and all these chemicals cannot be combined entirely with fibres during the dyeing process. These high concentrations of chemicals in effluents interfere with the natural water resources, cause severe toxicological implications on the environment with a dramatic impact on human health. This article reviewed the various metal-containing dye types and their heavy metal ions pollution from entryway to the wastewater, which then briefly explored the effects on human health and the environment. Graphene-based absorbers, specially graphene oxide (GO) benefits from an ordered structured, high specific surface area, and flexible surface functionalization options, which are indispensable to realize a high performance of heavy metal ion removal. These exceptional adsorption properties of graphene-based materials support a position of ubiquity in our everyday lives. The collective representation of the textile wastewater's effective remediation methods is discussed and focused on the GO-based adsorption methods. Understanding the critical impact regarding the GO-based materials established adsorption portfolio for heavy metal ions removal are also discussed. Various heavy-metal ions and their pollutant effect, ways to remove such heavy metal ions and role of graphene-based adsorbent including their demand, perspective, limitation, and relative scopes are discussed elaborately in the review.

A tunable broadband graphene-based metamaterial absorber in the far-infrared region

This paper reports a new design of a broadband absorber composed of graphene, dielectric, and gold layers. The designed absorber has four absorbent modes close to each other, which results in the formation of broadband absorption. The relative bandwidth, a key parameter to assess the bandwidth improvement, shows a significant increase in the proposed design compared to similar structures published in recent years. The numerical results also reveal this metamaterial absorber can be used for applications in the far-infrared frequency range due to choosing optimized dimensions and the graphene Fermi level. Unlike other graphene-based metamaterials, which require complicated structures to be able to attain broadband absorption, the physical structure of the proposed design has a relatively simple fabrication process. For further investigations, the effect of split geometry on the absorption spectrum is studied. Also, the use of graphene in this metamaterial absorber provides dynamic adjustability through electrostatic doping in order to tune the amount of absorption. This characteristic has been studied by changing the graphene Fermi level. This feature can be widely used in electro-absorption switches and modulators.

Graphene-based dual-band tunable perfect absorber in THz range

ABSTRACT We design and simulate a tunable dual-band perfect absorber. This absorber is composed of a circular graphene layer with an ellipse cut out in the centre, separated by a dielectric layer in the middle and a gold mirror at the bottom. Results reveal that there exist two perfect absorption peaks in the terahertz band. The position of the absorption peak can be changed by adjusting the Fermi level and geometric parameters of the graphene pattern. Furthermore, the absorber is insensitive to the polarized incident light with a wide incident angle. Functionalities are achieved based on a simple and symmetrical single-layer graphene pattern, which can greatly reduce the manufacturing costs. The proposed absorber can be applied in the field of graphene-based photonic devices, such as detectors, sensors, and filters.

Independently Tunable Multipurpose Absorber with Single Layer of Metal-Graphene Metamaterials

This paper reports an independently tunable graphene-based metamaterial absorber (GMA) designed by etching two cascaded resonators with dissimilar sizes in the unit cell. Two perfect absorption peaks were obtained at 6.94 and 10.68 μm with simple single-layer metal-graphene metamaterials; the peaks show absorption values higher than 99%. The mechanism of absorption was analyzed theoretically. The independent tunability of the metamaterial absorber (MA) was realized by varying the Fermi level of graphene under a set of resonators. Furthermore, multi-band and wide-band absorption were observed by the proposed structure upon increasing the number of resonators and resizing them in the unit cell. The obtained results demonstrate the multipurpose performance of this type of absorber and indicate its potential application in diverse applications, such as solar energy harvesting and thermal absorbing.

A wideband and polarization-insensitive graphene-based metamaterial absorber

The concept of a wideband and polarization-insensitive absorber based on graphene for terahertz (THz) applications is presented in this paper. To broaden the absorption bandwidth, we have combined two concentric rectangular rings with cross-shaped strips. These are located on a silicon substrate. The analysis of the proposed absorber is based on the impedance matching theory and finite difference time domain (FDTD) methods. The simulated results show that the proposed absorber has a wideband absorption. The obtained bandwidth for absorption coefficients of 0.9 and 0.7 are 16% and 20.3%, respectively over the range of 2.47–2.9 THz and 2.39–2.93 THz. The bandwidth and absorption performance of the absorber can be adjusted by the control of the conductivity of graphene by applying an external DC-bias voltage. Due to the symmetrical design of the structure, the presented absorber is polarization insensitive. Furthermore, due to having a cross-strip structure, the performance of the proposed absorber is insensitive to the angle of the radiant wave. Thus, the proposed absorber can be considered a suitable candidate for various THz applications.

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.

Tunable terahertz perfect absorber with a graphene-based double split-ring structure

In this paper, we describe a tunable perfect absorber based on a graphene metamaterial. It consists of a square split graphene ring and a round split graphene ring. Using silicon as the base, gold, silicon dioxide, and graphene are placed layer by layer in sequence. The results of simulations performed using CST Microwave Studio indicate that the double split-ring structure possesses two absorption peaks at 10.96 THz and 12.71 THz, with absorption efficiencies of 99.7% and 99.4%, respectively, which approach perfect absorption. The permittivity of graphene can be controlled by the plus gate voltage, which results in dynamic control over the absorption peaks. Furthermore, owing to the small impact on the resonance frequency, the relaxation time may be applied to manipulate the absorption peaks. In addition, we also discuss how different structural parameters affect absorption. Lastly, we conclude that the proposed graphene-based absorber has a wide-angle incoming characteristic and great potential in infrared, filter, and terahertz detection.

Graphene-Based Tunable Wideband Metamaterial Absorber With Polarization Insensitivity and Wide Incident Angle

A graphene-based tunable wideband metamaterial absorber with polarization insensitivity and wide-incident-angle is presented in this paper. The results show that the absorption is over 90% from 5GHz to 8GHz when the fermi level of graphene is 0.5eV, and the absorption can be tuned by electrically changing the Fermi level of graphene. The mechanism of wideband and tunable absorption is explained by calculating the normalized input impedance and monitoring the surface current. Finally, the results adequately verify that the absorption of metamaterial absorber has the advantages of polarization insensitivity and wide-incident-angle. Thus, it holds great potential application value on many fields such as electromagnetic stealth, electromagnetic shielding, communication and so on.

A graphene-based dual-band THz absorber design exploiting the impedance-matching concept

A THz wave absorber is designed by exploiting a unique concept and method. Three layers including dielectric and graphene patterns are incorporated on top of a thick gold surface to ensure zero transmission of THz waves through the structure. A favorable dual-bias scheme is used for the periodic arrays of graphene ribbons in the first layer, while the second layer consists of periodic arrays of graphene disks. Also, a top layer consisting of a continuous graphene sheet is used to enhance the overall absorption. All the components are modeled via a circuit model approach developed based on passive circuit elements. This leads to an explicit impedance calculation for the structure, paving the way to fully predict the device behavior. By applying impedance-matching theory, the structural parameters are tuned to obtain perfect dual-band absorption at frequencies of 5.5 and 8.5 THz. Extensive simulations verify the robustness of the structure versus variations in its geometrical parameters, while a great ability to tune the absorption peaks is achieved via variation of the chemical potentials. Such robust and tunable absorbers are attractive for the design of optical sensors, detectors, and modulators.