Surface Impedance Of Graphene Research Articles

Graphene-based plasmonic nanoantenna with trapezoidal structure in THz band

Mainly manipulating emitted waves via plasmonic resonance is a fundamental characteristic of optical antennas. Plasmonic nanoantennas are used for terahertz (THz) radiation caused by optical interaction with surface resonance. The designed nanoantenna is such that the plasmonic resonance of the nanoantenna changes by controlling the graphene chemical potential (μc) from 0 eV to 0.2 eV. Although a dielectric layer separates the trapezoidal graphene sheets, an essential difference in the scattering parameters and the radiation pattern is observed since an external direct current bias changed the graphene surface impedance to about 860 Ω, which is the optimal value for reducing return loss in nano-systems. In other words, the plasmonic resonance frequencies can be tuned by the effect of a localised electrostatic field. Also, the trapezoidal graphene-based plasmonic nanoantenna has a high quality factor (Q-factor) from 2.35 to 5.71. Therefore, dynamic control, enhancing the input impedance and stable efficiency of graphene-based plasmonic nanoantennas, improves performance and matching at THz nano-systems for many applications of optoelectronic devices.

Attenuation‐tunable balanced bandpass filter based on graphene nanoplates

In this letter, a unique type of balanced bandpass filter (BPF) with high selectivity and tunable attenuation is proposed through the comprehensive design of a microstrip balanced BPF and graphene nanoplates. In order to achieve the passband response in differential-mode (DM) excitation and produce a wide and deep stopband in common-mode (CM) excitation, the balanced couple of lines are used in balanced BPF. The graphene nanoplates are loaded on the open ends of λ/4 stepped-impedance resonators to dissipate the electromagnetic field. By raising the bias voltage applied to the graphene nanoplates, the values of the graphene's surface impedance can be adjusted from high to low. The insertion loss of the balanced BPF can be indirectly tuned while maintaining the return loss at a comparatively low level. A prototype of the proposed gain tunable balanced BPF is fabricated and measured. The measured results show that a tuning range of insertion loss from 1.75 to 10.7 dB can be obtained for the DM circuit over the operating bandwidth centered at 1.84 GHz, without interrupting the signal rejection of the CM circuit.

Designing and Simulating of Tunable Microstrip Patch Antenna for 5G Communications Utilising Graphene Material

The patch antennas are considered one of the widespread antennas that are extensively utilized due to their capability to combine and unite with various devices, and their manufacturing cost is low, in addition to their low weight. On the other hand, in contrast to these advantages, these antennas are tainted by their narrow bandwidth and low efficiency, in addition to their weak gain. The use of high frequencies such as 60-GHz for indoor uses and the future of the next generation of wireless communications allows users to obtain a high data transfer speed of up to many gigabits per second but at the expense of losses in this frequency band, which must be rationed as much as possible through the antenna design. In this article, a Microstrip Patch (MP) antenna with the rectangular patch shape will be simulating and improved for 60-GHz applications based on graphene material. This material has amazing and unique characteristics one of which is the variable surface impedance which can be tuned via practised DC voltage on the material layer. This property allows constructing the tunable antennas that are considered as a good solution to eliminate the narrow bandwidth of the MP antennas. The introduced antenna structure comprises of fully copper radiated patch inside it two graphene slots are inserted, a substrate with 2.2 relative permittivity and 0.1 mm thickness, and a fully copper ground plane. The suggested antennas design is done by using the finite integration that is offered by the CST software. The obtained gain results for the simulated antenna is diverse among 6.19-4.6dBi for both states of the surface impedance (i.e. low graphene surface impedance and high graphene surface impedance).

Simulation and optimization of a tuneable rectangular microstrip patch antenna based on hybrid metal-graphene and FSS superstrate for fifth-generation applications

In this paper, a tuneable rectangular microstrip patch antenna (MPA) is simulated and optimized to operate in four frequency bands of the next generation of wireless communication systems. The proposed design incorporates a copper radiating patch with four implanted graphene strips for tuning purposes. The reconfigurable surface impedance of graphene can easily be altered by applying a DC voltage bias directly to the graphene strips, allowing the operating frequency of the antenna to be tuned as desired. The capability of the applied voltage to tune the operating frequency band of the proposed antenna is studied via computer simulation technology (CST) microwave studio (MWS). Frequency selective surfaces (FSSs) are introduced in order to improve the radiation parameters of the antenna. The operating frequency band of the tuneable rectangular MPA increases directly as the applied DC voltage bias is increased. Based on the simulation results, a tuneable rectangular MPA placed between two FSSs is proposed for fifth-generation applications.

Electrically tunable mantle cloaking utilizing graphene metasurface for oblique incidence

An analytic formulation to achieve mantle cloaking of a dielectric cylinder using scattering cancellation method for TMz polarized oblique incidence is presented. To obtain this goal, a closed form expression for the required surface impedance of the covering metasurface is derived. By properly tuning the chemical potential of graphene, its surface impedance is adjusted for the invisibility of the considered cylinder for arbitrary angles of the impinging plane wave. The scattering width reduction of 15.7 dB and 21.27 dB is obtained with 3-dB bandwidth of 18.7% and 10.6% at 1 THz and 1.5 THz, respectively for the incident angle of 60 degrees. Furthermore, by optimizing the surface impedance of graphene, finite cylinders can also be cloaked. The results indicate 11.4 dB and 13.5 dB radar cross section (RCS) reduction with 30% and 22% 3-dB bandwidth for the incident angles of 45 and 80 degrees, respectively. Full-wave simulations verify the analytical results. The effect of the cloaking is examined by comparing the input impedance of a pyramidal horn antenna when it is loaded with an array of bear and cloaked cylinders positioned in the aperture.

A Tunable Attenuator Based on a Graphene-Loaded Coupled Microstrip Line

In this article, a novel tunable attenuator based on graphene is proposed. It consists of a coupled microstrip line (CMSL) and two graphene-covered gaps in MSLs. By utilizing a biased voltage, the surface impedance of graphene can be tuned, and then, the reflective coefficient at two ports of the coupler can be correspondingly adjusted; therefore, the performance of the attenuator can be modified. The design procedure and the mechanism of the attenuator are demonstrated in detail. To verify the analysis, a prototype was fabricated and tested. According to the measured results, the fabricated attenuator presents the favorable attenuation tuning range from 3 to 20 dB at the operating frequency from 1 to 6 GHz, and it is just 38 mm $\times22$ mm in size. Besides, $\vert S_{11}\vert $ of the attenuator can always be maintained at a low level, and the phase of $S_{21}$ is substantially unchanged.

Dynamically Tunable Attenuator on a Graphene-Based Microstrip Line

In this paper, a novel type of dynamically tunable attenuator based on graphene is proposed. This type of attenuator is composed of one or two graphene sandwich structures (GSSs) and a microstrip line. The GSSs are spread on the substrate of microstrip line near the signal strip along the direction of propagation to dissipate electromagnetic field. By the application of biased voltage, the surface impedance of graphene can be tuned, and consequently, the insertion loss of microstrip line can also be adjusted while the return loss maintains relatively low level. A transverse equivalent network and a closed form of attenuation of each attenuator are also proposed, which can be utilized to analyze the performance of attenuator with different critical parameters of graphene in detail. According to the analysis, there is a maximum value of the attenuation as the surface impedance of graphene rises from 0 to $3000~\Omega /\Box $ . As an example, a prototype of each tunable microstrip line attenuator operating at gigahertz frequencies is fabricated and measured. The fabricated attenuator present favorable attenuation ranges from 3 to 15 dB, at the operating frequency from 9 to 40 GHz, and with a stable wideband attenuation corresponding to bias voltage changing from 0 to 4.0 V.

Graphene-Based Dynamically Tunable Attenuator on a Coplanar Waveguide or a Slotline

In this paper, we propose a novel type of dynamically tunable graphene-based attenuator fabricated on a coplanar waveguide (CPW) or a slotline. We obtain this type of attenuator by depositing a graphene sandwich structure (GSS) on a CPW or a slotline. The fabrication of this type of attenuator is much easier than that of another GSS-based attenuator. The attenuation of the attenuator can be controlled by the surface impedance of graphene which can be tuned by a bias voltage. To investigate the performance of the attenuator, a closed-form equation of attenuation based on a transverse equivalent network is derived. It can be manifested from the closed-form equation that there exists a maximum attenuation when the surface impedance of graphene reaches a certain value. It is very different from other types of attenuators. To verify the closed-form equation, we manufactured and tested a prototype of each attenuator. The measure results manifest that the attenuation can be varied from 3 to 15 dB at 9–40 GHz while the bias voltage increases from 0 to 4 V; meanwhile, the magnitude of S11 can always be smaller than −15 dB across the operating frequency. The test results also validate the role of the length of graphene and the spacing between the CPW and the GSS in the attenuator.

Performance of Graphene Plasmonic Antenna in Comparison with Their Counterparts for Low-Terahertz Applications

Graphene has attracted great interest for antenna applications because of its two-dimensional nature and superior electronic properties. Low losses, strong confinement, and high tunability properties of surface plasmon make this material as one of the suited material for terahertz applications. In this paper, we have investigated the surface plasmon polariton properties and plasmonic resonance of the graphene dipole antenna at low-terahertz frequency range and compared its radiation performance with that of carbon nanotube and copper, in order to find the suitability of these materials for THz antenna designing. Surface conductivity and surface impedance of graphene, carbon nanotube, and copper at terahertz band have been studied. The performances of these antennas are analyzed, and it was found that graphene plasmonic antenna better suits for making THz antennas.

Design of a graphene‐based patch antenna on glass substrate for high‐speed terahertz communications

AbstractFor the development of terahertz (THz) wireless communication systems, the antenna needs to be in micrometer size range and give high far‐field gain. Graphene is highly conductive, thermally and electrically; this article puts forward a transparent graphene‐based patch antenna with metal lobbing, it resonates nearly at 1 THz and has gain nearly 5 dB. It investigates the effect of chemical potential of graphene on the return loss and resonant frequency of the antenna, this would represent the most effective way to tune radiation characteristics by tuning graphene surface impedance. The performance of designed graphene antenna is compared with its metallic counterpart as a reference antenna, the former outperforms the latter in terms of spectral characteristics. Besides that, metallic thin films are prone to fail because of microcracks which give a compelling reason to switch to graphene antenna for future communication in the THz domain.

Singular integral equations in diffraction by multilayer grating of graphene strips in the THz range

Diffraction of the H-polarized electromagnetic wave by multilayer graphene grating in the THz range is considered. The scattered field is represented in the spectral domain via unknown spectral functions. The mathematical model is based on the graphene surface impedance and the method of singular integral equations. The numerical solution is obtained by the Nystrom-type method of discrete singularities. The scattering and absorption characteristics as functions of frequency and distance between layers are studied.

Modeling of Graphene Planar Grating in the THz Range by the Method of Singular Integral Equations

AbstractDiffraction of the$H$- polarized electromagnetic wave by the planar graphene grating in the THz range is considered. The scattering and absorption characteristics are studied. The scattered field is represented in the spectral domain via unknown spectral function. The mathematical model is based on the graphene surface impedance and the method of singular integral equations. The numerical solution is obtained by the Nystrom-type method of discrete singularities.

Graphene-based dynamically tunable attenuator on a half-mode substrate integrated waveguide

We demonstrate that a graphene-based attenuator on a half-mode substrate integrated waveguide (HMSIW) can achieve dynamically tunable attenuation. This attenuator is produced by depositing a graphene sandwich structure (GSS) on the substrate of a HMSIW. By applying a bias voltage on the GSS, the surface impedance of graphene and the attenuation of the attenuator can be tuned. A prototype was fabricated and measured to verify the analysis. The measurement results manifest that as the bias voltage increases from 0 V to 4 V, the attenuation can be increased from 3 dB to 14 dB at the frequency from 7.7 GHz to 19 GHz, while |S11| of the attenuator can always remain at a low level smaller than −20 dB.

Beam reconfigurable antenna for the THz band based on a graphene high impedance surface

In this paper a reconfigurable beam antenna is proposed for THz applications, based on a switchable Graphene High Impedance Surface (G-HIS) that acts as a reflector for a primary radiator. The Graphene-HIS structure composed of two layers of graphene cells arranged in a 5×5 array, the two planes of the array are separated by a thin silicon oxide layer; patches in the same row are connected together, to be biased by a common DC voltage to the entire row; this gives the ability to control the graphene conductivity σ. The results show that the shape of the radiation pattern can be changed by changing the voltage applied to each row of G-HIS array. The antenna was fabricated and characterized using a THz-TDS laser system. Measurements are in good agreement with simulations as far as the graphene surface impedance and the resonance frequency are concerned. The scanning reflection on the antenna surface due to the change on the applied voltage is presented to show the effect of biased graphene layer on the dipole properties.

An ultra-wideband pattern reconfigurable antenna based on graphene coating**Project supported by the National Natural Science Foundation of China (Grant Nos. 61661012, 61461016, and 61361005), the Natural Science Foundation of Guangxi, China (Grant Nos. 2015GXNSFBB139003 and 2014GXNSFAA118283), Program for Innovation Research Team of Guilin University of Electromagnetic Technology, China, and the Dean Project of Guangxi Key Laboratory of Wireless Wideband Communication

An ultra-wideband pattern reconfigurable antenna is proposed. The antenna is a dielectric coaxial hollow monopole with a cylindrical graphene-based impedance surface coating. It consists of a graphene sheet coated onto the inner surface of a cylindrical substrate and a set of independent polysilicon DC gating pads mounted on the outside of the cylindrical substrate. By changing the DC bias voltages to the different gating pads, the surface impedance of the graphene coating can be freely controlled. Due to the tunability of graphene's surface impedance, the radiation pattern of the proposed antenna can be reconfigured. A transmission line method is used to illustrate the physical mechanism of the proposed antenna. The results show that the proposed antenna can reconfigure its radiation pattern in the omnidirectional mode with the relative bandwidth of 58.5% and the directional mode over the entire azimuth plane with the relative bandwidth of 67%.

Graphene-based waveguide resonators for submillimeter-wave applications

Utilization of graphene covered waveguide inserts to form tunable waveguide resonators is theoretically explained and rigorously investigated by means of full-wave numerical electromagnetic simulations. Instead of using graphene-based switching elements, the concept we propose incorporates graphene sheets as parts of a resonator. Electrostatic tuning of the graphene surface conductivity leads to changes in the electromagnetic field boundary conditions at the resonator edges and surfaces, thus producing an effect similar to varying the electrical length of a resonator. The presented outline of the theoretical background serves to give phenomenological insight into the resonator behavior, but it can also be used to develop customized software tools for design and optimization of graphene-based resonators and filters. Due to the linear dependence of the imaginary part of the graphene surface impedance on frequency, the proposed concept was expected to become effective for frequencies above 100 GHz, which is confirmed by the numerical simulations. A frequency range from 100 GHz up to 1100 GHz, where the rectangular waveguides are used, is considered. Simple, all-graphene-based resonators are analyzed first, to assess the achievable tunability and to check the performance throughout the considered frequency range. Graphene–metal combined waveguide resonators are proposed in order to preserve the excellent quality factors typical for the type of waveguide discontinuities used. Dependence of resonator properties on key design parameters is studied in detail. Dependence of resonator properties throughout the frequency range of interest is studied using eight different waveguide sections appropriate for different frequency intervals. Proposed resonators are aimed at applications in the submillimeter-wave spectral region, serving as the compact tunable components for the design of bandpass filters and other devices.

Theoretical analysis of graphene nanoantennas with different shapes

AbstractIn this article, we present a numerical analysis of graphene nanoantennas with rectangular, elliptical, triangular, and circular geometries in terahertz band. We model the electromagnetic scattering of these planar structures by the method of moments with the surface impedance of graphene. We analyze the absorbing cross section and the resonances of nanoantennas for different, sizes, chemical potential, temperature, and incident angle. The obtained results can be useful to design efficient nanoantennas for terahertz wireless communications. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1019–1024, 2014

Integral Equation Analysis of Plane Wave Scattering by Coplanar Graphene-Strip Gratings in the THz Range

The plane wave scattering and absorption by finite and infinite gratings of free-space standing infinitely long graphene strips are studied in the THz range. A novel numerical approach, based on graphene surface impedance, hyper-singular integral equations, and the Nystrom method, is proposed. This technique guarantees fast convergence and controlled accuracy of computations. Reflectance, transmittance, and absorbance are carefully studied as a function of graphene and grating parameters, revealing the presence of surface plasmon resonances. Specifically, larger graphene relaxation times increases the number of resonances in the THz range, leading to higher wave transmittance due to the reduced losses; on the other hand an increase of graphene chemical potential up-shifts the frequency of plasmon resonances. It is also shown that a relatively low number of graphene strips ( >10) are able to reproduce Rayleigh anomalies. These features make graphene strips good candidates for many applications, including tunable absorbers and frequency selective surfaces.

Non-contact characterization of graphene surface impedance at micro and millimeter waves

The experimental characterization of the surface impedance of monolayer graphene at micro and millimeter wave frequencies is addressed. Monolayer graphene is transferred on a substrate stack, which is placed in the cross-section of a rectangular waveguide. In the fundamental mode, this setup is equivalent to a TE-polarized plane wave impinging under oblique incidence on an infinite graphene sheet, and similarly, the surface impedance of the graphene is a simple lumped element in a transmission-line model, that exactly represents the electromagnetic problem under study. Using this model, we propose a technique based on transmission matrices to accurately extract the surface impedance. The method is able to relax the influence of the substrates tolerances by taking advantage of the graphene infinitesimally small electrical thickness. It can also account for any gap between the sample and the test waveguide, thereby allowing to disregard graphene-metal contact resistance issues. The approach has been successfully applied to characterize graphene samples at X and Ka bands. The extracted surface impedances fit the Drude conductivity model in the absence of interband transitions, and provides a wideband frequency independent resistance (in the range of KΩs) and a weak inductive behavior (in the range of pHs).