Graphene Disks Research Articles

Dynamic tunable triple-band terahertz perfect absorber based on graphene metamaterial

This work developed a graphene-based perfect absorber with dynamic tuning capabilities. This absorber consists of a metal substrate, a layer of graphene square rings, a layer of graphene disks, and two layers of SiO2 dielectric sandwiched between them. Simulation results demonstrate absorption peaks at 1.23 THz, 4.2 THz, and 6.38 THz, with respective absorption rates of 99.9 %, 99.8 %, and 98.3 %. By analyzing the distributions of the electric field and surface current at the central frequencies of the three absorption peaks, the excitation mechanism of each absorption peak was discussed. It was found that by adjusting graphene's chemical potential, the absorption spectrum can be actively controlled. Furthermore, the absorber is insensitive to polarization and can sustain the stability of the absorption spectrum within an incident angle range of 0∼35°. This work has the potential to advance the research of multi-band terahertz absorbers, dynamically tunable devices, and other graphene-based photonic devices.

CO2 sensing via periodic Array of graphene disks

This work aims to introduce a gas detection structure based on stacked layers of graphene-spacer-metal. The presented structure consists of three stacked layers. The first layer is the periodic arrays of graphene disks on top of a typical dielectric such as TOPAS or KAPTON with a known refractive index. Then the second layer provides an air gap opening room for a sample environment. Finally, the third layer includes the dielectric and a continuous graphene sheet. All the exploited elements are modeled by circuit element while the equivalent impedance of the whole structure is calculated. The impedance matching concept is also exploited to obtain absorption power based on the calculated impedance. Additionally, full-wave simulation is performed to investigate the circuit modeling accuracy. Achieving a perfect match for absorption versus frequency verifies the developed method's robustness. Furthermore, comprehensive and ample simulation results are reported to highlight the sensitivity and reliability of the proposed gas detector.

A tunable graphene dual mode absorber for efficient terahertz radiation absorption and sensing applications

This paper presents a novel plasmonic graphene absorber designed for the terahertz frequency range, utilizing a dual mode optical filter configuration. The absorber consists of a layered periodic array comprising gold, SiO₂ and graphene. Key components include a graphene disk and four strategically positioned graphene stripes on a SiO₂ substrate. An underlying gold layer enhances the absorption efficiency by serving as a reflector. The structure is numerically simulated using the 3D finite difference time domain (FDTD) method. A comprehensive parametric study has been conducted to optimize the absorber's performance. The simulation results demonstrate near perfect absorption at 4.16 THz and 5.88 THz. This innovative design enables efficient absorption of terahertz radiation due to the plasmonic resonance effects of the graphene components. Additionally, the absorption frequency can be dynamically adjusted by altering the chemical potential of graphene through applying an external bias voltage. The tailored geometry and material composition result in a compact absorber with high absorption values and tunability. Detailed performance analysis through numerical simulations demonstrates the absorber's potential for applications in sensing, imaging, and communication systems operating in the terahertz frequency range.

CSRR and EBG loaded wideband THz dielectric resonator MIMO antenna for nano communication and bio-sensing applications

In this paper, a wideband Graphene-inspired dielectric resonator THz MIMO antenna is designed. The antenna has dimensions of 56 × 56 × 3.6 μm³ and is designed on a Rogers RO3035 substrate with a relative permittivity of 3.6 and loss tangent of 0.0015. This antenna works in the range of 6.0 THz-12.5 THz (70.76 %) with a peak gain of 7.68 dBi and radiation efficiency (>70 %) is suitable for use in medical imaging and THz wireless near-field applications. A CSRR is loaded to obtain band notch characteristic for avoiding interference between nearby wireless devices in the range of 10.6–10.8 THz. EBG was also introduced with a patch antenna to reduce surface wave loss and improve isolation, resulting in a high front-to-back ratio (FBR) in the range of 10–12.5 THz. Without the graphene disk, the silicon-based DRA did not achieve significant gain. At a chemical potential of 0.5 eV and a relaxation time of 0.1 ps, the proposed DRA demonstrated good antenna properties. By varying the chemical potential and relaxation time, frequency agility was easily achieved. A Graphene disk having a height of 3 μm is placed on a Silicon (εr = 11.1) based cylindrical DRA to provide high gain and improve the impedance bandwidth, achieving wide bandwidth from 6.0 THz to 12.5 THz. The proposed two element antenna performance is evaluated by parameters such as gain, return loss, isolation between two antenna elements, and diversity parameters like Envelope correlation coefficient ECC<0.1, Directive Gain >9.5 dB, Total Active reflection Coefficient >10 dB and Avg. Channel capacity loss <0.35bps/Hz so that the proposed antenna is suitable for wideband nano/optical communication in IoT-6G. Furthermore, the antenna is suitable for biological sensing applications due to its average sensitivity and FOM for hemoglobin and urine of 805.33GHz/RIU and 805.55GHz/RIU, respectively, and 3.37 and 10.55.

Magnetic Dipole and Magnetic Quadrupole Scattering Enhanced Graphene‐based Tunable Plasmonic Metasurface‐ Design and Sensor Applications

In this paper, we have proposed a graphene-based tunable plasmonic sensor in the far-infrared region. Our structure consists of patterned graphene disks on a SiO2 gap dielectric structure. On a SiO2 substrate, a monolayer of graphene is placed. On top of that, with a gap dielectric structure, graphene is disk patterned, forming a 2D metastructure. At the resonance wavelength where the dominating electric dipole due to plasmonic resonance is suppressed, the scattering is enhanced due to the magnetic dipole and magnetic quadrupole. The parameter tuning of the proposed device can tune this scattering. We have shown a dominating tunable magnetic quadrupolar scattering at some geometrical parameters. Moreover, graphene has a tunable chemical potential, allowing us to tune the resonance wavelength up to 30 µm. We have shown two tuning schemes to this range with the advantage of a double graphene structure. Along with the interplay of the multipole analysis of these plasmonic resonances, we have shown practical application to sensing. A chemical solvent sensor for detecting hazardous solvents like benzene and nitrobenzene has been proposed, considering the experimental dispersion of these compounds. The chemical solvent sensor has shown a maximum sensitivity of 1307.716 nm/RIU. We also have shown a potential application for high-sensitivity sensing where the refractive index changes only a fraction compared to the base parameter of the air environment, reporting a maximum sensitivity of 4830.95 nm/RIU.

Sub-Sharvin Conductance and Incoherent Shot-Noise in Graphene Disks at Magnetic Field.

Highly doped graphene samples show reduced conductance and enhanced shot-noise power compared with standard ballistic systems in two-dimensional electron gas. These features can be understood within a model that assumes incoherent scattering of Dirac electrons between two interfaces separating the sample and the leads. Here we find, by adopting the above model for the edge-free (Corbino) geometry and by computer simulation of quantum transport, that another graphene-specific feature should be observable when the current flow through a doped disk is blocked by a strong magnetic field. When the conductance drops to zero, the Fano factor approaches the value of F≈0.56, with a very weak dependence on the ratio of the disk radii. The role of finite source-drain voltages and the system behavior when the electrostatic potential barrier is tuned from a rectangular to a parabolic shape are also discussed.

Switchable THz wave absorber based on disks and its complement graphene surfaces

A new geometrical vision for a graphene-based THz wave absorber is investigated in this work. A high-performance THz absorber is proposed to exploit complementary conventional periodic arrays of graphene disks. The equivalent circuit model is modified to take account of the complement pattern. The absorber works in two different operational modes based on chemical potential values. One mode shows absorption around 1 THz while the other one expresses perfect absorption at 6 THz and 9 THz. Such a considerable shifting ability via gate stimulations makes the proposed absorber an ideal block for reconfigurable metasurface. The comparison between the circuit model and full-wave simulation verifies an excellent match while ample sensitivity analysis are reported to show the reliability of the proposed switchable THz absorber.

Switchable dual ultra-broadband perfect absorber based on graphene and vanadium dioxide hybrid metamaterials

The phase transition property of vanadium dioxide (VO2) makes it an attractive field in temperature-controlled chips. In this paper, a microstructure based on a graphene disk and a ring-shaped VO2 hybrid metamaterial is proposed to achieve switchable dual ultra-broadband perfect absorption in the terahertz region, which is analyzed by the phase transition property of VO2 and the dynamic electrical regulation of graphene. When the graphene disk is not present, the absorption intensity can reach up to 97.07%. When the graphene disk is present and respectively interacts with the VO2 metallic and insulated phases, the results exhibit an ultra-broadband perfect absorption (&gt;90%) from 1.620 THz to 4.533 THz and from 1.506 THz to 3.576 THz, respectively, where the bandwidths are as high as 2.913 THz and 2.070 THz, respectively. Adjusting the Fermi level of graphene and the VO2 conductivity allows the absorption intensity and bandwidth to be effectively controlled, where the fractional bandwidth from 81.46% to 94.69% and a high modulation depth of 95.09% can be achieved. These results suggest that dual ultra-broad perfect absorption can be dynamically switched within a single absorber and has various modulation means, which are expected to be developed in applying multifunctional modulators.

Cutting nanodisks in graphene down to 20 nm in diameter

A direct focused He+ beam direct machining is presented to fabricate solid-state nano-disk at the surface of a graphene multilayer micro-flake deposited on an Au/Ti/sapphire surface. At irradiation doses larger than 5.0 × 1017 ions cm−2 and with a beam size well below 1 nm, graphene disks down to 20 nm in diameter have been machined with for nano-disk down to 50 nm in diameter, a central hole for preparing the positioning of a rotation axle. The local heat generated by this irradiation is inducing a partial graphene amorphization and deformation, leading to a complete graphene nano-disk vaporization at doses larger than 5 × 1018 ions cm−2. A dry transfer printing technique followed by a graphene surface cleaning was used to transfer the nano-disks from its initial surface to a fresh and clean surface. Tapping mode atomic force micrograph have been recorded to follow the vaporization as a function of the He+ dose to confirm the graphene solid-state nano-disk fabrication limit to about 20 nm with this process.

Closed-form analytical model for plasmonic metasurfaces consisting of graphene disks

In this paper, a closed-form analytical model is proposed for the fundamental plasmon mode of a metasurface of graphene disks. First, we derive an equivalent admittance for the graphene metasurface at the normal incidence of a plane wave using the Rayleigh expansion. This admittance is then expressed as a series R-L-C circuit obtained by analyzing the integral equation that governs the surface current on the graphene disks. Finally, by using the equivalence substitution method, the equivalent circuit for this structure is fully analyzed. The accuracy of the model is verified by comparison with the results of full-wave simulations. Examples of applications are provided to predict tunable transmission resonances and design broadband absorbers, thus demonstrating their convenience for practical device design.

Multi-band THz plasmonic meta-surface absorber based on dual bias graphene disks: bio sensing application

Multi-band THz plasmonic meta-surface absorber based on dual bias graphene disks: bio sensing application

Adjustable graphene disk‐based THz absorber for biomedical sensing: Theoretical description

AbstractAs a basic building block, the THz wave absorber has intense imaging, sensing, and nondestructive testing applications. There are several methods for tuning THz absorbers, including electricity modulation, light modulation, mechanical tuning, using phase change materials, liquid crystal, flexible materials, MEMS technology, and thermally tuning vanadium dioxide. The choice of tuning method depends on the specific application and the desired performance characteristics of the THz absorber. In this work, we report a theoretical description of mechanically tunable THz absorber based on overlapping periodic arrays of graphene nano‐disks. The basis of this work is based on the movement of a dielectric surface covered on both sides with graphene disks. This surface moves on a fixed plane while the distance between these two surfaces is free space. Also, the fixed surface consists of a relatively thick layer of gold at the bottom, dielectric on it, and graphene disk patterns on the dielectric. Now, by moving the movable surface in the horizontal direction, it is possible to adjust the amount of absorption in different frequencies of the terahertz (THz) band. Additionally, an equivalent RLC circuit model is developed and theoretical results match with simulated data. The proposed mechanically tunable THz absorber can be exploited in various emerging applications such as sensing due to its capability of covering all of the THz gap and beyond with multiple absorption peaks.

Large Angular Momentum States in a Graphene Film

At energy lower than 2 eV, the dispersion law of the electrons in a graphene sheet presents a linear dependence of the energy on the kinetic momentum, which is typical of photons and permits the description of the electrons as massless particles by means of the Dirac equation and the study of massless particles acted upon by forces. We analytically solve the Dirac equation of an electron in a graphene disk with radius of 10,000 atomic units pierced by a magnetic field and find the eigenenergies and eigenstates of the particles for spin up and down. The magnetic field ranges within three orders of magnitude and is found to confine the electron in the disk. States with a relatively large total angular momentum exist and can be considered in a vorticose condition; these states are seen to peak at different distances from the disk centre and can be used to store few bit of information.

An ultra-thin meta-material including graphene patterns: Coupling application

Here a novel ultra-thin meta-material structure is proposed, including periodic arrays of graphene rings, disks, and ribbons and SiO2 dielectric as spacer between graphene patterns layers at the terahertz (THz) range. The introduced device can couple electromagnetic waves by considering reflection and transmission channels as outputs. Electromagnetic wave coupling depends on the parameters design and the device thickness. The proposed structure can couple electromagnetic waves in multi-band and close frequencies including 2 THz, 4 THz, 6 THz, 7.5 THz, and 9.5 THz. By considering the impedance matching concept, an equivalent circuit model (ECM) is developed for the proposed meta-material. Also, the device stability is investigated in various physical coefficients, geometrical parameters, and incident wave angles to ensure optical applications such as sensors, indoor communications, security, and medical imaging.

Tuning of magnetosplamon coupling between graphene scatterers for the optimal design of adjustable metasurfaces

The resonance characteristics of magnetically-biased graphene micro-scatterers are thoroughly investigated in the present work using both eigenvalue and full-wave solvers. Initially, the graphene surface conductivity is presented in a tensor form due to the application of a magnetostatic bias field, which is perpendicular to the material’s surface. Then, the simple case of a graphene disk scatterer is examined, and a properly modified eigenvalue formulation is utilized to extract the plasmonic fundamental frequencies. The validity of the modal analysis is verified via a full-wave analysis that involves a plane-wave propagation and the extraction of the subsequent absorption cross-section utilizing the Finite-Difference Time-Domain method. Additionally, the dependence of a single disk scatterer resonances with the magnetostatic bias is evaluated, highlighting that as the bias field is increased, every edge mode degenerates into two sub-modes with an augmented difference between the resonant frequencies. Finally, the plasmonic coupling between adjacent scatterers is studied considering a periodic arrangement, similar to a metasurface, indicating the additional coupling modes as well as the adjustability of the properties with multiple degrees of freedom.

CPA-induced Hz to THz broadband absorber with switchable perfect absorption between radio-microwave and THz frequency spectrum

The coherent perfect absorption (CPA) occurring in the graphene sheet suspended in air can be utilized to develop an ultrathin, ultra-broadband absorber working in the frequency range from a few hertz (Hz) to terahertz (THz) with perfect absorption. A graphene sheet is studied to induce the CPA to cover radio, microwave and lower THz frequency ranges. A graphene resonator able to provide the surface plasmon resonance (SPR) is combined with the graphene sheet to provide CPA at either side of a thin dielectric layer forms metamaterial structure with the cavity and enhances the absorption bandwidth in the THz region by creating a resonance near quasi-CPA frequency. A dielectric silicon resonator is embedded in the structure, which creates dipolar resonances between the resonances obtained by the formed cavity between the graphene sheet and resonator. This enhances the absorption level in the THz region. The absorption bandwidth is further enhanced to 7 THz by including a graphene disc at the top of the silicon resonator. Thus, the multiple multi-order resonances occurring in the silicon dielectric and SPR of graphene resonators are merged with the phenomena of CPA occurring in the graphene sheets to extend the CPA bandwidth in the THz regime. The doping level of graphene or its tunable Fermi energy based on the applied DC electric field provides the tunability in the total obtained absorption bandwidth. The symmetric structure provides polarization-insensitive behavior with an allowed incident angle of more than 45° with more than 90% absorption.

Nested graphene disks patterned THz wave absorber: Bio sensing vision

A three-layer adjustable THz wave absorber is introduced in this paper. The structure's geometry is heavily based on a combination of periodic graphene disks and their complementary patterns. Also, the middle layer is considered an air gap while the graphene patterns are stacked into TOPAS filler. Additionally, a golden back reflector is placed at the bottom of the absorber to ensure a blocked transmission channel. This configuration is modeled exclusively by passive circuit elements, while full wave simulation is also performed. Also, a simple genetic-based algorithm is exploited to optimize design parameters in numerical values, including geometrical and stimulation values. In addition, a mechanical tuning setup is suggested that can interestingly change absorption peaks. The frequency shift of absorption peaks can be up to 5 THz, which makes the proposed absorber an ideal basic building block for several applications ranging from sensing to indoor communication. Furthermore, the discussion section explains the potential applications of the proposed absorber for bio-sensing by filling the air gap with biological samples.

Analytical modeling of terahertz graphene metasurfaces

Control over the optical response of graphene metasurfaces enables many promising applications in terahertz devices, including beam filters, polarizers, and absorbers. In this paper, we provide an analytical theory that enables us to use equivalent circuit models to simulate these responses. Closed-form expressions are provided for their associated scattering spectra, including an infinity of parallel R-L-C circuits for each metasurface of arbitrary morphology. These models’ accuracy is verified by comparing their results with those that are derived from full-wave simulations. Moreover, applications of these models are presented to anticipate both tunable optical response in interacting graphene disks and refractive index sensing via the response of graphene squares to ambient media. These models open up a new way of controlling the structure’s performance and refining it to meet our application-specific requirements.

Meta-surface filter for visible frequency range based on meta-materials

To enhance the efficiency of exposure in greenhouses during specific cultivation periods, it is essential to design a meta-face that effectively filters the green part of visible light. This targeted filtering function will enable optimal control of the light spectrum, resulting in better cultivation conditions and increased productivity. Leveraging innovative concepts and advanced methods, a highly efficient meta-surface design aimed at filtering the green portion of the visible light spectrum is proposed. The proposed structure comprises periodic arrays of graphene disks and rings strategically positioned on both sides of a silicon oxide substrate. This straightforward coated layer configuration offers a practical solution for greenhouses and controlled agriculture applications, facilitating improved light management and tailored growth conditions. Through two separate simulation paths, the validity and accuracy of our proposed approach were investigated. Both theoretical analysis and simulation results demonstrate that the proposed structure attenuates the green part of visible light. Filtered output waves prove to be highly beneficial for indoor cultivation, during the flowering period, offering improved control over light conditions. The design methodology relies on an equivalent circuit model and impedance matching criteria. Additionally, full-wave simulation is performed to verify the effectiveness of the employed modeling. According to the simulation results, the proposed meta-surface effectively filters the green part of visible light, while allowing the transmission of the red spectrum.

Design of terahertz metamaterial absorbers with switchable absorption functions utilizing thermal and electrical dual-modulation strategies.

This work demonstrates a dual-functional tunable terahertz metamaterial absorber based on thermally controllable vanadium dioxide (VO2) and electrically tunable graphene. The switchable absorption functions could be obtained in the same metamaterial, which consists of alternating stacked cross-cut graphene disks (CGDs) and VO2 square rings (VSRs) separated by an ultra-thin dielectric film placed on a continuous gold mirror. The metallic state of VSRs is the dominant factor for the broadband absorption function, resulting in a broadband absorption of 4.746 THz. Based on this, the Fermi energy level of CGDs increases to 0.7 eV, which could broaden the absorption bandwidth to 5.398 THz. When the VSRs are in the insulating state, CGDs dominate the absorption, and the suggested device switches to the dual-band absorption function. These two absorption peaks appear to be larger than 97% and their frequencies could be dynamically controlled by the Fermi energy level of CGDs. In addition to the excellent absorption characteristics of dynamic switching of two different functions, polarization insensitivity and large-angle tolerance are also advantages of this work, which could provide new insights and guidance for the study of dynamically tunable metamaterial absorbers.