Periodic Arrays Of Graphene Ribbons Research Articles

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.

Graphene-based multi-layers THz absorber: Circuit model representation

Leveraging both concept and method, a unique THz wave absorber is presented. Three layers include dielectric and graphene patterns that are exploited on top of a thick golden surface to ensure zero THz waves transmitting via the structure. Auspicious dual bias scheme is used for periodic arrays of graphene ribbons in the first layer while the second layer consists of periodic arrays of graphene disk. Also, the top layer is a continuous graphene sheet to enhance the whole absorption. All involved components are modeled via the developed circuit model approach as passive circuit elements. This leads to an exclusive impedance calculation of the structure, paves the way to the fully predictable behavior of the device. Thanks to impedance matching theory, the structure parameters are tuned to obtain perfect dual-band absorption at 5.5THz and 8.5THz frequencies. Ample simulations verify the robustness of the structure versus geometrical parameters while a great capability of tuning absorption peaks is achieved via chemical potentials variation. Such a robust and tunable absorber is attractive for designing optical sensors, detectors, and modulators.

Graphene-based terahertz metamaterial absorber for broadband applications

Terahertz (THz) metamaterial absorbers are promised as one of the appropriate candidates for the biosensing and imaging novel devices. In this work, a wideband THz absorber consists of two periodic arrays of graphene ribbons (PAGRs) and one graphene continuous sheet (GCS) in a form of Fabry-Perot resonator is presented. Analytical circuit model of PAGRs and GCS in addition to a developed transmission line theory is exploited to model the proposed structure as RLC circuits. The concept of impedance matching with the aid of circuit principles besides a genetic algorithm (GA) are implemented to reduce the reflected waves from the structure. As a result, a THz absorber with absorption over 98% in the frequency range of 6.45–7.35 THz is achieved. The use of GCS as a coupling element to boost the absorption bandwidth is investigated. The results of this work is in a high demand for the THz sensing and imaging applications and it can be fabricated through chemical vapor deposition (CVD).

A tunable ultra-broadband terahertz absorber based on two layers of graphene ribbons

In this paper, a novel configuration of a tunable ultra-broadband terahertz (THz) absorber based on two layers of graphene ribbons and two different kinds of substrate materials has been proposed. The recently suggested equivalent circuit theory of Periodic Arrays of Graphene Ribbons (PAGRs) with a developed transmission line model is utilized to design the structure of the THz absorber. Also, an equivalent series RLC branch is considered for each layer of PAGRs. By adopting the impedance matching approach, the real part of the input impedance of the proposed structure is tuned to be approximately matched to the free-space impedance in a wideband THz region. Then, the imaginary part of the input impedance is adjusted to near zero around the central frequency for increasing the proposed absorber's bandwidth. As a result, the normalized bandwidth of 70% absorption has reached up to 118% with only just two layers of graphene ribbons. Based on the purpose of this paper, the bandwidth of the proposed absorber has been controlled by the chemical potentials of the graphene ribbons.

Ultra-wideband terahertz graphene absorber using circuit model

In this work, a novel solution for designing an ultra-wideband terahertz graphene-based absorber by exploiting the periodic arrays of graphene ribbons (PAGRs) and graphene continuous sheet (GCS) is presented. Here we develop and extend a transmission line method to describe the absorber structure as circuit elements. Regarding impedance-matching concept and using analytical circuit model of PAGRs and GCS which recently have been proposed, different PAGRs have been used to gain ultra-wide bandwidth. As a result, a bandwidth of 90% absorption is extended up to 130% and 146% of the central frequency with three and four layers of graphene, respectively. Excellent performance of the proposed method regarding computation time (showing more than five orders of magnitude reduction in runtime) and memory sources besides producing results with an acceptable agreement with the results of full-wave simulations (with an error less than 5%), nominates it as a promising approach to design and simulate other sub-wavelength graphene-based devices and; moreover, since the proposed structure is a multi-layer one, it can be manufactured by chemical vapor deposition.

Multi-band circuit model of tunable THz absorber based on graphene sheet and ribbons

Terahertz (THz) absorbers due to the high potential of providing reliable applications in modern devices and technology are increasingly being investigated. Recently, a dual-band THz absorber based on a graphene sheet and ribbons has been proposed in which the results were obtained using Full-wave simulations and its authors have not provided any specific design method. In this work, we have proposed and investigated that tunable THz absorber by its circuit model. A developed transmission line method beside the analytical circuit model of graphene continues sheet and periodic arrays of graphene ribbons are used to obtain analytical expressions for the circuit model. Here, first of all, we have compared the results of the proposed method with the results of that paper and secondly, using impedance matching concept we have proposed multi-band tunable THz absorber with near-unity absorption by the same structure. Then, we have compared the results of our circuit method with the Full-wave simulations. In either case, our proposed method in addition to excellent performance in terms of runtime and memory sources, has an acceptable agreement with the results of the Full-wave simulations. The proposed method is general and can be applied to design and simulate the other sub-wavelength structures.

Ultra-broadband terahertz absorber based on graphene ribbons

Terahertz (THz) absorbers have been verified to be a promising candidate for the biosensing and imaging modern devices. In this paper, an ultra-broadband THz absorber based on periodic arrays of graphene ribbons (PAGRs) is presented. The recently proposed analytical circuit model of PAGRs besides a developed transmission line theory is used to describe the absorber structure as circuit elements. Using circuit theory and impedance matching concept, the input impedance of the structure is adjusted to be closely matched to the free space in a wide frequency range. As a result, the bandwidth of 90% absorption extended up to 100% of central frequency. The results show that the Full-wave simulations (while using Finite Element Method) and circuit model approach verify each other. The proposed structure and its performance can be a useful structure and be utilized in filters and sensors.

Study of backward waves in multilayered structures composed of graphene micro-ribbons

In this paper, the multilayered structure composed of periodic arrays of graphene ribbons separated by the dielectric slabs is investigated for both the transverse electric (TE) and transverse magnetic (TM) polarizations. The transmission spectra show a stopband region between the low-THz frequency passband region and the higher-frequency one for TM polarization due to dual capacitive-inductive nature of the surface impedance corresponding to arrays of graphene ribbons. For TE polarization, the surface impedance is pure inductive; therefore, a passband region can be seen after the low-THz frequency stopband region. The in-plane permittivities are calculated based on the scattering parameters retrieval method. The negative permittivities are obtained in stopband regions corresponding to Bloch evanescent modes. This structure allows the propagation of backward waves in negative permittivity regime for TM polarization.

Ultra-Sharp Transmission Resonances in Periodic Arrays of Graphene Ribbons in TE Polarization

When illuminated by TM polarized waves, periodic arrays of graphene ribbons are known to exhibit plasmonic resonances due to their dual inductive-capacitive nature. It is demonstrated here that even in TE polarization, resonances can be observed in these structure. These resonances, which are of nonplasmonic origin, are explained by means of a circuit model. It is shown that, for a certain frequency range, arrays of graphene ribbons have both capacitive and inductive properties, which lead to an ultra-sharp inductor-capacitor resonance. The banw idth of this resonance can be as narrow as ~0.0002 nm at a wavelength of 630 nm. The resonance can also be viewed as the grating excitation of a TE mode on graphene. The resonance properties in terms of different parameters are also invedtigated.

Design of ultra-broadband graphene absorber using circuit theory

I propose a novel method for designing a broadband THz absorber by using periodic arrays of graphene ribbons on a Salisbury-screen-like structure. The recently proposed analytical circuit model of graphene arrays is used for obtaining analytical expressions for the input admittance of the proposed device. The input admittance is then adjusted to be closely matched to the free space in a wide frequency range. Consequently, it is demonstrated that a bandwidth of 90% absorption can be extended up to 100% of the central frequency with only one layer of patterned graphene.

Broadband single-layered graphene absorber using periodic arrays of graphene ribbons with gradient width

We demonstrate that a broadband single-layered graphene absorber can be obtained in the THz range using periodic arrays of chemically doped graphene ribbons with gradient width, supported on a dielectric film placed on a thick piece of metal. The working bandwidth of 90% absorption for this structure is as high as 1.3 THz with a central frequency of 3 THz. The broadband operation mechanism is a result of the varying continuous plasmon resonances occurring in graphene ribbons with gradient width. The operation wavelength can be expanded to the far-infrared range.

Electrically controlling the polarizing direction of a graphene polarizer

We theoretically demonstrate a polarizer with an electrically controllable polarizing direction in the far infrared range using two orthogonal periodic arrays of graphene ribbons, which have different widths and are supported on a dielectric film placed on a thick piece of metal. The operation mechanism originates from the polarization-dependent resonant absorption of the two orthogonal graphene ribbons, which can be respectively controlled with different external bias voltages. The operation wavelength can be expanded to terahertz (THz) radiation.

Fourier modal method formulation for fast analysis of two-dimensional periodic arrays of graphene

Recently, an approximate boundary condition [Opt. Lett.38, 3009 (2013)] was proposed for fast analysis of one-dimensional periodic arrays of graphene ribbons by using the Fourier modal method (FMM). Correct factorization rules are applicable to this approximate boundary condition where graphene is modeled as surface conductivity. We extend this approach to obtain the optical properties of two-dimensional periodic arrays of graphene. In this work, optical absorption of graphene squares in a checkerboard pattern and graphene nanodisks in a hexagonal lattice are calculated by the proposed formalism. The achieved results are compared with the conventional FMM, in which graphene is modeled as a finite thickness dielectric layer. We show that for the same truncation order, computation time can be reduced to one-ninth by the proposed formulation in comparison with the conventional FMM. Furthermore, the convergence rate is increased. Therefore, thanks to the improved convergence rate and reduced computational cost for a given truncation order, the computational time is saved more than 100 times for relative error of less than 1%. This is crucially important in analyzing two-dimensional periodic structures of graphene by the FMM.

Fast convergent Fourier modal method for the analysis of periodic arrays of graphene ribbons

Li's Fourier factorization rules [J. Opt. Soc. Am. A13, 1870 (1996)] should be applied to achieve a fast convergence rate in the analysis of diffraction gratings with the Fourier modal method. I show, however, that Li's inverse rule cannot be applied for periodic patterns of graphene when the conventional boundary condition is used. I derive an approximate boundary condition in which a nonzero but sufficiently small height is assumed for the boundary. The proposed boundary condition enables us to apply the inverse rule, leading to a significantly improved convergence rate. A periodic array of graphene ribbons is in fact a special type of finite-conductivity strip grating, and thus the proposed approach is also applicable to these kinds of structures.

Surface plasmon enhanced absorption and suppressed transmission in periodic arrays of graphene ribbons

Resonance diffraction in the periodic array of graphene microribbons is theoretically studied following a recent experiment [L. Ju et al., Nature Nanotech. 6, 630 (2011)]. Systematic studies over a wide range of parameters are presented. It is shown that a much richer resonant picture would be observable for higher relaxation times of charge carriers: More resonances appear and transmission can be totally suppressed. The comparison with the absorption cross-section of a single ribbon shows that the resonant features of the periodic array are associated with leaky plasmonic modes. The longest-wavelength resonance provides the highest visibility of the transmission dip and has the strongest spectral shift and broadening with respect to the single-ribbon resonance, due to collective effects.