Graphene Ribbon Arrays Research Articles

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).

Reconfigurable multifrequency and wide-angle directional beaming of light from a subwavelength metal slit with graphene metasurfaces.

Directional beaming of light from a subwavelength slit surrounded with phase-gradient metasurfaces can be realized with very high efficiencies, but the available optical proposals suffer from narrow bandwidth after the fabrication process and cannot be reconfigured dynamically with switchable frequencies. Here we demonstrate reconfigurable multifrequency directional beaming by placing a graphene ribbon array at two sides of a subwavelength metallic slit. Each graphene ribbon works with the metallic film to fulfill the magnetic response, and the resultant reflection phase can cover full 0-2π range by changing the chemical potential of graphene. The on-axis and off-axis directional beams can be formed by properly arranging the chemical potentials of graphene ribbons to serve as meta-couplers between surface waves and propagating waves. Thanks to the negligible interaction between vertically stacked graphene sheets, the beaming frequencies and angles can be broadened by use of double layers of the graphene ribbon array.

Roles of structural and chemical defects in graphene on quenching of nearby fluorophores

Oxidation scanning probe lithography was used to systematically fabricate graphene/defective graphene ribbon arrays on a graphene sheet. The tribological, structural, and chemical natures of the defects created by o-SPL were characterized using lateral force microscopy (LFM), micro-Raman (μ-RS) and micro-X-ray spectroscopy (μ-XPS), respectively. While LFM revealed all fabricated patterns exhibited increase in friction, μ-RS and μ-XPS analysis showed that a transition of defect type from structural to chemical defects occurred in the defective graphene ribbons at a threshold voltage (8.5 V). Subsequently, micro-photoluminescence (μ-PL) quenching spectroscopy was measured for a thin layer of polystyrene containing isolated fluorophores (MEH-PPV) spin cast on the ribbon patterns. Whilst the quenching efficiency was suppressed more significantly as defect concentration increased, for pure structural defects, the μ-PL lifetime varied linearly with defect concentration and then saturated to a constant as chemical defects dominated. Among all oxygen related functional groups, the presence of CO bonds led to greatly reduced quenching efficiency while simultaneously reducing the strong blue spectral shift seen for pure graphene with the spectrum at higher CO bond concentration approaching to pristine MEH-PPV. This work demonstrated that local chemical and structural composition in 2D quencher strongly affects the quenching efficiency and dynamics.

Tunable Graphene Split-Ring Resonators

A split-ring resonator is a prototype of meta-atom in metamaterials. Though noble metal-based split-ring resonators have been extensively studied, up to date, there is no experimental demonstration of split-ring resonators made from graphene, an emerging intriguing plasmonic material. Here, we experimentally demonstrate graphene split-ring resonators with deep subwavelength (about one hundredth of the excitation wavelength) magnetic dipole response in the terahertz regime. Meanwhile, the quadrupole and electric dipole are observed,depending on the incident light polarization. All modes can be tuned via chemical doping or stacking multiple graphene layers. The strong interaction with surface polar phonons of the SiO2 substrate also significantly modifies the response. Finite-element frequency domain simulations nicely reproduce experimental results. Our study moves one stride forward toward the multi-functional graphene metamaterials, beyond simple graphene ribbon or disk arrays with electrical dipole resonances only.

Tunable Terahertz Graphene-Based Absorber Design Method Based on a Circuit Model Approach

In this paper, a simple, fast, and novel method for designing a tunable terahertz absorber with arbitrary central frequency and desired fractional bandwidth is presented. The proposed absorber consists of a single layer periodic array of graphene ribbons (PAGRs), placed a quarter wavelength from a metallic ground, separated by a dielectric material. An analytical circuit model of the terahertz absorber is used to obtain analytical expressions for the input impedance of the proposed device. Then, a simple expression for determining the value of capacitance and the resonance conditions of the RLC circuit is used to achieve a terahertz absorber with arbitrary central frequency and desired fractional bandwidth. The proposed method is applicable for the design of both narrowband and broadband absorbers, with only one layer of graphene ribbons. Also, the presented method is applicable for designing ultra-wideband absorbers using multiple layered PAGRs. Full-wave numerical simulation is performed to verify the accuracy and validity of the presented method. Excellent performance of the proposed method in terms of computation time and memory resource and providing the desired terahertz absorber characteristics shows that our method is promising as a design approach for sensing, imaging and filtering applications.

Strong circular dichroism enhancement by plasmonic coupling between graphene and h-shaped chiral nanostructure.

Circular dichroism (CD) is useful in polarization conversion, negative refraction chemical analysis, and bio-sensing. To achieve strong CD signals, researchers constantly break the symmetry of nanostructures. However, how to further enhance the CD based on a new mechanism has become a new challenge in this field. In this work, a hybrid plasmonic chiral system composed of an array of graphene ribbons (GRs) over h-shaped sliver chiral nanostructures (HSCNs) is theoretically investigated. Results demonstrate that the plasmonic coupling between HSCNs and GRs results in different enhanced absorptions for different circularly polarized lights. The absorbance of right circularly polarized light is enhanced to perfect absorption; the absorption of left circularly polarized light is enhanced weakly. It leads to the CD effect of HSCNs@GRs approaching 88%. The loss distributions of HSCNs and HSCNs@GRs reveal that the absorption is enhanced and transferred from HSCNs to GRs. Moreover, the current distributions of HSCNs@GRs are simplified to equivalent LC resonant circuits, which can qualitatively explain the change of CD signals by tuning geometrical parameters of HSCNs@GRs. The findings of this work provide a new method of enhancing chirality and benefit the design of graphene-based chiral optoelectronic devices.

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.

Analytical and rigorous method for analysis of an array of magnetically-biased graphene ribbons

A sheet of graphene under magnetic bias attains anisotropic surface conductivity, opening the door for realizing compact devices such as Faraday rotators, isolators and circulators. In this paper, an accurate and analytical method is proposed for a periodic array of graphene ribbons under magnetic bias. The method is based on integral equations governing the induced surface currents on the coplanar array of graphene ribbons. For subwavelength size ribbons subjected to an incident plane wave, the current distribution is derived leading to analytical expressions for the reflection/transmission coefficients. The results obtained are in excellent agreement with full-wave simulations and predict resonant spectral effects that cannot be accounted for by existing semi-analytical methods. Finally, we extract an analytical, closed form solution for the Faraday rotation of magnetically-biased graphene ribbons. In contrast to previous studies, this paper presents a fast, precise and reliable technique for analyzing magnetically-biased array of graphene ribbons, which are one of the most popular graphene-based structures.

Terahertz Quarter Wave-Plate Metasurface Polarizer Based on Arrays of Graphene Ribbons

We propose a novel graphene-dielectric-based metasurface for manipulating the polarization of the incident light in the terahertz regime. The proposed structure comprised two orthogonally oriented periodic array of graphene ribbons (PAGRs) which are separated by a dielectric spacer and deposited on an Au-backed dielectric substrate. Based on the transmission line theory, an equivalent model with excellent accuracy is suggested for the proposed structure. By leveraging the simplicity of the model, we design a three-state quarter wave plate that is able to dynamically switch the polarization of the reflected wave to linear, right-, and left-hand polarizations while keeping the reflected amplitude remarkably high. Thanks to the simplicity of the model, the proposed structure can be easily used for designing other metasurfaces for light propagation control applications in imaging, sensing, and telecommunication.

High-efficiency tunable plasmonically induced transparency-like effect in metasurfaces composed of graphene nano-rings and ribbon arrays and its application.

In this paper, a plasmonically induced transparency (PIT)-like phenomenon in a metasurface composed of a periodic graphene ring and ribbon arrays is studied in the terahertz region. We used the Lorentz oscillator model to analyze the metasurface physically and theoretically. This PIT-like effect can be tuned by alternation of the chemical potential and dimension of the nano-graphene ring and ribbon. The resonance frequency of the PIT-like phenomenon is not sensitive to the incident lightwave angle. As an application of the structure, a refractive index sensor is proposed and simulated. Furthermore, we propose a metasurface composed of a double ring and graphene ribbon to realize the PIT-like effect with three dips. Our results express an appropriate approach for the expansion of mid-infrared absorbers and sensors.

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.

Efficient terahertz transmission modulation in plasmonic metallic slits by a graphene ribbon array.

Extraordinary optical transmission is the widely known phenomenon of enhanced transmission of waves through subwavelength periodic metallic apertures. Here, we propose efficient terahertz transmission modulation in subwavelength metallic slits by a graphene plasmonic ribbon array. The extraordinary optical transmission through the subwavelength metallic slits can be tuned by coupling with the plasmonic resonance of graphene ribbon array, resulting in a deep transmittance depression. Numerical simulations show that maximal transmission modulation depth of 98.99% can be obtained at 1.03THz via this mechanism.

Tunable dual-band perfect metamaterial absorber based on a graphene-SiC hybrid system by multiple resonance modes

We propose dynamically tunable dual-band perfect absorption of light in a composite structure, which consists of a graphene ribbon array coated on a dielectric layer, a silicon carbide (SiC) film and a distributed Bragg reflector (DBR). Two absorption peaks reaching 98.8% at 10.87 μm and 97.5% at 12.4 µm are observed in the hybrid structure. The dual-band absorption enhancement can be explained by two different mechanisms: one is attributed to the graphene localized surface plasmons (GLSP), and the other is originated from the excitation of Tamm plasmon polaritons (TPP) at the interface between SiC and DBR. Moreover, the high absorption peaks can be modulated by altering the geometrical parameters or dynamically controlling the chemical potential of graphene ribbons. More significantly, we numerically and theoretically investigate the hybridization between GLSP mode and TPP mode, which is characterized by the anti-crossing behaviour of the two modes. The mode splitting phenomenon is a typical Rabi splitting. In addition, we also study the coupling characteristics based on graphene surface plasmons (GSP) mode and TPP mode. This work offers a new paradigm for enhancing light–matter interaction through multiple resonance modes, and the proposed device could provide potential applications in MIR plasmonic devices, such as absorbers, detectors, and modulators, etc.

Frequency-Reconfigurable Wide-Angle Terahertz Absorbers Using Single- and Double-Layer Decussate Graphene Ribbon Arrays.

We propose and numerically demonstrate two novel terahertz absorbers made up of periodic single- and double-layer decussate graphene ribbon arrays. The simulated results show that the proposed absorbers have narrowband near-unity terahertz absorption with ultra-wide frequency reconfiguration and angular stability. By tuning the Fermi level of graphene ribbons, the over 90% absorbance peak frequency of the absorber with single-layer graphene structure can be flexibly adjusted from 6.85 to 9.85 THz for both the transverse magnetic (TM) and transverse electric (TE) polarizations. This absorber with single-layer graphene demonstrates excellent angular stability with the absorbance peaks of the reconfigurable absorption bands remaining over 99.8% in a wide angle of incidence ranging from 0 to 70°. The tuning frequency can be significantly enhanced by using the absorber with double-layer graphene structure from 5.50 to 11.28 THz and 5.62 to 10.65 THz, approaching two octaves under TM and TE polarizations, respectively. The absorbance peaks of the reconfigurable absorption band of this absorber for both polarizations maintain over 70%, even at a large angle of incidence up to 70°. Furthermore, an analytical fitting model is also proposed to accurately predict the absorbance peak frequencies for this variety of absorbers. Benefitting from these attractive properties, the proposed absorber may have great potential applications in tunable terahertz trapping, detecting, sensing, and various terahertz optoelectronic devices.

Investigation of multiband plasmonic metamaterial perfect absorbers based on graphene ribbons by the phase-coupled method

We develop an original phase-coupled method to realize multispectral metamaterial near-unity absorbers based on spatially separated graphene ribbon arrays with mid-infrared plasmonic resonances. The results both from the coupled-mode theory and finite-difference time-domain simulations reveal that in addition to the single-band absorption enabled by the bi-layer identical ribbon arrays, the outstanding dual-band perfect absorption is observed with the change in the phase between bi-layer ribbons only by varying the spacer thickness. The spectral positions of absorption peaks are tuned handily by small changes in ribbon widths and chemical potentials of graphene. Moreover, the triple-band absorber is achieved handily by the same principle and such absorbers are robust for nor-normal incident angles. The transfer matrix method is also utilized to uncover further the underlying physics of the phased-coupled-induced multispectral absorbers. Theoretical analysis are in excellent agreement with numerical calculations. The phase-coupled method thus provides new opportunities for obtaining multi-channel metamaterial perfect absorbers.

A tunable dual-band THz absorber based on graphene sheet and ribbons

A tunable terahertz dual band absorber consisted of graphene ribbon arrays and a graphene sheet with near-unity absorption is proposed and studied in this letter. With the coupling enhancement of graphene ribbon and graphene sheet, a perfect dual band absorber with absorption over 99% at 3.67 THz and 5.88 THz is achieved. Moreover, the principal of the coupling enhancement is analyzed. By optimizing the parameters of the structure, not only the absorption can be enhanced, but also the center of absorbing frequency could be shifted. Another approach to adjust the absorber–tuning the chemical potential of graphene which is more convenient and timely is investigated. Furthermore, the sensing of the refractive index is emulated by changing the refractive index of the medium on the top. Results show a broad application prospect of the absorber proposed in this paper.

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.

Low-loss forward and backward surface plasmons in a semiconductor nanowire coated by helical graphene strips

In the long-wavelength approximation, the effective conductivity tensor is introduced for graphene ribbons (strips) placed periodically at the interface between two media. The resulting conducting surface is considered as a coating for semiconductor nanowire. For the hybrid waves of such nanowire the dispersion equations are obtained in explicit form. Two types of surface plasmons are found to exist: (i) the modified surface plasmons, which originate from the ordinary surface plasmons of a graphene-coated semiconductor nanowire, and (ii) the spoof plasmons, which arise on the array of graphene ribbons and may possess forward-wave and backward-wave dispersion. It is revealed that the spoof surface plasmons are low-loss ones, and their frequencies, field-confinement and group velocities can be tuned widely by adjusting the coil angle and width of helical graphene strips.

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.