Graphene Metamaterials Research Articles

Tunable broadband metamaterial absorber with single-layered graphene arrays of rings and discs in terahertz range

A tunable single-layered broadband metamaterial graphene absorber in the THz range is investigated in this paper. The absorber is made up of the ring and disc of the graphene, a metallic ground plane bottom layer and a polyimide spacer layer. The simulated results show that the relative bandwidth of 90% absorbance is 81.44% from 1.15 to 2.73 THz under the normal incident angle. In addition, the absorber is insensitive to the polarization and has a wide range of the incidence for the TE mode and TM mode. Moreover, the absorbance of the absorber can be flexibly changed from 0 to 1 by controlling the Fermi energy from 0 to 1.0 eV.

Thermal Modulation of Plasmon Induced Transparency in Graphene Metamaterial

Thermal Modulation of Plasmon Induced Transparency in Graphene Metamaterial

石墨烯超材料微结构材质的制备及其在吸波涂层改进设计上的应用

石墨烯超材料微结构材质的制备及其在吸波涂层改进设计上的应用

Dynamically Tunable Fano Resonance Based on Graphene Metamaterials

We introduce a remarkably simple coplanar nanostructure consisting of two longer and one shorter graphene nanorods to support a Fano resonance, which arises from the coupling between bright mode and dark mode by breaking the geometrical symmetry. Simulation results, performed with the finite-difference time-domain method, show that the resonant frequency of the bright mode in the transmission spectrum can be independently tuned by changing the lateral offset of the shorter graphene nanorod, while changing its longitudinal offset can independently adjust the resonant frequency of the dark mode. Furthermore, Fano resonance spectrum can also be regulated actively by altering the Fermi energy of graphene as well as refractive index of the surrounding media. This letter proposes a unique modulation strategy of Fano resonance in metamaterials, assisting in developing potential in applications of sensor devices and some other plasmonic functional devices.

Flexible properties of THz graphene bowtie metamaterials structures

Tunable propagation properties of graphene asymmetrical bowtie metamaterials (MMs) structures have been investigated in the terahertz regime, including the effects of graphene Fermi levels, structural parameters and operation frequencies. Because of its thin film thickness, the strong resonant curves of the proposed graphene MMs structures are dominated by the plasmonic mode instead of the Fabry-Perot mode for the metal structures. Compared with existing tunable graphene devices, the sharp Fano resonant curve manifests a large Q-factor of more than 40. In addition, as the width of graphene bowtie aperture increases, the resonant frequency dip shifts low frequency, and the resonant amplitude and figure of merit increase. The results are very helpful in order to understand the tunable mechanisms of graphene components and design high sensitivity functional devices, sensors, modulators, and antenna.

Hybridization-induced dual-band tunable graphene metamaterials for sensing

The infrared absorption spectrum due to structural vibrations in molecules has been widely used to resolve chemical identification. However, this method is limited by the weak molecule-light interaction. Graphene plasmon, having strong confinement and large field enhancement, provides a promising way to increase their interactions. Here we propose tunable hybridization-induced graphene nanostructures for sensing application. Our results reveal that when the symmetry of the graphene disk is broken by introducing a small circular defect, dual-band resonance can be excited at mid-infrared frequencies. The dual-band resonance peaks are produced by plasmon hybridization effect between two simple structures: graphene disks and circular defects. We investigate the influence of the size and position of the circular defect on the dual-band resonance peaks. We further explore the dual-band resonance peaks for sensing, and the sensitivity can reach 550 cm−1/RIU and the FOM can reach 20.4. Compared to the original graphene disk structure, the results show stronger resonance intensity and higher frequency sensitivity. Our findings provide a new platform for sensing, and moreover the dual-band resonance structure feature enables us to promote the development of multi-substance detection.

Dual broadband absorber based on graphene metamaterial in the terahertz range

In this paper, a dual broadband terahertz (THz) graphene metamaterial absorber (MA) is presented. The absorber consists of patterned graphene, a gating layer, and gold ground separated by a SiO2 substrate. Multiple resonances and broadband absorption are obtained simultaneously with a simple asymmetric electrical split ring structure. The numerical calculations indicate that there are two broad absorption bands (with an absorptance larger than 95%) from 1.4 THz to 1.9 THz and 4.5 THz to 5.1 THz, respectively. Under the regulation of the graphene chemical potential, the absorption strength can be controlled electrically from 95% to 10%. Furthermore, it possesses a good tolerance of polarization and incidence angles. The design scheme provides a new perspective to design dual broadband absorbers and is scalable to develop various graphene absorbers at other frequencies.

Design of a tunable dual-band terahertz absorber based on graphene metamaterial

Based on the tunable electrical and optical properties of graphene, we propose a tunable dual-band graphene-based metamaterial terahertz (THz) absorber. It successively consists of two layers of graphene-ring arrays and a metal ground plate separated by two dielectric layers. Simulation results indicate that either of the absorber’s two absorption bands can be tuned over a broad frequency range while the other absorption band remains unchanged. The designed tunable structure can readily achieve perfect absorption at nearly any two frequency bands dynamically in the THz regime with wide incidence angle and polarization independence. It can find potential applications in photodetectors, solar cells, coherent thermal emitters, and microbolometers.

Tunable plasmon-induced transparency based on asymmetric H-shaped graphene metamaterials**Project supported by the Key Science and Technology Research Project of Henan Province, China (Grant Nos. 162102210164 and 1721023100107) and the Natural Science Foundation of Henan Educational Committee, China (Grant No. 17A140002).

We propose and numerically demonstrate a tunable plasmon-induced transparency (PIT) phenomenon based on asymmetric H-shaped graphene metamaterials. The tunable PIT effect is realized through varying the applied polarization angles rather than changing the structure geometry. By simply adjusting the polarization angle, the transmission spectra can be controlled between the switch-on state and switch-off state. The physical mechanism of the induced transparency is revealed from magnetic dipole inductive coupling and phase coupling. Importantly, by varying the Fermi energy of the graphene or the refractive index of the substrate, the resonant position of the PIT can be dynamically controlled and the maximum modulation depths can reach up to 60.7%. The sensitivity (nm/RIU) of the graphene structure, which is the shift of resonance wavelength per unit change of refractive index, is 5619.56 nm/RIU. Moreover, we also extend our research to the x-axis symmetric H-shaped structure, and the tunable PIT transmission window can also be realized. The physical mechanism of the induced transparency is revealed from the electric dipole hybridization coupling. Our designed H-shaped graphene-based structures is a promising candidate for compact elements such as tunable sensors, switches and slow-light devices.

Analogue of dual-controlled electromagnetically induced transparency based on a graphene metamaterial

Analogue of dual-controlled electromagnetically induced transparency (EIT) based on graphene metamaterial is proposed and investigated. The structure consists of a simple graphene patch array connected by horizontal wires. The phase coupling induced by the oblique illumination results in the EIT-like spectral response in the investigated frequency range. By mechanically adjusting the incidence angle, the phase delay of the field distribution between adjacent graphene patches varies and thus the resonance frequency as well as the amplitude of transparency peak is modulated. Moreover, by electrically tuning Fermi energy through gate bias, the transparency window, especially the adjacent transmission dips, are manipulated as the surface conductivity of graphene is altered. This work provides a new modulation strategy for fast and convenient control of the EIT effect in metamaterial and raises prospects in designing tunable compact terahertz devices such as buffers and modulators with graphene material.

Tunable high Q-factor terahertz complementary graphene metamaterial

Based on the complementary graphene asymmetric double bars patterns, the tunable Fano resonances with large Q-factors have been investigated in the terahertz regime, including the effects of Fermi levels, structural parameters and operation frequency. The results reveal that compared with existed graphene tunable devices, the Fano resonant curve is very narrow and indicates a large Q-factor of about 60. The strong Fano resonant curves can be convenient tailored. As Fermi level increases, the amplitude of the Fano dip decreases, and the resonant peak position shifts to high frequency. The amplitude modulation depth (MD) of the Fano curve is more, about 90%, if the Fermi level changes in the scope of 0.2–1.0 eV. With the increase of the sample refractive index, the resonant Fano dip shifts low frequency, and the dip amplitude MD can reach more than 40%. The results are very helpful to understand the tunable mechanisms of graphene based Fano systems and to design high sensitivity functional devices, e.g. sensors, modulators, and antenna.

Tunable multiband band-stop filter based on graphene metamaterial in THz frequency

ABSTRACTIn this paper, a tunable multiband band-stop filter based on single-layer and few-layer doped periodically graphene rings is designed at THz wavelength. The performance of the proposed structure is calculated by using the finite-difference time-domain method. The metamaterial structure is comprised of the concentric graphene ring resonators printed on substrate layer of silica medium. The multiband band-stop filter can be achieved by adjusting the numbers of graphene ring resonator. More importantly, a dynamically tunable filter can be designed by adjusting the applied voltage or chemical potential doping to manipulate graphene’s Fermi energy. The z-component electric field is investigated to explain the physical mechanism of the transmission. In addition, the proposed filter has the advantage of independence to polarization and insensitivity to the angles of the incident wave. Our design can have potential applications in various fields of tunable photonic devices, sensors and multi-band communication.

Independently tunable dual-spectral electromagnetically induced transparency in a terahertz metal–graphene metamaterial

We theoretically investigate the interaction between the conductive graphene layer with the dual-spectral electromagnetically induced transparency (EIT) metamaterial and achieve independent amplitude modulation of the transmission peaks in the terahertz (THz) regime. The dual-spectral EIT resonance results from the strong near field coupling effects between the bright cut wire resonator in the middle and two dark double-split ring resonators on the two sides. By integrating monolayer graphene under the dark mode resonators, the two transmission peaks of the EIT resonance can exhibit independent amplitude modulation via the shifting of the Fermi level of the corresponding graphene layer. The physical mechanism of the modulation can be attributed to the variation of damping factors of the dark mode resonators arising from the tunable conductivity of graphene. This work shows great prospects in designing multiple-spectral THz functional devices with highly flexible tunability and implies promising applications in multi-channel selective switching, modulation and slow light.

Multi-mode tunable absorber based on graphene metamaterial

In this paper, a novel design of metamaterial absorber (MA) with multi modes and tunability characteristics in terahertz (THz) range is presented. The MA consists of patterned gold-graphene on the top layer, graphene sheet and gold ground separated by SiO2. The chemical potential of patterned graphene and graphene sheet can be tuned independently. Thus, the regulation flexibility of MA is greatly enhanced. For demonstration, the proposed MA is investigated using circuit analogy method and the distributions of the electric field are also discussed in detail to explore underlying mechanism. Numerical calculations are stimulated in a commercial full-wave solver and indicate that the proposed MA possesses excellent absorption properties. There are three perfect absorption (≥99.2%) modes at 2.4 THz, 4.2 THz and 7.2 THz respectively. Under the regulation of chemical potential, the proposed MA could serve as single-frequency or dual-frequency absorber without change of physical structure. When working as single-frequency absorber, the MA could be regulated between different modes during a large frequency range and the absorption peaks could also be regulated within the same mode during a relative smaller frequency range. Furthermore, the MA has two absorption peaks at 2.4 THz and 7.2 THz (≥95%) simultaneously as dual-frequency absorber. The proposed MA shows promising application potentials due to the flexible modulation and functions and this work provides a new perspective for the design of tunable THz absorbers based on graphene.

Dynamic plasmon-induced transparency modulator and excellent absorber-based terahertz planar graphene metamaterial

We have investigated an obvious plasmon-induced transparency (PIT) that results from the destructive interference between the bright and dark modes in a novel planar metamaterial structure. Two different kinds of graphene, respectively, play the parts of optical bright and dark modes. We have investigated the spectral location and line shape and find that they can be successfully tuned by shifting the Fermi energy. We have theoretically studied its resonance mechanism, and the numerical simulation data are well verified by the theoretical analytical results. Furthermore, the absorption rate is up to 50%. Thus, the narrow PIT phenomena render our nano plasmonic system ideal for an excellent modulator and novel absorber.

Broadband tunable terahertz polarization converter based on a sinusoidally-slotted graphene metamaterial

A new wideband sinusoidally-slotted graphene-based cross-polarization converter (CPC) is proposed in this paper. The proposed polarization converter can realize a broadband terahertz polarization conversion from 1.28 to 2.13-THz with a polarization conversion ratio (PCR) of more than 0.85. Taking advantage of the gradient width modulation of the graphene-based unit structure, the continuous plasmon resonances are excited at the edges of the sinusoidal slot. Therefore, the proposed converter can achieve a broadband polarization conversion in a simplified structure. Furthermore, the polarization conversion characteristics of the CPC are insensitive to the incident angle. The PCR remains more than 0.85 with little bandwidth degradation even as the incident angle increases to as high as 50°. More importantly, the operating bandwidth and the magnitude of the PCR can be tuned easily by adjusting the chemical potential and the electron scattering times of the graphene. In a way, we believe this kind of graphene-based polarization converter can enrich the polarization conversion community for realizing broadband and tunable polarization conversion.

Tunable plasmon induced transparency based on bright–bright mode coupling graphene metamaterial

A graphene-based metamaterial structure with tunable plasmon induced transparency (PIT) effect is numerically and theoretically researched in this paper. Both the collinear and parallel graphene strips serve as bright mode and a PIT window emerges due to the weak hybridization between them. The transparency window can be tuned by not only adjusting the geometry parameter of the structure but also changing the chemical potential of the graphene strips. Different from the reported tunable PIT system, the spectra width and resonance strength can be tuned with the PIT peak at a fixed frequency in this research. Moreover, the Q factor can be affected by the distance between two bright modes and a figure of merit (FOM) as high as 15.20 can be achieved in this research. These properties expand the application for PIT phenomenon in mid-infrared metamaterial devices, such as filter, modulator and sensor.

Active graphene metamaterial absorber for terahertz absorption bandwidth, intensity and frequency control

A novel graphene-based metamaterial absorber with actively tunable bandwidth, intensity and frequency, working at the terahertz (THz) spectral region, was designed and numerically investigated in this paper. The actively controlled absorption characteristics in the graphene absorber were achieved by integrating two identical-sized graphene disc elements to construct a supercell, in which two elements of all unit cells were connected respectively by the electrical isolation graphene wires to form selectively electrostatic doping. The surface current distributions and the circuit model analysis were conducted to reveal the absorption mechanism and predict the tuning mechanism. Moreover, by selectively tuning two gate voltages to change the Fermi energy reconfiguration state of two graphene elements, the absorption bandwidth, intensity and frequency of the metamaterial absorber could be actively controlled. In addition, a striking switching contrast was obtained by switching the reconfiguration state of the two discs. Therefore, this work paves a pathway for the realization of actively controlled terahertz waves based on electrically reconfigured graphene metamaterials.

Tuning thermal conductivity of porous graphene by pore topology engineering: Comparison of non-equilibrium molecular dynamics and finite element study

Tuning thermal conductivity of porous graphenes has attracted much interest in the thermal management of nanoelectronics devices due to the promising multifunctional properties of engineered nanomaterials. To explore the potential of tuning thermal properties of monolayer porous graphenes in multiple scales, non-equilibrium molecular dynamics (NEMD) and finite element method (FEM) are implemented to manipulate their thermal conductivity and temperature distribution by the engineering of pore topology. Results indicate that the thermal conductivity of porous graphenes can be significantly lower than a pristine graphene. The thermal conductivity reduction is attributed to phonon scattering at the boundaries of defects described by the phonon density of states analysis. It is found that the thermal conductivity and the temperature distribution of a porous graphene can be desirably tuned by the simultaneous engineering of relative density, pore topology, and pore orientation. Then, the effect of unit cell periodicity on the thermal conductivity of periodic porous graphenes, called phononic graphene or graphene metamaterial, is explored. Finally, comparing the results of continuum mechanics approach through the implementation of FEM and NEMD simulation presents the advantages of NEMD for predicting the thermal conductivity of engineered porous graphenes with characteristic length of lower than 50 nm.

Multiple plasmonic resonance excitations on graphene metamaterials for ultrasensitive terahertz sensing

A three-dimensional graphene metamaterial structure consisting of dielectric pillar arrays covered by graphene monolayer with metal layer underneath is proposed and investigated, which can realize the efficient excitations of multiple plasmonic resonance modes under incident terahertz (THz) wave. The multiple plasmonic resonance modes are highly sensitive to the change of analyte refractive index and thickness, and the frequency sensitivity of each resonance mode can reach up to 1.402, 1.687 and 1.643 THz per refractive index unit, respectively. By controlling the Fermi level of graphene, the resonance frequency can be tuned dynamically. Moreover, the proposed structure has three absorption peaks with absorptivity more than 95%, it is polarization-insensitive and can perform very well in a wide incident angles for transverse electric and transverse magnetic waves. This proposed structure can be utilized for active THz plasmonic devices such as multiple channel sensors and absorbers.