Graphene Metamaterials Research Articles

Highly Sensitive THz Refractive Index Sensor Based on Folded Split-Ring Metamaterial Graphene Resonators

A highly sensitive absorption-based sensor based on folded split-ring metamaterial graphene resonators (FSRMGRs) is designed, and its biomedical application in terahertz (THz) spectrum is investigated. The sensor has a nearly perfect absorption, with a spectral absorption coefficient of 99.75% at 4 THz and an average Q-factor of 13.76. The resonance peak frequency is sensitive to the refractive index (RI) of the test medium (analyte), and a fairly high sensitivity of 851 GHz/RIU has been obtained. The specifications of the sensor can be tuned by an external DC-bias voltage applied to the graphene layer. According to the obtained results, the developed absorber appears to be a good candidate bio-sensing applications.

Multiband tunable perfect metamaterial absorber realized by different graphene patterns

In this paper, the absorption performance of a proposed metamaterial (MM) absorber based on a three-layer graphene structure working in the terahertz (THz) frequency band is studied. By using different types of combined graphene patterns, dual-band, tri-band, and quad-band absorption can be achieved. In the case of tri-band absorption, three absorption peaks with absorption rates of 99.7%, 99.9%, and 99.9% can be found at frequencies of 4.64 THz, 6.45 THz, and 9.71 THz, respectively. In addition, the proposed structure is polarization independent and has the absorption characteristic of wide incident angles. The frequency and the intensity of the absorption peaks can be adjusted by changing the chemical potential and the relaxation time of the graphene and the structural parameters. Therefore, we believe that the proposed graphene MM absorber structure provides flexible design ideas for a multibandwidth MM perfect absorber, and the proposed absorber also can be applied to subwavelength integrated sensors and optoelectronic devices in the terahertz range.

A terahertz graphene metamaterial based on polarization-insensitive plasmon-induced transparency for sensing application

A graphene metamaterial based on polarization-insensitive plasmon-induced transparency (PIT) in the terahertz region is theoretically presented. The graphene metamaterial comprises a cross-shaped and four other identical graphene strips, and a distinct narrow PIT window resulted from the bright-bright coupling is obtained when the electric field is along both x and y polarization directions. Surface current distributions and the two-particle model are used to elucidate the physical mechanism of the PIT effect, where the theoretical results of the two-particle model agree well with the simulation transmission spectra. In addition, the amplitude and resonant frequency of the PIT window can be modulated by tuning the Fermi level of graphene without varying the geometric parameters. Moreover, a high group delay of 1.545 ps can be achieved at the fixed Fermi level of 1 eV. As a function of the terahertz (THz) sensor, the PIT structure exhibits excellent sensing performances with the maximum sensitivity of 0.8 THz per refractive index unit (RIU) and figure of merit (FOM) of 17.28 RIU−1. Therefore, these features indicate that this work paves the way for designing polarization-insensitive and high-performance PIT sensors in the THz region.

Small-Angle Ultra-Narrowband Tunable Mid-Infrared Absorber Composing from Graphene and Dielectric Metamaterials

We report a small-angle ultra-narrowband mid-infrared tunable absorber that uses graphene and dielectric metamaterials. The absorption bandwidth of the absorber at the graphene Fermi level of 0.2 eV is 0.055 nm, and the absorption peaks can be tuned from 5.14803 to 5.1411 μm by changing the graphene Fermi level. Furthermore, the resonance absorption only occurs in the angle range of several degrees. The simulation field distributions show the magnetic resonance and Fabry–Pérot resonance at the resonance absorption peak. The one-dimensional photonic crystals (1DPCs) in this absorber act as a Bragg mirror to efficiently reflect the incidence light. The simulation results also show that the bandwidth can be further narrowed by increasing the resonance cavity length. As a tunable mid-infrared thermal source, this absorber can possess both high temporal coherence and near-collimated angle characteristics, thus providing it with potential applications.

Plasmon Resonances of Graphene-Dielectric-Metal Structures Calculated by the Method of Recurrence Relations

Graphene supports surface plasmons in the terahertz range, and compared with noble-metal plasmons, they show an extreme level of field confinement and relatively long propagation distances, with the advantage of being highly tunable via electrostatic field. Nevertheless, its interaction with light is normally rather weak. To obtain a more powerful capability of excite plasmons, a combination of graphene and artificial structures (metamaterials) present a powerful tunability for enhancing light-matter interaction. These features make graphene metamaterials a promising candidate for plasmonics and surface plasmon resonance for biological sensors. In this work, we study the plasmon spectra in a finite number of graphene layers on a metallic-dielectric substrate surrounded by materials with different dielectric constants. It is shown that using standard electromagnetic boundary conditions and solving the recurrence relation (a suitable alternative to transfer matrix method) for the coefficients of the electric potential between graphene layers, an explicit effective dielectric function of the metamaterial can be obtained giving the plasmon dispersion relations. It is found that the metal-dielectric-layered graphene structure supports both high-energy optical plasmon oscillations and out-of-phase low-energy acoustic charge density excitations. Experimentally, the Kretschmann configuration can be used to excite the surface plasmon resonances. It is based on the observation of a sharp minimum in the reflection coefficient versus angle (or wavelength) curve.

Graphene Metapixels for Dynamically Switchable Structural Color.

Structural coloration providing vibrant and tailored colors enables broad applications. Existing strategies of structural coloration either use resonances or diffraction induced by arrayed nanostructures with element sizes at a wavelength scale or are based on interference from vacuum-deposited large-area thin films. It is extremely challenging to achieve full color pixels with diffraction-limited resolution without sophisticated multiple-step nanostructure fabrication or externally applied field control. Realization of dynamically switchable full color displays with diffraction-limited resolution is even harder. This work demonstrates a structural color strategy with developed anisotropic graphene metapixels. The anisotropic optical property is given by the intrinsic birefringence of the layered structure of graphene metamaterials, and each metapixel is spatially encoded by direct laser printing with diffraction-limited resolution (250 nm). The colors can be dynamically and instantly switched by controlling the scattering of the light source to excite different modes based on the strong anisotropic optical properties of the graphene metapixels. The low-cost large-scale fabrication method allows experimental demonstration of a large-area (4 in.) flexible full color optical switchable display. Such a simple, effective and flexible method promises broad practical applications in color display and color image sensing related fields.

Hierarchical kirigami-inspired graphene and carbon nanotube metamaterials: Tunability of thermo-mechanic properties

Tuning and programming the multiphysical properties of advanced materials are of critical importance for developing the next generation of adaptable multifunctional metamaterials. This study demonstrates the tunability of thermo-mechanical properties of graphene sheets and carbon nanotubes by inspiring from hierarchical kirigami mechanical metamaterials. The theoretical investigation, multiscale simulation, and experimentation show that the thermo-mechanical properties of nano-architected kirigami metamaterials can be tuned by altering geometrical parameters and introducing the hierarchical cutting patterns. Additionally, the thermal conductivity of kirigami-inspired graphene and carbon nanotube metamaterials can be regulated by an external mechanical tension. We develop closed-form formulations for predicting the mechanical behavior of kirigami graphene sheets and carbon nanotubes. Molecular dynamics and finite element simulations are conducted to evaluate theoretical predictions. By analyzing and comparing the results from atomistic and continuum-based simulations, the effect of length scale on the thermo-mechanical properties is explored. We realize that the stress–strain response, thermal conductivity, and buckling-induced 3D patterns of nano-architected graphenes can be programmed by utilizing kirigami building blocks, nano-architectural hierarchy, and heterogeneous material design.

Broadband plasmon-induced transparency modulator in the terahertz band based on multilayer graphene metamaterials.

In this study, multilayer graphene metamaterials comprising graphene blocks and graphene ribbon are proposed to realize dynamic plasmon-induced transparence (PIT). By changing the position between the graphene blocks, PIT phenomenon will occur in different terahertz bands. Furthermore, PIT with a transparent window width of 1 THz has been realized. In addition, the PIT shows redshifts or blueshifts or disappears altogether upon changing the Fermi level of graphene, and hence a frequency selector from 3.91 to 7.84 THz and an electro-optical switch can be realized. Surprisingly, the group index of this structure can be increased to 469. Compared with the complex and fixed structure of previous studies, our proposed structure is simple and can be dynamically adjusted according to demands, which makes it a valuable platform for ideas to inspire the design of novel electro-optic devices.

Triple-band tunable terahertz perfect absorber based on graphene metamaterial

To further reduce the structural complexity of high-performance modulation devices in the terahertz region, we propose and demonstrate a triple-band tunable metamaterial perfect absorber, which consists of a patterned graphene and Au ground plane spaced by Si dielectric layer. The graphene surface plasmon resonance at the terahertz region is used to couple the patterned graphene with the electric field and provide electric dipole resonance to form multiple absorption peaks. Numerical results indicate that the amplitude of the absorption peaks is large than 99.9% at 0.489 THz, 1.492 THz, and 2.437 THz, respectively. In addition, due to the amplitude of the absorption peaks that can be controlled by the Fermi level via externally applied bias voltage, it means that we are able to switch the structure between reflector and absorber at their corresponding absorption bands. However, the symmetric unit cell structure of the proposed absorber is the inherent reason for the excellent polarization-insensitive, and the broadband absorption of the absorber maintains excellent absorption performance over a wide range of incident angles. Therefore, the designed graphene-based terahertz metamaterial function device has great potential in modulation and sensing.

Triple plasmon-induced transparency in graphene and metal metamaterials and its anomalous property

A silicon-graphene-silica periodic graphene metamaterial, comprising of a graphene-strip, graphene-block, and graphene-ribbon, is proposed to realize dynamic triple plasmon-induced transparency (PIT). Coupled mode theory is employed to explain this phenomenon, and the results are in close agreement with the finite-difference time-domain simulation. It is interesting to note that the triple-PIT can also be realized by using silver instead of graphene and silicon layer. When the Fermi level of the graphene is 0.75 eV, the difference of the triple-PIT based on the two structure types is easily analyzed because the resonant frequencies are effectively coincident. As a result, the structure of graphene exhibits better absorption characteristics compared with that of silver, however, it decreases with increasing Fermi levels within graphene. Thus, the higher Fermi level of graphene is not suitable for the absorbers. Furthermore, the anomalous property of graphene triple-PIT that is attributed to the interaction way of the dark mode differing from the bright mode is analyzed compared with the silver triple-PIT, and it can be used to compensate for the defects of silver PIT. This study is significant for the absorber for which the graphene is utilized. The insights gained from the comparison between metal and graphene are crucial to the design of the metamaterials for which graphene and metal are used.

Tunable Ultra-Broadband Terahertz Waveband Absorbers Based on Hybrid Gold-Graphene Metasurface

In this paper, we propose a tunable ultra-broadband terahertz absorber based on hybrid gold-graphene resonator to achieve an ultra-broadband, ultra-thin, and tunable metasurface, which has not only the high absorption rate of gold metamaterials but also the tunable properties of graphene metamaterials, compared with the traditional single gold-based and graphene-based methods. By simulating the response frequencies by the finite element method, we found that the absorber achieves a broadband 2.68-7.48 THz range (absorption>80%) absorbance with a relative bandwidth of 94.5%, where the center frequency is f c=5.08THz. The physical mechanism of the absorber was analyzed using the electric field of the surface plasmon resonance, due to the design structure presents periodic geometric symmetry, the polarization angle and large angle incidence both present better advantages. The dynamic tuning of the metamaterial absorber is achieved by changing the Fermi energy level of graphene by adjusting the bias voltage to effectively control the metamaterial absorption intensity and resonant frequency. Our proposed absorber has important application prospects in sensing, optical communication, detection, and optical devices.

Electromagnetically Induced Transparency-Like Terahertz Graphene Metamaterial With Tunable Carrier Mobility

In this work, a graphene-based planar terahertz electromagnetically induced transparency-like (EIT-like) metamaterial, composed of two horizontally symmetric mono-layer graphene micro-ring resonators (MRR) and a vertical mono-layer graphene micro-strip resonator (MSR), has been proposed. The transparent window was observed in the y polarization direction. By changing the carrier mobility of the graphene, the transparent window could be opened and closed while the position of the EIT-like window remained unchanged. The physical mechanism of the EIT-like effect was explained by the distribution of the electric field on E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>z</i></sub> and the three-level <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Lambda $ </tex-math></inline-formula> -type system. The theoretical fitting results based on the Lorentz oscillator model and the S-parameter inversion method were consistent with the numerical simulation results. In addition, the performance of the metamaterial for detecting the refractive index of the surrounding medium was analyzed. Numerical simulation showed that when the FOM exceeded 8.0, the sensitivity was 1.6 THz/RIU. Finally, as the graphene carrier mobility increased, the group delay of the device increased. The delay time reached 1.49 ps, and the group index was as high as 400. The proposed design provides a feasible method for the development of optical switches, biochemical molecular detection, and slow light equipment.

Terahertz plasmonic sensing based on tunable multispectral plasmon-induced transparency and absorption in graphene metamaterials

Graphene surface plasmons have gained wide interest due to their promising applications in terahertz technology. In this paper, we propose an easily implemented monolayer graphene structure, and exploit its quadra resonance mode to achieve triple plasmon-induced transparency (PIT) and triple plasmon-induced absorption (PIA) effects. A uniform theoretical model with four resonators is introduced to elaborate the intrinsic coupling mechanism and examine the accuracy of simulated results. By altering the Fermi energy and the carrier mobility of the graphene, the proposed triple PIT (PIA) system exhibits a dynamically tunable property, and the absorption intensity can be controlled over a broadband frequency range. It is found that the absorption intensity of the triple PIA spectrum can be as high as 50% with four absorption bands, which is 20 times more than that of monolayer graphene. Besides, we further investigate the triple PIT system for terahertz plasmonic sensing applications, and it is shown that the highest sensitivity of 0.4 THz RIU−1 is reached. Thus, the triple PIT system we propose can be employed for multi-band light absorption and plasmonic optical sensing.

Tunable terahertz Dirac semimetal metamaterials

The tunable propagation properties of 3D Dirac semimetal (DSM) patterned metamaterial (MM) structures have been symmetrically investigated in the terahertz (THz) regime. The results demonstrate that the resonant properties are very sensitive to the thicknesses of DSM MMs, and hundreds of nanometers are required to excite strong resonant curves. The DSM MMs support both strong LC and dipolar resonances, quite different from graphene MM patterns which mainly depend on dipolar resonance. As the Fermi level increases, the resonant strength becomes stronger, and significant modulation can be achieved, e.g. the amplitude and frequency modulation depths of transmission curves are more than 99% and 80%, respectively. In addition, by utilizing asymmetrical resonators, a very sharp Fano resonant peak is achieved with a large Q-factor of more than 25, for which the figure of merit is about 20. The results are very helpful to understand the tunable mechanisms of DSM devices and design novel THz plasmonic components, such as modulators, filters, and sensors.

Tunable ultra-narrowband mid-infrared absorber with graphene and dielectric metamaterials

We report a tunable ultra-narrowband mid-infrared absorber with graphene and dielectric metamaterials. At the graphene Fermi level of 0.2 eV in the mid-infrared regime, the absorber has an absorption bandwidth of 0.033 nm which is at least one order of magnitude smaller than those reported with graphene in the mid-infrared regime. In addition, by changing the graphene Fermi level from 0.2 eV to 0.6 eV, the absorption peaks of the absorber can be shifted from 4.468529μm to 4.464453μm with bandwidth less than 0.05 nm in the tuning Fermi level range. The simulation results of electromagnetic field distribution show that the guided-mode resonance occurs at the resonance absorption peak. As a thermal source, the corresponding spatial coherence length is 212.4 mm. The tunable ultra-narrowband mid-infrared absorption characteristics indicate that our absorber has the potential applications in the tunable coherent emission of thermal source and the tunable filtering.

Graphene Multilayer Photonic Metamaterials: Fundamentals and Applications

AbstractGraphene is given high expectation due to their unique properties and advantages and has revolutionized different research fields and leads to enormous applications. However, monolayer graphene shows limited absorption due to the ultrathin thickness, which compromises the optical modulation capability. Therefore, graphene metamaterials consisting of alternatively stacked graphene and dielectric layers or graphene layer combined with conventional metamaterial structures are proposed as artificially structured materials to exploit both graphene's unique material properties and nanostructure capability of metamaterials to achieve strong optical modulations. The recent breakthrough in experimental fabrication of graphene metamaterials, in particular the solution‐based method which allows large‐area fabrication of graphene metamaterials with monolayer‐thickness controllability and nanometer surface roughness, enables extraordinary optical properties and opens various new possibilities both in research and manufacturing. In this comprehensive review, the concept of graphene metamaterials, the unique optical properties, and the enabling fabrication methods are demonstrated. The multifunctionality of graphene metamaterials offer a novel platform for manufacturing innovative optical and photonic devices with the advantages of broad working bandwidth, high efficiency, fast response, tunability, integrability, and low energy consumption. The review provides insights to future innovations for the booming graphene metamaterials and metaphotonic devices in the disciplines of optics, nanophotonics, and optoelectronics.

Graphene Derivatives and Metamaterials: Structure - Properties Relations Obtained by Atomic-Scale Modeling

Graphene Derivatives and Metamaterials: Structure - Properties Relations Obtained by Atomic-Scale Modeling

Tunable broadband terahertz absorber based on graphene metamaterials and VO2

A tunable broadband terahertz (THz) absorber is designed in this paper, which is composed of graphene, silica medium and phase change material Vanadium dioxide (VO 2 ). By simultaneously changing the fermi level of graphene and conductivity of VO 2 , the dual-controlled absorber can be realized. The results show that the full width at half maximum of the absorption spectrum is 1.03 THz, the bandwidth with the absorptance above 90% is 0.65 THz, and the absorptance is close to 100% at 1.2 THz. Moreover, the absorptance, transmittance and reflectance of the absorber can be dynamically adjusted from 0 to 99%, 0–65%, and 0–74%, respectively. The large tunability allows the proposed absorber to be used as THz tunable filters, sensors, switches and modulators. • A broadband metamaterial absorber has been proposed based on the patterned graphene resonators. • Dual control and tunability is realized based on graphene array and phase change material vanadium dioxide. • The proposed structure can be dynamically tuned to act as an absorber or reflector. • The structure can be made to work in transmission or absorption mode by changing the conductivity of vanadium dioxide.

Tunable polarization-independent and angle-insensitive broadband terahertz absorber with graphene metamaterials.

In this paper, we design a polarization-independent and angle-insensitive broadband THz graphene metamaterial absorber based on the surface plasmon-polaritons resonance. Full-wave simulation is conducted, and the results show that the designed metamaterial absorber has an absorption above 99% in the frequency range from 1.23 THz to 1.68 THz, which refers to a very high standard. Furthermore, the absorber has the properties of tunability, and the absorption can be nearly adjusted from 1% to 99% by varying the Fermi energy level of the graphene from 0 eV to 0.7 eV. In the simulation, when the incident angles of TE and TM waves change from 0° to 60°, the average absorption keeps greater than 80%. The proposed absorber shows promising performance, which has potential applications in developing graphene-based terahertz energy harvesting and thermal emission.

Active manipulation of dual transparency windows in dark–bright–dark mode coupling graphene metamaterial

Active manipulation of dual transparency windows for EIT metamaterial in dark–bright–dark coupling mode, which is constructed by three graphene patterns, including graphene strip and two asymmetrical double-split graphene ring resonators, is numerically and theoretically investigated in terahertz regime. Simulation results illustrate that the causation of dual-windows EIT phenomenon is the near field coupling of two adjacent graphene patterns in the unit cell which acted as dark and bright mode respectively. In addition, dual transparency windows of EIT can be actively and selectively tuned to realize red or blue shift via varying of the Fermi energy of the corresponding dark mode element in the unit cell. Meanwhile, when the Fermi energies of dark mode elements are shifting inversely, dual transparency windows could be actively modified to single transparency window or even disappeared.