Graphene Resonators Research Articles

The Comparative Analysis of Graphene Nano-based and Copper Nano-based Patched Antenna using HFSS

Patch material plays a significant role in controlling the performance and resonating frequency of patch based antennas. In this work, we compared graphene and copper nano-based patch antennas in terms of return loss, bandwidth, gain, directivity and voltage standing wave ratio (VSWR). The simulation is carried out using high frequency structure simulator HFSS. The substrate material used is Silicon dioxide. The antenna is designed to operate in the THz frequency band of (1-15THz) with the fundamental frequency of 5.5THz. The result shows that, the classical metallic antennas with copper material resonant at lower frequency of 3THz while graphene resonant at the higher frequency of 7THz, this is due to high electrons mobility in graphene than in copper. Graphene based patch antenna achieves maximum return loss of -24.4555dB with the corresponding VSWR of 1.0413. The maximum gain of 7.1943dB is achieved with a bandwidth of 522.3GHz; this shows better antenna performance than copper except the bandwidth. The contemporary copper antenna attains return loss of -14.7028dB with the corresponding VSWR of 3.2336. The gain of 4.6219dB is achieved with a bandwidth of 1188.5GHz. With this result, it can be seen that graphene is a suitable choice and can replace copper patch material for patch antennas for wireless applications in terahertz frequency band.

Vibration Characteristics of Graphene nano resonator as mass sensor

The decrease in the physical dimensions of the devices have sought the attention of research community due to quick response, high sensitivity and sturdiness of the devices. These devices in the form of nano resonators have been extensively used as sensors to detect the entity at submicron level as well as to identify the properties of matter at submicron level. With the process like chemical vapour deposition, lithography technique as well as mechanical exfoliation techniques, it has become possible to produce materials which are 2D in nature. The excellent mechanical and electrical properties of graphene as well as its complete plain geometry advocate it as an ideal material for the development of sensors used for identifying the object at nano level. Here an attempt is made to analyse the vibration characterization of graphene resonator in the form of membrane to understand the shift in the frequency by adsorbing the mass and change in the temperature. The tobacco mosaic virus is considered as a mass adsorbed onto the graphene nano ribbon based membrane. Along with the adsorption of the mass, the effect of variation in temperature is also introduced to observe the shift in the natural frequency of the graphene membrane based resonator.

Dynamically tunable slow light characteristics in graphene based terahertz metasurfaces

We propose a graphene based metasurface exhibiting dynamic slow light behavior via electromagnetically induced transparency (EIT) effect in the terahertz regime. The unit cell of the metasurface consists of bright and dark graphene rod resonators in a planar orthogonal configuration. The onset and tunability of the EIT effects are investigated through manipulating the near field electromagnetic coupling among the bright and dark resonators. An analytical method based on three level Lorentz oscillator model is employed to validate the simulated results. Further, we investigate the dynamic tunability of EIT effect by altering the Fermi energy of the graphene resonators. A blue shift of 0.59 THz in the EIT transparency peak is observed as the Fermi energy is increased from 0.3 eV to 1.0 eV. Similar dynamic tunability is investigated for the slow light characteristics (i.e., group delay, group index & group velocity) along with the delay bandwidth product. It is observed that an increase in the Fermi energy ultimately leads to an increase in group delay, group index & delay bandwidth product with their corresponding maximum values as 0.76 ps, 22.78 & 0.14 respectively for Fermi energy as 1.0 eV. For this case, the normalized group velocity reaches a minimum value of 0.04, thus, confirming the dynamic tunability of slow light behavior in the studied metasurfaces. Our findings can lead to the development of active slow light components like modulators, sensors, filters, buffers, ultrafast switches, and compact delay lines etc. in the terahertz domain.

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.

The Effect of Edge Mode on Mass Sensing for Strained Graphene Resonators.

Edge mode could disturb the ultra-subtle mass detection for graphene resonators. Herein, classical molecular dynamics simulations are performed to investigate the effect of edge mode on mass sensing for a doubly clamped strained graphene resonator. Compared with the fundamental mode, the localized vibration of edge mode shows a lower frequency with a constant frequency gap of 32.6 GHz, despite the mutable inner stress ranging from 10 to 50 GPa. Furthermore, the resonant frequency of edge mode is found to be insensitive to centrally located adsorbed mass, while the frequency of the fundamental mode decreases linearly with increasing adsorbates. Thus, a mass determination method using the difference of these two modes is proposed to reduce interferences for robust mass measurement. Moreover, molecular dynamics simulations demonstrate that a stronger prestress or a higher width–length ratio of about 0.8 could increase the low-quality factor induced by edge mode, thus improving the performance in mass sensing for graphene resonators.

Enhancing the sensitivity of a transmissive graphene-based plasmonic biosensor

A biosensor platform based on the plasmonic resonance of graphene in the terahertz (THz) range (0.1 to 10 THz) is designed and investigated. The initial design is to create a nanofluidic channel as a sensing layer in the substrate of a biosensor grounded by metal. The sensor consists of a rectangular graphene patch over the substrate, which can be fed by either an external near-field source or an antenna. The presence of molecules in the nanosensing layer causes small changes in the channel's properties, detectable through the scattering parameters of the designed biosensor. Since biomolecules are poorly absorbed in the initial biosensor, it can be grounded by a graphene sheet that is the same size as the graphene sheet over the substrate, which results in a performance improvement of the biosensor. It is shown that, by increasing the number of graphene sheets between the ground and the patch, high absorption occurs that enhances the sensitivity of the initial surface plasmon resonance THz biosensor. With varying refractive index of the sensing layer (Ns) in the range of 1.3-1.6, the proposed biosensors are investigated and compared with the initial biosensor. It is shown that by applying a graphene sheet between the two graphene sheets in the substrate, a maximum sensitivity of 8470 nm/RIU is achieved, which is a significant improvement, and also a sensitivity improvement of 4130 nm/RIU is achieved at Ns=1.3. In the final section, it is shown that changing the substrate material from silicon (Si) to silica (SiO2) brings a significant sensitivity enhancement in the proposed biosensor with three graphene sheets, which accounts for 15410 nm/RIU in the best scenario.

Tunable s-SNOM for Nanoscale Infrared Optical Measurement of Electronic Properties of Bilayer Graphene.

Here we directly probe the electronic properties of bilayer graphene using s-SNOM measurements with a broadly tunable laser source over the energy range from 0.3 to 0.54 eV. We tune an OPO/OPA system around the interband resonance of Bernal stacked bilayer graphene (BLG) and extract amplitude and phase of the scattered light. This enables us to retrieve and reconstruct the complex optical conductivity resonance in BLG around 0.39 eV with nanoscale resolution. Our technique opens the door toward nanoscopic noncontact measurements of the electronic properties in complex hybrid 2D and van der Waals material systems, where scanning tunneling spectroscopy cannot access the decisive layers.

Mechanical resonance properties of porous graphene membrane; simulation study and proof of concept experiment

Mechanical resonance properties of porous graphene resonators were investigated by simulation studies. The finite element method was utilized to design the porous graphene membrane pattern and to calculate the mechanical resonance frequency and quality factor. The changes in the resonance frequency and quality factor were systematically studied by changing the size, number, and relative location of pores on the graphene membrane. Mass loss and carbon-carbon bond break were found to be the main competing parameters for determining its mechanical resonance properties. The correlation between the geometry and the damping effect on the mechanical resonance of graphene was considered by suggesting a model on the damping factor and by calculating the membrane deflections according to the pore location. Based on the simulation results, an optimal porosity and porous geometry were found that gives the maximum resonance frequency and quality factor. Suspended graphene with various number pore structures was experimentally realized, and their mechanical resonance behaviors were measured. The trend of changes in resonance frequency and quality factor according to the number of pores in the experiment was qualitatively agreed with simulation results.

Equivalent circuit model of graphene chiral multi-band metadevice absorber composed of U-shaped resonator array.

In this study, we have designed an equivalent circuit model (ECM) by use of a simple MATLAB code to analyze a single-layered graphene chiral multi-band metadevice absorber which is composed of U-shaped graphene resonator array in terahertz (THz) region. In addition, the proposed metadevice absorber is analyzed numerically by the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed device which is the first tunable graphene-based chiral metadevice absorber can be used in polarization sensitive devices in THz region. It is single-layered, tunable, and it has strong linear dichroism (LD) response of 94% and absorption of 99% for both transverse electric (TE) and transverse magnetic (TM) electromagnetic waves. It has four absorption bands with absorption >50% in 0.5-4.5 THz : three absorption bands for TE mode and one absorption band for TM mode. Proposed ECM has good agreement with the FEM simulation results. ECM analysis provides a simple, fast, and effective way to understand the resonance modes of the metadevice absorber and gives guidance for the analysis and design of the graphene chiral metadevices in the THz region.

Graphene Multiple Fano Resonances Based on Asymmetric Hybrid Metamaterial.

We theoretically investigate multiple Fano resonances in an asymmetric hybrid graphene–metal metamaterial. The multiple Fano resonances emerge from the coupling of the plasmonic narrow bonding and antibonding modes supported by an in-plane graphene nanoribbon dimer with the broad magnetic resonance mode supported by a gold split-ring resonator. It is found that the Fano resonant mode with its corresponding dark mode of the antibonding mode in the in-plane graphene nanoribbon dimer is only achieved by structural symmetry breaking. The multiple Fano resonances can be tailored by tuning the structural parameters and Fermi levels. Active control of the multiple Fano resonances enables the proposed metamaterial to be widely applied in optoelectronic devices such as tunable sensors, switches, and filters.

Enhanced tunable plasmonic resonance in crumpled graphene resonators loaded with gate tunable metamaterials.

Graphene devices have been widely explored for photonic applications, as they serve as promising candidates for controlling light interactions resulting in extreme confinement and tunability of graphene plasmons. The ubiquitous presence of surface crumples in graphene, very less is known on how the crumples in graphene can affect surface plasmon resonance and its absorption properties. In this article, a novel approach based on the crumpled graphene is investigated to realize broadband tunability of plasmonic resonance through the mechanical reconfiguration of crumpled graphene resonators. The mechanical reconfiguration of graphene crumples combined with dual electrostatic gating (i.e. raising the Fermi level from 0.2-0.4 eV) of graphene serves as a tuning knob enabling broad spectral tunability of plasmonic resonance in the wavelength range of 14-24 µm. The crumpled region in the resonators exhibits an effective trapping potential where it extremely confines the surface plasmonic field on the surfaces of crumples providing localized surface plasmon resonance at the apices of these crumples. Finally, to achieve near-unity absorption >99% at the resonance wavelengths (17 µm and 22 µm) crumpled graphene resonators are loaded with four ring shaped metamaterials which result in the enhanced near-field intensity of ≈1.4×106. This study delivers insight into the tunability of crumpled graphene and their coupling mechanism by providing a new platform for the flexible and gate tunable graphene sensors at the infrared region.

Surface modification of graphene by coupling with electron deficient radicals

In the present article, the surface modification of graphene by coupling with electron deficient nitroaryl radicals (NO2-Ar•) under microwave irradiation was reported. The NO2-Ar• species were generated insitu by reaction of nitroaniline derivatives with isoamyl nitrite as a diazotization agent, followed by decomposition of nitroaryl diazonium intermediates. The nitrogen-rich graphene flakes prepared by this procedure were characterized by energy dispersive X-ray (EDX) and elemental map. The morphology of graphene before and after surface modification was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman and Fourier transform infrared (FT-IR) spectroscopy. Additionally, density functional theory (DFT) calculations were carried out to investigate the optimized geometry, structural parameters, molecular orbitals, density of states (DOS), Raman, infrared (IR) and carbon-13 nuclear magnetic resonance (13C NMR) of graphene modified by a variety of NO2-Ar• species. Energy decomposition analysis (EDA) was performed for evaluating the contribution of dispersion, polarization, electrostatic and exchange energies to the interaction energy of nitroaryl radicals with graphene. Results showed that the surface modification of graphene by coupling with nitroaryl radicals leads to the significant changes in the morphological, structural, electronic, conductive and spectral properties of graphene. Moreover, the bonding nature between nitroaryl radicals and graphene is covalent.

Tunable Ultra‐High Q‐Factor and Figure of Merit based on Fano Resonance in Graphene–Dielectric Multilayer Corrugated Structure

AbstractHigh‐performance sensors have been a focus of the nanophotonics in recent years owing to their great potential for use in medical diagnosis and environmental remediation. Particularly, the sensing properties based on graphene have attracted intensive interest. Yet, low quality‐factor (Q‐factor) and figure of merit (FOM) have limited significant device performance. Here, a graphene–dielectric–graphene corrugated structure is proposed, the simulations show that by introducing the graphene the Q‐factor becomes tunable and can be significantly improved. The transmission and reflection spectra exhibit asymmetric Fano resonance with ultra‐narrow width that depends on the Fermi energy and modulation amplitude. Based on the high Q‐factor of the structure, the sensing properties of the structure are exploited when introducing sensing medium. For the shallow grating modulation, ultra‐high FOM is obtained. The proposed structure based on graphene has a promising potential for designing and fabricating novel refractive index sensors.

Active control of electromagnetically induced transparency analog in all-dielectric metamaterials loaded with graphene

Electromagnetically induced transparency (EIT) analog in all-dielectric metamaterials with a high quality factor provides an effective route to enhance light–matter interaction at the nanoscale. In particular, the active control applied to it enables great degree of freedom for spatial light modulation and thus promises functional device applications with high flexible tunability. Here, we load graphene into all-dielectric metamaterials and realize the remarkably high modulation depth in the transmission amplitude of the EIT resonance with the manipulation of graphene conductivity, via shifting the Fermi level or altering the layer number. The physical origin lies in the controllable light absorption through the interband loss of graphene in the near infrared. This work reveals a strategically important interaction mechanism between graphene and EIT resonance in all-dielectric metamaterials, and opens avenues in designing a family of hybrid metadevices that permit promising applications to light modulation, switching, slow light and ultrasensitive biosensing.

Tunable THz and Infrared Plasmonic Filters and Switches Based on Circular Graphene Resonator With 90$^\circ$ Bending of Output Port

Two types of novel graphene-based components, namely, filters and electro-optical switches in guided wave configuration are suggested and analysed. The filters differ from the known ones with collinear orientation of the input and output waveguides by geometry and symmetry. The components consist of a circular graphene disk and two nanoribbons oriented at 90° to each other in the plane of the graphene layer. The graphene elements are placed on a dielectric substrate. We show that change in symmetry leads to a drastic change in the properties of the components. The physical principle of the devices is based on the dipole, quadrupole and hexapole resonances in the graphene disk which define stop-band, pass-band and stop-band frequency characteristics, respectively. A combination of stop-band and pass-band filter properties by shifing the electronic Fermi level allows one to design a switch. Numerical simulations show that the suggested components have very small dimensions, good characteristics and provide a dynamic control of their central frequency via electrostatic gating.

Sideband ground-state cooling of graphene with Rydberg atoms via vacuum forces

We present a scheme leading to ground-state cooling of the fundamental out-of-plane (flexural) mode of a suspended graphene sheet. Our proposal exploits the coupling between a driven Rydberg atom and the graphene resonator, which is enabled by vacuum forces. Thanks to the large atomic polarizability of the Rydberg states, the Casimir-Polder force is several orders of magnitude larger than the corresponding force achieved for atoms in the ground state. By playing with the distance between the atom and the graphene membrane, we show that resolved sideband cooling is possible, bringing the occupation number of the fundamental flexural mode down to its quantum limit. Our findings are expected to motivate physical applications of graphene at extremely low temperatures.

Polarization-insensitive complementary metamaterial structure based on graphene for independently tuning multiple transparency windows**Project supported by the National Natural Science Foundation of China (Grant No. 61275174).

Polarization-insensitive multiple transparency windows are obtained with a graphene-based complementary metamaterial structure in terahertz regions, which is composed of two kinds of monolayer graphene perforated in shapes of a cross and four identical split rings that construct a resonator. The geometric parameters of resonators are different from each other. Numerical and theoretical results show that the quantum effect of Autler–Townes splitting is the key factor for appearance of transparency windows within the resonant dips. Further investigation demonstrates that by employing the fourfold-symmetry graphene complementary structure, polarization-independent transparency windows can be achieved. Moreover, multiple transparency windows can be separately manipulated over a broad frequency range via adjusting the chemical potential of the corresponding graphene resonators, and the bandwidth as well as resonance strength can also be tuned by changing the relative displacement between resonators each consisting of a cross and four split rings. The proposed metamaterial structure may be utilized in some practical applications with requirements of no polarization-varied loss and slowing the light speed.

Degenerate four-wave mixing in the THz regime with standing-wave graphene resonators

Graphene is one the most promising two-dimensional materials for functional electromagnetic components. Harnessing graphene’s high third-order nonlinearity, a standing-wave resonant system is proposed that realizes low-power and high-conversion-efficiency degenerate four-wave mixing in the THz regime. The proposed system is analyzed in depth, using a recently developed nonlinear framework based on the perturbation theory and temporal coupled-mode theory, which allows for efficient design, accurate results, and physical insight into the system performance. Following robust design guidelines derived from the developed framework, a clear design path is highlighted, covering two possible realizations of the coupling scheme using one or two waveguides as physical ports. The two systems are compared on the basis of input power and conversion efficiency performance metrics, accurately extracted taking into account all relevant nonlinear phenomena including the nonlinear resonance frequency shifts due to self- and cross-phase modulation in graphene, owing to the Kerr effect. The reported values of 10% conversion efficiency and sub-mW power requirements are highly promising for practical applications, highlighting the potential of graphene-based structures in the far-infrared.

A polarization- and angle-insensitive broadband tunable metamaterial absorber using patterned graphene resonators in the terahertz band

The emerging material of graphene has resulted in new options for the design of metamaterial devices with excellent performances, offering an attractive platform for electromagnetic absorption applications. To date, researches on patterned graphene-based metamaterial absorbers (MAs), have focused on achieving broadband absorption while maintaining many excellent properties. Here, we propose a broadband tunable MA with a perfect absorption based on a novel design of the combination of cross- and square-shaped graphene sheets. Numerical simulation results show that the bandwidth of the proposed broadband MA reaches 2.76 THz with the absorptivity greater than 90% for both the transverse electric (TE) and transverse magnetic (TM) modes, which exceeds most of broadband absorption results achieved by MAs in previous studies. A numerical analysis employing the equivalent circuit model for the designed structure is conducted to determine the influence of the structural parameters on the absorptivity and optimize the parameters. Results show that impedance matching is achieved between the input impedance and the free-space impedance at the absorption band. Detailed numerical simulations are conducted to calculate the surface loss density, electric field, and surface currents to obtain an understanding of the high absorption mechanism. Also, the designed MA exhibits excellent properties, such as tunability, polarization independence, incident angle insensitivity and outstanding tolerance to changes in the structural parameters, suggesting that the proposed absorber is a promising candidate for various applications in the terahertz band.

Broadband near-unity light absorption with anti-hermitian coupled stacked graphene metasurfaces

Enhancement of the interaction between monolayer or few-layers graphene and light is crucial to the potential applications of graphene-based optoelectronic devices. Recently, various nanostructured graphene has been proposed to achieve efficient optical absorption, whereas the bandwidth of the enhanced spectral absorption is always fairly narrow, which might be disadvantageous to their opt-electrical applications relying on light harvesting. In this work, we propose an effective method of constructing a stacked graphene-based metasurface to achieve broadband unity absorption. The main concept of the design is to realize the near-zero mutual coupling among graphene resonators based on the idea of anti-Hermitian coupling. This coupling will restrain the mode splitting due to the effect of direct near-field coupling between the neighboring graphene resonators, and thus facilitate the realization of a broadband absorption through the superposition of the individual resonance. In principle, the absorption bandwidth could be expanded further, and the proposed idea might also be used to design broadband absorber of other sub-wavelength nano-resonators.