Graphene Resonators Research Articles

Graphene-based dual-functional chiral metamirror composed of complementary 90\xb0 rotated U-shaped resonator arrays and its equivalent circuit model

An equivalent circuit model (ECM) using a MATLAB code to analyze a tunable two-layered graphene-based chiral dual-function metamirror, is proposed in this work. The investigated metastructure is composed of complementary U-shaped graphene resonator arrays in the terahertz (THz) region. The ECM analysis could be used for any two-layered chiral metastructure for any frequencies, containing resonators with a thickness less than λ/50. The characteristics of the proposed tunable metamirror were analyzed numerically using the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed metamirror can be used in polarization-sensitive devices in the THz region with simpler biasing without a need for ion gels or similar. It works as a broadband TE and multiband (four bands) TM mirror in the 0.3–4.5 THz bandwidth with a strong linear dichroism (LD) response (up to 96%). The designed mirror is a dynamically tunable, dual-functional structure, requiring only 90° rotation of the incident electromagnetic fields to switch between broadband and multiband spectral behavior making it a promising candidate for future THz intelligent systems. The proposed ECM is in agreement with the FEM results. The ECM analysis provides a simple, fast, and effective way to understand the metamirror’s behavior and guides for the design and analysis of graphene-based chiral metastructures in the THz region.

Nonreciprocal dynamically tunable power dividers by three (1x3) based on graphene for terahertz region

This paper presents two new types of dynamically tunable THz nonreciprocal power dividers based on graphene. These components fulfill division of the input power between three output ports. Besides, they possess isolation properties, i.e., a source connected to the input port is protected against unwanted reflections from the output ports. The components consist of a disk-shaped graphene resonator and six graphene waveguides connected to it. The graphene elements are deposited on a dielectric substrate. The resonator is subject to an external electric and DC magnetic field. The dipole mode resonance in the magnetized resonator is excited by surface plasmon–polariton waves propagating in the graphene waveguides. Numerical simulations and the analytical temporal coupled-mode theory are used in the analysis. The division level of the input power between the three output ports of dividers is about −8 dB and the isolation of the source in relation to the output ports is higher than −15 dB with the central frequency 7.45 THz and the bandwidth 3%. DC magnetic field is 0.29 T and Fermi energy is 0.15 eV. The central frequency of the dividers can be dynamically adjusted by an external electric field.

Enhanced Molecular Infrared Spectroscopy Employing Bilayer Graphene Acoustic Plasmon Resonator.

Graphene plasmon resonators with the ability to support plasmonic resonances in the infrared region make them a promising platform for plasmon-enhanced spectroscopy techniques. Here we propose a resonant graphene plasmonic system for infrared spectroscopy sensing that consists of continuous graphene and graphene ribbons separated by a nanometric gap. Such a bilayer graphene resonator can support acoustic graphene plasmons (AGPs) that provide ultraconfined electromagnetic fields and strong field enhancement inside the nano-gap. This allows us to selectively enhance the infrared absorption of protein molecules and precisely resolve the molecular structural information by sweeping graphene Fermi energy. Compared to the conventional graphene plasmonic sensors, the proposed bilayer AGP sensor provides better sensitivity and improvement of molecular vibrational fingerprints of nanoscale analyte samples. Our work provides a novel avenue for enhanced infrared spectroscopy sensing with ultrasmall volumes of molecules.

Zero-field spin resonance in graphene with proximity-induced spin-orbit coupling

We investigate collective spin excitations in graphene with proximity-induced spin-orbit coupling (SOC) of the Rashba and valley-Zeeman types, as it is the case, e.g., for graphene on transition- metal-dichalcogenide substrates. It is shown that, even in the absence of an external magnetic field, such a system supports collective modes, which correspond to coupled oscillations of the uniform and valley-staggered magnetizations. These modes can be detected via both zero-field electron spin resonance (ESR) and zero-field electric-dipole spin resonance (EDSR), with EDSR response coming solely from Rashba SOC. We analyze the effect of electron-electron interaction within the Fermi- liquid kinetic equation and show that the interaction splits both the ESR and EDSR peaks into two. The magnitude of splitting and the relative weights of the resonances can be used to extract the spin-orbit coupling constants and many-body interaction parameters that may not be accessible by other methods.

Self-Referenced Refractive Index Biosensing with Graphene Fano Resonance Modes.

A hybrid structure composed of periodic monolayer graphene nanoribbons and a dielectric multilayer structure was designed to generate a Fano resonance (FR). The strong interaction between the surface plasmon resonance of graphene and the dielectric waveguide mode results in the FR. The finite element method is utilized to investigate the behaviors of the FR, and it matches well with the theoretical calculations using rigorous coupled wave theory. The results demonstrate that the profile of the FR can be passively tuned by the period of the graphene nanoribbons and actively tuned by the Fermi level of the graphene. The decoupled nature of the FR gives it potential applications as a self-calibrated refractive index biosensor, and the sensitivity can reach as high as 4.615 μm/RIU. Thus, this work provides a new idea for an excellent self-referencing refractive index biosensor.

Ultrasensitive terahertz sensing with broadside coupled polarization insensitive graphene metamaterial cavities

The ubiquitous property of metamaterials to confine energy at deep-subwavelength scale paves the way to leverage strong light (or terahertz)-matter interactions towards the development of efficient, state-of-the-art sensing technologies at the terahertz (THz) frequencies. In two-dimensional active metamaterials, for instance, graphene metasurfaces; THz-matter interactions are largely governed by the near-field coupling, where strong field confinement is predominantly achieved within very small areas (compared to total device area), viz, in split gaps, narrow apertures etc. Here, we demonstrate a polarization insensitive broadside coupling mechanism in graphene- metamaterials, which is beneficial for realization of miniaturized, compact, ultra-sensitive THz thin film sensors with substantially large effective areas for strong THz-matter interaction. Proposed structure entails ultrathin layer of dielectric spacer (cavity) sandwiched between two “plus” shaped sub-wavelength graphene resonators. In order to investigate the sensing capabilities, obscure materials are placed in ultrathin cavity region to detect its refractive indices. A record high refractive index sensitivity of around 2.8 THz/RIU substantiates the utility of introducing the concept of broadside coupling in graphene metamaterials. Further, we employ the Lorentz oscillator based coupled mode theory in order to elucidate the numerical outcomes.

Dynamically Tunable Terahertz Plasmon-Induced Transparency Analogy Based on Asymmetric Graphene Resonator Arrays

Dynamically Tunable Terahertz Plasmon-Induced Transparency Analogy Based on Asymmetric Graphene Resonator Arrays

Squeeze-Film Effect on Atomically Thin Resonators in the High-Pressure Limit.

The resonance frequency of membranes depends on the gas pressure due to the squeeze-film effect, induced by the compression of a thin gas film that is trapped underneath the resonator by the high-frequency motion. This effect is particularly large in low-mass graphene membranes, which makes them promising candidates for pressure-sensing applications. Here, we study the squeeze-film effect in single-layer graphene resonators and find that their resonance frequency is lower than expected from models assuming ideal compression. To understand this deviation, we perform Boltzmann and continuum finite-element simulations and propose an improved model that includes the effects of gas leakage and can account for the observed pressure dependence of the resonance frequency. Thus, this work provides further understanding of the squeeze-film effect and provides further directions into optimizing the design of squeeze-film pressure sensors from 2D materials.

Graphene THz filter-switch dividers based on dipole-quadrupole and magneto-optical resonance effects.

We propose and analyze a multifunctional THz graphene-based component with graphene elements placed on a dielectric substrate. The structure of the device consists of a disc shaped resonator coupled to three graphene waveguides that excite the dipole or quadrupole resonance of surface plasmon polaritons in the resonator. The graphene resonator can be magnetized by a DC magnetic field. This device fulfills filtering of the input signal and can be used as a power divider and also as a switch. The division mechanism of the T-junction can be provided by application of a DC magnetic field or by changing the Fermi energy of the graphene resonator via an electrostatic field. Some peculiarities of the two mechanisms are discussed. Numerical simulations show that for a central frequency of 7.12 THz, devices in the OFF state have the two output ports isolated from the input port at a central frequency of about 27 dB provided by the dipole mode resonance. In the ON state and the division regime, the transmission to the output ports is around -(4÷5)dB in the 3-dB bandwidth of about 12%.

Ultrafast all-optical metamaterial terahertz switch by exploiting nanoparticle-enriched substrate nonlinearity and graphene resonators

The emergence of graphene as a new material with several extraordinary optical properties paves a way to introduce new electromagnetic devices in the terahertz (THz) band. In this paper, we have exploited the unique qualities of graphene in combination with the optical Kerr effect in order to propose a new THz switch. In the proposed structure, two graphene layers with circular and notch ring patterns are stacked to obtain two narrowband absorption peaks at the frequencies of 3.65 and 6.5 THz with high absorption values of 92% and 97%, respectively. The imposed pump power in interaction with the nanocomposite substrate provides a controlling mechanism that makes this structure an ideal candidate for all-optical devices in the THz regime. The obtained switching time of the proposed structure is 2 ps, and the maximum extinction ratio of 83% is achieved for the pump power of 273 mW. Moreover, the absorption spectrum of the switch can be manipulated by the chemical potential of graphene layers, which is useful for electro-optical modulation application. The application of the THz switch as an all-optical tunable filter is also studied in this paper.

Design of a type of broadband metamaterial absorber based on metal and graphene

In this paper, we demonstrate a kind of broadband metamaterial perfect absorber using both graphene and metal resonator elements. Through step by step design and simulation, wider absorption band from about 4.22 THz to 7.48 THz with average absorption rate up to 98.21% is achieved in the absorption spectrum. In addition, the absorber has characteristics of polarization insensitivity and wide incident angle due to its inherent rotational symmetry. Moreover, the absorption band can be adjusted by changing the chemical potential of the graphene. The superiorities of broadband, high absorption rate, polarization independent and wide-angle characteristics make it have potential application prospects in electromagnetic wave absorbing, signal sensing and detection, and other optoelectronic devices.

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.

Tunable graphene-based metasurface for an ultra-low sidelobe terahertz phased array antenna.

In this paper, we propose an all-solid-state, electrically tunable, and reflective graphene metasurface array that can generate a specific phase or continuous scanning between 0° and 352.5° in the terahertz band. By optimizing the structural parameters of the metasurface, the average reflectivity can reach 68.3%, and the maximum reflectivity variation range is only 30%. We also simulate the results that an electrically tunable terahertz phased array can be achieved by adjusting the Fermi levels of a monolayer graphene resonator. The maximum deflection of the reflected beam is 46.05°, and the resolution can be improved to 1.10°. It should be noted that the sidelobe energy only accounts for 1.06% of the main lobe energy, due to the slight change in reflectivity with the phase gradient.

Optical-Thermally Excited Graphene Resonant Mass Detection: A Molecular Dynamics Analysis.

In consideration of the presented optical-thermally excited resonant mass detection scheme, molecular dynamics calculations are performed to investigate the thermal actuation and resonant mass sensing mechanism. The simulation results indicate that an extremely high temperature exists in a 6% central area of the graphene sheet exposed to the exciting laser. Therefore, constraining the laser driving power and enlarging the laser spot radius are essential to weaken the overheating in the middle of the graphene sheet, thus avoiding being burned through. Moreover, molecular dynamics calculations demonstrate a mass sensitivity of 214 kHz/zg for the graphene resonator with a pre-stress of 1 GPa. However, the adsorbed mass would degrade the resonant quality factor from 236 to 193. In comparison, the sensitivity and quality factor could rise by 1.3 and 4 times, respectively, for the graphene sheet with a pre-stress of 5 GPa, thus revealing the availability of enlarging pre-stress for better mass sensing performance.

An Ultrahigh-Sensitivity Graphene Resonant Gyroscope.

In this study, a graphene beam was selected as a sensing element and used to form a graphene resonant gyroscope structure with direct frequency output and ultrahigh sensitivity. The structure of the graphene resonator gyroscope was simulated using the ANSYS finite element software, and the influence of the length, width, and thickness of the graphene resonant beam on the angular velocity sensitivity was studied. The simulation results show that the resonant frequency of the graphene resonant beam decreased with increasing the beam length and thickness, while the width had a negligible effect. The fundamental frequency of the designed graphene resonator gyroscope was more than 20 MHz, and the sensitivity of the angular velocity was able to reach 22,990 Hz/°/h. This work is of great significance for applications in environments that require high sensitivity to extremely weak angular velocity variation.

Imaging vibrations of electromechanical few layer graphene resonators with a moving vacuum enclosure

Imaging the vibrations of nanomechanical resonators means measuring their flexural mode shapes from the dependence of their frequency response on in-plane position. Applied to two-dimensional resonators, this technique provides a wealth of information on the mechanical properties of atomically-thin membranes. We present a simple and robust system to image the vibrations of few layer graphene (FLG) resonators at room temperature and in vacuum with an in-plane displacement precision of ≈0.20 μm. It consists of a sturdy vacuum enclosure mounted on a three-axis micropositioning stage and designed for free space optical measurements of vibrations. The system is equipped with ultraflexible radio frequency transmission lines to electrically actuate resonators. With it we characterize the lowest frequency mode of FLG resonators by measuring its frequency response as a function of position on the membrane and by extracting its effective mass. We use the background noise of the undriven vibrational spectrum to calibrate in-plane displacement. Finally, we measure the first three vibration modes of a resonator whose membrane is partially folded and find that folds locally suppress vibrations.

Broadband terahertz metamaterial absorber based on graphene resonators with perfect absorption

The applications of terahertz waves and metamaterials in electromagnetic wave absorbers are one of the key focus areas of current interdisciplinary scientific research. In this study, we propose a metamaterial absorber composed of graphene double-open rectangular ring and graphene strip cross structures. The experiment uses numerical analysis software to study the proposed absorber. Transverse electric waves were normally incident on the absorber from the plane port, where resonance coupling was achieved. With an increase in the incidence angle alpha, the trough in the middle of the absorption spectrum continued to deepen. The bandwidth that the spectral absorption maintains above 0.9 is 2.88 GHz (1.260–1.548 THz). The maximum spectral absorption has reached 99.9%, which is approximately perfect absorption. The absorber under transverse magnetic wave incidence also exhibited a bandwidth advantage. As the Fermi energy continued to increase, the absorption bandwidth first increased and then decreased, and reached the maximum at ef = 0.5 eV. Simultaneously, the relative absorption bandwidth also reached its maximum. By adjusting the Fermi level of graphene, dynamic tuning of the metamaterial absorber could be achieved. Adjustment of the Fermi level shifted the absorption range and absorption bandwidth, and helped in controlling the increase in the relative absorption bandwidth. The findings of this study can be of theoretical and engineering significance in the domains of thermal photovoltaics, solar cells, and sensors, among others.

The design and 3D simulation of a new high-speed half adder based on graphene resonators

In the field of optical technologies, modern optical scenarios such as integrated multi-operand optical gates are considered as very useful and effective substrates. In this paper, a new and unique design and three-dimensional simulation of a graphene-based integrated half adder by the use of ring resonators and central waveguides have been presented. The proposed structure includes two separate AND and XOR gates, and the presence of graphene in this design allows to tune the wavelength and to turn on or off the structure at the desired wavelength without changing the design parameters and only by changing the Fermi voltage of graphene. The half adder logic is controlled based on the output power of the CARRY and SUM ports at a central wavelength of 1550 nm. Additionally, the extinction ratio obtained for each of the CARRY and SUM ports has been equal to 21.76 dB and 18.73 dB, respectively. The average quality factor of the output of resonators for XOR and AND gates has been 873.75 and 1116.96, respectively. In the proposed structure, single-layer graphene was used because the graphene plasmonic effect was considered. The Extinction ratio in our work is better than the other works. Finite-difference-time-domain (FDTD) and coupled mode theory (CMT) have been the two methods used to extract and analyze the results of the output spectrum of the proposed structure, and the results of the output spectrum have been exactly the same in the two methods.

Coupled-Mode Theory for Graphene-Based Metasurfaces

In this article, a temporal coupled-mode theory (CMT) formalism is proposed to design graphene-based metasurface (MTS) structures. Such structures may be nonuniform and synthesized by numerous graphene resonators. To obtain a desired functionality, such MTSs may be designed, by performing multiple parametric analyses of the structure's geometric parameters. An accurate calculation of their response may be attained by a full-wave vectorial finite-element method (VFEM), by modeling graphene as a surface boundary condition; however, it requires high computational resources, especially for larger configurations. CMT offers an appealing time- and memory-efficient alternative, by solving a sparse system of equations, of particularly small size, fed by the results of simpler and smaller simulations, with the ability to take into account graphene dispersion. CMT may be used to design more complex MTS structures, by providing approximate and preliminary results, which shall be used as a starting point for the final FEM fine-tuning.

Phonon lasing with an atomic thin membrane resonator at room temperature.

Graphene has been considered as one of the best materials to implement mechanical resonators due to their excellent properties such as low mass, high quality factors and tunable resonant frequencies. Here we report the observation of phonon lasing induced by the photonthermal pressure in a few-layer graphene resonator at room temperature, where the graphene resonator and the silicon substrate form an optical cavity. A marked threshold in the oscillation amplitude and a narrowing linewidth of the vibration mode are observed, which confirms a phonon lasing process in the graphene resonator. Our findings will stimulate the studies on phononic phenomena, help to establish new functional devices based on graphene mechanical resonators, and might find potential applications in classical and quantum sensing fields, as well as in information processing.