Fermi Level Of Graphene Research Articles

Tunable plasmon-induced transparency with a dielectric grating-coupled graphene structure for slowing terahertz waves.

We present a tunable plasmon-induced transparency (PIT) structure that is composed of dielectric grating and a graphene system to manipulate terahertz (THz) waves. The graphene system consists of a graphene sheet and a graphene ribbon layer, with a spacer between them. By exploiting the diffraction coupling of THz wave with dielectric grating, graphene plasmonic resonance is efficiently excited on both graphene sheet and graphene ribbons. This leads to the surface plasmon mode of the graphene sheet and the localized plasmon mode of the graphene ribbons. The coupling between the two-plasmon modes via near-field destructive interference generates a strong PIT effect with slowing the group velocity of THz waves. A group delay over 0.2 ps and group index beyond 170 can be achievable. The group slowing effect is dynamically tunable with varying the Fermi level of graphene. The work suggests a promising scheme for on-chip graphene slow-wave devices at the THz regime.

Studies of the Dirac Point in a GO/P3HT Nanocomposite Thin-Film Phototransistor

Developing a numerical model for thin-film transistors has become significant for optoelectronic applications. In this study, we describe the shift of the Dirac point in a graphene oxide thin-film phototransistor doped with various ratios of poly (3-hexylthiophene) (P3HT) (0.01 and 0.05). According the electrical characteristics of graphene oxide/poly (3-hexylthiophene) thin-film transistors and based on the proposed model, we simulate the carrier concentration, the Fermi level (Ef), the mobility (μ), and the conductivity (σ) of charge carriers, the square resistance, and the Seebeck coefficient as a function of the applied gate voltage in the dark and under the illumination of 100 mW/cm2, using Matlab/Simulink. The results show that, when applying a negative gate voltage, the Fermi level of graphene will shift below the Dirac point, due to the electrical field effect induced by the P3HT molar ratios and the illumination effect. This shift is exhibited more obviously in the mainly simulated parameters, and can be explained by the molar ratios of P3HT, which modulate the displacement field to allow the opening of a transport band gap through a Colombian force created by the oxygen groups. This work can provide a theoretical basis for analyzing the characteristics of these components for application in the logic circuit domain.

Dual-band high impedance surface with graphene-based metasurfaces

High impedance surface (HIS) is an electromagnetic band gap (EBG) material that shows magnetic conductor surface properties within its band gap frequencies. The operation bandwidth is corresponding to the frequency range, where the reflection phase varies between +90∘ and −90∘. In this paper, a dual-band HIS is investigated based on planar periodic graphene arrays placed on a grounded dielectric substrate. Analytical circuit model of the graphene array together with the transmission line theory is employed to analyze the proposed structure. We demonstrate that the HIS bands can be adjusted by tuning the geometrical parameters of the structure and the Fermi level of graphene. The graphene-based HIS promises future applications in the low-profile, high gain and high efficiency antennas at THz frequencies.

Electrically Tunable Four-Wave-Mixing in Graphene Heterogeneous Fiber for Individual Gas Molecule Detection.

Detection of individual molecules is the ultimate goal of any chemical sensor. In the case of gas detection, such resolution has been achieved in advanced nanoscale electronic solid-state sensors, but it has not been possible so far in integrated photonic devices, where the weak light-molecule interaction is typically hidden by noise. Here, we demonstrate a scheme to generate ultrasensitive down-conversion four-wave-mixing (FWM) in a graphene bipolar-junction-transistor heterogeneous D-shaped fiber. In the communication band, the FWM conversion efficiency can change steeply when the graphene Fermi level approaches 0.4 eV. In this condition, we exploit our unique two-step optoelectronic heterodyne detection scheme, and we achieve real-time individual gas molecule detection in vacuum. Such combination of graphene strong nonlinearities, electrical tunability, and all-fiber integration paves the way toward the design of versatile high-performance graphene photonic devices.

The triboelectric microplasma transistor of monolayer graphene with a reversible oxygen ion floating gate

Using the adsorbates as a floating gate on graphene is an importance strategy for developing novel field effect transistor. However, O 2 , as the most common and active gas in air, has not been used as floating gate of graphene transistor, since its lowest unoccupied molecular orbital (LUMO) is higher than the Fermi level of graphene, blocking the formation of O 2 − ions. Here, the graphene transistor using O 2 − as a reversible floating ion gate has been reported by introducing triboelectric microplasma to activate the adsorption path, where O 2 molecule is activated into O 2 − ion before its adsorption. The adsorbed O 2 − with a concentration of 3.45 × 10 12 cm −2 has been realized in the experiment, acting as a negative floating gate to move down Fermi level and produce p-type doping of graphene. The floating gate of O 2 − is reversible, which can be erased by heating and the desorption barrier is calculated as 198 meV. The ab initio simulation shows that, the LUMO level of the adsorbed O 2 − is lowered to 0.85 eV below the Fermi level of graphene, which overcomes the barrier in the adsorption path with microplasma. The experiments results have demostrated that the triboelectric microplasma technology has potential applications in developing novel electronic and optoelectronic devices. This paper, the adsorption of O 2 − on monolayer graphene is achieved by introducing the triboelectric microplasma to generate a novel adsorption path, acting as a negative floating gate to move down Fermi level and produce p-type doping of graphene. The floating gate of O 2 − is reversible, which can be erased by heating and the desorption barrier is calculated as 198 meV. The ab initio simulation shows that, the LUMO level of the adsorbed O 2 − is lowered to 0.85 eV below the Fermi level of graphene, which overcomes the barrier in the adsorption path with microplasma. The technology has potential applications in developing novel electronic and optoelectronic devices of graphene. • The graphene transistor using O 2 − as a reversible floating ion gate has been reported by introducing triboelectric microplasma to activate the adsorption path. • The adsorbed O 2 − act as a negative floating gate to move down Fermi level and produce p-type doping of graphene. • The floating gate of O 2 − is reversible, which can be erased by heating and the desorption barrier is calculated as 198 meV. • The ab initio simulation shows that, the LUMO level is lowered to 0.85 eV below the Fermi level of graphene, which overcomes the barrier in the adsorption path with microplasma. • The technology has potential applications in developing graphene-based novel electronic and optoelectronic devices.

Simultaneous switching at multiple frequencies and triple plasmon-induced transparency in multilayer patterned graphene-based terahertz metamaterial

A novel multilayer terahertz metamaterial composed of double rectangle, vertical single rectangle, vertical double rectangle and single rectangle graphene layer is proposed. The dynamic adjustable triple plasmon induced transparency (PIT) is realized by coupling two bright modes and two dark modes, which is an especial synergy effect between two single-PIT. Coupled mode theory contained four resonators is employed to explain the triple-PIT, and the theoretical results exhibit excellent consistency with finite-difference time-domain. Surprisingly, the triple-PIT can evolve into a dual-PIT or a single-PIT only by changing the Fermi level of graphene, and the amplitude modulation degrees at the four resonance frequencies of the triple-PIT are 74.7%, 87.8%, 76.5%, and 77.7%, respectively. In addition, a simultaneous switching at multiple frequencies is realized by adjusting different Fermi levels. Therefore, this study not only lays the foundation for explaining phenomenon of the triple-PIT but also puts forward new ideas for the design of optoelectronic device.

Wide-Range Tunable Narrow Band-Stop Filter Based on Bilayer Graphene in the Mid-Infrared Region

In this paper, a tunable graphene-based band-stop filter in the mid-infrared region was demonstrated, which consists of periodic structure based on bilayer graphene nanoribbons. Due to the unique tunable Fermi level of graphene, the resonance wavelength of the filter can be tuned over a wide range (10020 nm–15100 nm) with the applied voltage varying from 1 V to 5 V, realizing selectivity and tunability. The results also confirm that the presented structure can realize a narrow-band selective filter with a 3 dB bandwidth of 40 nm. Furthermore, the trough transmittance of stopband is close to zero and the transmittance of passband remains above 96.7%. Moreover, this filter is quite robust to the incident angles. The stopband is still extremely narrow as the incident angle varies from 0° to 60°, realizing wide-angle filtering. In addition, the fabrication processing and fabrication error tolerance are investigated, and it has proved that the filter is quite tolerant to fabrication deviations. Hence, the work has potential applications in narrowband filters, active plasmonic switches, and sensors in the mid-infrared region.

Flexible control of absorption enhancement of circularly polarized light via square graphene disks

A narrowband absorber consisting of periodically patterned square graphene disks (SGDs) is proposed to achieve flexible control of the absorption enhancement of circularly polarized light (CPL) in the far-infrared region. It is shown that absorption of CPL can be enhanced by utilizing the double-cavity enhancement of edge graphene plasmons (EGPs) of the SGDs in both x and y directions. Perfect light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. By using the Fabry-Pérot (F-P) cavity model with a linear fitting method, the location of the absorption peak of CPL can be well estimated. The location of the absorption peak can be modulated by changing the Fermi level of graphene, while it can be kept almost the same even though the structural parameters such as period and the thickness of the dielectric spacer are significantly altered. Furthermore, by integrating multi-sized SGDs into the unit cell of the structure, multiple absorption channels of CPL with good absorption performances can be realized. As examples, two and three absorption channels with high peak absorptivity are demonstrated via double and triple SGDs, respectively.

A Polarization-Insensitive and Wide-Angle Terahertz Absorber with Ring-Porous Patterned Graphene Metasurface.

A broadband terahertz (THz) absorber, based on a graphene metasurface, which consists of a layer of ring-porous patterned structure array and a metallic mirror separated by an ultrathin SiO2 dielectric layer, is proposed and studied by numerical simulation. The simulated results show that the absorptivity of the absorber reaches 90% in the range of 0.91–1.86 THz, and the normalized bandwidth of the absorptivity is 68.6% under normal incidence. In the simulation, the effects of the geometric parameters of the structure on the absorption band have been investigated. The results show that the absorber is insensitive to the incident polarization angle for both transverse electric (TE) and transverse magnetic (TM) under normal incidence. In addition, the absorber is not sensitive to oblique incidence of the light source under TE polarization conditions, and has an approximately stable absorption bandwidth at the incident angle from 0° to 50°. The absorption band can be adjusted by changing the bias voltage of the graphene Fermi level without varying the nanostructure. Furthermore, we propose that a two-layer graphene structure with the same geometric parameters is separated by a dielectric layer of appropriate thickness. The simulated results show that the absorptivity of the two-layer absorber reaches 90% in the range of 0.83-2.04 THz and the normalized bandwidth of the absorptivity is 84.3% under normal incidence. Because of its excellent characteristics based on graphene metamaterial absorbers, it has an important application value in the field of subwavelength photonic devices.

Electronic and optical properties of borophene and graphene with an adsorbed ionic liquid: A density functional theory study

We present a comparative density functional theory study of the electronic and optical properties of graphene and borophene with an adsorbed ionic liquid. The Dirac cones of these two-dimensional nanostructures are not affected by the presence of the ionic liquid close to these sheets, not even in the borophene one in which a very small charge transfer to the ionic liquid anion is detected. However, Bader analysis and charge density calculations showed that the polarization of the ionic liquid induces a redistribution of the electric charge in both sheets as a consequence of the electromagnetic interaction between the anion and cation of the ionic liquid and the electrons on the two-dimensional materials, the effect being notably stronger in graphene than in borophene. Our calculations reveal that several changes take place in the dielectric constant and electron energy loss spectra, including shifts and damping of plasmon bands, related to electrostatically induced changes in the adsorbent surface electron charge distributions and to plasmon loss channels associated to excitations of electrons in the energy levels of the adsorbed ionic liquid species close to the Fermi level of the surface material. While only minor changes are registered in borophene plasmon spectra and in the IR and visible regions of the graphene spectra, important changes in the graphene ultraviolet C and X-ray spectra are registered due to the existence of IL electronic energy levels close to the Fermi level of graphene that are absent in borophene.

Broadband Adjustable Terahertz Absorption in Series Asymmetric Oval-Shaped Graphene Pattern

We propose a broadband adjustable graphene absorber in terahertz regime. A prototype is designed to verify the terahertz absorption characteristics by a layer of a periodic array of series asymmetric oval-shaped graphene patterns on polyimide and bottom layer of metal plate. The absorber with 92% absorption and the bandwidth ranging from 3.39 THz to 5.96 THz can be achieved. By changing the Fermi level of graphene, the bandwidth and absorption can be flexibly adjusted. The absorption remains above 92% in the range of 0~40o incident angle. Using these characteristics, the simple absorber has broad application prospects in the fields of filter and switch in terahertz region.

Graphene-bridged topological network metamaterials with perfect modulation applied to dynamic cloaking and meta-sensing.

Perfect state transfer of the bus topological system enables the sharing of information or excitation between nodes. Herein we report groundbreaking research on the transfer of the graphene-bridged bus topological network structure to an electromagnetic metamaterial setting, named "bus topological network metamaterials (TNMMs)." Correspondingly, the electromagnetic response imprints onto the topological excitation. We find that the bus-TNMMs display a perfect modulation of the terahertz response. The blue-shift of resonance frequency could increase to as large as 1075 GHz. The modulation sensitivity of the bus-TNMMs reaches 1027 GHz/Fermi level unit (FLU). Meanwhile, with the enhancement of modulation, the line shape of the reflection keeps underformed. Parabola, ExpDec1, and Asymptotic models are used to estimate the modulation of the resonance frequency. Besides, the bus-TNMMs system provides a fascinating platform for dynamic cloaking. By governing the Fermi level of graphene, the bus-TNMMs can decide whether it is cloaking or not in a bandwidth of 500 GHz. Also, the bus-TNMMs exhibit the immense potential for dynamically detecting the vibrational fingerprinting of an analyte. These results give a far-reaching outlook for steering dynamically the terahertz response with the bus-TNMMs. Therefore, we believe that the discovery of bus-TNMMs will revolutionize our understanding of the modulation of the electromagnetic response.

Active EIT-like Dependent on Polarization Angle with Graphene Metamaterial

An active electromagnetically induced transparency (EIT) analog adopting graphene as microstructure of metamaterial is investigated. The analog consist interconnected graphene strip (GS) and graphene split ring resonator (GSRR), which can act as bright and dark elements alternately. The distinct EIT-like effect can be generated by bright-dark mode coupling in x and y polarization directions. Further investigations reveal that EIT peak can be actively controlled via Fermi level of graphene, and theoretical analysis based on two particles mode express the reason. Moreover, notable EIT-like effect can also be generated in a wide range of polarization angles, and impact of polarization angle on EIT peak is also discussed in detail. These characteristic paves the way for the applications of EIT-like in sensor device.

Plasmonic biosensor based on excellently absorbable adjustable plasmon-induced transparency in black phosphorus and graphene metamaterials

Both black phosphorus (BP) and graphene metamaterials support surface plasmon resonance that is very sensitive to the local change of dielectric properties in the system. We design a novel BP ribbons and graphene sheet hybrid structures based on tunable mid-infrared plasmon-induced transparency. The destructive interference of two excited state modes in the structure leads to a pronounced induced transparent window, and the excellent absorption of the two channels is obtained. The transmission characteristics of the hybrid structures are theoretical calculation, and the results are in good agreement with the numerical simulation curve. The dynamic adjustment function can be realized by adjusting the carrier density of BP and the Fermi level of graphene, and the excellent absorption performance not affected within the adjustment range. Moreover, we found that the optical phase is rapid flips at the plasmon resonance position. Therefore, a sensing medium layer is added on top of the structure to implement a high-sensitivity plasmonic biosensor. This work may provide potential applications to plasmonic devices based on two-dimensional metamaterial.

Tuning of the polariton modes induced by longitudinal strong coupling in the graphene hybridized DBR cavity.

In this Letter, we construct a graphene hybridized distributed Bragg reflector (DBR) cavity, where spatially longitudinal strong coupling occurs between the Tamm plasmon polaritons (TPPs) existing around the graphene layer and the cavity mode (CM) existing in the DBR cavity. As a result, two hybrid polariton modes emerge, which contain both the TPP and the CM components. In the simulation, we demonstrate that the resonant frequencies and the damping rates of the polariton modes can be actively tuned by the graphene Fermi level and the incident angle of light. Besides, the coupling strength and the damping rates are also passively tuned by the pair number of the layers in the DBR. Theoretically, we analyze the TPP-CM strong coupling by the coupled harmonic oscillator equations, which help to explain the regulation process. The controllable TPP-CM longitudinal strong coupling with two absorption bands may achieve potential applications in developing graphene-based active optoelectronic and polaritonic devices in terahertz waves.

Analysis of infrared spectroscopy absorption characteristics of graphene transferred to grating structures

In this paper, we designed and fabricated a graphene transferred to grating structure with high absorption from 4 µm to 14 µm. The grating structure was composed of Au/SiO2/Au and the graphene was grown by chemical vapor deposition (CVD) in vacuum conditions. We have experimentally demonstrated that magnetic polaritons (MPs) and graphene plasmon were excited to achieve high absorption in six sets of graphene grating devices in the infrared region, which were in good agreement with the simulation results. We also discuss the effect of controlling the Fermi level of graphene on the absorption spectrum of the device. The research method can provide potential support for the application of graphene in infrared optoelectronics.

Investigation of graphene supported terahertz elliptical metamaterials

Abstract Based on a graphene asymmetric sdouble ellipse (ADE) structure, the tunable Fano resonance characteristics in the terahertz range have been symmetrically studied, including the effects of graphene Fermi levels, asymmetrical degrees, operation frequencies and structural parameters. The results show that due to the strong coupling between the incident THz waves and graphene ADE structures, a strong Fano peak can be observed by arranging double ellipse asymmetrically. The maximum peak of Fano resonance reaches 0.918, and its Q-factor is about 20. As the asymmetrical degree of the ADE structure increases, the Fano peak value increases, and the amplitude modulation depth reaches about 75%. As graphene level increases, the amplitude and frequency modulation depths reaches more than 31% and 11%, respectively. These results are very helpful to design novel tunable graphene devices with high Q-factor devices in the future, such as sensors, modulators, and filters.

Graphene Plasmonics in Sensor Applications: A Review.

Surface plasmon polaritons (SPPs) can be generated in graphene at frequencies in the mid-infrared to terahertz range, which is not possible using conventional plasmonic materials such as noble metals. Moreover, the lifetime and confinement volume of such SPPs are much longer and smaller, respectively, than those in metals. For these reasons, graphene plasmonics has potential applications in novel plasmonic sensors and various concepts have been proposed. This review paper examines the potential of such graphene plasmonics with regard to the development of novel high-performance sensors. The theoretical background is summarized and the intrinsic nature of graphene plasmons, interactions between graphene and SPPs induced by metallic nanostructures and the electrical control of SPPs by adjusting the Fermi level of graphene are discussed. Subsequently, the development of optical sensors, biological sensors and important components such as absorbers/emitters and reconfigurable optical mirrors for use in new sensor systems are reviewed. Finally, future challenges related to the fabrication of graphene-based devices as well as various advanced optical devices incorporating other two-dimensional materials are examined. This review is intended to assist researchers in both industry and academia in the design and development of novel sensors based on graphene plasmonics.

Dynamically tunable angular optical transparency induced by photonic topological transition in graphene-based hyperbolic metamaterials

Based on photonic topological transition (PTT), the dynamically tunable angular optical transparency of graphene-based hyperbolic metamaterials is theoretically and numerically investigated in mid-infrared region. The results indicate that an ultra-narrow angular optical transparency window with angular full width at half maximum of 0.8° appears near the transition point of PTT for TM waves, which is achieved by tailoring the metamaterial's equi-frequency surface from closed ellipsoid to open hyperboloid. In addition, we further propose the application of TM-pass/TE-reflect polarization beam splitter in the case of normal incidence. Besides, the broadband of angular optical transparency can be realized by changing the Fermi level of graphene.

Manipulating Charge and Energy Transfer between 2D Atomic Layers via Heterostructure Engineering.

Two-dimensional (2D) van der Waals heterostructures have attracted enormous research interests due to their emergent electrical and optical properties. The comprehensive understanding and efficient control of interlayer couplings in such devices are crucial for realizing their functionalities, as well as for improving their performance. Here, we report a successful manipulation of interlayer charge transfer between 2D materials by varying different stacking layers consisting of graphene, hexagonal boron nitride, and tungsten disulfide. Under visible-light excitation, despite being separated by few-layer boron nitride, the graphene and tungsten disulfide exhibit clear modulation of their doping level, i.e., a change of the Fermi level in graphene as large as 120 meV and a net electron accumulation in WS2. By using a combination of micro-Raman and photoluminescence spectroscopy, we demonstrate that the modulation is originated from simultaneous manipulation of charge and/or energy transfer between each of the two adjacent layers.