جستجوی مقالات مرتبط با کلیدواژه "graphene" در نشریات گروه "برق"

In this article, a refractive index absorber and sensor based on graphene metamaterials in the terahertz range is presented. In many studies, the dependence of the sensor on the radiation angle and polarization type has not been investigated, and usually complex structures have been used, the reason for this complexity is the lack of a systematic method and the reader is not convinced by the reasons of the geometry of the structure in the desired frequency band. In this article, advanced transmission line theory has been used to investigate the geometric structure and efficiency of terahertz sensors.The designed sensor has an alternating arrangement of graphene discs on a dielectric material and an integrated gold plate on the other side, which compared to other references has high accuracy in the selection of parameters and the geometry of its structure without the use of optimization algorithms has been calculated. The values maximum figure of merit (FOM) 24.5, the quality factor (Q-Factor) 80 and the sensitivity 1.57 THz⁄RIU have been obtained, which has improved significantly compared to previous works. Also, the simplicity of the structure, reduction of calculation time and memory, higher sensitivity and better stability are the advantages of this design compared to the previous methods.

In this paper, the Aluminum-doped graphene has been investigated and simulated based on the electron density functional theory as a sensor for nitrogen dioxide and hydrogen sulfide gases. The simulation results using SIESTA software demonstrate that the optimum doping is substitutional and occurs when a carbon atom in graphene is replaced with an Al atom. The dopant Al atom at the minimum relaxation energy is placed 1.85 Å higher than the graphene sheet and creates a bulge on the graphene surface. The different probable adsorption states of each NO2 and H2S gas were simulated and the results showed that the optimum adsorption occurred around the placement of dopant where the electron density is higher. Finally, the adsorption energy of NO2 and H2S molecules were obtained -3.45 eV and -0.87 eV, respectively. The resulting adsorption enthalpies, which demonstrate the formation of strong bonds, illustrate that doped graphene could serve as a suitable substrate for the fabrication of NO2 and H2S sensors.

In this study, a plasmonic infrared refractive index sensor (RI) based on the Metal-Insulator-Metal (MIM) structure by two gold nanorods in the top layer to measure the analytic refractive index has been proposed. Sensor characteristics such as Sensitivity (S), Full Width at Half Maximum (FWHM) and Figure of Merit (FOM) were calcuted 736.842 nm / RIU, 40 nm and 18.421, respectively.Since the two-dimensional nature of collective provocations in graphene makes plasmon imprisonment in graphen is more powerful than metals and in addition, the  metals does not ideally absorb biological molecules, A graphene layer has been added to increase the sensitivity of the sensor. Finally, the gold nanorod sensor using graphene layer between the nanorods and dielectric was investigated and the characteristics of the sensor with thickness of graphene layer, 2 nm and chemical potential of 0.7 eV have obtained as S = 778.571 nm / RIU, FWHM = 46 and FOM = 16.045.

In this paper, a unique symmetric graphene-based plasmonic nano-waveguide is proposed to guiding electromagnetic waves in the mid-infrared spectrum. The geometric structure of the waveguide consists of two symmetrical cylinders arranged around a rectangular substrate with a high refractive index. The lateral surfaces of both cylinders are covered with a graphene monolayer, and the cylinders and rectangular substrate are placed in a low refractive index environment. Here, the modal model properties of surface plasmon polaritons are investigated using the finite element method. The dependence of the propagation properties of the modes on the frequency of the incident wave, the graphene Fermi energy and the dimensions of the geometric structure of the waveguide has been calculated in detail. Due to the unique structure of the designed waveguide, the propagation length of the modes is approximately 4600 nm , the normalized mode area is ~〖10〗^(-6) and the figure of merit is nearly 500. According to the simulation results, this structure has a strong confinement the light subwavelength and high performance in the mid-infrared region for use in practical applications in photonic integrated circuits.

In this paper, we present an electro-optical modulator based on graphene absorption. In this structure, graphene is placed on aluminum oxide, the structure of which is repeated alternately. This alternating structure of graphene on aluminum oxide is located inside a semi-cylindrical cavity in a 1-dimensional photonic crystal nanobeam cavity. Unlike previous articles, which consisted of cylinder holes and different areas in the nanobeam cavity is formed by changing the radius of the holes, in this structure, semi-cylinder holes have been used and, different areas are formed by rotating these semi-cylinders. This type of crystal photonics has a high quality factor. Also, this type of modulator requires little space after construction, so they are a very good option for integrated circuits. In this paper, three-dimensional finite-difference time-domain method is used for analysis. In this modulator, it is observed that by changing the bias voltage, the amount of absorption peak and also its resonance wavelength are changed. As a result of these changes, we achieve a modulation depth of about 7dB. Our proposed structure can have potential applications in integrated optical circuits, especially in telecommunication frequencies.

In optoelectronic devices such as photodetectors, solar cells, modulators, filters, etc., the optical absorption coefficient is a key parameter in successful device operation. To this end, in this paper, a dual band absorber, consisting of a metal-graphene-dielectric-metal structure which can operate at communication bands is presented. First of all, the light absorption characteristic has been achieved by finite difference time domain method and then the influences of the thicknesses of top metallic structure, dielectric and substrate layers on the light absorption of the structure have been analyzed. The results revealed that utilizing the graphene monolayer and the plasmonic structure improves the light-graphene interaction and in result, the graphene layer absorption. It is shown that the proposed structure has the light absorption of 95% at the wavelength of 1310nm and the light absorption of 96% at the wavelength of 1675nm. Therefore, this absorber has the ability of operating at two optical communication bands (O-band and U-band) simultaneously.

This paper presents a graphene-based polarization-insensitive metamaterial perfect absorber at terahertz frequency range. The absorber consists of a graphene pattern layer at the top of the structure, a dielectric spacer layer, and an Au layer at the bottom of the structure. In the proposed structure, the ability of perfect absorption is investigated by the complete suppression of radiation and reflected light and the complete dissipation of incident energy. At the first, the proposed structure and its design process are verified and it is shown that in the three bands, the results of perfect absorption at frequencies of 2, 2.95 and 3.75 terahertz are achieved with absorption rates of 93.5%, 99.8% and 98.1%, respectively. Also, the physical mechanism of structure performance is investigated by the surface distribution of the electric field, as well as the geometric changes of the structure. In addition, the design of the proposed structure with graphene provides this advantage so that the resonant frequency can be adjusted by changing the Fermi energy level and graphene relaxation time without changing the proposed structure again. With the study, it is found that the proposed metamaterials perfect absorber is not sensitive to polarization and is more tolerant to the angle of incident radiation. Accordingly, the proposed broadband absorber in this paper has potential in imaging, detection, filtering, solar tracking and other applications.

In the last decade, optical integrated circuits, including modulators, have made significant progress in optical communications, imaging, and sensors. Among the active materials used in modulators, graphene and indium tin oxide (ITO) are some of the suitable options among the active materials for modulation action due to their epsilon-near-zero (ENZ) characteristics, speed, and considerable response. In this paper, by applying direct coupling of light, the structure of plasmonic modulator in three-dimensional mode has been designed. And by changing the thickness of ITO, HfO2 (hafnium oxide) layers and waveguide width, the structure is optimized. Optimization thickness of 3 nm for ITO, 5nm for HfO2 and 280nm for waveguide width is achieved. The results of three-dimensional simulations of this paper with appropriate coupling show that the insertion loss (IL) of three-dimensional mode have not changed and the extinction ratio (ER) of the modulator has been slightly reduced in comparison with two-dimensional mode .On the other hand, appropriate and optimal coupling has no effect on energy consumption.Three-dimensional results show that the proposed plasmonic modulator can achieve an extinction ratio of 13.9 dB, an insertion loss of 2.9 dB, modulation speed of 140.9 GHz and a very low power consumption of 1.51 fj/bit for a 1µm length of the modulator, at 0.5 V voltage and a wavelength of 1.55 µm. Our design demonstrates a considerable reduction in energy consumption and improvement in extinction ratio compared to previous works. Also, for a 2 µm length of the modulator, an extinction ratio of 27.76 dB, an insertion loss of 5.68 dB, modulation speed of 70.14 GHz and power consumption of 2.88 fj/bit is achieved.

Schottky-barrier-type graphene nanoribbon transistors (SB-GNRFET), despite their prominent characteristics compared to conventional transistors, have a relatively high off-current and a low Ion/Ioff ratio. In this paper, a new structure of SB-GNRFET is presented in which the gate of the transistor is divided into two parts. A constant voltage is connected to the gate located on the drain side, and the gate located on the source side is the main gate of the transistor. The proposed SB-GNRFET is simulated using non-equilibrium Green functions-based numerical simulator under different geometric and physical characteristics and in biases. The simulation results show Ion/Ioff ratio improvement of up to 6.7-fold at VDS = 0.8 V. At this voltage the ratio has increased from 1.2 in the normal SB-GNRFET transistor to 8.01 in the new transistor and the off current has been reduced from 5 µA to 0.7 µA. Also at VDS = 0.6 V, as the supply voltage, the Ion/Ioff ratio increased from 3.97 to 15.8 and the off current decreased from 0.63 µA to 0.16 µA.

The design and simulation of optical switches is of great importance due to the optimal use of the bandwidth and bandwidth available in telecommunication networks. The subject of this article includes the design and simulation of microprocessor resonators and all-light switches with graphene adjustment. In fact, this paper evaluates the optical switches based on the microstructure of the resonator and their electro-optical adjustment by applying voltage to the applied graphene layer. The designed switch has a high scale due to its micrometer size. In fact, in this article, by designing different micro-circuit structures of resonators, optical switch with decreasing coefficient and wide bandwidth has been designed.

A seamless graphene inverter including graphene nanoribbon field effect transistor (GNRFET) and graphene interconnect is proposed. The seamless structure is suggested to eliminate the ohmic, schottky, and parasitic resistances in the junction of the traditional interconnects with the Gate, Source and Drain of GNRFET. After that, using the circuit models of the graphene devices that are used in the proposed structure, transfer matrix model of the proposed seamless graphene inverter is calculated and extracted. All of the capacitive, inductive and scattering effects are included in the assumed circuit models of the GNRFET - graphene interconnect and consequently in the overall matrix model of the seamless graphene inverter. Elimination of the ohmic, schottky and parasitic resistances causes to improve in the working speed of the proposed inverter. Extraction of the transfer matrix model of the seamless graphene inverter and calculation of its step time response, relative stability and frequency bandwidth confirms this improvement. The advantage of the transfer matrix model of the proposed inverter is that any change in the physical parameters of the graphene nanoribbons that are used in the structure can be included in the model and one can analyze the effect of it in all of the technology nodes. Using the circuit model and the extracted transfer matrix, anyone can evaluates various stability analyses such as Nyquist, Bode and Nichols together with the time-frequency responses of the graphene seamless inverter used in very large scale integrated (VLSI) circuits.

In this paper, graphene has been designed for tunable scattering manipulation of a dielectric cylinder. The goal is changing scattering properties of a dielectric cylinder to ones of another cylinder with desired radius. For this purpose, scattering coefficients of the covered and target cylinders have been achieved and equated. Required surface impedance of the graphene in order to obtain the considered goal in the frequencies of 3 and 4 THz has been derived. By properly tuning the chemical potencial of the graphene, caused by induced voltage, this goal has controllably been achieved. Total scattering widths of the bare, target and covered cylinders are obtained from numerical simulations and analytical calculations.

In this paper, for the first time, the role of graphene-based emerging technology has been investigated in digital adder circuits. The aims are to decrease power consumption and enhance security against power analysis attack. Static and current mode logic (CML) styles are employed to design one, four, and eight-bit adders in silicon and graphene technologies. In simulations, a SPICE-compatible model for graphene transistors and a PTM model for silicon FINFET transistors are used. Results reveal that the graphene-based static adders show the least power consumption. For an 8-bit adder, power overshoots and standard deviation from power traces confirm that a graphene-based CML (G-CML) adder is the most robust scheme against power analysis attack among the static and CML designs. Eventually, a new hybrid approach is proposed by providing a circuit in which security is enhanced by creating an irregularity in the power trace and this makes the data detection more difficult. Accordingly, an adder based on the proposed approach yield higher security with a different pattern of a power trace and also contains lower power consumption than pure CML-based adders.

Since graphene discovery, designing and implementation of electronic circuits have been developed in the THz and optical frequencies to achieve ultrafast responses. Thus, electromagnetic shielding due to its protection effects against disturbances caused by adjacent elements has emerged as a vital issue in circuit designing. In this paper, two types of electromagnetic shields are proposed in the THz regime. Regarding the adjustability of graphene’s conductivity, one can easily tune the frequency response in order to adapt it with the frequency range in which their circuit works. In order to accelerate the computation of shield efficiency factor, firstly an equivalent circuit model is proposed as a closed-form expression, and then shield efficiency can be achieved using transmission line model. Comparisons indicate that the results derived from the proposed method are in high accordance with those of CST-MWS commercial software. Finally the effects of the number of layers, the thickness of SiO2 layers, the width of graphene ribbons, the gap between the two individual cells, Fermi energies, and oblique incident on the shielding effectiveness of the proposed structure are discussed in details for interesting readers.

In this paper, by using the equivalent electrical circuit method, an analytical study for the electrical and dissipated energy densities in graphene is presented. In the first step, electronic excitations on the graphene surface are described by an infinitesimally thin layer of electron fluid, ignoring the electrons completely. Then, general expressions of electrical and dissipated energy densities are obtained by using a simple equation of motion for an electron of the electron fluid (that is subjected to a time dependent external electric field) in conjunction with the equivalent electrical circuit method. In the next step, in the range of high frequencies, by means of the conductivity formula of the system that is recently presented, the problem is investigated in a two-fluid model.