Exceptional advancement has been made in the development of graphene optical nanoantennas. They are incorporated with optoelectronic devices for plasmonics application and have been an active research area across the globe. The interest in graphene plasmonic devices is driven by the different applications they have empowered, such as ultrafast nanodevices, photodetection, energy harvesting, biosensing, biomedical imaging and high-speed terahertz communications. In this article, the aim is to provide a detailed review of the essential explanation behind graphene nanoantennas experimental proofs for the developments of graphene-based plasmonics antennas, achieving enhanced light-matter interaction by exploiting graphene material conductivity and optical properties. First, the fundamental graphene nanoantennas and their tunable resonant behavior over THz frequencies are summarized. Furthermore, incorporating graphene-metal hybrid antennas with optoelectronic devices can prompt the acknowledgment of multi-platforms for photonics. More interestingly, various technical methods are critically studied for frequency tuning and active modulation of optical characteristics, through in situ modulations by applying an external electric field. Second, the various methods for radiation beam scanning and beam reconfigurability are discussed through reflectarray and leaky-wave graphene antennas. In particular, numerous graphene antenna photodetectors and graphene rectennas for energy harvesting are studied by giving a critical evaluation of antenna performances, enhanced photodetection, energy conversion efficiency and the significant problems that remain to be addressed. Finally, the potential developments in the synthesis of graphene material and technological methods involved in the fabrication of graphene-metal nanoantennas are discussed.

A Review on the Development of Tunable Graphene Nanoantennas for Terahertz Optoelectronic and Plasmonic Applications

Large research articles based on graphene have been published in various areas since 2007 due to its unique properties. Among these designed structures, graphene-based plasmonic waveguides are one of the interesting subjects in communications. In this paper, a historical review of plasmonic graphene waveguides is presented. The graphene-based waveguides can be divided into two main categories 1- rectangular and 2- cylindrical waveguides. In both categories, graphene can be only electrically biased or magnetically and electrically biased simultaneously where the conductivity of graphene becomes a tensor in the second case. Therefore, we have four main categories for plasmonic graphene waveguides which are studied and discussed more precisely in this paper.

Plasmonic Graphene Waveguides: A Literature Review

In order to improve the operating bandwidth and achieve a compact design of the combined antenna, the Klopfenstein impedance taper method is adopted to implement the structure of the combined antenna in this paper. The effects of Klopfenstein taper with different parameters are compared and analyzed with simulation method. Also, the quantity of the magnetic moment of the combined antenna is optimized to improve the low-frequency performance. The relationship between the performance and the structural parameters for the combined antenna is concluded based on the research results. Finally, an optimized combined antenna with length of 30 cm is designed and measured. The impedance bandwidth [voltage standing wave ratio (VSWR) $\lambda $ , is reduced to 0.18. The weighted radiation efficiency within 0–7 GHz is 93.7%. Both numerical and experimental results show that the antenna exhibits uni-directional patterns within the frequency range of 0.5–7 GHz. With a short ultra-wideband pulse excitation, gain of the antenna is measured and derived within the spectrum of the excitation, i.e., 0–3 GHz. The corner frequency of the antenna gain is about 200 MHz, which is close to that of VSWR. Moreover, the radiation efficiency is 76.1%, and the pulse fidelity factor is 0.82 with the applied excitation.

Design and Optimization of High-Power UWB Combined Antenna Based on Klopfenstein Impedance Taper

Ternary MoS2/graphene (G)-TiO2 photocatalysts were prepared by a simple hydrothermal method. The morphology, phase structure, band gap, and catalytic properties of the prepared samples were investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, UV-vis spectrophotometry, and Brunauer-Emmett-Teller surface area measurement. The H2 production efficiency of the prepared catalysts was tested in methanol-water mixture under visible light. MoS2/G-TiO2 exhibited the highest activity for photocatalytic H2 production. For 5 wt.% and 1 wt.% MoS2 and graphene (5MT-1G), the production rate of H2 was as high as 1989 μmol h. The catalyst 5MT-1G showed H2 production activity that was ~ 11.3, 5.6, and 4.1 times higher than those of pure TiO2, 1GT, and 5MT, respectively. The unique structure and morphology of the MoS2/G-TiO2 photocatalyst contributed to its improved hydrogen production efficiency under visible light.

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Synthesis and Characterization of MoS 2 /Graphene-TiO 2 Ternary Photocatalysts for High-Efficiency Hydrogen Production under Visible Light

A novel compact ultrawideband biconical antenna with the impedance bandwidth of 200-965 MHz for VSWR less than 2.35 is proposed. Two metal disks and cylinders are loaded around a traditional biconical antenna, which can form two magnetic dipoles. The broadband impedance matching, miniaturization, and omnidirectional radiation patterns are achieved through the combination of the biconical antenna and two magnetic dipoles. The average H -plane pattern omnidirectionality is less than 1.0 dB in the operating band.

Design of a compact biconical antenna loaded with magnetic dipoles

In this paper, simulation and measurement results of 0.7-7 GHz double-ridged guide horn (DRGH) antenna with coaxial input feed section is presented. This antenna, due to the large frequency band required by standards, is appropriate to be used in electromagnetic compatibility (EMC) testing and antenna measurement as a transmitter. A step by step method for designing DRGH antenna is given. A suitable taper for the ridges in the horn is designed and its impedance variations along the horn are shown. In addition, a new structure for the electrical field probe in the feed section is introduced by which a shift-down of lower frequency to 0.7 GHz is achieved. A sensitivity analysis is done on the parameters of the proposed structure. Other parameters of the feed section are also investigated and optimized values are obtained. Finally, the proposed antenna has been fabricated and measurement results show good agreement with the simulation.

Design and implementation of 0.7 to 7 GHz broadband double-ridged horn antenna

In this letter, design and fabrication of an ultrawideband combined antenna are presented. Actually, this element is a combination of a TEM horn antenna and some magnetic dipoles. The combination of radiation fields due to these elements causes reduction in reactive energy around the antenna, and with this method, the antenna matching in a wide frequency band becomes possible. The bandwidth is further improved by tapering the radiation part of the antenna exponentially. Also, proposing a novel transition structure from coaxial cable to the antenna, the discontinuity the wave faces is reduced and, finally, a bandwidth of 180 MHz-30 GHz is obtained for this structure. The fabrication results for this antenna verify simulation results. The plots of return loss and radiation pattern at several frequencies will be presented. Finite integral-based CST software is utilized for simulation.

Design and Fabrication of a Novel Ultrawideband Combined Antenna

This paper focuses on the analysis of microstrip structures with graphene material as the conducting strip using two numerical methods, spectral domain technique and method of lines. The conductivity of graphene sheet is modeled in the analytical method using a tensor matrix. The results obtained from the above-mentioned methods are compared with those of commercial COMSOL modeling software. The structure is parametrically studied to investigate the effects of chemical potential and magnetic field bias on its behavior. The considered structures demonstrate attractive characteristics such as tunablility of dispersion and characteristic impedance as functions of chemical potential and magnetic bias, which can be used in new microwave applications. These characteristics have not been observed in the ordinary microstrip line structures.

Analysis of Graphene-Based Microstrip Structures

In this paper, the method of lines is extended to analyze 2-D graphene-based multilayered structures. The graphene plates with tensor conductivity may cover partially or totally the interfaces between two consecutive layers. In this paper, the impedance and admittance transformations through the graphene-contained interfaces are developed. Also, the characteristic equation of the graphene-contained multilayered structure is obtained by matching the tangential fields at the graphene sheet. To verify the obtained relations, the following structures are analyzed using the extended method of lines: graphene-based microstrip lines, graphene-based striplines, and graphene-based parallel-plate waveguides. The results of the analysis of the graphene-based microstrip line are compared with the results obtained from the spectral domain approach and COMSOL software, whereas the results of the analysis of the graphene-based stripline and parallel-plate waveguide are compared with those of COMSOL software, which show a good agreement. The conductivity of graphene can be adjusted by varying parameters such as the bias of electric and magnetic fields perpendicular to the graphene sheet. Also, a parametric study is carried out on these parameters by which the characteristics of the analyzed structures can be controlled. The tunable structures are vastly used in new microwave applications.

Analysis of Two Dimensional Graphene-Based Multilayered Structures Using the Extended Method of Lines

In this paper, the design of an array of wideband combined antenna elements is presented. A TEM horn antenna is combined with magnetic dipoles to obtain a wideband element with a bandwidth from 180MHz to 30GHz. The element is then miniaturized in order to be placed in a two by two array, and then the optimized values of the horizontal and vertical spacings are calculated. To enhance the array bandwidth, a lens is placed in front of each element. This leads to an increase in the bandwidth from 0.2 to 10GHz while no grating lobes appear over the bandwidth. The combined element is fabricated and the simulation results are verifled by the measured data. Furthermore, simulation results for the return loss, radiation patterns and antenna gain for the 2 £ 2 array are presented.

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Ali Mehrdadian

Papers

Design and implementation of 0.7 to 7 GHz broadband double-ridged horn antenna

Design and Fabrication of a Novel Ultrawideband Combined Antenna

Analysis of Graphene-Based Microstrip Structures

Analysis of Two Dimensional Graphene-Based Multilayered Structures Using the Extended Method of Lines

Design of a uwb combined antenna and an array of miniaturized elements with and without lens