DESIGN AND ANALYSIS OF GRAPHENE PLASMONIC ANTENNAS FOR TERAHERTZ APPLICATION

Abstract

Acute shortage of spectrum in the lower frequency range and benefit of using higher frequencies led the researchers to think of using beyond gigahertz frequency and towards terahertz (THz). In recent years, THz technology has gained more research interest because of its growing number of applications in spectroscopy, medical, earth and space science, defense, communication, material characterization, sensing and imaging. The reason of so much interest in the THz technology is due to the non-ionizing nature, high penetration with low attenuation, high resolution imaging capability of this signal. Although the THz communication system is in its infancy, but looking to the future, definitely it will have a huge impact on the societal advances. As the THz antennas play a significant role to achieve the better performance in the THz technology advancement, researchers have already engaged themselves for the design and analysis of effective THz antennas. The present thesis is an attempt in this direction. The work done in the thesis is to find the answer to some of these key questions in THz antenna area: (i) why to choose graphene for design of THz antennas? (ii) how to design effective graphene plasmonic antennas? (iii) how to improve the electromagnetic performance of graphene based antennas? and (iv) how to use graphene based antenna arrays for beam-switching applications? The thesis is organized as follows. Chapter-1 presents an overall introduction of the work done in the thesis. It starts with an introduction of plasmonic antennas and its requirement in the present day scenario. This chapter also sets the research objective for the thesis. In Chapter-2 of the thesis, a state-of-the-art overview of the design of THz antennas made-up of different materials including copper, CNT and graphene is given. Metal based THz antennas have their own limitations. This fact boosts the use of carbon materials, such as, CNT and graphene for making THz antennas. Based on the literature survey it was found that, Graphene is the latest material used for THz antenna design. Although the literature survey reveals many THz antennas, but why to use a specific material for design of THz antennas and which material is best suitable for this is viii missing from the literature. In order to find a suitable material for THz antenna design, the electromagnetic performance of possible materials that has been reported in the literature for THz antenna design, such as metal, graphene, and CNT has to be tested. This particular task has been carried out in chapter-3. Performance of THz antennas is carried out by analyzing their material properties and behavior at THz. Results reveal that the antenna made-up of graphene has better performance in terms of radiation efficiency, directivity and miniaturization. For this reason, a thorough analysis has been done for the graphene plasmonic antenna. The phenomenon of surface plasmon polariton (SPP) was used in order to give reasoning for the use of suitable material for THz antenna applications. The concept and analysis of a graphene plasmonic antenna over SiO2/Si substrate at THz band are presented in the chapter-4. The performance enhancement of the antenna is proposed by dynamically controlling the surface conductivity of graphene using electric field effect. The controlling ability of graphene via gate voltage enables frequency reconfiguration of the antenna. The performance merits of the antenna in terms of its high directivity, low reflection coefficient, stable input impedance and high miniaturization have been presented in this chapter. Further, a dual-band reconfigurable bilayer graphene plasmonic antenna over SiO2/Si substrate at THz band is proposed in this chapter. The dual band reconfiguration of antenna is achieved by dynamically controlling the conductivity of upper layer and lower layer of graphene using electric field effect. The design and analysis of graphene plasmonic THz antenna with pattern reconfigurable capability is presented in Chapter-5. The core concept of famous Yagi-Uda antenna is used in this design. Two different antennas with either 2-beam or 4-beam switching capability is designed. Antenna-I is able to switch the beam in 90 directions, whereas for Antenna-II, the switching directions are 0, 90, 180. Furthermore, this pattern reconfigurability is observed over a range of frequency leading to simultaneous pattern and frequency reconfigurable nature of the antenna. The reconfigurability is obtained by changing the graphene conductivity through its chemical potential. Chapter-6 presents the concept of using graphene for performance enhancement of a planar metal THz antenna. The antenna is implemented by using a parasitic layer ix of graphene sandwitched between the copper metal radiator and silicon dielectric layer. The design is inspired from the tunable conductivity behavior of graphene that can be achieved by applying a DC bias voltage across it. The obtained metal THz antenna has the advantages of low cross polarization, provision of enhancement of efficiency, besides the frequency reconfigurable behavior. Finally, the research contribution with concluding remarks and future scope is summarized in chapter 7. In summary, the thesis contributes towards the development of effective THz antennas for THz applications. Frequency reconfigurable and pattern reconfigurable graphene plasmonic THz antenna, and frequency reconfigurable metal THz antenna using graphene have been designed, analyzed and presented in the thesis.