GRAPHENE BASED PLASMONIC INTEGRATED CIRCUITS FOR TERAHERTZ APPLICATIONS

Abstract

Graphene has enticed a wide number of researchers due to its remarkable optical and electronic properties, which have been exploited rapidly in the field of nano-photonics and plasmonics. It has unique magnetic properties due to its unusual band structure and large number of charge carriers behaving as Dirac Fermions equivalent to 2D gases in metals, leading to Berry's phase and quantum Hall effect in graphene supporting TE, TM and Q-TEM modes. It needs excitation of graphene plasmon polaritons, demanding sophisticated technology and instrumentation. As compared to the plasmon polaritons in inert metals, graphene has many distinguishably valuable features like high field confinement, lower losses and longer propagation lengths. The potential applications of graphene lie in the area of modulators, phase-shifters, surface cloaking, filters, antennas, etc. in the terahertz frequency regime. This thesis mainly focuses on the process of developing graphene plasmonic devices from the concept of nanoscale wireless links, with specific emphasis on synthesizing data from numerical simulation into an accurate, predictive understanding of nanoscale THz (0.1 to 10 THz) phenomena. The wireless communications demand ultra-high bandwidth at high data rates in the order of Tbps. The optical communications allow transmission of high bandwidths, but wireless communication technology is not advanced enough to provide transmitter and receiver integrated circuits in THz regime. The transmission of data rates upto 100 Gbps has been achieved until now. But, in order to achieve higher data rates with ultrafast communication speeds, we require high-speed communication links. Therefore, we move forward to the graphene based terahertz integrated circuits, which provide high data rates with additional features of compactness, tunability and easy-to-synthesize methods. This will accomplish the need of miniaturized transmitter and receivers working at the nanoscale. It provides single-band or multi-band operations sustaining the increased demand of multiple functions using the same system. Multi-band systems can be implemented by parallel, switchable, or concurrent configurations. The concurrent multiband front end provides different standard compatibility with a single circuitry only; hence reduce the circuit size and the power consumption. An attempt is made to an extent to implement this concept in photonic integrated circuits. Probably due to the fact that most of the photonic integrated circuits are developed by using optical waveguides. These optical waveguides operate in TE or TM modes viii and have a limited single mode operational bandwidth. In order to obtain multiband operation, we require large single mode operational bandwidth of the waveguide so that no higher order mode can start propagating in the desired two or more than two frequency bands. This can easily be achieved, if waveguide operates in TEM or Quasi TEM mode. Here, quasi-TEM nature of modes is supported by the graphene based plasmonic waveguides. This waveguide can be modeled with the help of a two-wire transmission line operating in quasi TEM mode. The graphene plasmonic THz link concept is still in its infancy. The design of individual building blocks is missing in the literature. Small efforts have been made to design these devices; therefore, there is a strong need to address this issue, i.e. design of building blocks for nanoscale wireless links so that the concept can be realized in future. The research work reported in this dissertation deals with the design and analysis of graphene plasmonic waveguide based integrated circuits for future photonic transceiver links. The constituents of this link such as antennas, diplexers, filters, couplers and waveguide sections have been investigated and reported in this dissertation work. This thesis consists of six chapters. Chapter 1 deals with the motivation for carrying out this research work along with detailed literature review for identifying research gaps. On the basis of identified research gaps, the research problem to be solved during the course of PhD dissertation has been defined. Chapter 2 of this thesis is devoted for the electromagnetic analysis of graphene plasmonic waveguide and its variants. The transmission line characteristics such as characteristic impedance, propagation constant, effective dielectric constant, etc. have been obtained using full wave electromagnetic simulation tool CST microwave studio 2012 version. In all simulations, graphene has been modeled using its surface impedance model with the help of Kubo's formula. Chapter 3 of this dissertation describes the graphene plasmonic waveguide based resonators such as ring resonator, and modified complementary split ring resonators. Further, dual pass band photonic circuits have been designed using band pass filters. The full wave simulations have been carried out using commercial software tool CST microwave studio. Chapter 4 of this thesis deals with the design and analysis of filter based diplexer and directional coupler. The diplexer design involves two band-pass filters (BPFs) designed using graphene ix plasmonic SWR. The incident power is split into two parts with a broadband power splitter. This diplexer can simultaneously act as a wavelength-division-multiplexer/demultiplexer. Chapter 5 explains the design and analysis of an antenna at terahertz frequencies. The antenna is designed with two graphene plasmonic stepped width resonator (GPSWR) connected with a gap. The simulated return loss and radiation pattern of the antenna is also provided in the chapter. Finally, the conclusions and future scope for further research work have been presented in Chapter 6 of this dissertation.