GRAPHENE PLASMONICS-BASED ON-CHIP TERAHERTZ ANTENNAS

Abstract

Worldwide research on THz communications has been actively investigated in various fields of sensing and imaging, radio astronomy, high-resolution spectroscopy, biology/medicine and wireless communication measurements. With the significantly wider segments of unused bandwidth, the THz region (0.3 THz to 10 THz) is quite capable of providing high-capacity data transmission of 100 GB/s or more. Moreover, THz region can complement millimeter-wave communications to design high-speed wireless LAN and wireless PAN for the next generation. Due to the absorption and scattering by clouds, rain, dust, and other environmental factors, it cannot sustain long-range communication. Therefore, THz communication can only use for short range communication. A thorough literature survey reveals the use of graphene to design the different plasmonic devices such as nanoantennas, switches, filters, modulators, phase shifters and couplers in the THz region. In order to design high speed plasmonic devices, we therefore move on to graphene-based THz integrated circuits. For the noble metals, only TM-polarized waves can excite the surface plasmon polariton waves at metal-dielectric interface. However, recent research has revealed that in graphene, the SPP waves can be excited for both TE and TM modes. Furthermore, graphene attains the SPP waves in the THz region in contrast to novel metals that attain it in the optical region. The graphene plasmons wavelength is about two orders of magnitude lower than the free space wavelength of similar frequency. Therefore graphene-based antennas are designed in the subwavelength scale. Consequently, they are highly miniaturized than the metallic antenna. Besides this, graphene has other advantages, such as dynamically changing the surface conductivity via varying electrostatic bias voltage, moderate losses and higher propagation length. This thesis primarily focuses on developing the graphene plasmonics-based on-chip THz antennas for different applications such as low-profile dipole antenna, dual-band antenna with low-profile, high gain antenna to compensate for the high atmospheric losses in the THz region, frequency and pattern reconfigurable antenna that allow dynamic spectrum allocation and beam steering in the desired direction and circularly polarized antenna to overcome multipath fading and polarization mismatch brought on by antenna misalignment. The graphene-based low-profile dipole antenna consists of two graphene patches to form the bow-tie dipole and the wideband artificial magnetic conductor (AMC). Using AMC surface, the total profile of the antenna has been achieved up to 0.09 times of free space wavelength (0.09𝜆0) at the 0° operating frequency of 2.5 THz. In another approach, a hybrid surface combination of AMC and PEC has been used instead of AMC surface to enhance the radiation performance by reducing unwanted current distribution. The graphene-based low-profile dual-band antenna consists of the metasurface-based 4 x 4 AMC configuration and the square graphene patch driven through the aperture coupling. The fundamental TM10 mode of graphene patch excites the first resonance frequency, while the TM10 and antiphase TM20modes of metasurface simultaneously excite the second wide frequency band. The proposed antenna with a profile height of 0.12𝜆0 at a center frequency of 1.14 THz achieves a gain from 7.06 dB to 10.4 dB in the first band and an average gain of 10 dB in the second band. The dielectric lens feed by the graphene-based antenna has high gain, narrow beamwidth, broadside radiation pattern and stable input impedance. Directivity achieved through simulation is up to 7.3 dBi with radiation efficiency of 70% and 21.2 dBi with radiation efficiency of 65% for feed antenna and dielectric lens antenna respectively. The performance of dielectric lens antenna has also been studied as the function of feed element, relative permittivity of lens material, radius of lens and off-axis displacement tolerances. The reconfigurable radiation pattern graphene antenna consists of a circular graphene patch and four graphene fan-shaped parasitic elements. Ten different operational states of radiation patterns can be obtained by controlling the conductivity of parasitic elements. The main beam is rotated from -45° to +45° with a step increment of 90° in the azimuth plane. Simulated gain has been achieved up to 4.68 dBi with a total efficiency of 62%. The graphene-based circularly polarized antenna consists of a truncated graphene patch and a cross aperture single series feed. The proposed antenna can resonate in the broad range of frequencies from 1.22 to 1.44 THz at chemical potential from 0.5 to 1.0 eV. In this range of chemical potential, the axial ratio of the antenna is less than 3 dB. The maximum gain of the antenna has been achieved up to 4.6 dBi with 62% radiation efficiency at the chemical potential of 1eV.