Efficient Antennas for Terahertz and Optical Frequencies.

Abstract

The inherent low conductivity of metals and low skin depth at terahertz (THz) frequencies and above decrease antennas radiation efficiency at such high frequencies. In addition to the high surface impedance, due to the small dimensions of metallic antennas at THz frequencies and above can further reduce the radiation efficiency. Nevertheless, attention has been drawn to metallic antennas that are designed to operate at optical frequencies due to their ability to create substantial field confinement and enhancement at their terminals. In this thesis, a highly conductive nanomaterial, Bundled Carbon Nanotubes (BCNTs) is examined to overcome the low efficiency of metallic antenna at THz frequencies. Due to their axial current, BCNTs are modeled by a 2D anisotropic resistive sheet having a tensor surface resistivity. Using a numerical method, the radiation efficiency of antennas consisting of BCNTs and gold are compared and it is concluded that BCNTs should be packed up about 1,000 times more than the current density of BCNTs to outperform gold at 2 THz. Efficient nanoantennas near infrared (IR) frequencies are also studied to enhance the performance of uncooled IR detectors and thermophotovoltaics (TPVs) power transducers. A gold bowtie dipole antenna topology loaded with a low bandgap indium gallium arsenide antimonide (InGaAsSb) p-n junction is investigated for this purpose. Through optimized arrangements, it is shown that a large array of flexible load bowtie nanoantennas can produce an efficient TPV system that can absorb 95% of the incident power. Similarly, a focal-plane array of nano-bowtie antennas used as an uncooled IR detector is demonstrated to enhance the sensitivity of the detector by a factor equal to the field enhancement factor, approximately 23 when compared to a detector made from a thick layer of the same material, InGaAsSb. Finally, a more advanced antenna topology using a cross tapered-bowtie antenna for detecting circularly polarized (CP) IR signals is designed and its perfect CP property is verified experimentally for the microwave range. A conceptual full‐Stoke's vector polarimetric imager using focal planar arrays of the nanoantennas with vertical, horizontal, 45°-tilted, and right-hand circular polarization is proposed.