Design and Fabrication of Fractal Photoconductive Terahertz Emitters and Antenna Coupled Tunnel Diode Terahertz Detectors

Improved terahertz (THz) photoconductive antennas that are able to both emit higher THz radiation power and detect lower THz radiation signal levels than the traditional THz photoconductive antennas are presented. Also, different classes of fractals are investigated to realize whether the enhanced THz radiation properties of fractal photoconductive THz emitters are universal to all fractals. Finally, the effect of self-similarity of the antenna structure on its performance is studied.;New classes of metal-insulator-metal (MIM) junctions that possess extreme asymmetry and nonlinearity in their current-voltage (I-V) characteristics are investigated. Metal-insulator-insulator-metal (MIIM) diodes incorporating different electron tunneling mechanisms under forward and reverse bias are implemented through the introduction of a cascaded potential barrier. A high I-V nonlinearity, up to 10 times that of a conventional MIM, was obtained for identical metal electrodes. As the next step, the tunneling mechanism is further altered. For this, resonant tunneling transport is incorporated into a metal-insulator-insulator-insulator-metal (MIIIM) diode. This is analogous to a tunnel diode (normal tunneling) with the mechanism of a resonant tunneling device. It is envisioned that high speed THz rectifiers, ultrahigh bandwidth frequency mixers, diodes, and electron tunnel transistors having extraordinary I-V nonlinearity can be realized.;Next, the tunnel diodes are integrated into the antenna-coupled detectors for detecting free space THz radiation. The fabrication process for the MIIM, and MIIM diodes having an integrated antenna has been presented. To accomplish this, the knowledge gained from designing and fabricating optimized THz fractal antennas and highly nonlinear MIIM and MIIIM diodes is utilized for fabricating ultrafast THz detectors.;Finally, the capabilities of THz spectroscopy for characterizing and studying the time dynamics of devices and complicated systems were investigated. To accomplish these tasks, a versatile optical setup for all-THz time resolved pump-probe spectroscopy was designed and tested. This THz setup is used to potentially characterize the temporal and spectral response of the fabricated antenna coupled MIIM and MIIIM diodes. Importantly, utilizing this setup and conventional THz time-domain spectroscopy (THz-TDS), the interaction of THz waves with complex media such as metal-dielectric composites was investigated. This was to expand the utilization of THz imaging from imaging inside dielectric materials into imaging more complex materials largely composed of highly opaque substances. The knowledge gained through a series of experiments was applied in utilizing THz-TDS for imaging dielectric objects embedded inside metallic media. This advancement of THz-TDS was successfully accomplished.