In most terahertz (THz) systems, the propagation of THz signals relies on metal or dielectric waveguides which suffer from high conductivity losses caused by the skin effect or dielectric losses resulted from the material absorption. Due to this reason, we propose and demonstrate a simple low-loss air-core tube strucutre for THz waveguiding. The simulation method we utilized is the finite-difference frequency-domain (FDFD) method with the perfectly matched layers (PMLs). The modal indices and propagation losses of the guided core modes on the THz tube waveguide are successfully obtained. The simulation results show that the guiding mechanism of the hollow tube waveguide is based on the antiresonant reflecting optical waveguide (ARROW) model. We also utilize a Fabry-Perot resonantor model to find out the resonance frequencies of the dielectric layer, which match well with the results of the FDFD method. By varying the core size, it is observed that the propagation losses are reduced when the core size is increased. The propagation losses can be reduced from 10-3 cm-1 (0.0043 dB/cm) to 10-4 cm-1 (4.34¡Ñ10-4 dB/cm). In addition, we can use the thin dielectric layer to provide a broad transmission band with £Gf = 0.13THz. We also propose a novel tube THz waveguide sensor. The influence of the thickness and material of the dielectric layer 2 are investigated. We can observe that the shift of the propagation loss peak is inversely proportional to the thickness of dielectric layer 2, which can be used as a thickness sensor with the sensing sensitivity being 0.125 GHz/£gm. On the other hand, the index of the dielectric layer 2 and the position of the propagation loss peak are in an exponential relationship. These properties of the tube waveguide can be applied in the dielectric-film sensing.

In this thesis we use three main approaches to enhance the performance of terahertz photoconductive switches (THz PC switches). We first propose two novel materials (GaBiAs and carbon nanotubes) for the substrate. The resulting enhancement in THz emission and reception are significant for GaBiAs. As thoroughly analyzed and addressed in Chapter 2, both the emission bandwidth and the emission amplitude of the device are improved by these materials. A systematic study of CNTs predicts 2 orders of magnitude enhancement in THz emission and one order of magnitude enhancement in THz reception. Experimental results for GaBiAs indicate 0.5 THz increase in bandwidth and 68% increase in the emitted THz wave amplitude. The bandwidth enhancement is in comparison to premium commercial devices. The optical excitation of the PC switch is studied and optimized next as the second enhancement approach (Chapter 3). The study presented in Chapter 3 provides an insight on the subwavelength dynamics of the optical excitation E-field at the edge of the electrodes. The study reveals that majority of the fast photocarriers are collected at the edge of the electrode in a subwavelength scale area. This insight leads to optimization of illumination profile and also the third enhancement approach, namely, the enhancement of electrode structure (Chapter 4). In Chapter 4 we have engineered the electrodes down to nanometer scale. This significantly enhances the optical excitation of the substrate and also overcomes the undesired properties of some substrate materials such as long carrier lifetime. Fabricated devices and fabrication processes are assessed in Chapter 5. Results (Chapter 6) highlight more than two orders of magnitude enhancement for nanostructures on GaAs. / Graduate / 0544