Electromagnetic waves in the terahertz spectral range have gained significant research focus due to their applications in various fields of science. To effectively generate and integrate terahertz waves in systems, appropriate waveguide design is critical. Conventionally waveguides have been used to control the propagation of electromagnetic waves. A waveguide with low loss and dispersion is always preferred. But achieving these characteristics is quite challenging especially if operating in the terahertz spectral range. There are inherent material and geometric limitations that exist for terahertz waveguides. It is therefore important to optimize the design to enable their use in applications efficiently. This thesis investigates the characteristics of three primary terahertz waveguides based on the underlying theory and results obtained from simulations. The three waveguides are parallel-plate waveguides, two-wire waveguides, and coplanar striplines. The work in this thesis mostly focuses on coplanar striplines, optimal for building a highly efficient commercial and portable terahertz system-on-chip (TSOC). The contribution of the thesis is around the use of different types of passive components mounted on a thin commercial Silicon Nitride membrane. A bias tee is introduced which is a combination of interdigitated electrodes and a meander inductor. The length of the interdigitated electrodes and the gap between them are 55 um and 5 um, respectively. The S21 parameter for this structure ranges from -24 dB/mm at near-zero frequencies to -0.8 dB/mm at 1 THz. This indicates that the designed bias tee can appropriately block low frequencies. Split-ring resonators are also used to act as band-stop filters. The resonant frequency of the resonator depends on the radii of the split-rings. In the optimized design, the internal radius of the outer ring is 25 um and the external radius of the inner ring is 20 um. This results in a narrowband band-stop filter with its resonant frequency centered at 701 GHz. The optimized final TSOC design discussed in this work uses these passive components placed on the Silicon Nitride membrane and is shown to have a total loss that is 3 dB/mm less than any of the previous work for terahertz frequencies. / Graduate

This thesis presents studies of novel terahertz photonic devices, including photoconductive optoelectronic devices and guided-wave components, aimed at the development of next-generation terahertz systems. In chapter 2, a scalable interdigitated THz transmitter is designed to increase the output power and compared with a conventional 50 μm coplanar transmitter. In chapter 3, we compare four different receivers with different antenna geometries in terms of bandwidth and sensitivity. Then we describe a photoconductive near-field detector with a subwavelength aperture and its system integration and characterization. In chapter 4, a parallel metal plate waveguide is designed with an integrated step inside the waveguide that can couple to higher order TM modes efficiently from the TEM mode. In this chapter, we also experimentally and numerically study a 2-dimensionally tapered parallel plate waveguide, by which a free-space THz beam can be focused into a deep subwavelength-scale volume. In chapter 5, a parallel thin dielectric film waveguide is used to explore the guiding mechanism of an antiresonant optical reflection waveguide. Cylindrical silica single capillaries and a microstructured capillary, which guide in a similar way, are characterized in terms of mode profiles and attenuation. In chapter 6, we study oblique transmission through freestanding thin nickel films, which are perforated with periodic conical hole arrays. Surface modes can be supported by both metallic surfaces with different nonlinear dispersion curves, which results in spectral interferences in a near-field region when the surface modes couple out of the waveguide into free space.

Terahertz waveguides are the focus of considerable research interest due to their potential for sensing, imaging and communications applications. Two of the most promising designs are the metal wire waveguide and the parallel-plate waveguide. The metal wire waveguide exhibits excellent low loss and low dispersion characteristics. However, the radiation is only weakly coupled to the wire and the beam extends a great distance from the waveguide, which can lead to high bending loss. In my research I show that this large beam extent also gives a high degree of flexibility in the geometry required to couple radiation into the waveguide or between waveguide sections. I also show that the traditional formalism of bending loss is incomplete, and that there is an optimum radius of curvature to reduce loss. The relationship between the beam extent and the radius of the wire presents the possibility of a tapered waveguide to confine the radiation as it propagates. I here present experimental data and simulations results to verify this subwavelength confinement at the tip of a tapered metal wire waveguide, which is of great interest for near-field imaging applications. The parallel-plate waveguide is another design frequently employed due to its low loss and low dispersion characteristics. Resonant structures may also be easily incorporated into the waveguide for sensing and filtering applications. One such structure is a single rectangular groove, which serves as a notch filter with a very narrow linewidth when the transverse-electric (TE) mode of the waveguide is excited, though its physical origin is poorly understood. In this work I present a detailed experimental and theoretical study of the rectangular resonant cavity in a TE-mode parallel-plate waveguide, particularly with respect to its potential as a microfluidic refractive index sensor. This study is extended to include the possibility of two grooves, in both coupled and non-coupled geometries, and their efficacy as multichannel or high-resolution single-channel microfluidic sensors.