Passive Terahertz Waveguide Elements: Loss Engineering and All-Dielectric Components for High-Frequency Applications

QC 20251126

Abstract

The practical deployment of terahertz systems for future applications requires a comprehensive toolkit of high performance, compact passive components that overcome several limitations innate to terahertz waves. A primary challenge is minimizing attenuation in signal routing, a problem especially critical when terahertz power is scarce. At the same time, effective signal management and device characterization require components that can efficiently absorb power to minimize reflections and crosstalk. This thesis addresses both of these needs, presenting novel low-loss all-silicon devices for terahertz applications and introducing advanced loss engineering techniques to create highly effective integrated and free space absorbers.

The first part of this thesis introduces high performance, all-silicon passive devices fabricated using silicon micromachining techniques. A perforation-free, mechanically robust planar parabolic reflector antenna is presented utilizing slab optics and total internal reflection to achieve a flat gain response over a broad bandwidth of 220-330 GHz. Furthermore, a very low crosstalk waveguide crossing is demonstrated by applying transformation optics to a Maxwell fisheye lens. This approach resolves the fundamental mode mismatch problem inherent to conventional lens designs, enabling dense and complex terahertz circuit integration.

The second part focuses on lossy structures for both dielectric and metallic waveguides. For open dielectric waveguides, ultrathin single-walled carbon nanotube films are deposited to create compact, broadband and reflectionless terminations without altering the geometry of the guide, drastically attenuating the propagating waves over short distances by evanescent field interaction. For enclosed hollow metallic waveguides, integrated absorbers are developed using highly porous nanomaterials, including randomly oriented and aligned graphene-coated nanofibers, as well as carbon nanotube aerogels. Characterized using a novel multi-band measurement methodology, these materials demonstrate broadband stealth and shielding performance across a wide frequency range (67-500 GHz). The investigation is also extended to free-space applications, demonstrating a hierarchically porous carbon-silica composite as a low reflectivity absorber in 140-220 GHz.

Collectively, this thesis expands the component toolkit for terahertz integrated systems, providing practical and high performance solutions for waveguiding, radiation and termination that are crucial for the next generation of high-frequency applications.

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