Robust Unclad Terahertz Waveguides and Integrated Components Enabled by Multimode Effects and Matched Slot Couplers

We address critical physical fragility issues associated with terahertz integrated all-silicon substrateless devices. This is necessary because, although the current state-of-the-art offers excellent electromagnetic performance, real-world deployment is currently held back by structural weaknesses. One such example is the input coupler, which has previously taken the form of an exposed taper that reduces core dimensions over several wavelengths, and is vulnerable to breakage. This is replaced with a compact subwavelength slot-waveguide coupler, which exploits reflection-cancellation as opposed to a progressive transition. The other key structural weakness is the in-plane integrated support that physically suspends the substrateless waveguide's core, and this is addressed with multi-mode effects that localize a field null to the point of contact with a solid supporting beam. The resultant robust waveguide platform exhibits a working relative bandwidth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>31%, which is sufficient for terahertz communications in standard allocated bands. Multimode effects are also exploited to realize an integrated photonic 2×2 splitter, which is incidentally the first demonstration of an integrated dielectric multimode interferometer splitter in the terahertz range.