Circular Waveguide Tapers for High Resolution Terahertz Spectroscopy

Abstract

Parallel-plate waveguides have shown their usefulness in regards to terahertz applications by greatly enhancing the ease and sensitivity at which spectroscopic measurements of molecular samples are made. However, coupling into these devices has been a challenge, and up until now has primarily relied on the quasi-optical method of placing high resistivity silicon lenses on the outsides of the waveguide. These lenses limit the scan length of the measurements and hence, the spectral resolution. They also introduce limits with regards to reproducibility, since aligning them to the parallel plate entrance/exit points presents an additional challenge. This work explores incorporating a horn-like coupling structure onto the plates themselves, creating a cylindrically-based method of coupling with no external or moving parts. This coupling is shown to be gap-independent, on the order of the quasi-optic method, and increases the length of the scans because it removes the reflections formerly caused by the silicon lenses. For spectroscopy applications this can theoretically enhance the resonance features of molecular samples. This also has good implications for the future of terahertz science, as it presents a mechanism by which terahertz radiation can be transmitted in a gap-independent, dispersionless fashion.