Mechanically robust cylindrical metal terahertz waveguides for cryogenic applications

As the ambition behind THz quantum cascade laser based applications continues to grow, abandoning free-space optics in favor of waveguided systems promises major improvements in targeted, easy to align, and robust radiation delivery. This is especially true in cryogenic environments, where illumination is traditionally challenging. Although the field of THz waveguides is rapidly developing, most designs have limitations in terms of mechanical stability at low temperatures, and are costly and complicated to fabricate to lengths > 1 m. In this work, we investigate readily available cylindrical metal waveguides which are suitable for effective power delivery in cryogenic environments, and explore the optimal dimensions and materials available. The materials chosen were extruded un-annealed and annealed copper, as well as stainless steel, with bore diameters of 1.75, 2.5, and 4.6 mm. Measurements were performed at three different frequencies, 2.0, 2.85 and 3.2 THz, with optimal transmission losses < 3 dB/m demonstrated at 2.0 THz. Additionally, novel optical couplers are also presented and characterised, with the ability to change the beam path by 90 with a coupling loss of just 2.2 dB whilst maintaining mode quality, or thermally isolate sections of waveguide with a coupling loss as low as 0.5 dB. The work presented here builds on previous work, and forms a comprehensive investigation of cryogenically compatible THz waveguides and optical couplers, paving the way for a new generation of systems to utilize THz QCLs for a host of low-temperature investigations.