To roughly 1.25 THz, the most sensitive and high-resolution heterodyne spectrometers are based on superconductor-isolate-superconductor (SIS) tunnel junction mixers. Beyond 1.25 THz, however, superconducting hot-electron-bolometer (HEB) mixers are considered a better option. Most HEB mixers operating above 2 THz couple the mixer element to a planar antenna on a dielectric substrate lens. Yet waveguide feeds boast better coupling efficiency to free space while naturally reducing the direct detection response of HEBs. It also is generally easier to implement compact, high-pixel-count, focal-plane arrays using waveguide technology. At NASA’s Jet Propulsion Laboratory, a combination of conventional metal machining and metallic micro-plating techniques has been used to construct a low-noise superconducting mixer operating near 2.7 THz.
The JPL team—Faouzi M. Boussaha, Jonathan H. Kawamura, Jeffery A. Stern, Anders Skalare, and Victor White—adopted an approach that combined ultraviolet (UV) lithography with micro-plating techniques to make the waveguide embedding circuit. The resulting low-noise, waveguide-based heterodyne mixer uses a superconducting NbN hot-electron bolometer (HEB) operating near 2.7 THz. The NbN nano-bridge is integrated with a gold-bowtie planar antenna on a silicon substrate that is merely ~2 to 3 μm thick.
The mixer chip is placed in a channel across the input waveguide. These chips are produced using high-purity, silicon-on-insulator (SOI) substrate. The mixer chips are suspended at a distance corresponding to the substrate thickness plus an air-cavity depth from the waveguide backshort. With this configuration, the effective dielectric constant of the silicon substrate is reduced from 11.9 to ~2.8. See “A Low Noise 2.7 THz Waveguide-Based Superconducting Mixer,” IEEE Transactions On Terahertz Science And Technology, May 2012, p. 284.