Terahertz-frequency waveguide HEB mixers for spectral line astronomy

We report on the development of waveguide-based mixers for operation beyond 2 THz. The mixer element is a superconducting hot-electron bolometer (HEB) fabricated on a silicon-on-insulator (SOI) substrate. Because it is beyond the capability of conventional machining techniques to produce the fine structures required for the waveguide embedding circuit for use at such high frequencies, we employ two lithography-based approaches to produce the waveguide circuit: a metallic micro-plating process akin to 3-D printing and deep reactive ion etching (DRIE) silicon micromachining. Various mixer configurations have been successfully produced using these approaches. A single-ended mixer produced by the metal plating technique has been demonstrated with a receiver noise temperature of 970 K (DSB) at a localoscillator frequency of 2.74 THz. A similar mixer, produced using a silicon-based micro-machining technique, has a noise temperature of 2000 K (DSB) at 2.56 THz. In another example, we have successfully produced a waveguide RF hybrid for operation at 2.74 THz. This is a key component in a balanced mixer, a configuration that efficiently utilizes local oscillator power, which is scarce at these frequencies. In addition to allowing us to extend the frequency of operation of waveguide-based receivers beyond 2 THz, these technologies we employ here are amenable to the production of large array receivers, where numerous copies of the same circuit, precisely the same and aligned to each other, are required.