We report on a low noise waveguide-based heterodyne mixer utilizing a superconductiang NbN hot electron bolometer (HEB) operating near 2.7 THz. The mixer is an NbN nano-bridge integrated with a gold bowtie planar antenna on an ultra-thin silicon substrate of similar to 2-3 mu m thickness. To produce the waveguide embedding circuit for use at such a high frequency, we adopted a novel approach combining UV-lithography and micro-plating techniques. The mixer response agreed precisely with model predictions, and we measured a minimum uncorrected DSB receiver noise temperature of 965 K at an LO frequency of 2.74 THz.

We develop balanced waveguide-based mixers operating at molecular transition frequencies of OH and HD lines expected respectively at 2.5 and 2.7 THz. The receivers use NbN HEB mixers and 90 quadrature hybrids. Beyond 1.5 THz, the realization, among others, of the waveguides by conventional machining fabrication techniques becomes complicated and novel techniques such as gold-plating and silicon micromachining technologies are used. Hence, waveguides as well as channels that hold mixers are not machined directly on a metal part as usual but defined on an intermediate piece made of a thick silicon substrate by means of UV-photolithography and gold microplating techniques. The concept would facilitate the downscaling for operation at higher frequencies, particularly around 5 THz for the detection of atomic oxygen OI line. We present in detail the main steps of the development and first experimental results.