Quasi-optical submillimeter-wave SIS mixers with NbN/A1N/NbN tunnel junctions

We report on low-noise heterodyne mixing with NbN/A1N/NbN tunnel junctions in the submillimeter-wave region for the first time. The receiver consists of the quasi-optical NbN SIS mixer with an integrated tuning circuit in which a radial short stub tuner is incorporated to resonate out the junction capacitance for RF matching and a balanced IF circuit for broadband IF matching. The prepared NbN SIS junction has a current density of 20 kA/cm and is about 1 I1111 in diameter, supplying a small coCJRN product (o)CJRN = 3 at 300 GHz). The junctions showed good dc I-V characteristics, excellent submillimeter-wave responses and sensitive heterodyne mixing responses. From 254 to 350 GHz the average receiver noise temperature measured using by the standard Y-factor method was about 250 K (DSB) at 5 K. The lowest receiver noise temperature, 200 K (DSB), was obtained at around 303 GHz. Comparing NbN/A1N/NbN and Nb/A10x/Nb tunnel junction performance with the same tuning circuit showed that the frequency dependence of the receiver noise temperature agreed well in the two receivers. These results suggest that our well-controlled NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.