Nb-based SIS mixer in the 800 GHz band

We have built a waveguide SIS mixer with Nb-based parallel-connected twin junctions at 800 GHz which is above the gap frequency of Nb (690 GHz). Heterodyne mixing was performed at 810 GHz and a receiver noise temperature of 580 K (DSB), which includes noise contributions due to a vacuum window and a beam splitter, was achieved using Nb/AlO/sub x//Nb junctions with Nb striplines. The performance of this mixer is quite good and it is shown that efficient mixing in Nb-based SIS junctions is still possible above the gap frequency, SIS mixers with Al striplines have also been fabricated and good DC I-V characteristics have been observed. Heterodyne measurements of these mixers are now under preparation.