Performance of Terahertz Waveguide SIS Mixers Employing Epitaxial NbN Films and Nb Junctions

We have designed, fabricated, and tested terahertz waveguide superconductor-insulator-superconductor (SIS) mixers with Nb/AlO_x/Nb junctions and NbN/SiO₂/Al microstriplines. The NbN ground plane used was an epitaxial film grown on a single-crystal MgO substrate. A two-junction tuning circuit, which was directly placed at the feed point of a bow-tie waveguide probe without impedance transformers, was used. A full-height waveguide, hammer-type choke filter, and zero-depth backshort were adopted for the design of the microstrip-waveguide transition structure. The Nb junctions in which the current density was 6.5 kA/cm² showed good I-V characteristics, yielding a subgap-to-normal-state resistance ratio greater than 20. The results obtained for the mixer performance showed a receiver noise temperature of 440 K (DSB) at 1 THz; the results were corrected for losses in the beam splitter and the vacuum window. A detailed analysis of the mixers suggested that the waveguide mixers composed of epitaxial NbN films and Nb junctions were effective in the terahertz band.