Performance of All-NbN quasi-optical SIS mixers for the terahertz band

We have designed, fabricated, and tested terahertz superconductor-insulator -superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel j unction and an epitaxially grown NbN/MgO/NbN microstripline, The junction a s a distributed element was 0.8-mum wide and 2.4-um long, and its current d ensity was 50 kA/cm(2). The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring l ayer. An investigation of the mixer in our receiver configuration showed fl at noise characteristics from 870 to 960 GHz, The lowest receiver noise tem perature of about 550 K was obtained at 909 GHz, including a 9-mum-thick My lar beam splitter loss and other optical losses. These characteristics sugg est that SIS mixers with the self-compensated NbN/AlN/NbN junctions and the NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-n oise operations at terahertz frequencies.