Superconducting mixer elements for terahertz frequencies

Frequency mixing in the THz range is of great importance for the reception of the very weak signals appearing particularly in radio astronomy. Since several years Nb/Al-AlOx/Nb Superconductor-Insulator-Super-conductor (SIS) tunnel junctions are very successfully in use. However, the energy gap of the superconductor limits the frequency of mixer application to twice the gap frequency. In this work a process was developed to integrate for the first time NbN/MgO/NbN SIS junctions into Al strip lines resulting in working devices with receiver noise temperatures of 1450 K, 1700 K, and 3300 K at frequencies of 600 GHz, 680 GHz, and 850 GHz, respectively.