Performance of quasi-optical SIS mixer with NbN/AlN/NbN tunnel junctions and NbN tuning circuit at 760 GHz

An all-NbN quasi-optical superconductor–insulator–superconductor (SIS) mixer for the terahertz band has been designed and tested. This mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, an NbN twin-slot antenna, and NbN tuning circuits. The size of the NbN/AlN/NbN junction was about 0.5 μm in diameter, and the current density was about 30 kA/cm². The junctions showed good d.c. I–V characteristics, with a high gap voltage of about 5.4 mV and a small sub-gap leakage current. The double side band (DSB) receiver noise temperature, measured by the standard Y-factor method, was about 2700 K at 761 GHz. This value is much higher than the theoretical sensitivity based on the experimental I–V curve calculated using Tucker’s quantum theory of mixing. The difference between experimental and theoretical mixer performance may result from large RF losses in the tuning circuit of NbN fabricated on SiO.