Development of low-noise terahertz SIS mixers with high current density NbN/AlN/NbN tunnel junctions

We report on progress in the development of high current density NbN/AlN/Nb N tunnel junctions for application as submillimeter wave SIS mixers. A ultr a-high current density up to 120 kA/cm(2), roughly two orders of magnitude larger than any reported results for all-NbN tunnel junctions. was achieved in the junctions. The magnetic field dependence and temperature dependence of critical supercurrents were measured to investigate the Josephson tunne ling behaviour of critical supercurrents in the high-J(c) junctions. We hav e developed a low-noise quasi-optical SIS mixer with the high-current densi ty NLN/AlN/NbN junctions and two-junction tuning circuits which employ Al/S iO/NbN microstriplines. The tuning characteristics of the mixer were invest igated by measuring the response in the direct detection mode by using the Fourier Transform Spectrometer (FTS) and measuring the response in the hete rodyne detection mode with the standard Y-factor method at frequencies from 670 to 1082 GHz, An uncorrected double sideband receiver noise temperature of 457 K (12hv/k(B)) was obtained at 783 GHz.