High-current density, submicron-sized superconductor-insulator-superconductor niobium junctions are made with optical lithography. With a novel process junctions as small as 0.8 μm2 are routinely made using a controlled under-etch with the quality parameter Vm up to 67 mV. The junction quality is found to be independent of the amount of under-etch. Used in a 345 and 490 GHz waveguide mixer, noise temperatures have been obtained as low as 131 K double-side band (DSB) and 120 K (DSB), respectively.
The noise and gain of a heterodyne waveguide mixer employing Nb/A&O,/Nb superconducting tunnel junctions with an on-chip integrated tuning element are measured and analyzed at 680-750 GHz and at 840 GHz. The lowest receiver noise temperatures are 400 K (double side band) at 720 GHz and 1500 K (3000 K including the beam splitter loss) at 840 GHz. We compare data of the pumped I-V curves with the quantum theory of mixing and demonstrate good agreement at frequencies well above the gap frequency.
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