Waveguide-Type SIS Receiver Using All-NbN Technique

Abstract

This paper describes the first experimental results for a waveguide-type all-NbN superconductor-insulator-superconductor (SIS) heterodyne mixer on an MgO substrate designed to operate over the gap frequency of Nb. The mixer consists of an NbN/MgO/NbN junction, which has a length of one wavelength at 880GHz as a tuning circuit, an NbN/MgO/NbN microstrip as a λ/4 impedance transformer, and an RF choke filter. The mixer chip was designed using a high-frequency-structure simulator. Its return-loss and embedding-impedance characteristics were examined using a 180-times-scaled mixer model. By optimizing the cutting and polishing processes for the MgO substrate, we were able to fabricate the mixer chip with an accuracy of less than 5 μm. We succeeded in mounting the chip on a mixer block and in estimating the receiver noise temperature. The uncorrected minimum double-sideband receiver noise temperature was 740 K at 824 GHz. A comparison of the receiver noise temperature in a quasi-optical SIS mixer fabricated on the same wafer as the waveguide mixer showed that input noise was the major contributor to receiver noise in the waveguide mixer.