Heterodyne Mixing and Direct Detection with High Temperature Josephson Junctions

We have examined various properties of high characteristic frequency YBCO superconductor-normal-superconductor (SNS) Josephson junctions that are important to their performance as low-noise THz frequency mixers. Without far-infrared laser illumination, the microwave frequency noise temperature of our lowest noise device shows good agreement with the predictions of the resistively shunted Josephson model in applicable regions of bias. It has a maximum noise temperature of 36±4 K at a physical temperature of 4 K. When illuminated with a 404 GHz far-IR laser local oscillator (LO) and a chopped 77 K blackbody signal, strong modulation of the 1 GHz IF noise power is observed. However, certain features of the modulated IF power signal strongly suggest that a large fraction of it is not true heterodyne detection. The spurious component is probably due to direct detection of the broadband hot load/cold load signal. We believe that reliable measurement of heterodyne performance will require narrowband signal sources.