LOW-NOISE SLOT ANTENNA SIS MIXERS

We describe quasi-optical SIS mixers operating in the submillimeter ba nd (500-750 GHz) which have very low noise, around 5 h nu/k(B) for the double-sideband receiver noise temperature. The mixers use a twin-slo t antenna, Nb/Al-Oxide/Nb tunnel junctions fabricated with optical lit hography, a two-junction tuning circuit, and a silicon hyperhemispheri cal lens with a novel antireflection coating to optimize the optical e fficiency We have flown a submillimeter receiver using these mixers on the Kuiper Airborne Observatory, and have detected a transition of (H 2O)-O-18 at 745 GHz. This directly confirms that SIS junctions are cap able of low-noise mixing above the gap frequency.