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Abstract-We describe quasi-optical SIS mixers operating in the submillimeter band (500-750 GHz) which have very low noise, around 5 hu/kB for the double-sideband receiver noise temperature. The mixers use a twin-slot antenna, Nb/Al-Oxide/Nb tunnel junctions fabricated with optical lithography, a twojunction tuning circuit, and a silicon hyperhemispherical lens with a novel. antireflection coating to optimize the optical efficiency. We have flown a submillimeter receiver using these mixers on the Kuiper Airborne Observatory, and have detected a transition of Hzl'O at 745 GHz. This directly confirms that SIS junctions are capable of low-noise mixing abwe the gap frequency.

The performance of a broadband SIS receiver with no mechanical tuning elements was scale modelled and tested. The mixer mount had a broadband waveguide to microstripline transition consisting of a 4-step Chebychev single ridge transformer. The last step of the ridge connected the waveguide to a microstripline circuit. The on-chip circuit consisted of a microstripline, which transmitted the rf and the local oscillator signals to the SIS mixer. Some chips included a thin film strip inductor in parallel with the mixer. Both the SIS element and the inductor had rfgrounds provided by 90" radial stubs. The inductor tuned out the junction capacitance to allow operation over the full frequency band. The SIS element can be a single junction or series array using Nb/AlO,/Nb mlayer tunnel junctions with areas as small as 0.5 pm2 and V (2mV) = 39 mV. Preliminary results indicated a DSB receiver n&e temperature of 65-80 K across the band measured at 4.4 K with an internal cryogenic rf hodcold source and with Tif= 21 K. With another device, we achieved a mixer noise temperature of 35 K at 100 GHz, increasing to 45 K at 79.5 and 110 GHz. Coupled mixer gain of up to +3 dB and negative dynamic resistance on the first photon step were observed. The lowest noise temperature was obtained for an untuned single junction mixer at 80 GHz; TR = 41 K and TM = 20 K were measured.

The radiant output of a noise tube (C. P. Clare Model TN-167), designed for the 90-140-GHz (3.3-2.1-mm) frequency range, has been compared with that from mercury lamps over the wavelength region from 0.4 to ~6 mm. Lamellar grating and Michelson Fourier transform spectrometers were used in conjunction with He cooled bolometers of NEP from 10(-12) to 10(-14) W/(H(2))(1/2) to measure relative spectral irradiance. With this instrumental arrangement, the radiant power emitted by the noise tube was observed to be less than that from a mercury lamp, at least to a 3-mm wavelength, but it produced less source noise than an ac operated mercury lamp. When the noise tube operating current was reduced, the spectral irradiance peak shifted to longer wavelengths.

Lownoise 1 THz niobium superconducting tunnel junction mixer with a normal metal tuning circuit