We present the development of a low noise 1.2 THz and 1.4 THz SIS mixers for heterodyne spectrometry on the Stratospheric Observatory For Infrared Astronomy (SOFIA) and Herschel Space Observatory. This frequency range is above the limit for the commonly used Nb quasi particle SIS junctions, and a special type of hybrid Nb/AlN/NbTiN junctions has been developed for this project. We are using a quasi-optical mixer design with two Nb/AlN/NbTiN junctions with an area of 0.25 m 2 . The SIS junction tuning circuit is made of Nb and gold wire layers. At 1.13 THz the minimum SIS receiver uncorrected noise temperature is 450 K. The SIS receiver noise corrected for the loss in the LO coupler and in the cryostat optics is 350-450 K across 1.1-1.25 THz band. The receiver has a uniform sensitivity in a full 4-8 GHz IF band.The 1.4 THz SIS receiver test at 1.33-1.35 THz gives promising results, although limited by the level of available LO power. Extrapolation of the data obtained with low LO power level shows a possibility to reach 500 K DSB receiver noise using already existing SIS mixer.

We present a low noise SIS mixer developed for the 1.2 THz band of the heterodyne spectrometer of the Herschel Space Observatory. The device of this design will be among the first SIS mixers flown in space at Herschel. The developed SIS mixer has a quasi-optical design, with a double slot planar antenna and an extended spherical lens made of pure Si. The SIS junctions are Nb/AlN/NbTiN with the critical current density of about 30 KA/cm2 and with the junction area of a quarter of a micron square. For the ease of the Josephson current suppression, the SIS junctions are diamond-like shaped. The simultaneous suppression of the Josephson currents in the two SIS junctions is a particular advantage of the used shape. A low loss Nb/Au micro-strip transmission line is used for the first time in the mixer circuit well above the gap frequency of Nb.Using the new mixer we demonstrated the minimum uncorrected Double Sideband receiver noise of 550 K (Y=1.34). The minimum receiver noise corrected for the local oscillator beam splitter and for the cryostat window is 330 K, about 6 hν/k, apparently for the first time in the THz frequencies range.

We report on a fabrication process that uses SOI substrates and micromachining techniques to form wide-IF SIS mixer devices that have suspended metal beam leads for rf grounding. The mixers are formed on thin 25 m membranes of Si, and are designed to operate in the 200-300 GHz band. Potential applications are in tropospheric chemistry, where increased sensitivity detectors and wide-IF bandwidth receivers are desired. They will also be useful in astrophysics to monitor absorption lines for CO at 230 GHz to study distant, highly redshifted galaxies by reducing scan times. Aside from a description of the fabrication process, electrical measurements of these Nb/ Al-AlN x / Nb trilayer devices will also be presented. Since device quality is sensitive to thermal excursions, the new beam lead process appears to be compatible with conventional SIS device fabrication technology.

We report on the design and performance of a 530-GHz balanced SIS mixer, the first balanced mixer in this frequency range. This quasi-optical balanced mixer utilizes a cross-slot antenna on a hyperhemispherical substrate lens with eight superconductor-insulator-superconductor (SIS) junctions and a 180 lumped element IF hybrid circuit. The local oscillator (LO) and the radio frequency (RF) signal, orthogonal in polarization to each other, are coupled to the mixer using a wiregrid polarizer. The noise performance of the mixer is excellent, giving an uncorrected receiver noise temperature of 105 K (DSB) at 528 GHz.

We report on balanced SIS receivers covering the astronomical important 180−720 GHz submillimeter atmospheric window. To facilitate remote observations and automated spectral line surveys, fully synthesized local oscillators are employed. High-current-density Nb-AlN-Nb superconductinginsulating-superconducting (SIS) tunnel junctions are used as the mixing element. The measured double-sideband (DSB) 230 GHz receiver noise temperature, uncorrected for optics loss, ranges from 50 K at 185 GHz, 33 K at 246 GHz, to 51 K at 280 GHz. In this frequency range the mixer has a DSB conversion gain of 0 ± 1.5 dB. The measured 460 GHz doublesideband receiver noise temperature, uncorrected for optics loss, is 32 K at 400 GHz, 34 K at 460 GHz, and 61 K at 520 GHz. Similar to the 230 GHz balanced mixer, the DSB mixer conversion gain is 1 ± 1 dB. To help optimize performance, the mixer IF circuits and bias injection are entirely planar by design. Dual-frequency observation, by means of separating the incoming circular polarized electric field into two orthogonal components, is another important mode of operation offered by the new facility instrumentation. Instrumental stability is excellent supporting the LO noise cancellation properties of the balanced mixer configuration. In the spring of 2012 the dual-frequency 230/460 SIS receiver was successfully installed at Caltech Submillimeter Observatory (CSO), Mauna Kea, HI.Index Terms-Superconducting-Insulating-Superconducting (SIS) mixer, balanced mixers, amplitude noise rejection, Wilkinson in phase power combiner, AlN tunnel barrier, heterodyne receiver, high-current-density, multiple Andreev reflection (MAR), broadband waveguide transition, system stability, Allan variance, synthesized local oscillator (LO), quantum noise limit.