A low noise NbTiN-based 850 GHz SIS receiver for the Caltech SubmillimeterObservatory

We have developed a niobium titanium nitride (NbTiN) based superconductor-i nsulator-superconductor (SIS) receiver to cover the 350 micron atmospheric window. This frequency band lies entirely above the energy gap of niobium ( 700 GHz), a commonly used SIS superconductor. The instrument uses an open s tructure twin-slot SIS mixer that consists of two Nb/AlN/NbTiN tunnel junct ions, NbTiN thin-film microstrip tuning elements, and a NbTiN ground plane. The optical configuration is very similar to the 850 GHz waveguide receive r that was installed at the Caltech Submillimeter Observatory (CSO) in 1997 . To minimize front-end loss, we employed reflecting optics and a cooled be amsplitter at 4 K. The instrument has an uncorrected receiver noise tempera ture of 205K DSB at 800 GHz and 410K DSB at 900 GHz. The degradation in rec eiver sensitivity with frequency is primarily due to an increase in the mix er conversion loss, which is attributed to the mismatch between the SIS jun ction and the twin-slot antenna impedance. The overall system performance h as been confirmed through its use at the telescope to detect a wealth of ne w spectroscopic lines.