DC AND TERAHERTZ RESPONSE IN Nb SIS MIXERS WITH NbTiN STRIPLINES

It is expected that the integration of NbTiN striplines with Nb-based SIS junctions will extend the range of operation of sensitive SIS mixers up to the energy gap of NbTiN (— 1.2-1.25 THz) to meet the requirements of the HIFI instrument for the Far-Infrared Space Telescope (FIRST). We have fabricated prototype RS waveguide mixers in which Nb/A1-A10,INb SIS junctions are integrated with NbTiN and Al wiring layers. DC current-voltage measurements of these devices show that the voltage gap is somewhat lower and the sub-gap current is somewhat higher for devices with two NbTiN wiring layers in comparison with those in which the top wiring layer is Al (Vgap = 2.7 mV vs. 2.8 mV, and R 2.0/RN — 32 vs. 35). Additionally, the gap voltage of the NbTiN stripline devices is further suppressed by the application of 890-1040 GHz local oscillator power during heterodyne measurements. These effects are attributed to the trapping of heat in the Nb junction electrodes due primarily to quasi-particle trapping at the Nb/NbTiN interface. FTS measurements result in what is believed to be the first reported observation of direct detection response in a NbTiN stripline device above 1.1 'THz. From an analysis of the resonance frequencies of several devices, it appears that the frequency response of the NbTiN striplines is predicted reasonably well by the MattisBardeen theory for the conductivity of a superconductor in the extreme anomalous limit. Furthermore, we present what is believed to be the first reported heterodyne measurements of NbTiN stripline devices in the 0.9-1 THz range. These measurements yield a receiver sensitivity of TN = 830 K at 930 GHz at a bath temperature of 3.3 K, after correction for beamsplitter losses.