PUSHING THE OPERATING RANGE OF SIS MIXERS INTO THE THZ REGIME

In recent years, responding to the need of the astronomical community, superconductor-insulator-superconductor (SIS) mixers have been develo ped to operate at THz frequencies. Best results are obtained by using niobium-based tunnel junctions with well-matched low-loss normal metal striplines, which are found to be less lossy than niobium striplines operated above the gap frequency of niobium (700 GHz). Niobium nitride tunnel junctions with a magnesium oxide tunnel barrier, although havi ng reasonable-quality current-voltage characteristics, have been found to be unsuitable owing to excess shot noise resulting from pinholes i n the barrier.