Fourier transform spectrometer studies (300-1000 GHz) of Nb-based quasi-optical SIS detectors

Three Nb/AlO/sub x//Nb SIS detectors, designed to operate in the 400-550, 550-700, and 600-750 GHz bands, have been studied in direct detection mode using a Fourier-transform spectrometer. All three detectors were of quasi-optical type and had on-chip-integrated-fixed tuned SIS junctions. The tuning ranges of the detectors were selected to cover the interesting region around the superconducting gap frequency of Nb (about 700 GHz). Measurements show detector responses at frequencies above the gap frequency, i.e., up to /spl ap/920 GHz, and that cooling the detectors to 3.1 K improved the direct detection responses about 15% below 700 GHz and about 50% for frequencies up to 800 GHz, compared to the responses at 4.2 K. The 500 GHz SIS detector was also studied in a 440-520 GHz heterodyne receiver set up. Good agreement between modeled tuning circuit characteristics, tuning range of the mixer and the direct detection response bandwidths were found. However, it is essential that the dispersion of the field penetration depth into the superconductor is included in the modeling of the tuning circuits when the detector is operated at frequencies above the superconducting gap.<>