Direct detection at submillimetre wavelengths using superconducting tunnel junctions

Superconducting tunnel-junction direct detectors are considered in some detail. For frequencies below twice that of the gap there is some bias voltage for which the input impedance is real, the responsivity quantum limited, and the dynamic range high. A susperconducting detector saturates for two reasons: intrinsic saturation due to the relative increase in two-photon tunnelling processes, and extrinsic saturation due to the input match changing with bias voltage. The responsivity of a detector with a resistive RF source is least sensitive to bias-voltage changes and has the greatest dynamic range when operating with a sloping load line. In the case of an inductive source, the dynamic range can be higher than the intrinsic saturation rate would suggest. Ideally, superconducting tunnel-junction detectors should be biased in a constant-voltage mode. If the responsivity is to be depressed by no more than a few percent, the photon step should have a height which is no more than one quarter of the total current turn-on at the gap. Superconducting direct detectors can be used to make precise and well-calibrated optical measurements at submillimetre wavelengths.