DIRECT-DETECTION AT SUBMILLIMETER WAVELENGTHS USING SUPERCONDUCTING TUNNEL-JUNCTIONS

Superconducting tunnel-junction direct detectors are considered in som e detail. For frequencies below twice that of the gap there is some bi as voltage for which the input impedance is real, the responsivity qua ntum limited, and the dynamic range high. A susperconducting detector saturates for two reasons: intrinsic saturation due to the relative in crease in two-photon tunnelling processes, and extrinsic saturation du e to the input match changing with bias voltage. The responsivity of a detector with a resistive RF source is least sensitive to bias-voltag e changes and has the greatest dynamic range when operating with a slo ping load line. In the case of an inductive source, the dynamic range can be higher than the intrinsic saturation rate would suggest. Ideall y, superconducting tunnel-junction detectors should be biased in a con stant-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 m ore than one quarter of the total current turn-on at the gap. Supercon ducting direct detectors can be used to make precise and well-calibrat ed optical measurements at submillimetre wavelengths.