Abstract:

The far-infrared (FIR) properties of niobium cat-whisker point contacts were studied using radiation from an optically pumped FIR laser. The reproducible behaviour of junctions with excellent high-frequency performance allowed a measurement of the FIR frequency dependence of the strength of the a.c. Josephson effect. The shape of the laser-induced steps was used to measure the effective noise temperatures, which increase with bias voltage in agreement with a heating model of metallic constrictions. The high-quality junctions were tested as frequency-selective, incoherent FIR detectors, with the d.c. bias in the vicinity of the incipient laser step. The response was found to be linear in the laser power, and the best measured responsivity at 604 GHz was 2 × 105 V/W, while the best noise equivalent power was 10p−13 W/Hz, with a 450 Hz chopping frequency. The NEP is limited by the voltage noise in the junction, which was found to have an approximately  frequency dependence. The detector performance is degraded considerably at higher FIR frequencies. Also studied was the low-laser-power behaviour of the I–V curves near the critical current, which may be of importance for mixing applications with external local oscillators.

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