Heterodyne Detection with Superconducting Point Contacts and Enhanced Heterodyne Signals from Tightly Coupled Contacts

Several modes of operation of point contacts as heterodyne detectors in the frequency range 0 to 1010 Hz were investigated experimentally. A point contact in a superconducting loop, under the application of two externally generated ac fields, behaved as expected on the basis of a model given previously. Likewise, a point contact in a partly resistive loop, in which an externally generated signal is heterodyned with the Josephson oscillation of the contact, gave the expected results. In both cases the heterodyne signal amplitude exhibits a decaying oscillatory (Bessel-function-like) dependence on applied signal level; the heterodyne power at the first peak being approximately 0.1φ02f0/L, where f0 is the heterodyne frequency, L the loop inductance, and φ0 the flux quantum. On the other hand, an anomalously large heterodyne signal was obtained from a pair of closely coupled Josephson-oscillating contacts. No explanation of this result is offered, but it appears that a sort of amplification of the heterodyne signal is effected when the Josephson frequencies are larger than the heterodyne frequency.