High-frequency properties of microbridge and point contact Josephson junctions

Abstract

Recent experiments by the Harvard Superconductivity Group concerning the response of variable‐thickness microbridges to high harmonics of microwave radiation and the response of selected Nb point contacts to far‐infrared laser radiation are reviewed. Gap structure on the dc I‐V characteristics of the various devices can be used to recognize those junctions with the best high‐frequency performance. Microbridges which show large numbers of microwave‐induced steps to voltages as high as 4.2 mV have been studied, although their poor coupling and low impedance has not yet allowed the observation of steps induced by submillimeter radiation. Selected Nb point contacts which have better coupling and higher resistance have been used to study the steps induced by harmonics of 604 GHz (496 μm) laser radiation to as high as 12.5 mV, and by higher fundamental frequencies up to 2.52 THz (119 μm) at 5.2 mV. Such contacts have been used to measure the intrinsic rolloff of the ac Josephson effect beyond the superconducting energy gap frequency.