TERHERTZ SHAPIRO STEPS IN HIGH-TEMPERATURE SNS JOSEPHSON-JUNCTIONS

We have studied the far infrared behavior of high-T(c) superconductor- normal metal-superconductor (SNS) microbridges with T(c) > 85 K and cr itical current-resistance products (I(c)R(N)) as high as 10 mV at 4 K. These are the highest I(c)R(N) products reported to date for microfab ricated Josephson junctions of any material. The junctions were integr ated at the feeds of planar log-periodic antennas made from Au thin fi lms. The junctions had dc normal state resistances R(N) between 6 and 38 OMEGA, reasonably well matched to the antenna's estimated RF impeda nce of 53 OMEGA. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltag es n(hf/2e), n = 1, 2, ...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at vol tages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K, respectiv ely. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared me asurements performed on high T(c) junctions. Measurements of the power , frequency, and temperature dependence of the Shapiro steps are prese nted and discussed in the context of a resistively and capacitively sh unted junction (RCSJ) model. A value of 4.5 fF for the junction capaci tance is inferred from the hysteresis of the slightly underdamped curr ent-voltage characteristics.