Terahertz 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 critical current-resistance products (I_c R_N) as high as 10 mV at 4 K. These are the highest I _cR_N products reported to date for microfabricated Josephson junctions of any material. The junctions were integrated at the feeds of planar log-periodic antennas made from Au thin films. The junctions had dc normal state resistances R_N between 6 and 38 Ω, reasonably well matched to the antenna´s estimated RF impedance of 53 Ω. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltages n(hf/2e), n=1,2,...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at voltages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K respectively. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared measurements performed on high T_c junctions. Measurements of the power, frequency, and temperature dependence of the Shapiro steps are presented and discussed in the context of a resistively and capacitively shunted junction (RCSJ) model. A value of 4.5 fF for the junction capacitance is inferred from the hysteresis of the slightly underdamped current-voltage characteristics