BROAD-BAND RESPONSE OF THE QUASI-PARTICLE CURRENT IN A SUPERCONDUCTING TUNNEL JUNCTION

We have measured the linear response of the quasiparticle current in a Nb/AlOx/Nb junction from 75-200 GHz. Picosecond pulses of millimeter wave radiation were generated by illuminating a photoconductive switch with a mode-locked laser. The pulses were coupled to the junction thr ough an impedance-mismatched, planar lithographed antenna. The broadba nd response of the superconductor-insulator-superconductor junction wa s measured by monitoring changes in the de current induced by interfer ing two electrical pulses at the junction as a function of the time de lay between them. The data are in agreement with the linear theory of photon-assisted tunneling.