A Experimental Study of the Frequency Dependence of the Josephson Current in Josephson Tunnel Junctions.

An experimental study has been made of the frequency dependence of the Josephson pair current in high current density Nb/Nb_2O_5 /Sn Josephson tunnel junctions. In these experiments, high current density Josephson tunnel junctions were used as voltage tunable local oscillators to provide the probe radiation. The generator junctions were capacitively coupled to the detector junctions, which provided efficient, broad bandwidth coupling. Initial experiments were performed with generators fabricated from junctions with Sn counter electrodes and Nb base electrodes. To increase the generator power and bandwidth, later experiments used refractory NbC_ xN_{1-x }/MgO/NbC_ xN _{1-x} junctions incorporated into a high speed (f > 1 THz) oscillator/detector circuit. The experimental results indicated that below the gap sum frequency, the frequency response of the pair current I_ j(omega) was well predicted by the Werthamer theory, but above the gap sum, an exponential dependence on frequency (I_ j(omega) ~ e^{-alphaomega }, hbar > Delta_1 + Delta _2) was observed, in disagreement with the 1/omega dependence predicted by theory. The extinction coefficient alpha was found to be a function of both the tunnel junction gap sum and the critical current density. The results of this study indicate that non-capacitively shunted, high critical current density Josephson tunnel junctions may be used as sources of sub-millimeter wave radiation with an amplitude equal to the tunnel junction I_ cR product, out to the gap frequency of the tunnel junction. For junctions with NbC_ xN_ {1-x} base and counter electrodes, this represents a maximum operating frequency of greater than 2.5 THz with an oscillation amplitude V_ g ~ 3mV.