Frequency conversion using the quasiparticle nonlinearity of Nb-Al2O3-Nb tunnel junctions

The frequency conversion effects in a tunnel junction made from niobium and an aluminum interlayer have been investigated experimentally. The normal resistance of the junctions was 1-10 Ohms over a junction area 5-50 sq microns in size. The ratio of the resistance below the elbow-shaped gap structure to normal resistance was in the range 5-25 to one. Estimates of the frequency conversion efficiency in the region of the elbow-shaped gap structure were obtained based on analysis of the typical voltage-current characteristic at a temperature of 4.2 K. It is shown that the steepness of the voltage-current characteristic near the gap structure may make it possible to increase the conversion efficiency and to achieve a quantum regime even in the presence of gap smearing.