Investigation of the high-frequency properties of superconducting point contacts in the far infrared

A theoretical and experimental investigation was made of the high-frequency properties of Nb–Nb superconducting point contacts (SPC's) operating under nonequilibrium conditions with a high critical current I_c and low normal-state resistance R_N. The possibility of using these contacts in raetrological applications in the near infrared was studied. On exposure of these SPC's to external radiation, current steps corresponding to the harmonics of the external radiation frequency were readily observed on their current-voltage characteristics at voltages exceeding the limiting value for niobium SPC's. In this case, the frequency was subject to a capacitative limitation. Frequency multiplication up to 8×10¹²Hz (~38μ) was observed at bridges having dimensions not exceeding 50 nm. Investigations were made of the dependences of the step-like structure of the current-voltage characteristics of SPC's on the external radiation power, and frequency. The dependences of the intermediate-frequency signal amplitude and width on the bias voltage and power of the external signals on exposure of SPC's to two radiation sources were also studied. A theoretical analysis showed that, in frequency multiplication and mixing by SPC's operated under nonequilibrium conditions, allowance must be made for the action of both the selfradiation and its harmonics on the SPC.