Josephson-Effect Far-Infrared Detector

Abstract

Superconducting point contacts, bridges, and tunnel junctions are all capable of carrying a zero‐voltage current; i.e., no voltage appears across the device until the current exceeds a finite critical value. The critical value of the zero‐voltage current is changed when microwave radiation is incident on such a junction. Thus such a device can be used as a detector of radiation. Experiments have been carried out demonstrating that this detection mechanism extends well into the far‐infrared, is highly sensitive, and has inherently high speed. Junctions were formed by pressing together the ends of two superconducting wires, one of which was flat, the other pointed. The spectral response of such point contact junctions was studied by using them as detectors in a far‐infrared Fourier transform spectrometer in which they were irradiated by broadband, incoherent radiation. The response of Nb–Nb junctions was found to extend to frequencies above 40 cm⁻¹ (λ−1 (λ=4 mm) yielded a value of 5×10⁻¹³ W for the noise equivalent power in a one‐cycle bandwidth and showed the junction detector could follow a pulsed signal which had a risetime of 10 nsec. Preliminary experiments using a monochromatic laser source at 32.2 cm⁻¹ showed the appearance of constant‐voltage steps in the voltage‐current characteristic of the junction, as is well known at microwave frequencies. These experiments demonstrate the existence of the Josephson effect at frequencies up to and beyond the superconducting energy gap, and show that over this range of frequencies, Josephson junction detectors exhibit both high sensitivity and high speed when compared with other helium‐temperature far‐infrared detectors.