Niobium Josephson junction bolometers for optical detection in the visible-infrared region

The response of Nb Josephson junctions fabricated on different substrates, silicon and amorphous glass, is measured under optical illumination at several conditions of light intensity and light chopping frequency. The linearity of the response on the optical power extends over five orders of magnitude. The signal of the junction on silicon is 2 orders of magnitude lower than that of the junction on amorphous glass, but at least one order of magnitude faster, as the time constant of a directly irradiated junction on silicon is lower than 10 mu s. The signal dependence on chopping frequency of the junction on amorphous glass is typical of a strong thermal coupling between film and substrate, while for the junction on silicon it depends mainly on the thermal boundary resistance between film and substrate.