Analysis of NB Josephson junction properties under optical irradiation

The gap voltage shift of Nb Josephson junctions fabricated on different substrates is measured under optical illumination at several conditions of light intensity and chopping frequency. The signal dependence on the chopping frequency for junctions realized on glass substrates is bolometer-like with a strong thermal coupling between film and substrate. The responsivity of a junction of 50 mu m x 20 mu m is 50 V/W in vacuum and at liquid helium temperature, and its time constant is lower than 10 mu s. The linearity of the response on the optical power extends over five order of magnitude. The response of junctions fabricated on silicon substrates is flat versus the chopping frequency and about 100 times lower than a junction on glass with the same incident optical power. The shift of the gap at temperatures between 1.2 K and 4.2 K has been measured to separate the nonequilibrium effects from the heating effects.