Optical responses of the switching currents in Al and Nb Josephson junctions

TL;DR

This study investigates how optical radiation affects the switching currents in Al and Nb Josephson junctions at ultra-low temperatures, revealing differences in sensitivity and the dominance of thermal effects, with implications for photon detection applications.

Contribution

The paper experimentally compares optical responses of Al and Nb Josephson junctions and discusses methods to enhance their optical responsivity for photon detection.

Findings

Al-junction has higher optical sensitivity than Nb-junction.

Minimum detectable radiation powers are 8 pW for Al and 2 nW for Nb.

Radiation-induced thermal effects dominate the optical responses.

Abstract

We experimentally demonstrated the optical responses of the switching currents in two types of Josephson tunnel junctions: Al/AlOx/Al and Nb/AlOx/Nb. The radiation-induced switching current shifts were measured at ultra-low bath temperature (T~16 mK). It is observed that the Al-junction has a more sensitive optical response than the Nb-junction, which is as expected since Al electrode has a smaller superconducting gap energy. The minimum detectable radiation powers with the present Al-junction and Nb-junction are 8 pW (corresponding to 8*10^5 incoming photons in one measurement cycle) and 2 nW respectively. In addition, we found that the radiation-induced thermal effects are dominant in the observed optical responses. Several methods are proposed to further improve the optical responsivity, so that the josephson junction based devices could be applicable in photon detections.