Responsivity of Ta-based STJ's to visible light as a function of Al trapping layer thickness

Superconducting tunneling junction devices are being developed as energy resolving photon counting imagers. We have fabricated a series of Ta/Al/AlOx/Al/Ta devices with variable Al layer thickness, dAl, and measured their response to a weak visible photon source. Single photon sensitivity is observed. We model our system with a set of modified Rothwarf-Taylor equations including quasiparticle losses, trapping, (back)tunneling and calculate the charge responsivity as a function of dAl with one fitting parameter. The quasiparticle loss time for Al and Ta is found to be τl(Al)=35 μs and τl(Ta)=0.14 μs in our devices, respectively