Future optical detectors based on Al superconducting tunnel junctions

TL;DR

This paper reports on the development and characterization of ultra-low temperature Al superconducting tunnel junctions for photon detection, highlighting the effects of different contact materials on quasiparticle dynamics and detector performance.

Contribution

It introduces high-quality Al superconducting tunnel junctions with Nb or Ta contacts, analyzing their quasiparticle loss times and implications for photon detector performance.

Findings

Nb contacts show quasiparticle loss times of 5-80 microseconds.

Ta contacts exhibit quasiparticle loss times over 200 microseconds.

Detectors demonstrate response to light wavelengths from 250 to 1000 nm.

Abstract

Superconducting tunnel junctions are being developed for application as photon detectors in astronomy. We present the latest results on the development of very high quality, very low critical temperature junctions, fabricated out of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic demagnetisation refrigerator. The contacts to the top and base electrodes of these junctions are fabricated either out of Nb or Ta, which has strong implications on the loss time of the quasiparticles. The Nb contacted junctions show quasiparticle loss times varying between 5 and 80 usec, depending on the device size. The bias range of the Nb-contacted junctions is limited to the range 0-100 uV, because of the set-in of strong non-equilibrium quasiparticle multiplication currents at higher bias voltages. The Ta-contacted junctions, on the other hand, show quasiparticle loss times in excess of 200 usec. These long loss times lead to very strong quasiparticle multiplication, which prevents the stable biasing of the junctions even at very low bias voltages. Junction fabrication and characterisation are described, as well as the response of the detectors to monochromatic light with wavelengths varying from 250 to 1000 nm. The energy resolution of the detectors is discussed.