6 research outputs found
Future optical detectors based on Al superconducting tunnel junctions
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.Comment: 12 pages, 10 figure
Optical photon detection in Al Superconducting Tunnel Junctions
We report on the successful fabrication of low leakage aluminium
superconducting tunnel junctions with very homogeneous and transparent
insulating barriers. The junctions were tested in an adiabatic demagnetisation
refrigerator with a base temperature of 35 mK. The normal resistance of the
junctions is equal to ~7 uohm cm2 with leakage currents in the bias voltage
domain as low as 100 fA/um2. Optical single photon counting experiments show a
very high responsivity with charge amplification factors in excess of 100. The
total resolving power (including electronic noise) for 500 nm photons is equal
to 13 compared to a theoretical tunnel limited value of 34. The current devices
are found to be limited spectroscopically by spatial inhomogeneities in the
detectors responseComment: 3 pages, 5 figure
Dynamics of nonequilibrium quasiparticles in a double superconducting tunnel junction detector
We study a class of superconductive radiation detectors in which the
absorption of energy occurs in a long superconductive strip while the redout
stage is provided by superconductive tunnel junctions positioned at the two
ends of the strip. Such a device is capable both of imaging and energy
resolution. In the established current scheme, well studied from the
theoretical and experimental point of view, a fundamental ingredient is
considered the presence of traps, or regions adjacent to the junctions made of
a superconducting material of lower gap. We reconsider the problem by
investigating the dynamics of the radiation induced excess quasiparticles in a
simpler device, i.e. one without traps. The nonequilibrium excess
quasiparticles can be seen to obey a diffusion equation whose coefficients are
discontinuous functions of the position. Based on the analytical solution to
this equation, we follow the dynamics of the quasiparticles in the device,
predict the signal formation of the detector and discuss the potentiality
offered by this configuration.Comment: 16 pages, 5 figures Submitted to Superconducting Science and
Technolog