We have shown that superconducting tunnel junctions based on
niobium or hafnium may have considerable potential as photon counting optical
and ultraviolet detectors. The basic experimental
feasibility has now been demonstrated, and the key features of such a
detector can be summarised as follows:
(a) photon counting operation with no readout noise, and with minimal
dark current contribution;
(b) an inherent spectroscopic capability at ultraviolet, optical, and
infrared wavelengths, with tunnel noise-limited resolutions
ranging from at 100 nm and
at 500 nm for
materials with
from
;
(c) high quantum efficiency of 50 per cent or more over the range
. A higher
efficiency over a more restricted waveband could be achieved through the
introduction of appropriate anti-reflection coatings;
(d) high speed, limited primarily by the processing
electronics, leading to a high count-rate capability, and high
resolution time datation (sub-ms) of individual photon events;
(e) inherent discrimination against background events through the
application of energy and risetime discrimination.
An illustrative list of astronomical applications for which
such a detector could play a major role, either at optical wavelengths from
the ground ( nm), or at ultraviolet and optical wavelengths
(
nm) from space, would be:
(a) faint object broad-band spectroscopy which, if coupled to a panoramic
imaging capability, would lead to the
direct determination of the spectral energy distribution
and redshift of every object detected in the field either through the
observation of continuum discontinuities (Lyman edge) or through the
observation of line centroids;
(b) highly efficient spectrophotometry of variable objects, with time
resolution well below 1 ms;
(c) order separation when coupling a strip of STJs to a high resolution
echelle-type spectrograph;
(d) speckle imaging, speckle spectroscopy, adaptive optics,
interferometric fringe detection, and other atmospheric correction
techniques requiring high time resolution.
An STJ array is already being considered as a possible European-contributed camera for Hubble Space Telescope (STECF 1995), and the benefits of the time and energy resolution for a space interferometer have also been considered (Perryman & Peacock 1995).
Acknowledgements
The authors acknowledge the technical support of A. van Dordrecht (ESTEC), R. Venn of Cambridge Microfab Ltd (UK) and D. Goldie of Oxford Instruments Ltd (UK).