The superconducting tunnel junction has been extensively investigated as a
photon detector in the X-ray region of the spectrum where the number of
quasiparticles generated after photoabsorption are times
larger than in the optical.
Peacock et al. (1996a,b) have recently reported the first
experimental demonstration of photon counting at optical and
ultraviolet wavelengths using an STJ. Single photon spectra from
monochromated ultraviolet and optical photons covering the waveband
were obtained by illuminating a
device. This symmetrical device, which was constructed of
aluminium-proximised niobium layers
(
operated at a temperature of 0.4 K, is shown schematically in
Fig. 2 (click here). After removal of the contribution to the resolution from
electronic noise a device-limited resolution
was derived.
Spatial variations in the response of the detector, similar to those
reported by Verhoeve et al. (1996) and
Lumb et al. (1995) at X-ray
wavelengths, may account for the small degradation in resolution
compared with that expected theoretically.
Recent performance improvements have been brought about by reducing
the noise associated with the
electronics, and by back illumination through the sapphire substrate
onto the high quality epitaxial Nb film (cf. Fig. 2 (click here)), leading to a
wavelength coverage extended to (the short wavelength
limit is simply limited by the experimental conditions).
The device-limited resolution
is shown in Fig. 1 (click here) for
this Nb-based device. A resolution
of 45 nm at
was
obtained, very close to the theoretical value
for Nb shown
in Fig. 1 (click here).
Figure 3 (click here) illustrates the single photon charge spectra, Q, obtained from this Nb device when illuminated with photons having a wavelength of 250 and 1000 nm.
Figure 3: Single photon charge spectrum of the device
(a histogram of the number
of photons giving a charge output Q from the detector) when
illuminated by a monochromatic source of photons of wavelength
250 nm (top) and 1000 nm (bottom). The charge is in arbitrary units.
The difference in the charge output versus wavelength
provides the intrinsic energy resolution.