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4. Photon detection at optical wavelengths

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 tex2html_wrap_inline1231 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 tex2html_wrap_inline1233 were obtained by illuminating a tex2html_wrap_inline1235 device. This symmetrical device, which was constructed of aluminium-proximised niobium layers tex2html_wrap_inline1237 (tex2html_wrap_inline1239 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 tex2html_wrap_inline1241 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 tex2html_wrap_inline1243 (the short wavelength limit is simply limited by the experimental conditions). The device-limited resolution tex2html_wrap_inline1245 is shown in Fig. 1 (click here) for this Nb-based device. A resolution tex2html_wrap_inline1247 of 45 nm at tex2html_wrap_inline1249 was obtained, very close to the theoretical value tex2html_wrap_inline1251 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.

  figure358
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.


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