On the detection of single optical photons with superconducting tunnel junction

We report the detection of individual optical and ultraviolet photons using a different approach to photon detection based on a superconducting tunnel junction. A 20 x 20 mu m(2) junction, employing a 100 nm niobium film and operated at a temperature of similar to 0.4 K, has been used to detect individual photons with inherently high quantum efficiency (>45%) over a broad wavelength range (between 200 and 500 nm), yielding high temporal (sub-ms) resolution, spatial resolution determined by the junction size, under conditions of minimal dark current, and in the absence of read noise. The quantum efficiency is limited by surface reflection, and could be improved by the deposition of antireflection coatings. The theoretical wavelength response range continues into the far UV and soft x-ray region, and is presently limited beyond 500. nm largely by the available signal processing electronics. The device intrinsically functions at very high incident photon rates-with count rates of order similar to 10 kHz or higher being feasible and again currently limited primarily by the signal processing electronics-thus providing a correspondingly enhanced dynamic range by several orders of magnitude compared with previous panoramic photon counting detectors. The measured charge output from the device is highly linear with photon energy resulting in an optical photon detection system with intrinsic spectral resolution, related to the critical temperature of the junction material and, in the current device, providing a limiting spectral resolution of about 50 nm. It is realistic in the future to envisage that these devices could be packaged into arrays, with the resulting system characteristics offering advantages over detectors based on semiconductors. (C) 1997 American Institute of Physics.