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 superconduct ing tunnel junction. A 20 x 20 mu m(2) junction, employing a 100 nm ni obium film and operated at a temperature of similar to 0.4 K, has been used to detect individual photons with inherently high quantum effici ency (>45%) over a broad wavelength range (between 200 and 500 nm), yi elding high temporal (sub-ms) resolution, spatial resolution determine d by the junction size, under conditions of minimal dark current, and in the absence of read noise. The quantum efficiency is limited by sur face reflection, and could be improved by the deposition of antireflec tion coatings. The theoretical wavelength response range continues int o the far UV and soft x-ray region, and is presently limited beyond 50 0. nm largely by the available signal processing electronics. The devi ce intrinsically functions at very high incident photon rates-with cou nt 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 detector s. The measured charge output from the device is highly linear with ph oton energy resulting in an optical photon detection system with intri nsic spectral resolution, related to the critical temperature of the j unction material and, in the current device, providing a limiting spec tral resolution of about 50 nm. It is realistic in the future to envis age that these devices could be packaged into arrays, with the resulti ng system characteristics offering advantages over detectors based on semiconductors. (C) 1997 American Institute of Physics.