A new noise source in superconducting tunnel junction photon detectors

We report on the development of an "all-in-one" detector that provides spec troscopy, imaging, photon timing, and high quantum efficiency with single p hoton sensitivity: the optical/UV single-photon imaging spectrometer using superconducting tunnel junctions. Our devices utilize a lateral trapping ge ometry. Photons are absorbed in a Ta thin film, creating excess quasipartic les. Quasiparticles diffuse and are trapped by Al/AlOx/Al tunnel junctions located on the sides of the absorber. Imaging devices have tunnel junctions on two opposite sides of the absorber. Position information is obtained fr om the fraction of the total charge collected by each junction. We have mea sured the single photon response of our devices. For photon energies betwee n 2 eV and 5 eV we measure an energy resolution between 0.47 eV and 0.40 eV respectively on a selected region of the absorber. We see evidence that th ermodynamic fluctuations of the number of thermal quasiparticles in the jun ction electrodes leads to current noise that far exceeds the expected shot noise of the de bias current, We believe that this may limit the resolution of our present generation of detectors at the operating temperature of 0.2 2 K.