Optical/UV single-photon imaging spectrometers using superconducting tunnel junctions

We present preliminary test results of optical/UV single-photon imaging spectrometers using superconducting tunnel junctions. Our devices utilize a lateral trapping geometry. Photons are absorbed in a Ta thin film, creating excess quasiparticles. Quasiparticles diffuse and are trapped by Al/AlOx/Al tunnel junctions located on the sides of the absorber. The Ta/Al interface does not overlap the junction area. Imaging devices have tunnel junctions on two opposite sides of the absorber. Position information is obtained from the fraction of the total charge collected by each junction. We have fabricated high-quality junctions with a ratio of subgap resistance to normal state resistance greater than 100 000 at 0.22 K. We have measured the single-photon response of our devices. For photon energies between 2 and 5 eV, we measure an energy resolution between 1 and 1.6 eV. We can estimate the number of pixels the device can resolve from the energy resolution. We find that these early devices have as many as 4 pixels per strip.