| We report progress on the ongoing development of optical/UV single-photon, 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. Devices designed for imaging 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. Using devices designed for large backtunneling gain, we have measured an energy resolution of 0.4 eV at 4.89 eV. The resolution in these devices is limited by thermodynamic fluctuations of the thermal quasiparticle number in the Al trapping layers. We predict that this previously unconsidered noise source should be important in any backtunneling device with "deep" traps. We also report preliminary measurements of a second generation of detectors designed to eliminate this noise and, consequently, backtunneling gain. These devices need small junctions, of order 1 mum2, for best noise performance. With currently available amplifiers, the resolution of these devices should approach the intrinsic limits of creation and trapping statistics, and exceed the resolution of devices with backtunneling. |
