Resolution limitation due to phonon losses in superconducting tunnel junctions

We report greatly enhanced energy resolving power of Ta/Al superconducting tunnel junction (STJ) photon detectors. The improvement in resolution has led to the observation of features in the energy dependence of the resolving power, which are not predicted by the currently prevailing formulas. We show that these effects are related to phonon noise generated during energy down-conversion in the superconducting electrodes, and the dependence of phonon energy loss on the distance of a photon absorption site from the escape interface. The predictions of the model agree well with experimental data in the energy range 0.6-6 eV.