Quasiparticle energy relaxation in superconducting tunnel junctions used as photon detectors

We exploit the results of recent work (Poelaert, Ph.D. Thesis, University of Twente, 1999) showing that energetic quasiparticles in a superconductor relax to an energy, called balance energy, larger than the superconducting energy gap, despite of the large density of states available at the gap. This feature may have a major impact on the performance of superconducting tunnel junctions used as photon detectors. We show how the balance energy and the parameters for photon detection are sensitive to the superconducting proximity effect. In particular, we show that the balance energy also exists for non-proximized structures, obeying the BCS theory.