Efficiency Of Quasiparticle Creation In Proximized Superconducting Photon Detectors

In previous work using thin superconducting films as photon detectors it has been assumed implicitly that the quasiparticle yield in proximized superconducting bilayers should be the same as for a pure superconducting layer with the same energy gap. The reasoning is that, following the energy down conversion cascade, the resultant quasiparticles will all finish up at the edge of the density of states, which has the same energy throughout the whole structure regardless or whether it is pure or proximized. In this paper we show that, although the energy gap is the same, the actual density of quasiparticle states may vary considerably across a proximized structure, with a secondary peak at the energy of the higher gap material. Our calculations indicate that this peak can give rise to the generation of excess subgap phonons through which a larger portion of the original photon energy is lost from the quasiparticle system. The associated lower quasiparticle yield effectively reduces the responsivity of the proximized detector and affects the limiting energy resolution. The predictions have been confirmed by experimental results obtained with a distributed read out imaging detector (DROID) in which the response to photons absorbed in a pure Ta layer and in a Ta/Al proximized structure could be compared directly. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3141840]