The role of phonon processes in the performance of superconducting tunnel junctions used as photon detectors

Phonons play an important role in the photon detection process in a superconducting tunnel junction (STJ). The absorption of a photon in a STJ produces a cloud of charge carriers (quasiparticles). Phonons can thereafter be generated by relaxation of quasiparticles down to lower energy states, or by recombination of two quasiparticles into a Cooper pair. These phonons can be re-used to produce new quasiparticles or excite existing quasiparticles, or can be lost from the system. The balance between the re-use and the loss of phonons can affect the performance of the STJ, in terms of charge output, energy linearity and resolution. The attention is particularly focussed on the possibility for quasiparticles to significantly increase their energy via multiple tunneling, before they have time to relax by phonon emission. A model involving the basic balance equations is used to account for the significant non linear response as a function of photon energy observed in Nb-Al-AlO _x-Al-Nb STJs.