Heat Pulse Studies of Quasiparticle Dynamics in Superconducting Tunnel Junction Photon Detectors

Non-equilibrium phonons provide a valuable method for studying the interactions of carriers in low temperature photon and particle detectors, and hence of characterising their performance. Superconducting tunnel junctions (STJs) are widely used as sensitive photon detectors throughout the electromagnetic spectrum [1]. We describe investigations of the response of STJ detectors in which the effect of an absorbed photon is simulated by a heat pulse with a known phonon frequency spectrum. The equivalent photon energy of the heat pulse is calculated from the number of phonons in the spectrum with energies above the STJ energy gap. In contrast to direct photon detection the technique has high temporal and spatial resolution, and in addition has the advantage that the input pulse energy can be varied continuously over a wide range.