Thermalisation in metals on a sub‐picosecond time scale

A theoretical model of the transient reflectivity of metals based on dynamics of down-converting electron and phonon cascades is developed in order to interpret recent pump-and-probe experiments in conventional superconductors and proximised structures. We show that the standard two-temperature model is totally inadequate to describe the response to pump excitation in the time domain up to several picoseconds. We also consider in detail the extended pre-equilibration regime for which the two-temperature model apparently does provide a credible description, despite failing to include realistically the essential characteristics of the quasiparticle and phonon spectral distributions. Based on our model we interpret recent experimental data on pump-and-probe studies of Nb films, and we discuss the important features of the down-conversion process in relation to various types of cryogenic detectors