Tracking the Time Evolution of the Electron Distribution Function in Copper by Femtosecond Broadband Optical Spectroscopy.

Multitemperature models are nowadays often used to quantify the ultrafast electron-phonon (boson) relaxations and coupling strengths in advanced quantum solids. To test their applicability and limitations, we perform systematic studies of carrier relaxation dynamics in copper, a prototype system for which the two-temperature model (TTM) was initially considered. Using broadband time-resolved optical spectroscopy, we study the time evolution of the electron distribution function, f(E), over a large range of excitation densities. Following intraband optical excitation, f(E) is found to be athermal over several 100 fs, with a substantial part of the absorbed energy already being transferred to the lattice. We show, however, that the electron-phonon coupling constant can still be obtained using the TTM analysis, provided that the data are analyzed over the time window where the electrons are already quasithermal, and the electronic temperature is determined experimentally.