Spectroscopic study of optically induced ultrafast electron dynamics in gold

Using a supercontinuum pulse as a probe, we have measured the transient reflectivity spectra of a thin film of gold for different values of the pump-probe time delay. The wavelength λₓ at which the measured transient reflectivity changes sign has been found to depend upon the time delay, leading to bipolar time resolved signals. The time dependence of λₓ has been shown to be consistent with calculations that take into account the full dependence of the reflectivity upon the electron occupation number, and to contradict qualitatively a model in which the signal is assumed to be directly proportional to the occupation number. The shift of λₓ has been found to persist at time delays that are much longer than the time required for the electrons to thermalize. Therefore the bipolar reflectivity signals do not necessarily contain a contribution from nonthermalized electrons, as has been previously assumed.