Nonlinear optical study of the Fermi-surface oscillation in gold induced by acoustic-phonon excitation

Using a two-color ultrafast optical pump-probe technique with an amplified Ti:sapphire femtosecond laser system, we study the heat-induced structural dynamics in gold by monitoring both the fundamental and third harmonic generation (THG) signals of the probe beam. An amplitude oscillation, with a period of tens of picoseconds, is clearly observed in the THG signal, but absent in fundamental. In contrast to previous understandings from a purely mechanical deformation point of view, we demonstrate that this oscillation in THG directly results from a periodic modulation of the Fermi level in gold driven by acoustic-phonon excitation following ultrafast laser heating.