Ultrafast Dynamics of Electron Thermalization in Gold

Time-resolved surface second-harmonic generation (SHG) is used to probe electron relaxation processes in gold following intense laser excitation at 1.55 eV. For the first time, an electron temperature ( $T_e$) dependent enhancement in the SHG signal is clearly observed at $T_e$ above 0.7 eV, which is shown to relate to the thermalization of nonequilibrium hot electrons. Therefore, the relaxation dynamics of the transient nonequilibrium electrons in the high $T_e$ regime is directly resolved by monitoring the time evolution of the SHG signal.