Non-thermalized electrons in metals, featured by deviation from the Fermi-Dirac distribution, have recently shown potential to facilitate realization of ultrafast photonic devices such as all-optical modulators. Dissection of non-thermalized electron dynamics and its influence on optical response of metals is therefore essential for optimization of photonic design. The sub-picosecond dynamics of non-thermalized electrons in gold, excited by femtosecond laser, is studied in this work combining electron Boltzmann transport equation and femtosecond spectroscopy. It is found that there are significant differences among the ultrafast reflectance signals detected with different probe photon energies in sub-picosecond timescale, which result from the evolution of non-thermalized electrons and cannot be described by temperature-based models. For the probe photon energy far away from the interband transition threshold (ITT) of gold, the transient reflectance signal is featured by short leading and tailing durations. While for the probe photon energy close to the ITT, the transient reflectance change exhibits much slower dynamics. By calculating the dynamics of the nonthermalized electron energy distribution, the origin of the ultrafast optical response is revealed.
The analysis in this study could provide insights on electron-photon coupling to guide the design of ultrafast photonic devices based on metals.