Ultrafast hot-electron dynamics in silicon

Ultrafast (150 fs) time-resolved photoelectron spectroscopy is used to directly observe the energy relaxation of optically excited electrons in silicon. Conduction band electrons fit a thermal distribution by 120 fs pump-probe delay. The initial cooling rate of the excited distribution is found to be extremely fast, followed by an electron-phonon thermalization time of approximately 1 ps, and an overall much slower cooling rate as the electrons lose energy. Here, we also report a new effect in two-photon photoemission unique to ultrashort laser pulses. Our model and results demonstrate this effect to be a sensitive new monitor of electron dynamics.