  The relaxation dynamics of hot carriers in silicon (100) is studied via a
novel holistic approach based on phase-resolved transient absorption
spectroscopy with few-cycle optical pulses. After excitation by a sub-5 fs
light pulse, strong electron-phonon coupling leads to an ultrafast momentum
relaxation with time constant of 10 fs. The thermalization of the hot carriers
occurs on a time constant of 150 fs, visible in the temporal evolution of the
collision time as extracted from the Drude model. We find an increase of the
collision time from 3 fs for the shortest timescales with a saturation at
approximately 18 fs. Moreover, the optical effective mass of the hot carrier
ensemble evolves on ultrafast timescales as well, with a bi-exponential
decrease from 0.7 $m_e$ to about 0.125 $m_e$ and time constants of 4 fs and 58
fs. The presented information on the electron mass dynamics as well as the
momentum-, energy-, and collision-scattering times with unprecedented time
resolution is important for all hot carrier optoelectronic devices.
