Direct observation of electron relaxation in intrinsic GaAs using femtosecond pump-probe spectroscopy

The evolution of a nonequilibrium photoinjected electron population is investigated independently of the hole population using femtosecond pump-probe transmission in an unconventional configuration with the absorption saturation measured from the unoccupied spin-orbit-split valence band. Ultrafast electron relaxation is observed: within 40 fs the excited electrons are redistributed down to the band gap even for densities as low as 10¹⁷ cm^-3. Thermalization of the electron distribution is achieved already 200 fs after the end of the pump pulse. The electron temperature can be obtained as a function of time and an absorption increase observed at long times is attributed to a Fermi-edge singularity.