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 p ump-probe transmission in an unconventional configuration with the abs orption saturation measured from the unoccupied spin-orbit-split valen ce band. Ultrafast electron relaxation is observed: within 40 fs the e xcited electrons are redistributed down to the band gap even for densi ties as low as 10(17) cm(-3). Thermalization of the electron distribut ion is achieved already 200 fs after the end of the pump pulse. The el ectron temperature can be obtained as a function of time and an absorp tion increase observed at long times is attributed to a Fermi-edge sin gularity.