Abstract
We present a combined experimental and theoretical study of ultrafast thermalization of high-energy carriers photogenerated by femtosecond laser excitation in GaAs and InP. Luminescence up-conversion is used to monitor the spectral and temporal evolution of the carrier distribution with a time resolution of about 100 fs. A rapid redistribution of electrons and holes over a wide energy range is found within the first 100 fs after excitation. The experimental results are analyzed by Monte Carlo simulations including a molecular-dynamics scheme to describe the carrier kinetics. We show that the Coulomb interaction among carriers is responsible for the initial ultrafast thermalization.