The dynamics of optically generated carriers in GaAs is investigated measuring transmission changes with ultrafast time and high spectral resolution. A novel two-color 15 fs Ti:sapphire laser system allows the observation of the femtosecond kinetics of energetic carrier distributions at excitation densities as low as ${10}^{15}{\mathrm{cm}}^{-3\mathrm{}}$. For the first time, LO phonon emission of highly excited nonequilibrium electrons is directly monitored. The contribution of the hole distribution to the data and the influence of carrier-carrier scattering is studied.

• Received 16 October 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.1545