Ultrafast dynamics of electronic excitations in semiconductors

Coherent nonlinear polarizations and non-equilibrium distributions of hot electrons are studied in femtosecond experiments with bulk GaAs. Both excitonic and free-carrier components contribute to the third-order polarization close to the bandgap and show a distinctly different time behavior. Phase relaxation during the femtosecond photogeneration process leads to a significant broadening of the initial carrier distribution in comparison to the power spectrum of the optical pulses. The experimental results are in excellent agreement with a Monte Carlo solution of the semiconductor Bloch equations.