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. B oth excitonic and free-carrier components contribute to the third-orde r polarization close to the bandgap and show a distinctly different ti me behavior. Phase relaxation during the femtosecond photogeneration p rocess leads to a significant broadening of the initial carrier distri bution in comparison to the power spectrum of the optical pulses. The experimental results are in excellent agreement with a Monte Carlo sol ution of the semiconductor Bloch equations.