Femtosecond time-resolved five-wave mixing at silicon surfaces

Abstract

The ultrafast dynamics of photoexcitations at silicon surfaces is investigated using asurface-sensitive purely optical technique. In the experiments, the diffracted secondharmonic generated by sequences of ultrashort laser pulses is detected as a function of thetime delay between the pulses. It is demonstrated that this five-wave-mixing technique canbe used to measure the temporal evolution of the optical polarization and the photoexcitedpopulations at the surface. The experimental results can be reproduced by numericalsolutions of optical Bloch equations. The theoretical analysis allows one to investigatewhich dephasing times and relaxation processes are compatible with experiment.Furthermore, it is outlined how one can describe optical nonlinearities at surfaces usinga microscopic theory within the framework of semiconductor Bloch equations.