ULTRAFAST RABI OSCILLATIONS OF FREE-CARRIER TRANSITIONS IN INP

We report on the first observation of Rabi oscillations of continuum t ransitions in semiconductors, employing two-color transmission spectro scopy with a time resolution of 10 fs. Up to two Rabi cycles are obser ved in bulk InP for peak intensities exceeding 4 GW/cm(2). Extremely f ast phase relaxation due to carrier-carrier scattering results in a st rong damping of the oscillations. Surprisingly, the measurements clear ly show a decrease of the dephasing rates during the excitation pulse. We attribute this phenomenon to an increase of Pauli blocking and pos sibly the build-up of screening.