Rabi oscillations in semiconductor multiwave mixing response

Abstract

We studied the semiconductor response with respect to high intensity resonant excitation on short-time scale when the contribution of the Fermi statistics of the electrons and holes prevails. We studied both the single- and double-pulse excitations. For the latter case we considered the time evolution of the multiwave mixing exciton polarization. The main difference between the excitation by a single pulse or by two noncollinear pulses is that the Rabi oscillations of the multiwave mixing response are characterized by two harmonics. Analyzing the operator dynamics governed by the external excitation we found that there are three invariant spin classes, which do not mix with the evolution of the system. Two classes correspond to the bright exciton states and one contains all dark states. We found that the dynamics of the classes are described by six frequencies and the Rabi frequencies are only two of them (one for each bright class). We discuss the effect of the dispersion of the electrons and holes and the Coulomb interaction describing the semiconductor by the semiconductor Bloch equation (SBE). We show that if initially the system is in the ground state then the SBE preserves the invariant spin classes, thus proving the absence of the dark excitons in the framework of this description. We found that due to the mass difference between holes of different kind additional Rabi frequencies, two of those present in the operator dynamics should appear in the evolution of the exciton polarization.