Oscillations in the differential transmission of a semiconductor microcavity with reduced symmetry

The nonlinear optical response of wire-shaped microcavities in the strong-coupling regime has been studied by pump-and-probe experiments. Due to electron-hole exchange interaction the lower polariton branch splits into a doublet. The asymmetry required for the exchange splitting arises from uniaxial strain release caused by the lateral patterning of the cavity. In the low excitation density regime the differential transmission in the lower polariton branch increases due to Pauli blocking by localized excitons. In contrast to a planar cavity, at high excitation the transmission signal of the wire structures shows an oscillation in time, which is observed for cocircular, as well as anticircular pump-and-probe configuration, whereas it is suppressed for linear polarization that is directed along or normal to the wire axis. The oscillation arises from a macroscopic, spin-coherent population in the lower polariton branch. The spin orientation of this population precesses due to the exchange splitting of the polariton states. Spin coherence of the system is observed beyond the polariton lifetime of the nonexcited cavity, due to an induced transfer of long-lived excitons into the macro-occupied polariton ground state.