Inverse-phase Rabi oscillations in semiconductor microcavities

We study experimentally the oscillations of a nonstationary transient signal of a semiconductor microcavity with embedded InGaAs quantum wells. The oscillations occur as a result of quantum beats between the upper and lower polariton modes due to strong exciton-photon coupling in the microcavity sample (Rabi oscillations). The detection of a spectrally resolved signal has allowed for a separate observation of oscillations at the eigenfrequencies of two polariton modes. Surprisingly, the observed oscillations measured at the lower and upper polariton modes have opposite phases. We demonstrate theoretically that opposite-phase oscillations are caused by a pump-induced modification of polariton Hopfield coefficients, which govern the ratio of exciton and photon components in each of the polariton modes. Such behavior is a fundamental feature of the quantum beats of coupled light-matter states. In contrast, the reference pump-probe experiment performed for pure excitonic states in a quantum well heterostructure with no microcavity revealed in-phase oscillations of the pump-probe signals measured at different excitonic levels.