Exciton–polaritons in a ZnO-based microcavity: polarization dependence and nonlinear occupation

Abstract

We report on the occupation of the lower exciton–polariton branch in a ZnO-based microcavity as a function of the detuning between the exciton and the uncoupled cavity-photon mode and on the optical excitation density. We emphasize the difference in the dispersion and occupation of the lower polariton branch as a function of the linear polarization of the emitted light. For the negative detuning regime, we found an energy splitting between the transverse electric (TE)- and transverse magnetic (TM)-polarized states at in-plane wave vectors between 0.4×107 m−1 and 1.2×107 m−1, which is caused by the polarization dependence of the dispersion of the uncoupled cavity-photon mode. The maximum energy splitting of about 6 meV was observed for a detuning of about Δ=−70 meV. From the integrated photoluminescence peak, we deduce the occupation of the lower polariton branch as well as the scattering rates of exciton–polaritons into the lower polariton branch. We found that the energy splitting causes an enhanced scattering of exciton–polaritons into the lower polariton branch for the TM-polarized light compared with that of the TE-polarized light. By varying the excitation density, we observe a superlinear growth of the lower polariton branch occupation for negative and intermediate detuning regimes. For an accumulation of exciton–polaritons in the ground state at low temperatures (T=10 K), we found an intermediate detuning regime (−20 meV<Δ<+20 meV) as the optimum. With increasing temperature, this optimum detuning range shifts to larger negative values.