Biexciton-polariton coupling mediated by dark states

Multi-exciton correlations shape the photo-induced response of nanostructured materials, particularly when interactions are enhanced by light confinement. Here multidimensional coherent spectroscopy is used to quantify biexciton and exciton-polariton dynamics in a semiconductor microcavity. One- and two-quantum spectra, which are dominated by polariton-related contributions, also include a polarization-dependent biexciton feature whose magnitude and spectral coordinates depend on the detuning between the cavity mode and the exciton resonance. Comparison of spectra acquired using collinear and noncollinear experimental geometries indicates that excitation wavevector plays no significant role in this behavior. The measured energy dispersion and cavity enhancement are not compatible with uncoupled biexcitons or bipolaritons. To explain the measurements, an indirect biexciton-photon coupling model is introduced whereby biexcitons are formed from dark excitons that Coulomb couple to the bright-exciton fraction of the polaritons. The model addresses inconsistent observations of biexcitons previously reported in semiconductor microcavities and is generalizable to any material where optically dark excitons contribute to light-matter interaction. Our results suggest a mechanism to access long-lived dark excitons through multi-exciton correlations in strongly coupled systems.