Abstract
The coherent dynamics of an optically driven exciton--photon--bipolariton system is studied in bulk CdS. Applying backward-scattering four-wave mixing (FWM) spectroscopy, we analyze the two-photon polarization associated with excitonic molecules and determine the dephasing time ${T}_{2}\ensuremath{\simeq}4.8 \mathrm{ps}.$ We demonstrate that the molecule-mediated FWM signal persists at positive time delays even in the ${\ensuremath{\chi}}^{(3)}$ limit, due to the spatial pinning of coherent polaritons induced by the second pulse. By generalizing the bipolariton model to uniaxial semiconductors, we calculate ${\ensuremath{\tau}}_{\mathrm{rad}}^{\mathrm{XX}}\ensuremath{\simeq}2.8 \mathrm{ps},$ justify the ${T}_{2}{=2T}_{1}$ limit, and prove the bipolariton representation for excitonic molecules in CdS.
Published Version
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