Observation of Autler-Townes splitting of biexcitons in CuCl

Wave vector selective ac Stark effect of the ${\mathrm{\Gamma }}_1$-biexciton state is clearly observed in the steady state regime. The biexciton line splits into two lines with resonant pumping between ${\mathit{Z}}_3$ exciton polariton and the biexciton state. From the splitting energy, we have obtained the relevant dipole moment of 4.2e Å. Observed energy of the dressed state coincides with the value derived from the model with strict energy and quasimomentum selection rule. This implies that the dressed state is established with the coherent mixing of delocalized Bloch states.