Coulomb correction to the dressed exciton in an inorganic-organic layered semiconductor: Detuning dependence of the Stark shift

We report the detuning dependence of the excitonic optical Stark shift in the inorganic-organic layered semiconductor (C₆H₅C ₂H₄NH₃)₂PbI₄ as a typical material in which the effective Coulomb interaction is strong. We show the polarization dependence of the detuning spectra, i.e., the shifts as functions of continuously changing pump photon energy, over the range of 350 meV below the exciton resonance. It is found that in the opposite-circularly and colinearly polarized configurations for pump and probe lights, the shift changes its sign at 50 meV below the exciton resonance and is resonantly enhanced around it. With larger detuning, in any polarizations, blue shifts are much smaller than predicted by the dressed exciton model. We present a tentative model, which is an extended dressed exciton model including the excitonic molecule and unbound biexcitons phenomenologically. The model reproduces the experimental result quite well.