Photon-dressed biexciton-mediated anomalous optical Stark effect in CsPbBr3 nanocrystals

Intense optical pumping below bandgap creates photon-dressed states, which interact with and result in the shift of electronic levels, known as optical Stark effect (OSE). The OSE persists only for pump pulse duration implying the coherent response. Previous researches in semiconductors have shown that coherent OSE is evident only at cryogenic temperatures. Only recently lead halide perovskites (LHPs) were found to possess room temperature OSE. However, despite strong exciton-exciton interactions known in LHPs, OSE is described in non-interacting framework that treats an exciton like isolated two-level systems. The present work demonstrates anomalous OSE in interacting regime at room temperature in CsPbBr3 nanocrystals having exciton resonance (E-0) at 2.40 eV using circularly polarized transient absorption spectroscopy. The results show that photon dressing for same pump-probe polarization (sigma(+)sigma(+)) lead to observation of blueshift of exciton resonance across all pump detuning energies (omega = E-0 - h omega; h omega is pump photon energy). On the contrary, under opposite pump-probe polarization (sigma(+)sigma(-)), the strong exciton-biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency. The hallmark observations under sigma(+)sigma(-) result from large biexciton binding energy of similar to 71 meV and exciton-biexciton transition dipole moment of similar to 25 Debye. Further, we demonstrate unusual crossover from linear to nonlinear dependence of the OSE as a function of the drive photon frequency. Our systematic study reveals crucial information on unexplored many body interacting regimes of coherent manipulation in LHPs, important for exotic quantum phase applications like quantum information processing and ultrafast switching.