Ultrafast Photo-Induced Absorption Change in Inorganic-Organic Multiple Quantum Well Compound

We have performed pump-and-probe measurements to investigate ultrafast excitonic nonlinearity expected in an inorganic-organic compound (C₆H₅C₂H₄NH₃)PbI₄, which has huge exciton binding energy (220 meV) and oscillator strength (0.5 per formula unit) by virture of the small dielectric constant in the barrier layer (the dielectric confinement or image charge effect). When the pump photon energy is set at the exciton level at 2.37 eV or above the band gap, the absorption decreases immediately after the pump due to the phase space filling effect and retrieves in a few ps. The retrieval time is ascribed to the ultrafast radiative lifetime of the exciton level. The second exciton absorption at 2.39 eV exhibits induced absorption when the pump is at the first exciton band, which may provide a hint to the origin of the band.