Intensity-Driven Mott-Like Transition in a 2D Ruddlesden-Popper Perovskite

Two-dimensional Ruddlesden-Popper-type halide perovskites have emerged as promising cutting-edge nonlinear optical materials due to their strong excitonic effects and carrier-density-dependent optical responses. In this work, we investigate the nonlinear optical properties and ultrafast carrier dynamics of (BA) ${}_2{\mathrm{PbI}}_4$ under infrared excitation, resonant with the two-photon energy of its excitonic transition. Using a nonlinear femtosecond Z-scan measurement with a 1030 nm pump pulse, we observe a pronounced intensity-dependent evolution of the nonlinear absorption coefficient and the refractive index. With increasing pump intensity, a switch from saturable absorption to reverse saturable absorption emerges, suggesting an intensity-driven Mott-like transition. To elucidate the underlying carrier dynamics, intensity-dependent transient absorption spectroscopy was performed under the same excitation intensities. This reveals a long-lived charge carrier population above the estimated Mott density, reflecting a clear phase transition from an exciton-dominated regime to a dense electron-hole plasma regime. Moreover, such an intensity-driven, high, and switchable nonlinear optical response, along with a Mott-like transition under infrared pulse excitation, makes the 2D (BA) ${}_2{\mathrm{PbI}}_4$ perovskites suitable for all-optical operation in the infrared regime.