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

ABSTRACT 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) 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) perovskites suitable for all‐optical operation in the infrared regime.