Phase Purification Optimizes Photophysical Properties in Quasi‐2D Perovskites for Ultra‐Low Threshold Amplified Spontaneous Emission

Quasi‐two‐dimensional (quasi‐2D) perovskites now are promising candidate materials for obtaining laser diodes and microlaser media, requiring phase‐purification as an effective strategy to minimize energy loss and enhance optical gain. However, the intrinsic mechanism by which purified phase purity leads to high photophysical properties remains understudied. Here, by employing phenylethylammonium trifluoroacetate (PEATFA) for phase purification, it is demonstrated that phase‐purified perovskites possess suppressed exciton‐phonon coupling, prolonged gain lifetime and enhanced nonlinear absorption property. At low temperature, photoluminescence reveals that the phase‐purified quasi‐2D perovskite film has weakened quantum confinement and suppressed exciton‐phonon coupling. During the establishment of stimulated emission, the enhanced energy transfer efficiency enables sufficient carrier accumulation in higher‐dimensional phases, prolonging the gain lifetime to 98 ps. After phase purification, perovskite films exhibit better optical properties in nonlinear regime with higher two‐photon absorption (TPA) coefficient and lower saturation absorption intensity. Finally, phase‐purified films achieve optimal amplified spontaneous emission (ASE) performance with a remarkably low threshold of 3.6 µJ cm −2 . This work reveals the intrinsic mechanism of phase purification regulating photophysical properties, providing a scientific basis for understanding that phase purification treatment enhances the gain characteristics of perovskite materials.