Multiexciton Dynamics and Lattice Responses Associated with Amplified Spontaneous Emission in Metal Halide Perovskites

Amplified spontaneous emission (ASE) in metal halide perovskites occurs under intense photoexcitation, where multiexciton (ME) states contribute to population inversion. Here, we investigate how A-site compositional modification regulates ASE in FAPbBr3 films partially substituted with methylammonium (MA). The ASE threshold varies nonmonotonically with MACl additive concentration despite nearly linear bandgap shifts. Fluence-dependent transient absorption signals were decomposed into odd-order nonlinear responses, confirming that the picosecond component emerging at high fluence originates from ME states. The ME lifetime exhibits a pronounced MACl concentration dependence that closely follows the ASE threshold. Coherent phonon analysis reveals reduced phonon frequencies at intermediate MA concentrations, consistent with slower lattice modulation and prolonged ME lifetimes. MA substitution also modifies film morphology and reduces void-like features associated with optical scattering. These results show that ASE performance is jointly governed by ME relaxation dynamics and photon propagation losses.