Reversible phase segregation in mixed-halide perovskites enabled by continuous-wave and femtosecond excitation strategies.

Mixed-halide perovskites offer continuously tunable bandgaps for micro- and nanophotonic applications, yet their performance is severely limited by light-induced phase segregation and spectral instability. Here, we investigate the phase-segregation dynamics and its reversibility in Br-I mixed-halide perovskite microplatelets under continuous-wave (CW) and femtosecond (fs) excitation. Under CW illumination, canonical iodine-rich phase segregation is observed, followed by a power-dependent anomalous reversal associated with polaron-mediated lattice strain homogenization. In contrast, fs excitation induces a distinct segregation-reversal behavior characterized by a pronounced pump-power threshold. By comparatively investigating phase-segregation reversal under dark storage, high-power CW laser irradiation, and femtosecond (fs) excitation, we find that fs-induced reversal occurs at a faster rate than in dark conditions and shows enhanced stability relative to that under high-power CW illumination. To explain this phenomenon, we introduce a field-assisted dissociation model, supported by CW-fs synergistic excitation experiments and density functional theory (DFT) calculations. These investigations demonstrate that the ultrahigh transient electric fields generated by femtosecond pulses can directly influence the potential energy surface of Pb-I bonds. Frontier orbital analysis and field-dependent potential energy calculations reveal field-assisted Pb-I bond softening, which destabilizes iodine-rich domains and enables a rapid and structurally complete segregation reversal. As a result, the system is reset to a kinetically pristine mixed state, from which subsequent CW illumination follows a slow segregation process analogous to the initial evolution.