Long‐Range Coherent Interference From Thermally Imprinted Quasi‐2D Perovskite Fabry–Pérot Microcavities

Microcavity lasing in solution‐processed 2D halide perovskites remains a central challenge because intrinsic phase heterogeneity, defect‐assisted scattering, and ambiguous optical feedback obscure the physical origin of coherence. It remains unresolved whether the long‐range coherence reported in quasi‐2D perovskite lasers arises from intrinsic cavity‐mediated photonic feedback or from extrinsic effects, such as interference contributions. Here, we demonstrate that a synergistic combination of alkali‐metal‐assisted phase engineering and thermal imprint‐induced recrystallization enables high‐quality‐factor (High‐ Q) Fabry–Pérot microcavity lasing with long‐range coherence in quasi‐2D perovskites. Incorporation of sodium bromide into PEA 2 Cs n −1 Pb n Br 3 n +1 quasi‐2D perovskites effectively modulates the distribution of n ‐phases by reducing nonradiative low‐ n components and suppressing intrinsic crystal defects. The recrystallization process was facilitated by thermal imprinting and enhanced orientational ordering in the selected phases. These advances enabled the formation of low‐optical‐loss FP microcavities that supported lasing with a quality factor reaching up to Q ≈ 6890. Angle‐resolved photoluminescence, combined with parity‐resolved Young's interference measurements, directly revealed the longitudinal mode order and phase locking, providing unambiguous evidence of photonic coherence across the entire microcavity. The observed lasing is attributed to enhanced wavefunction overlap and rapid exciton recombination enabled by spatial confinement within the quasi‐2D perovskite layers, which collectively promote photonic lasing over alternative mechanisms. These findings address a key ambiguity in quasi‐2D perovskite laser physics by positioning thermally imprinted microcavities as scalable and physically interpretable platforms for on‐chip coherent light sources.