Non‐Invasive Groove Engineering Enables External‐Cavity‐Free Single‐Mode Polarized Lasers in Perovskite Microplates

Dynamic modulation of microcavity lasers is crucial for achieving stable single‐mode operation with high monochromaticity, compact footprint, and directional emission. However, the emergence of higher‐order transverse modes with increasing cavity dimensions has frustrated efforts to scale up cavities while maintaining stable single‐mode operation. To date, no suitable physical mechanism has been identified that enables cavity size‐independent single‐mode lasing. Herein, a breakthrough non‐invasive technique is developed for in situ engineering of natural nano‐grooves in large‐size (20 × 20 µm) perovskite microplates (MPs) through kinetically controlled vapor‐phase epitaxy, achieving unprecedentedly precise mode control for optimal single‐mode lasing performance. Critically, these well‐defined grooves can alter the lasing resonance modes within the microlasers, enabling the transition from whispering gallery mode (WGM) to Fabry‐Pérot (F‐P) mode. The optical anisotropy induced by engineered groove structures also achieves polarized lasing emission with a high degree of polarization approaching 0.65. Additionally, numerical simulations elucidate that the external‐cavity‐free, high‐quality (≈2660), and single‐mode F‐P lasing behavior under large‐size conditions arises from the coupling of multiple cleavage cavities, which is induced by broken grooves in perovskite MPs morphology. This study pioneers a mode‐selective modulation approach that enables scalable polarization engineering in large‐area single‐mode microlasers, fundamentally advancing multifunctional integration in photonics and high‐density optoelectronic systems.