In‐Plane Tailoring of Colloidal Quantum Well Enables Continuous‐Wave Polariton Lasing

ABSTRACT Efficient continuous‐wave (CW) green lasers are hindered by the poor performance of conventional semiconductors in the so‐called “green gap”. Colloidal quantum wells (CQWs) offer giant optical gain and solution processability, yet suffer from exciton spatial dilution under sustained excitation. Here, we leverage an in‐plane exciton‐engineering strategy that tailors the lateral geometry of CdSe/CdSeS core/alloyed‐crown CQWs toward the intrinsic exciton coherence. This symmetric design minimizes the exciton migration distance from crown to core and confines the exciton population within the coherent area, thereby enhancing the local exciton density available for optical gain, reducing the threshold exciton density to 0.0022 /nm 2 . Integrating the optimized CQWs into a vertical distributed Bragg reflector microcavity yields CW polariton lasing in the green with an ultralow threshold of 70.1 W/cm 2 , several orders of magnitude lower than conventional colloidal semiconductor lasers. This work exploits in‐plane exciton engineering as a promising route to solution‐processable, low‐threshold coherent light sources and a step toward practical CW polariton lasers.