Exciton–Polariton Effect and Polarized High‐Quality Lasing in 1D CsPbBr₃‐CsPb₂Br₅ Perovskite Planar Microcavities via Solution‐Driven Soft Thermal Imprinting

High‐quality laser modes (Q‐factor of ≈6520) are achieved through the self‐assembly of 1D metal halide perovskite (MHP) planar microwires (MWs) with a preferred orientation using a solution‐driven soft thermal imprinting technique. The coexistence of CsPbBr 3 and CsPb 2 Br 5 crystal phases within these MHP MWs, supported by well‐matched heterointerfaces and improved resistance to environmental degradation, underpins their high exciton binding energy and the realization of exceptional laser quality factors in Fabry–Pérot (FP) resonators. Strong exciton‐polariton coupling is demonstrated across various wire lengths, with Rabi splitting energies ranging from 145 to 180 meV, as revealed by a modified Lorentz oscillator model. This rapid increase in the group refractive index near the excitonic transition further exemplifies the energy‐band dispersion inherent to exciton‐polaritons. These novel structures, which function as microcavities, also yield waveguide modes that exhibit an exceptionally high degree of linear polarization. By leveraging these light‐matter interactions and waveguide architectures, this work paves the way for cost‐effective, solution‐processed perovskite photonic devices with high‐quality, linearly polarized lasing and optical nonlinearity applications.