(color online) Polariton lasing based on perovskites. (a) Microcavity consisting of a planar F–P resonator embedded in two reflectors. Excitons in an active layer and cavity photons can strongly couple with each other to generate new quasiparticles, exciton–polaritons. (b) Mechanism of ideal polariton lasing. Under high-energy excitation, polaritons are created and then leave the UPB for the LPB via phonon emission. Next, they leap down mainly through polariton–phonon scattering before reaching the bottleneck region and through polariton–polariton scattering after reaching the bottleneck to relax continuously to the final state k_∥ = 0.^[90] (c) Energy–wave vector (E–k) dispersion curves of CH₃NH₃PbBr₃ (MAPbBr₃) micro/nanowire with a width of 0.32 μm and length of 3.66 μm. The L–T splitting energy (ΔE_LT) is about 33 meV, which shows the strong coupling strength. The inset shows a normalized electric field distribution |E|² at the cross-section of each corresponding wire.^[15] (d) Lasing emission spectra (green line) and spatially resolved PL spectra (blue line) of a MAPbBr₃ wire in the same condition. The red dots displayed in wave vector space with integer values of π/L_z are F–P peaks extracted from spatially resolved PL spectra, which can be fitted with the polariton dispersion curve (red line).^[15] (e) Schematic diagram of the microcavity structure, where CsPbCl₃ NPs are embedded in two DBR mirrors consisting of 7 and 13 HfO₂/SiO₂ pairs, respectively.^[32] (f) Angle-resolved PL spectrum at high pump fluence of 1.3P_th. Almost all areas around the final state k_∥ = 0 are occupied, indicating the realization of the whole polariton condensation.^[32]