(color online) (a) Schematic of hexagonal WGM perovskite of CH₃NH₃PbI_3−aX_a (X = I, Br, Cl) and the corresponding lasing mode.^[52] (b) Schematic of small perovskite lasers based on a WGM cavity with square shape (left panel). The PL emission image from the CsPbBr₃ NP above the threshold; the green color shows PL out-coupling from the NP (middle panel). The simulation of the electric field distribution for the square perovskite cavity; the pattern shows that the WGM is supported by the cavity, and the four corners show the stronger out-coupling than that at any other place (right panel).^[53] (c) Schematic of a CsPbX₃ (X = Cl, Br, or I) NP on mica substrate pumped by 400 nm laser excitation (left panel). Emission spectra with pump fluence increasing from 1.0 μJ·cm⁻² to 3.2 μJ·cm⁻² (right panel). The FWHM is ∼ 0.15 nm and the “S” curve shows a process from spontaneous emission to lasing. After photoexcitation with pump fluence below P ≈ 0.8 P_th and above threshold (≈ 1.2P_th), TRPL decay kinetics shows an ∼ 3.0 ns spontaneous emission decay process and a <30 ps lasing process, respectively.^[53]