(color online) (a) Schematic of photonic crystal (left panel), and the band structure of the photonic crystal (right panel). (b) SEM image of 2D photonic crystal nanostructured pattern (left panel), and lasing output from perovskite photonic crystal with the pitch in a range of 430–460 nm. Single mode lasing is observed separately at 768.4 nm, 777.3 nm, 788.1 nm, and 795.8 nm (right panel).^[8] (c) Schematic diagram of GaN-based DBRs (left panel), and the single mode lasing spectrum varying with pump fluence at 778.4 nm and FWHM of 0.24 nm.^[78] (d) Cross-sectional scanning electron micrograph of the DBR laser architecture, consisting of a CH₃NH₃PbI₃ (MAPbI₃) film deposited on an 80 nm thick etched alumina grating on a sapphire substrate (left panel), and input–output curve showing the process from spontaneous emission to lasing. The threshold is about 17 kW·cm⁻², and the inset indicates that the emission spectrum above the threshold is strongly TE-polarized.^[80]