  Various topological laser concepts have recently enabled the demonstration of robust light-emitting devices that are immune to structural deformations and tolerant to fabrication imperfections. Current realizations of photonic cavities with topological boundaries are often limited by outcoupling issues or poor directionality and require complex design and fabrication that hinder operation at small wavelengths. Here we propose a topological cavity design based on interface states between two one-dimensional photonic crystals with distinct Zak phases and demonstrate a lithography-free, single-mode perovskite laser emitting in the green. Few monolayers of solution processed all-inorganic cesium lead halide perovskite quantum dots are used as ultrathin gain medium. The topological laser has planar design with large output aperture, akin to vertical-cavity surface-emitting lasers (VCSELs) and is robust against variations of the thickness of the gain medium, from deeply subwavelength to thick quantum dot films. This experimental observation also unveils the topological nature of VCSELs, that is usually overlooked in the description of conventional Fabry-Perot cavity lasers. The design simplicity and topological characteristics make this perovskite quantum dot laser architecture suitable for low-cost and fabrication tolerant vertical emitting lasers operating across the visible spectral region. 
Perovskite quantum dot topological laser