Topology describes properties that remain unaffected by smooth distortions. Its main hallmark is the emergence of edge states localized at the boundary between regions characterized by distinct topological invariants. This feature offers new opportunities for robust trapping of light in nanoand micro-meter scale systems subject to fabrication imperfections and to environmentally induced deformations. Here we show lasing in such topological edge states of a one-dimensional lattice of polariton micropillars that implements an orbital version of the Su-Schrieffer-Heeger Hamiltonian. We further demonstrate that lasing in these states persists under local deformations of the lattice. These results open the way to the implementation of chiral lasers in systems with broken time-reversal symmetry and, when combined with polariton interactions, to the study of nonlinear topological photonics.Topological phase transitions in condensed matter have been extensively studied over the last decade. A key manifestation of these transitions is the emergence, at the frontier between materials exhibiting distinct topological phases, of localized states that are unaffected by disorder. One example of this topological protection is provided by chiral edge states at the surface of topological insulators that allow unidirectional transport immune to backscattering 1 .Initially proposed by Haldane and Raghu 2 , the idea of extending these topological arguments to the realm of photonics has recently triggered considerable efforts to engineer optical devices that are unaffected by local perturbations and fabrication defects 3 . For example, topological properties have been used to create polarizationdependent unidirectional waveguides 4 , optical delay lines with enhanced transport properties 5 , backscatteringimmune chiral edge states 6-9 , and protected bound states in parity-time-symmetric crystals 10,11 .The emergence of edge states at the boundary between materials with distinct topological invariants provides an efficient way to create localized photonic modes whose existence is protected by topology 9 . Lasing in these kind of modes would then be robust against fabrication defects, local deformations caused by temperature or other unstable ambient conditions, and long term degradation, all of which would eventually result in the modification of the local optical potential 12 . The main difficulty that has prevented the observation of lasing in topological modes is the need to implement topological lattices in media exhibiting optical gain. In this sense, microcavity polaritons, mixed quasiparticules formed from the strong coupling between cavity photons and quantum well excitons 13 , provide a unique platform: they allow for low-threshold lasing 14,15 , even at room temperature 16,17 , and the engineering of topological properties in lattices of resonators 18,19 .In this work we report lasing in topological edge states of a 1D lattice of coupled semiconductor micropillars. The lattice implements an orbital version of the Su-Schrieff...

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Topological photonics seeks to control the behaviour of the light through the design of protected topological modes in photonic structures. While this approach originated from studying the behaviour of electrons in solid-state materials, it has since blossomed into a field that is at the very forefront of the search for new topological types of matter. This can have real implications for future technologies by harnessing the robustness of topological photonics for applications in photonics devices. This Roadmap surveys some of the main emerging areas of research within topological photonics, with a special attention to questions in fundamental science, which photonics is in an ideal position to address. Each section provides an overview of the current and future challenges within a part of the field, highlighting the most exciting opportunities for future research and developments.

Using contactless electroreflectance, we determined the band gap of the two known phases of epitaxial ZnSnP2. Induced by small changes in Sn/Zn flux ratio during epitaxy, the order-disordered transition between the chalcopyrite and sphalerite phases reduces the band gap by 300 meV. The chalcopyrite ordered phase, unambiguously identified from x-ray diffraction, exhibits a band gap of 1.683 eV at 293 K. The band gap of the disordered sphalerite phase is 1.383 eV. Using the volume-averaged order parameter measured on the chalcopyrite sample, we find that its morphology is best described by the presence of perfectly ordered domains inside a disordered matrix.