Topological states of light represent counter-intuitive effects at boundaries of optical systems that originate from the properties of the bulk. Being defined by bulk properties, such boundary states are protected from perturbations of the boundaries themselves, showing promise to increase robustness of photonic circuitries to certain types of imperfections and disorder. In two-dimensional photonic crystals, topological bulk-boundary correspondence conventionally links 2D bulk modes with 1D waveguiding modes (edge states). Recently, the bulk-boundary correspondence was generalized to higher-order effects, in which the 2D modes are defining 0D cavity modes (corner states). The authors of this article propose and study theoretically a coupled system that combines both a 1D topological waveguide and 0D topological cavity within a single photonic crystal slab. The underlying physics relies on a recent suggestion for generalization of the Su-Schrieffer–Heeger (SSH) model from one to two dimensions. The authors compare directly the topologically protected system with its more conventional, topologically trivial counterpart. The topological system demonstrates superior optical properties and considerably higher robustness against deliberately added defects in contrast to its more conventional, topologically trivial counterpart.
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