Dutch researchers at AMOLF and TU Delft have seen light propagate in a special material without reflections. The material, a photonic crystal, consists of two parts that each have a slightly different pattern of perforations. Light can propagate along the boundary between these two parts in a special way: It is “topologically protected,” and therefore does not bounce back at imperfections. Even when the boundary forms a sharp corner, the light follows it without a problem.
Source:
https://phys.org/news/2020-02-topology-propagation-photonic-crystal.html