Observation of topological corner states in a D4-symmetric square lattice of split-ring resonators

Topological states offer an increased versatility in disorder-robust localization of electromagnetic waves at the edges and corners of photonic structures. In most of the cases, such properties are achieved due to the appropriate lattice symmetry. Here, by contrast, we explore an alternative design strategy where the topological states in a simple square lattice are tailored due to the orientation of non-centrosymmetric split-ring resonators comprising the meta-structure. We numerically predict the emergence of the nontrivial topological properties and confirm our prediction by fabricating the structure and observing the localized edge and corner states experimentally.