Topological states in arrays of optical waveguides engineered via mode interference

Abstract: Photonic structures with topologically nontrivial bands are usually designed by arranging simple meta-atoms, ideally, single-mode ones, in a carefully designed photonic lattice with symmetry that guarantees the emergence of topological states. Here we investigate an alternative option that does not require complex lattice geometry but instead relies on the tuning of the parameters of the individual meta-atoms to achieve the degeneracy of the modes with different symmetry. As an illustrative example, we consider a one-dimensional array of equidistant identical periodic nanophotonic waveguides supporting degenerate modes with strongly asymmetric near field profiles giving rise to the coupling modulation. Exploiting this feature, we demonstrate that the proposed system supports topological edge modes and can be viewed as a generalization of the paradigmatic Su-Schrieffer-Heeger model, reducing to it for the suitable parameter choice. Our results thus provide an avenue to engineer topological states via mode interference which further expands the plethora of topological structures available in photonics being especially promising for nonlinear topological systems.