Topological Transition Enabled by Surface Modification of Photonic Crystals

Effective mass in the Dirac equation model is generally applied to condensed-matter systems for describing the state of electrons and determining the topological character. In these systems, the sign change of the effective mass is accompanied by a topological transition. Here we demonstrate that the band-gap topological property of a photonic crystal can also be related to a pseudo effective mass that is constructed by mapping Maxwell's equations to a Dirac-like equation. Then, we propose an efficient method to achieve a sign change of the pseudo effective mass by surface modification, resulting in a topological transition of the photonic crystal. Such a topological transition is confirmed by the emergence of a zero-dimensional topological bound state. We experimentally verify the theoretical predictions via colloidal photonic crystals, where a thin surface coating can lead to the topological transition. Our research provides a new avenue for the manipulation of topological properties of optical systems.