Three-dimensional all-dielectric photonic topological insulator

The discovery of two-dimensional topological photonic systems has transformed our views on the propagation and scattering of electromagnetic waves, and the quest for similar states in three dimensions is open. Here, we theoretically demonstrate that it is possible to design symmetry-protected three-dimensional topological states in an all-dielectric platform, with the electromagnetic duality between electric and magnetic fields being ensured by the structure design. Magneto-electrical coupling plays the role of a synthetic gauge field that determines a topological transition to an ‘insulating’ regime with a complete three-dimensional photonic bandgap. We reveal the emergence of surface states with conical Dirac dispersion and spin-locking, and we numerically confirm robust propagation of the surface states along two-dimensional domain walls with first-principles studies. The proposed system represents a table-top platform capable of emulating the relativistic dynamics of massive Dirac fermions and the surface states can be interpreted as Jackiw–Rebbi states bound to the interface separating domains with particles of opposite masses. The theoretical study of a 3D photonic topological metacrystal based on an all-dielectric metamaterial platform shows robust propagation of surface states along 2D domain walls, making it a promising solution for photonics applications. The proposed metacrystal design might also open the way for the observation of elusive fundamental physical phenomena.