Helical metasurfaces based on topological surface states in three-dimensional photonic topological insulators.

Topological photonics expands the landscape of artificial electromagnetic materials and provides a variety of responses via robust boundary modes. Three-dimensional photonic topological insulators are predicted to host robust spin-momentum-locked surface states. However, their all-dielectric experimental realization has remained a fundamental challenge. Here we demonstrate a practical realization of a three-dimensional all-dielectric photonic topological insulator. We show a complete photonic topological bandgap as well as gapless topological surface states trapped on open boundaries of topological systems. The coupling of these states to the radiative continuum offers opportunities for controlling the emission of electromagnetic waves. We unveil that open interfaces in three-dimensional photonic topological insulators behave as effective metasurfaces and show that the helical nature of topological surface states supported by the interfaces enables control over far-field emission via the pseudo-spin degree of freedom. Further structuring of the topological interfaces provides further enhancement of such effective metasurfaces by offering control over far-field radiation patterns and directionality of the surface state emission.