Advances in Metasurfaces: Topology, Chirality, Patterning, and Time Modulation

Advances in metasurface design has given rise to the unique ability of manipulating the phase and propagation of electromagnetic (EM) waves. The application of topological protection, chirality, and other concepts borrowed from condensed matter physics have yielded a rich new source of structures with attractive features. Topological metasurfaces show that symmetry can be used to create unique EM modes that are, for instance, immune to many types of scattering. Similar modes can also be created using defects in homogeneous materials, essentially creating chiral waveguides with polarization-locked propagation. Such defects may have topological protection in real space and/or k -space and lead to modes without global changes to the lattice. This review article explores recent results in chiral and topological metasurfaces, primarily consisting of printed metal patterns, but also planar dielectric structures, at microwave, optical, and acoustic frequencies. The exciting potential applications include nonreciprocal structures, self-collimating waves based on diffusive transport, defect-based amorphous structures, time-modulated metasurfaces, and new kinds of chiral waveguides that exhibit unidirectional propagation without requiring a surrounding periodic structure.