Topological Metasurfaces

Recent developments in photonic topological insulators have shown that symmetry can be used to create unique electromagnetic states such as unidirectional waveguides in which modes are protected from scattering. Further research has shown that similar states can be created using defects in homogeneous materials, essentially creating chiral waveguides with polarization locked propagation. While topological protection usually refers to the band structure, it has recently become clear that such defects can have topological protection in real space as well and can lead to states that cannot be created nor destroyed, except in pairs, without global changes to the lattice. Similar isolated states can also be created as corner modes in higher order topological insulators. Much of this is not widely known within the microwave community, but these structures are will find increasing importance in the coming years for a variety of applications.