Fig. 1: Photonic topological band structures.
a “True” one-way topological boundary mode (magenta line) with absence of a back-scattering channel for any frequency in the topologically-protected bandgap. For a given frequency \({\omega }_{0}\), any momentum mismatch \(\Delta k\) induced by disorder cannot couple to a backward channel. b Symmetry protected boundary modes with pseudo-spin-polarized one-way transport, where symmetry reducing defects may lead to spin-flip and back-scattering into the backward mode. \(\Delta k\) is the change in momentum due to a defect-induced scattering.
