Strong transmission and reflection of edge modes in bounded photonic graphene.

The propagation of linear and nonlinear edge modes in bounded photonic honeycomb lattices formed by an array of rapidly varying helical waveguides is studied. These edge modes are found to exhibit strong transmission (reflection) around sharp corners when the dispersion relation is topologically nontrivial (trivial). An asymptotic theory is developed that establishes the presence (absence) of typical edge states, including, in particular, armchair and zigzag edge states in the topologically nontrivial (trivial) case. In the presence of topological protection, nonlinear edge solitons can persist over very long distances.