  Slow-light waveguides can strongly enhance light-matter interaction, but
suffer from narrow bandwidth, increased backscattering, and Anderson
localization. Edge states in photonic topological insulators resist
backscattering and localization, but typically cross the bandgap over a single
Brillouin zone, meaning that slow group velocity implies narrow-band operation.
Here we show theoretically that this can be circumvented via an edge
termination that causes the edge state to wind many times around the Brillouin
zone, making it both slow and broadband.
