Slow light mediated by mode topological transitions in hyperbolic waveguides

We show that slow light in hyperbolic waveguides is linked to topological transitions in the dispersion diagram as the film thickness changes. The effect appears in symmetric planar structures with type II films, whose optical axis (OA) lies parallel to the waveguide interfaces. The transitions are mediated by elliptical mode branches that coalesce along the OA with anomalously ordered hyperbolic mode branches, resulting in a saddle point. When the thickness of the film increases further, the merged branch starts a transition to hyperbolic normally ordered modes propagating orthogonally to the OA. In this process, the saddle point transforms into a branch point featuring slow light for a broad range of thicknesses, and a new branch of ghost waves appears. (C) 2020 Optical Society of America