Slow light mediated by mode topological transitions in hyperbolic waveguides

TL;DR

This paper investigates how topological transitions in mode dispersion within hyperbolic waveguides enable slow light phenomena, revealing new mode behaviors and ghost wave emergence as the film thickness varies.

Contribution

It demonstrates the occurrence of topological mode transitions and slow light in hyperbolic waveguides due to changes in electrical thickness and mode coalescence.

Findings

Topological transitions occur in the dispersion diagram as film thickness increases.

Slow light is observed over a broad range of thicknesses.

Ghost waves emerge at branch points during mode transitions.

Abstract

We show that symmetric planar waveguides made of a film composed of a type II hyperbolic metamaterial, where the optical axis (OA) lays parallel to the waveguide interfaces, result in a series of topological transitions in the dispersion diagram as the film electrical thickness increases. The transitions are mediated by elliptical mode branches, which, as soon as they grow from cutoff, coalesce along the OA with anomalously ordered hyperbolic mode branches, resulting in a saddle point. When the electrical thickness of the film increases further, the merged branch starts a transition to hyperbolic normally ordered modes with propagation direction orthogonal to the OA. In this process, the saddle point is transformed into a branch point where a new branch of Ghost waves appears and slow light is observed for a broad range of thicknesses.