From ray tracing to waves of topological origin in continuous media

TL;DR

This paper links topological wave phenomena in continuous media to ray tracing and phase space analysis, providing a physical interpretation of topological invariants like the Chern number through wave packet trajectories.

Contribution

It introduces a ray tracing approach to interpret the topological bulk-interface correspondence in continuous media, specifically for equatorial shallow water waves.

Findings

Chern number arises from wave packet quantization in phase space

Ray trajectories reveal topological properties of wave modes

Physical interpretation of topological invariants in continuous media

Abstract

Inhomogeneous media commonly support a discrete number of wave modes that are trapped along interfaces defined by spatially varying parameters. When they are robust against continuous deformations of parameters, such waves are said to be of topological origin. It has been realized over the last decades that such waves of topological origin can be predicted by computing a single topological invariant, the first Chern number, in a dual bulk wave problem that is much simpler to solve than the original wave equation involving spatially varying coefficients. The correspondence between the simple bulk problem and the more complicated interface problem is usually justified by invoking an abstract index theorem. Here, by applying ray tracing machinery to the paradigmatic example of equatorial shallow water waves, we propose a physical interpretation of this correspondence. We first compute ray trajectories in a phase space given by position and wavenumber of the wave packet, using Wigner-Weyl transforms. We then apply a quantization condition to describe the spectral properties of the original wave operator. We show that the Chern number emerges naturally from this quantization relation.