From ray tracing to topological waves in continuous media

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 topological. It has been realized over the last decades that the existence of such topological waves 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. 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