Topological photonics.

By translating these concepts into photonics, we design optical structures whose bands inherit the same topological invariants. The lattice we build with light is, in the right limit, the lattice the butterfly came from.

The hallmark is a chiral edge mode — light that travels in only one direction along the boundary, carrying information without ever scattering backward.

Inject a photon at the edge of a topological photonic lattice. It moves along the perimeter, in one direction only, regardless of where you put it in. The bulk stays dark; the edge does all the work.

The punchline

Now break the lattice. Remove a site from the edge. In any ordinary system, this would be a defect — a place where light scatters, gets stuck, leaks away.

In a topological system, the edge mode just reroutes. It finds a new boundary, traces around the gap, and continues its one-way journey. The topology of the band structure forbids backscattering — the chip can be sloppy, and the physics still works.

A photonic lattice — coupled ring resonators in a 2D grid.

Why I find this compelling

The promise of topological photonics is that the geometry of a chip can do work that previously required exquisite fabrication tolerances. If a device works because of its shape rather than the perfection of its parts, then we can build photonic systems at scale — and that's the door this field opens.