Science 349, 622–624 (2015)

Graphene has a range of fascinating electronic properties that arise from the massless relativistic nature of the electrons. Photonic graphene — the optical equivalent with electrons in place of light — has equally intriguing properties, but these systems are both intrinsically two dimensional. What about their three-dimensional counterparts?

Graphene-like crystals are zero-bandgap materials whose dispersion relation is linear at high-symmetry Dirac points. To maintain this linear relationship in 3D is tricky, as the points, which are known as Weyl points, become unstable. Ling Lu and co-workers, however, have now confirmed their earlier predictions and observed Weyl points in a photonic crystal.

To make Weyl points, you need to break parity–time symmetry. To do this, Lu et al. used a double-gyroid structure that breaks inversion symmetry (parity) while maintaining time-reversal symmetry. The resulting Weyl points are topologically protected, and were observed using microwave transmission measurements. And there is some evidence that this approach could be extended to optical wavelengths.

Aside from opening the door to 3D topological photonics, these observations compliment simultaneous reports of Weyl points in the solid state, bringing the 85-year Weyl search to an end.