Conducting electron states at the edge of an insulating material are robust against perturbation — and a rich treasure trove of physics is also promised by these 'topological insulator' systems. Observing some of these effects in condensed-matter systems, however, is made difficult by material impurities. Instead, Mohammad Hafezi and colleagues have now demonstrated photonic topological edge states: their approach provides an optical test-bed for the physics of topology and its effects.
Hafezi and his team created an array of optical ring microresonators on a silicon substrate. The resonators were connected in such a way that a photon could hop from one to its nearest neighbour, and in doing so the photon would acquire a phase that was dependent on whether it was hopping forwards or backwards. The Hamiltonian describing this system is analogous to that of two-dimensionally confined electrons in a magnetic field, and thus it should exhibit the same edge states.
And indeed it does: the researchers imaged light inserted into one corner of the array and confirmed that it propagated along the edges, even when defects were intentionally introduced.