Photonic Floquet topological insulators

An experimental realization of a photonic topological insulator is reported that consists of helical waveguides arranged in a honeycomb lattice; the helicity provides a symmetry-breaking effect, leading to optical states that are topologically protected against scattering by disorder. One of the hottest fields of condensed-matter research is that of topological insulators. They exist in electronic states that are robust against disorder owing to the topological protection provided by the underlying electronic structure. Their potential practical importance lies in their ability to control and manipulate electron waves without scattering. An interesting question is whether it would be possible to make a topological insulator for light. The answer is yes, and here Mordechai Segev and colleagues demonstrate the first experimental realization of a photonic topological insulator, which consists of helical waveguides arranged in a honeycomb lattice. The helicity is crucial, providing a symmetry breaking effect leading to topological insulator properties. The authors demonstrate one-way edge states that are protected from scattering. Topological insulators are a new phase of matter1, with the striking property that conduction of electrons occurs only on their surfaces1,2,3. In two dimensions, electrons on the surface of a topological insulator are not scattered despite defects and disorder, providing robustness akin to that of superconductors. Topological insulators are predicted to have wide-ranging applications in fault-tolerant quantum computing and spintronics. Substantial effort has been directed towards realizing topological insulators for electromagnetic waves4,5,6,7,8,9,10,11,12,13. One-dimensional systems with topological edge states have been demonstrated, but these states are zero-dimensional and therefore exhibit no transport properties11,12,14. Topological protection of microwaves has been observed using a mechanism similar to the quantum Hall effect15, by placing a gyromagnetic photonic crystal in an external magnetic field5. But because magnetic effects are very weak at optical frequencies, realizing photonic topological insulators with scatter-free edge states requires a fundamentally different mechanism—one that is free of magnetic fields. A number of proposals for photonic topological transport have been put forward recently6,7,8,9,10. One suggested temporal modulation of a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge states10. This is in the spirit of the proposed Floquet topological insulators16,17,18,19, in which temporal variations in solid-state systems induce topological edge states. Here we propose and experimentally demonstrate a photonic topological insulator free of external fields and with scatter-free edge transport—a photonic lattice exhibiting topologically protected transport of visible light on the lattice edges. Our system is composed of an array of evanescently coupled helical waveguides20 arranged in a graphene-like honeycomb lattice21,22,23,24,25,26. Paraxial diffraction of light is described by a Schrödinger equation where the propagation coordinate (z) acts as ‘time’27. Thus the helicity of the waveguides breaks z-reversal symmetry as proposed for Floquet topological insulators. This structure results in one-way edge states that are topologically protected from scattering.