title: Photonic Floquet topological insulators.
abstract: Topological insulators are a new phase of matter, with the striking property that conduction of electrons occurs only on their surfaces. 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 waves. One-dimensional systems with topological edge states have been demonstrated, but these states are zero-dimensional and therefore exhibit no transport properties. Topological protection of microwaves has been observed using a mechanism similar to the quantum Hall effect, by placing a gyromagnetic photonic crystal in an external magnetic field. 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 recently. One suggested temporal modulation of a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge states. This is in the spirit of the proposed Floquet topological insulators, 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 waveguides arranged in a graphene-like honeycomb lattice. Paraxial diffraction of light is described by a Schrödinger equation where the propagation coordinate (z) acts as 'time'. 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.
authors.affiliation: Department of Physics and the Solid State Institute, Technion - Israel Institute of Technology, Haifa 32000, Israel. mcrworld@gmail.com
authors.forename: Mikael C
authors.initials: MC
authors.lastname: Rechtsman
authors.forename: Julia M
authors.initials: JM
authors.lastname: Zeuner
authors.forename: Yonatan
authors.initials: Y
authors.lastname: Plotnik
authors.forename: Yaakov
authors.initials: Y
authors.lastname: Lumer
authors.forename: Daniel
authors.initials: D
authors.lastname: Podolsky
authors.forename: Felix
authors.initials: F
authors.lastname: Dreisow
authors.forename: Stefan
authors.initials: S
authors.lastname: Nolte
authors.forename: Mordechai
authors.initials: M
authors.lastname: Segev
authors.forename: Alexander
authors.initials: A
authors.lastname: Szameit