Photonic Floquet topological insulators

摘要

凝聚态物质研究最热门的领域之一是关于拓扑rn绝缘体的研究。它们存在于因其电子结构所提rn供的拓扑保护而不易产生无序的电子态。它们rn潜在的实用意义在于其能够在不发生散射的情rn况下控制和操纵电子波。一个有趣的问题是：rn是否有可能做出一种对光绝缘的拓扑绝缘体?rn答案是肯定的。在这项研究中，MordechairnSegevTJE．其同事演示了一种光子拓扑绝缘体的rn首次实验实现，它由以蜂窝状格子排列的螺旋rn形波导组成。螺旋形在这里非常关键，它提供rn了一个打破对称性的效应，从而导致产生拓扑rn绝缘体的性质。本文作者演示了受到保护而不rn会发生散射的单向边缘状态。%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 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 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 Schroedinger 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.