Observation of unidirectional backscattering-immune topological electromagnetic states

A hallmark of the quantum Hall effect, and a consequence of nontrivial topology of the electronic properties of systems manifesting this phenomenon, is the existence of so-called chiral edge states. These are a class of unique electronic states through which two-dimensional electron systems subject to a large magnetic field propagate in a unidirectional fashion, essentially unimpeded by scattering. Recent theoretical studies predicted that analogues to such electron 'one-way' edge modes could be realized for electromagnetic waves in photonic crystals, materials with periodic variations of the refractive index. Zheng Wang and colleagues now experimentally realize such photonic chiral edge states in a designed magneto-optical photonic crystal and demonstrate their unidirectional and scattering-protected nature. The finding of such photonic chiral edge states may enable new classes of photonic devices as well as new experimental realizations of the quantum Hall phenomenon in classical and bosonic systems. The quantum Hall effect arises in two-dimensional electron systems and is characterized by current being carried by electrons along the edges of the system, in so-called chiral edge states (CESs), as a consequence of nontrivial topological properties of the bulk electronic band structure. Recently, it was theoretically predicted that electromagnetic analogues of CESs could be observed in photonic crystals; here, this is experimentally demonstrated. One of the most striking phenomena in condensed-matter physics is the quantum Hall effect, which arises in two-dimensional electron systems1,2,3,4 subject to a large magnetic field applied perpendicular to the plane in which the electrons reside. In such circumstances, current is carried by electrons along the edges of the system, in so-called chiral edge states (CESs). These are states that, as a consequence of nontrivial topological properties of the bulk electronic band structure, have a unique directionality and are robust against scattering from disorder. Recently, it was theoretically predicted5,6,7 that electromagnetic analogues of such electronic edge states could be observed in photonic crystals, which are materials having refractive-index variations with a periodicity comparable to the wavelength of the light passing through them. Here we report the experimental realization and observation of such electromagnetic CESs in a magneto-optical photonic crystal7 fabricated in the microwave regime. We demonstrate that, like their electronic counterparts8,9,10,11,12,13, electromagnetic CESs can travel in only one direction and are very robust against scattering from disorder; we find that even large metallic scatterers placed in the path of the propagating edge modes do not induce reflections. These modes may enable the production of new classes of electromagnetic device and experiments that would be impossible using conventional reciprocal photonic states alone. Furthermore, our experimental demonstration and study of photonic CESs provides strong support for the generalization and application of topological band theories to classical and bosonic systems, and may lead to the realization and observation of topological phenomena in a generally much more controlled and customizable fashion than is typically possible with electronic systems.