Ideal Weyl points and helicoid surface states in artificial photonic crystal structures

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Science  02 Mar 2018:
Vol. 359, Issue 6379, pp. 1013-1016
DOI: 10.1126/science.aaq1221

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Exploring photonic topology

Scattering topological effects are being explored in a variety of electronic and optical materials systems owing to their robustness against defects (see the Perspective by Özdemir). Yang et al. designed and fabricated an ideal optical analog of a three-dimensional Weyl system. Angular transmission measurements revealed four Weyl points at the same energy, as well as the signature helicoidal arcs associated with such an exotic topological system. Zhou et al. theoretically proposed and experimentally demonstrated the formation of a topologically protected bulk Fermi arc. They attributed the formation of the arc to the topological nature of paired exceptional points (points at which gain and loss in the system are matched). Photonic crystals may provide a powerful platform for studying exotic properties of topological electronic systems and may also be used to develop optical devices that exploit topological properties of light-matter interactions.

Science, this issue p. 1013, p. 1009; see also p. 995


Weyl points are the crossings of linearly dispersing energy bands of three-dimensional crystals, providing the opportunity to explore a variety of intriguing phenomena such as topologically protected surface states and chiral anomalies. However, the lack of an ideal Weyl system in which the Weyl points all exist at the same energy and are separated from any other bands poses a serious limitation to the further development of Weyl physics and potential applications. By experimentally characterizing a microwave photonic crystal of saddle-shaped metallic coils, we observed ideal Weyl points that are related to each other through symmetry operations. Topological surface states exhibiting helicoidal structure have also been demonstrated. Our system provides a photonic platform for exploring ideal Weyl systems and developing possible topological devices.

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