Observation of non-Abelian nodal links in photonics

Abstract

In crystals, two bands may cross each other and form degeneracies along a closed loop in the three-dimensional momentum space, which is called nodal line. Nodal line degeneracy can be designed to exhibit various configurations such as nodal rings, chains, links and knots. Very recently, non-Abelian band topology was proposed in nodal link systems, where the nodal lines formed by consecutive pairs of bands exhibit interesting braiding structures and the underlying topological charges are described by quaternions. Here, we experimentally demonstrate non-Abelian nodal links in a biaxial hyperbolic metamaterial. The linked nodal lines threading through each other are formed by the crossings between three adjacent bands. Based on the non-Abelian charges, we further analyze various admissible nodal link configurations for the three-band system. On the interface between the metamaterial and air, surface bound states in the continuum (BICs) are observed, which serves as the symmetry-enforced derivative of drumhead surface states from the linked nodal lines. Our work serves as a direct observation of the global topological structures of nodal links, and provides a platform for studying non-Abelian topological charge in the momentum space.

18 pages, 4 figures

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Cited by in corpus (34)

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• Experimental observation of non-Abelian topological charges and bulk-edge correspondence
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• Hybrid topological photonic crystals
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• Observation of Non-Abelian Thouless Pump
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• Photonic nodal lines with quadrupole Berry curvature distribution
• Braiding Topology of Non-Hermitian Open-Boundary Bands
• Demonstration of non-Abelian frame charge flow in photonic crystals
• Hermitian Topologies originating from non-Hermitian braidings
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• Observing the nodal-line conversion determined by the relative homotopy
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• Topological Degeneracy Induced by Twisting
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• Nodal-line semimetals and their variance
• Emergence of Nodal-Knot Transitions by Disorder
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