Electromagnetic-dual metasurfaces for topological states along a one-dimensional interface

TL;DR

This paper demonstrates a simple method to realize topological states in metasurfaces by coupling surface modes, creating robust edge states at microwave frequencies, with potential for compact topological insulator applications.

Contribution

It introduces a straightforward approach to achieve topological states in metasurfaces through stacking unit cells, avoiding complex designs and narrow bandwidths of previous methods.

Findings

Stacked metasurfaces produce double Dirac cones and wide non-trivial bandgaps.

Robust gapless edge states are observed along a one-dimensional interface.

The approach is effective at microwave frequencies and suitable for compact applications.

Abstract

The discovery of topological insulators has rapidly been followed by the advent of their photonic analogues, motivated by the prospect of backscattering-immune light propagation. So far, however, implementations have mainly relied on engineering bulk modes in photonic crystals and waveguide arrays in two-dimensional systems, which closely mimic their electronic counterparts. In addition, metamaterials-based implementations subject to electromagnetic duality and bianisotropy conditions suffer from intricate designs and narrow operating bandwidths. Here, it is shown that symmetry-protected topological states akin to the quantum spin-Hall effect can be realized in a straightforward manner by coupling surface modes over metasurfaces of complementary electromagnetic responses. Specifically, stacking unit cells of such metasurfaces directly results in double Dirac cones of degenerate transverse-electric and transverse-magnetic modes, which break into a wide non-trivial bandgap at small inter-layer separation. Consequently, the ultrathin structure supports robust gapless edge states, which are confined along a one-dimensional line rather than a surface interface, as demonstrated at microwave frequencies by near field imaging. The simplicity and versatility of the proposed approach proves attractive as a tabletop platform for the study of classical topological phases, as well as for applications benefiting the compactness of metasurfaces and the potential of topological insulators.