Electromagnetic‐Dual Metasurfaces for Topological States along a 1D Interface

• Published in: Laser & photonics reviews Vol. 13; no. 10
• Main Authors: Bisharat, Dia'aaldin J., Sievenpiper, Daniel F.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.10.2019
• Subjects: 1D waves; Backscattering; bianisotropy; Cones; electromagnetic duality; Interlayers; Metamaterials; Metasurfaces; Microwave frequencies; Photonic crystals; Quantum Hall effect; Supports; Topological insulators; topological photonics; Topology
• ISSN: 1863-8880; 1863-8899
• DOI: 10.1002/lpor.201900126

The discovery of topological insulators was rapidly 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 (2D) 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 (TE) and transverse‐magnetic (TM) modes, which break into a wide nontrivial bandgap at small interlayer separation. Consequently, the ultrathin structure supports robust gapless edge states, which are confined along a one‐dimensional (1D) 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. A simple approach to realize photonic topological insulators (PTIs), which promise lossless wave‐transmission despite fabrication imperfections, is presented. The design is nothing but a flat metasurface (patterned plate) atop another of a complementary pattern. Hence, electromagnetic duality is exploited, yet without demanding any parameter fine‐tuning. Strikingly, the design outperforms existing PTIs in terms of bandwidth and energy concentration.