Bandwidth Enhancement of Topological Insulator Metasurfaces

In this paper, we investigate a photonic topological insulator metasurface utilising electromagnetic duality. Using strong magneto-electric cross-coupling, a complementary two-layer structure can open a band gap. The non-trivial band gap enables strong topological protection of bound surface waves, over a large bandwidth of mm-wave frequencies. We investigate the direct relationship between bandwidth and inter-layer separation of the two-layer structure, showing that by reducing the separation from 1mm to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$200\mu m$</tex> the fractional bandwidth is increased from 39.81% to 53.77%. In this study, we also investigate the effect of the number of unit cells on either side of the interface and its effect on transmission. We observe that topological protection is maintained from 5- to 2-cell width on either side of the interface, but below 2-cell the structure suffers from high losses in the band gap frequencies. We also note a negligible change in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{12}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{21}$</tex> for 5-cell to 2-cell width.