Broadband minimal footprint photonic topological waveguides enabled by strong magneto-electric coupling

Photonic topological insulator (PTI) metasurfaces are promising for integrated photonic circuits as well as microwave/mmWave applications such as advanced 5G (5G+) and SatCom. However, their adoption is hindered by limited bandwidth - especially at microwave/mmWave bands, crosstalk, and complex fabrication. In this work, we present an ultra-low-profile and easily manufacturable bianisotropic PTI metasurfaces at mmWave bands that achieve enhanced bandwidth with robust edge-state transmission. We also identify the minimum unit cell interface width required to maintain topological protection. The structure comprises two magneto-electrically coupled layers that support spin-Chern topological modes. By reducing the inter-layer separation, we demonstrate an exponential increase in fractional bandwidth, confirmed through full-structure
simulations and measurements. The structure supports tight in-plane and out-of-plane field confinement, requiring a minimum boundary width of just two unit cells (i.e., ≈0.74λ0 at 21.2 GHz centre frequency) on each side of the topological interface—well-suited for densely integrated systems. Experimental near-field measurements of the electric field magnitude, |E|, over the surface further confirm the localization of the edge modes along three different interface paths. These results highlight the potential of this PTI design for scalable, low-loss mmWave
waveguiding applications in complex, interference-prone electromagnetic package environments.