Transparent Topological Meta-Diplexer via Tightly Confined Valley Surface States.

Topological photonics offers a powerful platform for robust wave manipulation. However, conventional topological devices rely on opaque substrates, limiting their use in applications requiring optical transparency, such as smart windows and transparent electronics. Here, we demonstrate tightly confined valley surface states (VSSs) in an optically transparent topological metasurface platform based on a hollow-snowflake-wire design. The proposed platform simultaneously achieves over 85% visible transparency, deep-subwavelength vertical confinement, and a footprint half that of conventional topological designs. The resulting VSSs exhibit strong robustness against structural discontinuities and high routing flexibility. By tailoring the edge unit cells, we realize a compact topological meta-diplexer with spatially and spectrally separated channels. Communication experiments under 64-QAM modulation confirm robust in-band transmission with error vector magnitude (EVM) < 2 % and strong inter-channel isolation with EVM > 15 % for undesired channels. This work establishes an ultrathin transparent topological metasurface platform, paving the way toward integrated and multifunctional topological photonic systems in optically transparent media.