Highly Transmissive Silicon‐Based Metasurface for Multifunctional Terahertz Waveplates

High‐performance terahertz (THz) polarization converters are critical for next‐generation systems, yet conventional solutions often suffer from bulkiness or high losses. To address this, we present a numerically designed all‐dielectric metasurface consisting of a single‐layer array of silicon pillars. By leveraging form birefringence and Mie‐resonant responses, the anisotropic geometry enables precise phase retardation control. The device demonstrates robust multifunctionality: it operates as a high‐efficiency half‐wave plate (HWP) at 1.09 THz with a polarization conversion ratio greater than 99%, and as a quarter‐wave plate (QWP) at 0.9 and 1.22 THz. Notably, the metasurface maintains high transmittance (>84%) at all designated frequencies. This simple, single‐layer architecture offers a highly integrated, efficient solution for multifunctional THz photonics.