Broadband transmissive terahertz metasurface for simultaneous quad-mode OAM multiplexing

Multi-mode orbital angular momentum (OAM) multiplexing metasurfaces have emerged as promising devices for high-efficiency and flexible terahertz (THz) communication. In this work, we propose a cascaded three-layer polarization-selective transmissive OAM metasurface. The unit cell's top and bottom layers are polarizers realized by perpendicular parallel strips, while the middle layer consists of two pairs of ring and L-shape resonators to adjust the phase response. By properly arranging the metasurface units, four OAM modes (+1, -1, +2, and -2) can be simultaneously generated in different directions within the THz band. The design demonstrates excellent performance across a wide frequency range of 130-230 GHz, which is supported by full-wave simulations. The metasurface is fabricated using micro/nanofabrication techniques and experimentally characterized within the 130-210 GHz range due to the bandwidth limitation of the measurement setup. Experimental results show that the metasurface successfully generates and detects all four OAM modes at their respective radiation angles within this measured frequency range, with good agreement between simulations and measurements. These results confirm the effectiveness of the proposed design and indicate its strong potential for future high-capacity terahertz communication systems.