Wide-Angle Stable Transmissive Focusing Metasurfaces for OAM Vortex Waves in Wireless Communications.

Orbital angular momentum (OAM) vortex waves have become a promising technology for future 6G high-capacity wireless communications. However, the inherent propagation divergence and performance degradation under angular deviation severely restrict their practical applications. This paper proposes a transmissive focusing metasurface that maintains stable OAM vortex wave generation under angular deviation. By employing a quadratic phase distribution, the proposed design fundamentally eliminates the axial focal shift caused by oblique incidence and maintains stable focusing performance. A three-metal-layer transmissive unit is developed to achieve high cross-polarization transmittance and full 2π phase coverage. By combining the quadratic focusing phase and OAM spiral phase, the metasurface enables stable generation of high-purity focused OAM beams under angular deviation. A prototype is fabricated and experimentally characterized, which verifies the theoretical predictions under the designed incident angle and angular deviation scenarios. Furthermore, a wireless communication system based on the Universal Software Radio Peripheral (USRP) is constructed in this work, which achieves reliable transmission and demodulation verification of 16-quadrature amplitude modulation (16-QAM) signals. The proposed metasurface provides a feasible solution for short-to-medium distance robust transmission of OAM waves and shows good application potential in practical wireless communication systems.