Broadband vortex beam generating for multi-polarisations based on a single-layer quasi-spiral metasurface

A reflective metasurface based on the quasi-spiral element is proposed to achieve broadband orbital angular momentum vortex beams for multi-polarisations. The quasi-spiral element can provide linear and smooth phase responses covering about 360 degrees from 10 to 15 GHz with a relative bandwidth about 40% by adjusting the length of four phase delay lines. Additionally, the metasurface can generate vortex beams with optional charges in vertical, horizontal and circular polarisations which are determined by the polarisation of feed antenna. Finally,we have designed and fabricated a 24 x 24 cm(2) metasurface prototype. The simulated and measured results show that the reflective metasurface can generate broadband vortex beams with orbital angular momentum effectively. As the structure is simple, lightweight and easy to fabricate, the proposed method is applicable to generate broadband vortex beams for multi-polarisations.