High-efficiency ultra-broadband orbital angular momentum beam generators enabled by arrow-based fractal metasurface

Vortex beams carrying orbital angular momentum (OAM) are of great importance owing to the capacity to expand channel capacity of communication systems from microwave to optical regimes. In this work, a single-layer ultra-broadband meta-atom is designed by using the method of fractal. The reflected co-polarization amplitude of the arrow-based fractal meta-atom can exceed 0.9 in a frequency range from 6 to 19.7 GHz and the complete 2π phase control can be achieved by rotating the structure. Based on the proposed meta-atom, metasurface-based vortex beam generators are designed. As an illustrative example, a metasurface-based OAM beam generator with a mode of +1 is designed, fabricated, and measured. Both the experimental and simulated results match well with each other, proving the practicability of the proposed high-efficiency ultra-broadband metasurface.