Wavefront manipulation is of great importance in the development of science and technology, and the generation of vortex beam is one of the most effective methods to improve the channel capacity and communication accuracy. However, current available vortex beam generators suffer from complex structure, large size, especially narrow bandwidth. To solve these issues, we propose a general strategy to enhance the bandwidth of the vortex beam generator by using double-layered reflective metasurface. The well-optimized double-layered reflective element possesses dual resonance, which realizes complete phase coverage (larger than 360 degrees) and a flat slope of the reflection phase, resulting in a broad bandwidth (8.5-11.5GHz). A compact and low-profile vortex-beam generator is proposed by combining the parabolic phase and the spiral phase plate phase. Both near field and far field experiments are performed to demonstrate the predesigned effects. A pure vortex beam is observed clearly with a topological charge of m=1 in a wide frequency window of 3 GHz (8.5-11.5GHz). More importantly, the working efficiency of the vortex beam is better than 90% at center frequency 10 GHz. The findings in this paper motivate the realization of high-performance reflective metasurfaces and other functional metadevices.
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