Vortex beams with orbital angular momentum are characterized by a helical phase front and a phase singularity at the beam center. Such beams have various intriguing applications, such as optical communications, particle trapping, and high resolution imaging. Generating a vortex beam by exploiting the geometric phase has attracted great interest, due to its spin-to-orbital conversion feature and device versatility. To meet the requirements of applications such as optical communications and make the devices more user-friendly, vortex beam generation needs to be efficient in a large frequency range. The efficient bandwidth of the optical element is critical here, since the geometric phase is frequency independent. In this work, we design and fabricate a broadband efficient reflective vortex beam generator based on a metallic helix array, which has a uniform efficient co-circular-polarization reflection property. Vortex beams with arbitrary topological charge can be generated with such devices. The measured efficiency of our device is above 58.4%, with the relative bandwidth of 64.52%. Published by AIP Publishing.
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