High performance transmission-type meta-surface for generating terahertz focused vortex beams

Conventional optical devices that generate focused vortex beams (FVBs) often suffer from bulky size and integration challenges with other optical components. Here, we propose a novel approach using a transmission-type Pancharatnam-Berry (PB) meta-surface to generate nearly perfect focused terahertz vortex beams with opposite incident circular polarization. Our design leverages silicon pillars composed of two-layer structures exhibiting opposite symmetry Fabry-Perot resonances along two orthogonal directions, serving as high performance meta-atoms capable of both generating PB phase and achieving high transmittance. We have fabricated FVB samples comprising an array of all-dielectric silicon pillars with etched structure on both the front and back sides and demonstrated experimentally that our device can achieve exceptional focusing efficiency of 91.0% and maintains a high purity of 93.9% of designed FVB at THz frequencies. Moreover, we have numerically demonstrated that two complicated devices, including one FVB deflector (focusing efficiency of 94.4% and purity of 92.5%) and one dual-focal meta-surface (focusing efficiency of 92.8% and purity of 92.1%), both exhibit good performance-based on FVB design, which is in good agreement with our theoretical results. Our proposed method paves the way for the development of high-efficiency meta-surface integrated optical devices, demonstrating great potential in applications such as particle manipulation, high-dimensional information processing, and optical metrology.