Tight focusing of fractional-order topological charge vector beams by cascading metamaterials and metalens.

Vector beams have attracted widespread attention because of their unique optical properties; in particular, their combination with tight focusing can produce many interesting phenomena. The rise of 3D printing technology provides more possibilities for exploration. In this work, a cascading method involving a metamaterial and a metalens is used to generate a tightly focused field of vector beams in the terahertz band, which is prepared via 3D printing. As a proof-of-concept demonstration, a series of metamaterial modules capable of generating states of different orbital angular momentum are proposed by cascading with a metalens. The experimental results are in good agreement with the simulation results, fully verifying the feasibility of the scheme. The proposed design and fabrication strategy provides a new idea for the tight focusing of terahertz vector beams.