Manipulating structured terahertz beams by cascaded dielectric meta-systems

Higher-order vortex beams (VBs), including both scalar and vector ones, play important roles in future information technology due to their capabilities to significantly enhance information channels. However, generating high-order VBs and freely switching their orders remain challenging with conventional optical systems and single-layer metasurfaces. Here, we introduce a scheme to design cascaded meta-system consisting of multiple transmissive dielectric metasurfaces, which can perform "addition" and "subtraction" operations on the orbital angular momentum (OAM) carried by a VB. As a proof of concept, we first experimentally demonstrate a cascaded bifunctional meta-system operating in the terahertz (THz) regime for converting the input circularly polarized (CP) beams with different helicities to two scalar VBs with distinct topological charges, which offers an ideal platform to manipulate high-order VBs. We next fabricate two THz bifunctional cascaded meta-systems and experimentally demonstrate that the incident zero-order CP beam can be converted to a vector VB after passing through the first layer of the meta-systems, and subsequently restored to its original state after passing through the second layer. Our work establishes a highly compact and efficient platform for the flexible manipulation of both scalar and vector VBs, which significantly expands the wave-manipulation capabilities of single-layer metasurfaces and may inspire numerous applications in information storage and communication, bio-sensing, OAM detection, optical manipulations, and so on.