All-graphene geometric terahertz metasurfaces for generating multi-dimensional focused vortex beams

Simultaneously assembling the functions of lenses and phase vortices into geometric metasurfaces on an ultra-compact platform can potentially be used for light trapping and edge imaging, helping to enhance their applications in polarized optical systems. Here, we propose and theoretically investigate an all-graphene geometric metasurface operating in the terahertz (THz) band for generating multidimensional vortices. Focused scalar vortex beams with polarization-independent properties can be generated by introducing the superposition of two helical phases with the same topological charge within orthogonal circularly polarized (CP) channels. Focus shift can be further tolerated by tailoring the conventional helical phase distribution. Embedding polarization modulation into the proposed design enables the generation of multiple focused vortices with inhomogeneous polarization properties in the longitudinal direction. The typical conjugate phase is capable of generating aggregated vortices with vectorial characteristics by switching the incident polarization mode, and can be extended to arbitrary orders. Using the way in which the graphene Fermi energy was changed in the simulation provides strong evidence for dynamically tuning the focusing efficiency of the generated beam. Benefiting from the proposed method that manipulates the orthogonal CP components separately, the design scheme has greater degree of freedom and can find potential applications in meta-optics, high-tolerance edge imaging, and high-capacity optical communications.