Arbitrary manipulations of broadband terahertz focused Poincaré beams enabled by anisotropic silicon metasurfaces

Abstract

Focused Poincaré beams (FPBs) carrying inhomogeneous polarization properties describe the paraxial solution of the vector Helmholz equation in cylindrical coordinate systems, with potential for classical and quantum applications. Here, we theoretically demonstrate a generalized strategy for generating FPBs based on the spin decoupling mechanism. Silicon metasurfaces with high degrees of freedom allow the proposed design to generate vector beams carrying arbitrary topological charges in a broadband terahertz (THz) frequency range. By adopting a purely phase-based modulation approach, the single-layer polarization-multiplexed metasurface can act as a bridge between the conventional Poincaré sphere (PS) and the higher-order Poincaré sphere (HOPS), preserving the corresponding mapping relations. Benefiting from the independence of the on-demand tailored helical phases, we further evaluate the coaxial evolutionary behavior of FPBs, generating structured THz fields with incremental topological charges. It is foreseeable that this work will provide an efficient meta-platform for the generation of broadband THz structured fields and facilitate their application in information encryption and quantum scientific information.