Longitudinal Manipulation of Scalar to Vector Vortex Beams Evolution Empowered by All-Silicon Metasurfaces

Manipulating the trend of polarization profiles in the propagation direction, especially the evolution from homogeneous to inhomogeneous distribution, can provide subtle implements for light-matter interactions. The implementation of conventional stacked structures in the terahertz (THz) band to maneuver polarization states within a finite distance will result in large insertion losses due to inherent material absorption or Fabry-Perot resonance. Metasurfaces have proved to possess many extraordinary properties and propelled the integration of THz functional devices. Herein, the evolution from scalar vortex to vector vortex beams in the longitudinal direction is experimentally demonstrated using all-silicon metasurfaces generated by spin-polarization multiplexing encoding techniques. The coaxial universalized phase design scheme with extended focal length provides a paradigm for polarization manipulation that is not limited to the THz band. As a conceptual investigation of the working mechanism, a series of all-silicon samples are fabricated to experimentally evaluate the conversion capability from scalar to vortex beams, that is, homogeneous to inhomogeneous polarization profiles. The mode weight recorded in the effective region is utilized to further analyze the setting laws of topological charges in orthogonal channels, providing great opportunities for fundamental applications in emerging fields, such as singular optics and quantum science.