Manipulating twisted light beam through all-dielectric metasurfaces

Recently emerged metasurfaces, the two-dimensional (2D) counterpart of three dimensional (3D) metamaterials, gained significant attention in optics and photonics due to their less challenging fabrication requirements (compared to 3D metamaterials) and unique capabilities of wavefront manipulation by introducing abrupt phase shift. Realization of multiple functionalities in a single metasurface, is an intriguing perception to achieve further miniaturization and cost effectiveness. In this paper, we propose a polarization insensitive, highly efficient metasurfaces for the visible spectrum. For the design wavelength of 633nm, negligible absorption coefficient (k) and adequately large refractive index (n) of proposed hydrogenated amorphous silicon (a-Si:H) leads to considerably efficient and cost-effective solution towards metasurfaces design. Inherent property of cylindrical pillar to be polarization insensitive is exploited and 400 nm thick cylindrical nano-waveguide is opted as building block to construct the metasurface. A novel design strategy of achieving multiple functionalities from a single metasurface is proposed, where a combined effect of lensing and optical vortices with different topological charges at different focal planes is demonstrated for the proof of concept. Such unique design strategy of integrating multiple phases into a single device provides an innovative way of miniaturizing the optical devices and systems exhibiting versatile functionalities for on-chip applications.