Micron-scale light structuring via flat nanodevices

Miniaturized devices with multiple functionalities are exceedingly required in integrated optical systems. Flat nanostructures, named metasurfaces, provide fascinating boulevard for complex structuring and manipulation of light such as optical vortex generation, lensing, imaging, harmonic generation etc. at micron scale. Since, the performance of metal-based plasmonic metasurfaces is significantly limited by their optical absorption and losses, lossless dielectric materials (in the operational spectrum) provide decent alternative to attain higher efficiency. Here, a novel, polarization insensitive and highly efficient method for light structuring is demonstrated based on amorphous silicon (with subwavelength thickness of 400 nm) at an operational wavelength of 633 nm. The proposed phase gradient metasurface is based on circular cylindrical nanopillars of amorphous silicon exhibits two optical properties, the lensing and orbital angular momentum generation. The cylindrical nature of the pillar plays a pivotal role to make the overall structure as polarization insensitive. The proposed innovative methodology will provide an interesting road towards the development and realization of multi-functional ultrathin nanodevices which will find numerous applications in integrated photonics.