Rotationally tunable polarization-insensitive metasurfaces for generating vortex beams

The orbital angular momentum (OAM) of light has been applied to a variety of areas such as optical tweezers, interferometry, and high-resolution microscopy.^{1, 2} Metasurfaces, two-dimensional engineered structures with subwavelength features, give access to tailored functionalities through highly efficient phase shifting and polarization conversion. However, conventional designs with a single metasurface element produce vortex beams with fixed OAM of ℓ~ which limits the potential application areas. In this study, we propose and design a metasurface doublet lens structure having the property of generating variable modes controlled by the rotation angle. Inspired by Moir´e-lenses, the proposed structure consists of two all-dielectric metasurfaces where the second lens has the reverse phase profile compared to the first one. This causes the cancellation of the total phase shift at the nominal position. In our design, we rotate the second element with a discrete set of angles from 0 to 5.6 degrees with respect to the optical axis and obtain a set of the modes from ℓ = 0 to 4. We demonstrate that the structure converts the input plane wave to the vortex beams with OAM modes as a function of the rotation angle. We model the unit cell structure working at wavelength 532 nm a with circular cross-section, fixed height and variable radius titanium dioxide nanopillar on a fused-silica substrate. Nanopillar locations are distributed in a square lattice form with subwavelength periodicity which is suitable for conventional microelectronics fabrication methods. We believe our design can be used in optical trapping to detect different sizes of micro-particles and to create reconfigurable microoptomechanical pumps.