Sharp focus of a circularly polarized optical vortex at the output of a metalens illuminated by linearly polarized light
A three-level spiral metalens in an amorphous silicon film is designed. The metalens relief consists of two subwavelength gratings with a 220-nm period (for 633 nm wavelength) and depths of 90 and 170 nm. The metalens forms a left-hand circular polarized optical vortex with topological charge 2 when illuminated by a linearly polarized plane wave. The intensity distribution at a distance of 633 nm is in the form of a subwavelength circle, whereas the longitudinal projection of the Pointing vector has negative values on the optical axis, meaning that a backward energy flow occurs. Two subwavelength gratings with different depth act as quarter-wave plates, transforming linearly polarized light into circularly polarized light with a phase delay of λ/2. This metalens combines functionalities of three optical elements: a quarter-wave plate, a spiral phase plate, and a high-NA diffraction metalens (NA close to unity).