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 $\lambda/2$. 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).