Single metalens for generating polarization and phase singularities leading to a reverse flow of energy

Using Jones matrices and vectors, we show that a metasurface-based optical element composed of a set of subwavelength diffraction gratings, whose anisotropic transmittance is described by a matrix of polarization rotation by angle mphiv, where phiv is the polar angle, generate an mth order azimuthally or radially polarized beam, when illuminated by linearly polarized light, or an optical vortex with topological charge m, when illuminated by circularly polarized light. Such a converter performs a spin–orbit transformation, acting similarly to a liquid-crystal half-wave plate. Using the FDTD-aided numerical simulation, we show that uniform linearly or circularly polarized light passing through the above-described optical metasurface with m = 2 and then tightly focused with a binary zone plate generates an on-axis near-focus energy backflow comparable in magnitude with the incident energy. Notably, the magnitude of the reverse energy flow is shown to be the same when focusing a circularly polarized optical vortex with topological charge m = 2 and a light beam with the second-order polarization singularity