Sharp focusing of vector optical vortices using a metalens

Abstract

Using an FDTD-simulation, we numerically show that a spiral metalens with topological charge m = 1, numerical aperture 1, and a focal length equal to the incident wavelength produces a tight near-surface focal spot shaped as a circle for incident TE-waves or a doughnut for TM-waves. It is also demonstrated numerically and theoretically that in the intensity pattern of the resulting focal spot there are local (symmetric and asymmetric) regions, which can be located both at its center or on the periphery, where the energy flux has the opposite direction. For example, the reverse energy flux is found to occur at m = 1 and left-handed circular polarization or at m = 2 and right-handed circular polarization. This effect can be used for moving a microparticle in the opposite direction to the beam propagation.

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

“…The above analysis leads to a very interesting corollary. Let us first remind that the on-axis near-focus backflow of light energy was shown to occur when sharply focusing (i) a left-circular polarized optical vortex with m =2 [36,37] or (ii) a second-order azimuthally polarized light wave [38]. As can be seen from equations (3)-( 10), all four varieties of the light field that result in an on-axis energy backflow upon sharp focusing can be generated by means of a single optical element comprising the metasurface in equation (2) at m=2.…”

Hybrid Metasurfaces for Simultaneous Focusing and Filtering

“…Instead, metasurfaces introduce subwavelength nanostructures that allow controllable phase shifting of the propagating light. Metasurfaces can offer precise control over light intensity, phase, and polarization; thus, they have been widely utilized in the design and fabrication of optical metafilters 8,9 , metalenses [10][11][12][13][14] ,and in vortex beam generation [15][16][17] .…”