Abstract
Using a FDTD-aided numerical simulation we show that a sharp focal spot can be generated by a spiral metalens made of amorphous silicon, with unit topological charge and unit numerical aperture (NA = 1). The focal spot intensity is nearly round-shaped when using an incident TE-wave, and in the form of a doughnut for an incident TM-wave. We are the first to show theoretically and experimentally that the focal intensity pattern has local regions - located symmetrically or asymmetrically both at the center and on the periphery - where the energy flux is inversely directed. The discovered effect can be used for moving a microparticle in the opposite direction to the light propagation.
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