Flat-Top Focal Spot and Polarization Conversion Obtained in Tightly Focused Circularly Polarized Light

In this paper, using the Richards–Wolf equations, the focusing of circularly polarized light with flat diffractive lenses is considered. It is shown that, as the numerical aperture (NA) of the lens increases, the size of the focal spot first decreases and then begins to grow. The minimum focal spot is observed at NA = 0.96 (FWHM = 0.55 λ). With a further increase in the numerical aperture of the lens, the growth of the longitudinal component leads to an increase in the size of the focal spot. When a flat diffractive lens is replaced by an aplanatic lens, the size of the focal spot decreases monotonically as the numerical aperture of the lens increases. In this case, the minimum focal spot will be FWHM = 0.58 λ and, with a larger numerical aperture, NA = 0.99. We also reveal that, at the focus of a circularly polarized laser beam, different radius circles are observed to be centered on the optical axis, where polarization vectors rotate oppositely (clockwise and anticlockwise). This phenomenon of radius-dependent ‘spin’ separation may be interpreted as a manifestation of the radial spin Hall effect at the focus.