Exploiting the circular polarization of light to obtain a spiral energy flow at the subwavelength focus

Using Richards–Wolf formulas, we show analytically that when strongly focusing a circularly polarized axisymmetric non-vortex field (e.g., a Gaussian beam or zero-order Bessel–Gaussian beam), a spiral energy flow is produced around the sub-wavelength focal spot. This phenomenon can be explained by the conversion of spin angular momentum associated with the circular polarization state into the near-focus orbital angular momentum, with the on-axis angular momentum being strictly zero. It is also shown that a linearly polarized optical vortex with topological charge 2 produces an on-axis near-focus energy backflow whose amplitude (defined by the negative longitudinal component of the Poynting vector) is comparable with that of the incident beam.