Highly focused spirally polarized beams

The features of spirally polarized light beams focused by high-numerical-aperture systems are investigated in the nonparaxial regime by means of Debye theory with a multipole expansion technique. General expressions of the expanding coefficients are given, as well as the electric field distributions across the focal plane, Numerical examples are presented for the case of spirally polarized beams of the donut type, Comparisons with recent experimental results are also shown