Features of focusing of optical vortices using subwavelength elements with varying height of odd and even relief zones

In this work, the propagation of optical vortices with circular, radial, and azimuthal polarization through subwavelength ring gratings with standard and GRIN substrates using a finite difference time domain method is numerically simulated. It is shown that it is possible to select the polarization of laser radiation and parameters of the element in such a way that a long optical needle (up to 8.04λ, radial polarization), a tight focal spot (up to 0.4λ in diameter, circular polarization), single optical traps, and combinations thereof are generated on the optical axis.