Modeling a polarization microlens to focus linearly polarized light into a near-circular subwavelength focal spot

We propose a new version of the metalens, namely, a polarization microlens (PML) that focuses the linearly polarized laser light in a subwavelength circular spot. The PML consists of a set of binary subwavelength gratings, which are laid out in a parquet manner within the rings of a Fresnel zone plate. A phase shift of $\pi$, which the zone plate should introduce into the light field in passing the zone boundary, is implemented in the PML under study using two diffraction gratings adjacent to the zone boundary, which form oppositely polarized transmitted beams. It is shown that the PML relief depth can be reduced using a high refractive index material. For amorphous silicon, the PML relief depth can vary from 50 nm to 120 nm. In this case, a circular focal spot having a diameter smaller than the diffraction limit is formed at distances to the PML ranging from 200 nm to 1300 nm. The minimum focal spot diameter is 0.372 of a wavelength.