Subwavelength focusing of laser light of a mixture of linearly and azimuthally polarized beams

We investigated a four-zone transmission polarization converter (4-TPC) for a wavelength of 633 nm, enabling the conversion of a linearly polarized incident beam into a mixture of linearly and azimuthally polarized beams. It was numerically shown that by placing a Fresnel zone plate of focal length 532 nm immediately after the 4-TPC, the incident light can be focused into an oblong subwavelength focal spot whose size is smaller than the diffraction limit (with larger and smaller size, respectively, measuring $FWHM = 0.28\lambda$ and $FWHM = 0.45\lambda$, where $\lambda$ is the incident wavelength and FWHM stands for full-width at half maximum of the intensity). If after passing through a 4-TPC, light propagates in free space over a distance of 300 um before being focused by a Fresnel zone plate, the resulting focal spot was found to measure $0.42\lambda$ and $0.81\lambda$ (with the focal spot contributed to just by the transverse E-field components measuring $0.42\lambda$ and $0.59\lambda$). This numerical result was verified experimentally, giving a focal spot of smaller and larger size, respectively, measuring $0.46\lambda$ and $0.57\lambda$.