Sub-diffraction focusing by a meta spiral zone plate

Abstract

The sub-diffraction focus has been widely studied because of its wide application in the fields of optical lithography and super-resolution imaging. The methods based on the traditional refractive elements to achieve sub-diffraction focal spots face the problem of large volume, large mass and difficult alignment. Although diffractive optical elements can achieve the combination of multiple phase modulation functions, the polarization modulation cannot be achieved by such elements. As a planar element to achieve integrated modulation of phase and polarization, the metasurface has the potential to further miniaturize the sub-diffraction focusing system and eliminate alignment errors. In this article, we demonstrate the meta spiral zone plate, which converts the incident linearly polarized beam to an azimuthally polarized beam and focuses the latter into a sub-diffraction solid spot. The meta spiral zone plate with a diameter of 600 μm and a numerical aperture of 0.8 working at the wavelength 405 nm is designed and verified experimentally. The simulated full width at half maximum (FWHM) and measured FWHM of the focus spot are 0.6λ and 0.65λ. The simulated and measured efficiencies are 37.69% and 15.18%, respectively. Meanwhile, the effects of the incident beam's incident angle and polarization orientation on the focal spot are also investigated. Our proposal eliminates the alignment errors in the traditional methods and further simplifies the optical systems realizing sub-diffraction focusing, which may have great potential applications in super-resolution focusing and optical manipulation.