Analysis of the focal spot by variation of immersion medium for nanostructured metasurfaces

The refractive index of the immersion medium has a significant influence on the shape of the focal spot in the field of diffraction optics. For a refractive index of the immersion medium that varies from the designed one, the change in the focal properties of the diffractive optical elements needs to be verified. By combining the vectorial angular spectrum (VAS) theory with a genetic algorithm, multiannular nanostructured metasurfaces with super-resolution focusing abilities were designed with a linearly polarized beam in an oil immersion medium. The intensity distribution of the focusing field was calculated via the finite-difference time-domain, and the results agreed well with calculations using the VAS theory. The results of the theoretical calculations demonstrated an obvious shift of the focal spot and change in the spot size as the refractive index varied. The calculations showed that the refractive index had an impact on the focal properties of multiannular metasurfaces. This work provides theoretical guidance for super-resolution focusing and imaging.