Abstract
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.
Highlights
Breaking the Abbe diffraction limit is an important topic of research, and various methods and devices, such as near-field scanning optical microscopy,1 superlenses,2,3 plasmonic lenses,4,5 negative refractive index materials,6 nanoscale microspheres,7 and diffractive optical elements,8–11 have been used to realize this goal
NAmax, where z denotes the axial position of the focusing field; d0 restrains the full-width at half-maximum (FWHM) of the main focal spot, and the FWHM is used to characterize the size of the focal spot; r is the radial position; κ is a given coefficient; (κ − 1)d0 is the radial width of the dark region between the central main lobe and large surrounding side lobes of the main focal spot; the normalized maximum intensity is constrained to be less than 30% of the peak intensity of the central lobe; N is the total ring number of the multiannular metasurface (MAM); D denotes the diameter of the MAM; NA is the equivalent numerical aperture; and the optimal distance of the main focal spot is constrained between NAmin and NAmax
The results show that the differences between the results of vectorial angular spectrum (VAS) theory and the finite-difference time-domain (FDTD) method were fairly small
Summary
Breaking the Abbe diffraction limit is an important topic of research, and various methods and devices, such as near-field scanning optical microscopy, superlenses, plasmonic lenses, negative refractive index materials, nanoscale microspheres, and diffractive optical elements, have been used to realize this goal. The multiannular metasurface (MAM) is a new type of diffractive optical element. It was first proposed in 201210 and has been used to realize super-resolution focusing.. The refractive index of the immersion medium has a significant influence on the focusing properties of the diffractive optical elements. The relationship between the two deviations of the refractive index from the designed value is not clear, which restricts the practical application of MAMs. In this work, by combining the vectorial angular spectrum (VAS) theory and a genetic algorithm (GA), a vectorial design theory for MAMs with super-resolution is introduced.. By combining the vectorial angular spectrum (VAS) theory and a genetic algorithm (GA), a vectorial design theory for MAMs with super-resolution is introduced. The theoretical calculation results were rigorously tested via the 3D finite-difference time-domain (FDTD) method, and the two results were in good agreement
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