Design and studies on gradient index metasurfaces for broadband polarizationindependent, subwavelength, and dichroic focusing

This paper presents designs for and simulation studies on planar gradient index metasurfaces for polarizationindependent and dichroic subwavelength focusing for broadband applications. Polarization-independent lenses are designed based on dielectric (Si₃N₄ and TiO₂) gradient nanopillars. Dichroic metalenses are designed based on gradient aluminum nanohelices for helicity-dependent focusing of circularly polarized light. The helical shape is considered due to its sensitivity to circularly polarized light of a specific handedness depending upon the orientation of the helices in the lattice; this may help in 3D imaging. In the designed metalenses, the variation in the spatial dimension (fill factor) is in a gradient manner, which leads to directional bending of electromagnetic waves, and strong coupling between the bent electromagnetic waves leads to subwavelength focusing over the high numerical aperture. The designed metasurface can be materialized through multibeam interference using a combination of n plane beams and a nondiffracting Bessel beam of either zeroth or first order presented through the simulated irradiance profile and a proposed single-step experimental setup. The designed TiO₂-based metalens focuses the incident arbitrary or plane polarized light to a spot sized 0.314λ, at a wavelength of 635 nm, that is based on Si₃N₄, enabling polarization-independent subwavelength focusing over a broad (436-810 nm) wavelength range. Realization of these lenses will enable polarization-independent high-numerical-aperture focusing and super-resolution real-time imaging of biological samples.