Metalens for coaxial double wavelength focusing

We propose a metalens for coaxial double wavelength focusing. One focusing spot is a circular solid spot, and the other focusing spot is a doughnut-shaped spot that is circling the solid spot. The designed metalens was composed of a meta-molecular nanostructured cell array. Each meta-molecular nanostructured cell was divided into four squares. Two slots with exactly the same shape, but usually with the rotation angle measured clockwise from the positive x axis, are etched into the gold film in two diagonally connected squares. Another two slots with the same shape but with the rotation angle measured counter-clockwise from the positive x axis are etched into another two diagonally connected squares in the same cell. The lasers with two different wavelengths are transformed into right-handed and left-handed circularly polarized beams, respectively. The two sets of slots with different azimuthal rotations modulated the phases of incident right-handed and left-handed circularly polarized beams independently. The numerical simulation with finite-difference time-domain (FDTD) software was carried out, and the experimental verification was also implemented. Both the experimental result and the numerical simulation agree well with the theoretical design.