Metasurface configurations

To design a phase gradient metasurface, it is required to provide a complete $2\pi$ phase coverage by the building blocks, metaatoms, of metasurface. Three different techniques are mainly involved to realize $2\pi$ phase shift by dielectric metaatoms. The first mechanism, geometrical phase (Pancharatnam-Berry) phase, is based on the rotating of the metaatoms around the propagation direction of a circularly polarized incident light \cite{maguid2017multifunctional}. The second scheme is achieved by considering a high aspect ratio resonator with different cross-sections. These resonators can be considered as the truncated waveguides with Fabry–Pérot resonances \cite{kamali2016decoupling}. Finally, for the third approach, a low aspect ratio resonator is utilized to excite different orders of electric and magnetic Mie resonances. The spectral positions of Mie resonance modes are tuned by the changing of the metaatom cross-section. Then, the phase modulation of the transmitted wave is achieved by the tailoring of the spectral position of the Mie resonances. The advantage of this technique is the capability in providing multiple 0 to $2\pi$ phase coverages as a function of the metaatom size which can later benefit to design an achromatic metasurface \cite{yu2015high}. Moreover, a fine parametric design of metaatom provides a Kerker’s condition \cite{yu2015high}. In this work, due to the variation of the working wavelength the phase shifts are realized by a combination of Fabry–Pérot and/or Mie mechanisms \cite{li2019mechanisms}.