High-Efficiency Ultrathin Dual-Wavelength Pancharatnam-Berry Metasurfaces with Complete Independent Phase Control

Metasurfaces are planar structures that can offer unprecedented freedoms to manipulate electromagnetic wavefronts at deep-subwavelength scale. The wavelength-dependent behavior of the metasurface could severely reduce the design freedom. Besides, realizing high-efficiency metasurfaces with a simple design procedure and easy fabrication is of great interest. Here, a novel approach to design highly efficient meta-atoms that can achieve full 2 pi phase coverage at two wavelengths independently in the transmission mode is proposed. More specifically, a bilayer meta-atom is designed to operate at two wavelengths, the cross-polarized transmission efficiencies of which reach more than 70% at both wavelengths. The 2 pi phase modulations at two wavelengths under the circularly polarized incidence can be achieved independently by varying the orientations of the two resonators constructing the meta-atom based on Pancharatnam-Berry phase principle. As proof-of-concept demonstrations, three dual-wavelength meta-devices employing the proposed meta-atom are numerically investigated and experimentally verified, including two metalenses (1D and 2D) with the same focusing length and a vortex beam generator carrying different orbital angular momentum modes at two operation wavelengths. Both the simulation and experimental results satisfy the design goals, which validate the proposed approach.