Metasurfaces enabled dual-wavelength decoupling of near-field and far-field encoding

The metasurface is a platform with a small footprint and abundant functionalities.With propagation phase and geometric phase,polarization multiplexing is possible.However,different response behaviors of propagation phase and geometric phase to wavelength have not been fully employed to widen the capabilities of metasurfaces.Here,we theoretically demonstrate that metasurfaces can achieve near-field and far-field decoupling with the same polarization at two wavelengths.First,we found a set of pillars whose propagation phase difference between two wavelengths covers the full range of 2π.Then,by rotating pillars to control the geometric phase,the phase at both wavelengths can cover the full range of 2π.Finally,by means of interference principle,arbitrary independent coding for the near field and far field of dual wavelengths is realized.In addition,when the far-field function is focusing,the focused spot is close to the diffraction limit,and,when the NA of the lens is very small,the final output focal length is four times of initial input focal length.This work circumvents the strong wavelength-dependent limitation of planar devices and paves the way toward designing multi-wavelength and multi-functional metadevices for scenarios such as AR applications,fluorescence microscopy,and stimulated emission depletion microscopy.