**2. Design principle**

When electromagnetic wave propagates through the micro-structures, the wavefront shape of emergent wave is controlled by distributing the phase profile across the outputting surface. Generally, there are few steps to design a metalens. Firstly, the phase profile of the metalens aperture should be solved per desired. Secondly, the large amount of micro-structures is investigated to cover both the full phase variance and the dispersion at the same time. If the particular functionalities are needed, such as achromatic focusing, large numerical aperture (NA), the broader phase range is highly desired for the consideration of the dispersion margin as well. At last, the appropriate structures from the "phase library" is distributed accordingly to form the metalens, and the simulation of the whole metalens device with full wave simulator and experimental characterization are carried out to provide the full design circle of the metalens. The following parts will give the examples of typical metalens antenna applications with more design details.
