**2.2 Building the phase library of unit-cell**

There are two primary modes of metasurface from the unit-cell's viewpoint. One type of unit-cell could be treated as the truncated waveguide and the waveguide mode is excited accordingly. This model usually matches well for the case at high frequency, such as high terahertz and optical regime. Another is resonate mode which could be treated as resonating antenna array. In order to better understand the unit cell's phase variation behaviors in the sub-wavelength dimension and obtain the enough coverage for the potential design phase library, we analyze the waveguiding effect and resonance mode below.
