(a) Band structure of the proposed metasurface with period of P in both x-/y- directions and the band structure with period of P/2 in x-direction. The colour of the lines indicated the value of quality factor of the modes, while the area above light-lines represents radiation continuum. The green-dashed lines represent the modes that are folded above the lightline. (b) Illustration of Band-folding effect caused by period doubling.