Fig. 2. The conventional TiO₂ metasurfaces.

(A) Top-view SEM images of TiO₂ metasurfaces. Scale bars, 200 nm. The insets show the corresponding structural colors under a bright-field microscope. (B) Measured (solid lines) and numerically calculated (dashed lines) reflection spectra. From bottom to top, the lattice size decreases from 400 to 380, 370, 350, 330, 320, and 300 nm. The gap is kept at w = 70 nm. The insets are the recorded (left) and calculated (right) structural colors. (C) Corresponding structural colors of 15 metasurfaces in the CIE 1931 map.