a Schematic representations of steps in fabrication processes for preparing 3D SGM absorbers. b, c Photographs of b Cu foil and c SGM absorber, each having sample area of ~25 mm2. d Top-view SEM images of the SGM absorber at low and high magnification. e Measured reflectance and absorbance spectra of SGM absorbers having various hole widths (w) and periods (p): that is, w/p = 0.52/0.8 μm (red line) and w/p = 0.57/1 μm (blue line). The red dashed line and the blue dashed line are the simulated absorbance spectra of the H2 and H3 SGM absorbers, respectively.