Fig. 3. Multi-resonant high-Q metasurface with a gradient-thickness DBR.

a The left panel illustrates the working principle for FP cavity modes within a gradient-thickness DBR. The dimensions of the meta-atom are depicted in the right panel. Unit: nm. A 120-nm-thick SiO₂ layer is sandwiched between the nanostructure and the DBR substrate. b Simulated LCP-to-LCP reflection spectrum (black curve) and LCP-to-RCP reflection spectrum (blue curve) for a bare gradient-thickness DBR substrate and a multi-resonant metasurface, respectively. c Electric field distribution of the RCP component for the multi-resonant metasurface at 5 wavelengths under LCP illumination. d Cross-sectional electric field distribution for the multi-resonant metasurface, which are extracted from the region along the red dashed lines in (c). In the upper image, a schematic of the cross-sectional view is presented, with a black arrow indicating the position of the interface between the meta-atom and the SiO₂ spacer. The red curves result from multi-peak fitting for visual guidance.