Fig. 2: Linear and nonlinear optical responses of vertically stacked metasurfaces.

a Comparison of transmission spectra for single-layer (blue curve) and double-layer (red curve) structures with a 300 nm interlayer spacing. The Mie resonance in the single-layer design evolves into a transmission gap upon stacking, resulting in stronger field confinement. b Nonlinear simulation results showing the THG conversion efficiency for four configurations of a single-layer unpatterned film (1 layer UF, black curve), a two-layer unpatterned (2 layer UF, green curve), a single-layer metasurface (1 layer MS, blue curve), and a two-layer metasurface (2 layer MS, red curve), evaluated at fundamental wavelengths of 1278 and 1300 nm. The curves are normalized by dividing each value by the maximum efficiency observed across all cases, enabling direct comparison. The single-layer metasurface shows an enhancement of 10 compared to the unpatterned single-layer film, while the two-layer metasurface achieves an enhancement of 140 relative to the two-layer unpatterned configuration, and further demonstrates a ~10-fold increase compared to the single-layer metasurface at the band-edge wavelength. c Simulated transmission spectra of the two-layer structure for varying interlayer spacings of 200 nm, 300 nm, and 400 nm as a function of operating wavelength.