Fig. 1. Conceptual illustration of multi-resonant metasurfaces.

Schematic illustration for (a) an FP cavity, (b) an MIM metasurface, and (c) the proposed multi-resonant metasurface. To induce cavity modes in an FP cavity, the dielectric layer thickness (h₁) must exceed the wavelength of the incident light. The wavelength of the reflected beam is discrete because only those that satisfy the FP condition can be highly reflected. However, controlling the wavefront of the reflected beam using an FP cavity is challenging. For MIM metasurfaces, wavefront engineering can be achieved by optimizing the physical properties of the metallic meta-atom and adjusting the thickness of the dielectric spacer (h₂), leading to the attainment of either a relatively broad or narrow reflection band. It is crucial to maintain h₂ at an optically smaller scale than the wavelength for MIM metasurfaces. By replacing the metallic mirror with a gradient-thickness DBR, a multi-resonant high-Q feature is introduced, offering greater flexibility in modulating the spectral profile and enabling wavefront engineering at individual resonant wavelengths. In this case, the incident light is highly reflected at different interfaces within the DBR mirror, making the dielectric spacer thickness h₃(λ) dependent on the wavelength of incidence. Additionally, introducing a geometric phase through the rotation of the topmost nanostructures allows precise wavefront control at each resonant wavelength. The blue and brown blocks in (b) and (c) represent meta-atoms viewed from the top, each with varying sizes.