Fig. 2. Design of the multi-resonant meta-atom and off-axis focusing meta-mirror.

a The numerical reflection spectrum of the designed multi-resonant meta-atom. An Al nano-rod (length = 170 nm, width = 90 nm, thickness = 50 nm, period = 200 nm) array standing on a DBR substrate is optimized to possess four high-Q resonant peaks across the visible window. The circular cross-polarized reflection (b) and phase (c) as a function of structural rotation angle, presenting that the designed multi-resonant meta-atom satisfies the geometric phase conditions at four peak wavelengths. The inset in (b) represents the electric field intensity at 593 nm for various structural orientation angles. The inset in (c) shoes the schematic of the meta-atom. The thickness of dielectric spacer SiO₂ is 135 nm. d Ray-tracing calculations for the off-axis focusing meta-mirror (designed at a central wavelength of 593 nm with a focal length of 7.5 mm) based on the multi-resonant meta-atoms. Left images are the spot diagrams. Scale bars: 10 μm. The bottom right images show the phase distributions across the meta-mirror at 593 nm.