Fig. 1. Schematic for bilayer broadband achromatic metalens. It has the same focal plane over a large continuous wavelength region. Inset: Oblique view of a unit cell of a bilayer Si nanopillars with different heights h₁ = 850 nm, h₂ = 1500 nm, in-plane cross-sectional dimensions D_x = 420 nm, D_y = 190 nm and lattice constant P = 500 nm, on a SiO₂ substrate. Right: The top view of each layer.

Fig. 2. (a) Simulated polarization conversion efficiency of the top rectangular nanopillar as a function of wavelengths from 1000 nm to 1700 nm. Each nanopillar with 500 nm periods, in-plane cross-sectional dimensions D_x = 420 nm, D_y = 190 nm. (b, c) Transmission coefficient and phase map of the bottom cylindrical nanopillar with different diameters d as a function of wavelength from 1000 nm to 1700 nm. (d) Phase spectra for cylindrical nanopillar with four different diameters as a function of frequency.

Fig. 3. The phase profile of bilayer achromatic metalens. (a) The ideal phase profile at the λ_max. (b) The bottom layer normalized phase compensation profile over a wavelength region from 1000 nm to 1700 nm, at a step of 100 nm. (c) The required phase profile of top layer.

Fig. 4. Simulated verification of chromatic and achromatic metalens. (a, b) Numerical intensity profiles of broadband chromatic (a) and achromatic (b) metalens with NA = 0.15 at various incident wavelengths. The red dashed line indicates the position of the focal plane. (c, d) Normalized intensity profiles along the red dashed lines of (b). Scale bar: 6 μm.

Fig. 5. Performance of broadband achromatic metalens. (a) The focal length shift values of both chromatic and achromatic metalenses as a function of incident wavelength. (b) The FWHM of the focal spots as a function of indent wavelength. (c) The efficiency of achromatic metalens as a function of incident wavelength.

Fig. 6. The influence of structural deviations on the lensing quality. (a)– (d) The simulated focal intensity profiles of bilayer metalens with perfect alignment (a), misalignment of 500 nm (b), 1 μm (c) and 1.5 μm (d) between two metasurface layers at three different wavelengths.