Fig. 7. Metasurfaces for broadband dispersion control. (a) Schematic of an achromatic metalens that consists of titanium dioxide nanopillars. (b) The calculated and measured results of achromatic metalens demonstrate the focal length nearly remains the same from 490 to 550 nm. (c) SEM image of the broadband achromatic metalens in reflection configuration, where the integrated-resonant unit elements are employed. (d) Measured intensity profiles of the reflected beam along the axial planes at different wavelengths, showing negligible variation of focal length. (e) Calculated polarized conversion efficiency (red curves) and phase profile (blue curve) for an integrated resonant unit element, which is shown in the inset. (f and g) Full-color Eurasian eagle owl images formed by the achromatic metalens before and after color correction. (h) Unit cell design for broadband achromatic metalens in transmission configuration, which is composed of one or two TiO₂ nanofins with different geometric parameters. (i) SEM image of the fabricated sample for the design in (h). Scale bar: 500 nm. (j) Achromatic imaging by the fabricated metalens under a white light (470–670 nm) illumination. Scale bar: 200 μm. (a and b) Reproduced with permission.¹²⁴ (c and d) Reproduced with permission.³⁶ (e–g) Reproduced with permission.¹²⁵ (h–j) Reproduced with permission.¹²⁶.