Fig. 3. The experimentally fabricated TiO₂ metalens with NA = 0.24.

a The schematic of the fabrication process. A highly directional etching process was employed to fabricate TiO₂ nanostructures. b and c are the top-view SEM images of achromatic metalens with different resolutions. Four types of nanostructures can be clearly identified. d The corresponding tilt-view SEM image of the metalens. e and f are the intensity profiles of focal spots in x-z plane and x-y planes at different wavelengths. The bottom panels depict the intensity distributions along the diameter (solid lines) and the fitted curves (dashed lines). g The images of element-6, group-7 of the 1951 Unites States Air Force resolution target recorded by the achromatic metalens.