Modified cube phase for extended depth of focus in the visible range

We demonstrate a metalens based on a modified cubic phase term, featuring a symmetric phase distribution to achieve an on-axis point spread function (PSF). While the conventional cube phase mask is a foundational technique for extended depth of focus (EDoF), its inherent transverse asymmetry produces an off-axis focal distribution. In broadband applications, this off-axis shift induces severe lateral chromatic aberration. To overcome this limitation, we propose a fundamentally simple yet effective mathematical modification on the absolute-cube phase mask. By enforcing symmetry through an absolute value modification on the cube phase term, restores the extended focal profile back to the central optical axis. This direct on-axis alignment eliminates lateral chromatic shifts, ensuring that broadband spatial frequencies are symmetrically preserved and aligned across the operation range. This approach mitigates the chromatic aberrations of conventional EDoF designs while avoiding the high fabrication complexity associated with dispersion engineering. To validate this concept, we designed and fabricated three types of metalens: hyperbolic, cube, and absolute-cube. Each design features a numerical aperture of 0.268 and a nominal focal length of 90 m. By utilizing the absolute-cube phase profile, we successfully generated an extended depth of focus across the visible spectrum from 470 to 635 nm while remaining compatible with standard, mass-producible nanopillar geometries. The proposed method provides a robust and fabrication-friendly solution for broadband meta-optics, with potential for miniaturized microscopy, endoscopy and machine vision systems.