Achromatic Double-Helix Metalens for Broadband Structured Focusing in the Visible Range

Achromatic structured focusing in flat optics is important for compact depth-resolved imaging and three-dimensional localization, yet simultaneously achieving broadband operation and a depth-encoding focal field remains challenging. Here, we demonstrate an achromatic double-helix metalens operating across the visible range of 600-650 nm. The target phase profile combines an exact hyperbolic lens phase with a double-helix phase synthesized from Laguerre-Gaussian mode bases, whose coefficients are optimized by particle swarm optimization. Broadband implementation is achieved through a meta-atom library designed by jointly considering phase delay, group delay, and group delay dispersion. The fabricated device exhibits strong structural fidelity and experimentally reproduces the rotating two-lobe focal field at 625 nm as well as across the target wavelength range. This work extends achromatic metalenses beyond conventional focusing to broadband structured focusing and provides a compact platform for depth-resolved imaging.