Ultra‐Broadband Achromatic Cascaded Metalenses Based on Spatial‐Decoupling Design

ABSTRACT Achromatic focusing across the visible‐to‐near‐infrared (NIR) spectrum remains a fundamental challenge for single‐layer metalens due to material dispersion and phase‐coverage limits. We demonstrate an ultra‐broadband achromatic cascaded metalens that bypasses these constraints via a spectral splitting mechanism. By integrating a TiO 2 /SiO 2 dichroic mirror, the 400–1600 nm bandwidth is spatially decoupled into independent visible (400–900 nm) and NIR (900–1600 nm) channels, allowing for customized phase compensation in each layer. This architecture enables both bi‐directional spectral splitting achromatic focusing with reflection at 450–800 nm and transmission at 1190–1600 nm, as well as ultra‐broadband achromatic reflective focusing across 450–1600 nm. Full‐wave simulations demonstrate that the cascaded system achieves diffraction‐limited achromatic focusing within the spectral ranges of both operating modes. Particularly in the ultra‐broadband achromatic reflection focusing mode, the focal length shift exhibits a minor deviation with a standard deviation of 5.19% across the 450–1600 nm band, effectively suppressing the longitudinal chromatic aberration. Furthermore, numerical tolerance analysis demonstrates high tolerance to alignment errors, thereby validating the device's engineering feasibility for ultra‐compact multispectral imaging, efficient solar concentration, and broadband detection systems.