A broadband quarter-wave plate enabled by a dual-aluminium metastructure with process-realistic SiO₂ geometry

We report the design and experimental realization of a broadband quarter-wave plate (QWP) based on a dual-aluminum (Al) metastructure that incorporates the realistic geometry of a nonuniform SiO₂ overlayer. Structural parameters extracted from scanning electron microscopy were implemented in full-wave simulations to accurately reproduce the optical response of the fabricated device. This metastructure achieves stable π/2 phase retardation and efficient circular polarization conversion from the visible to the short-wave infrared region, depending on the incident polarization angle. The uneven SiO₂ morphology suppresses reflectance and improves polarization conversion efficiency compared with planar dielectric layers. Experimental results confirm the QWP operation with discrepancies within 5% at representative wavelengths of 520, 632, 785, and 1550 nm. This approach provides a robust and practical route toward fabrication-tolerant broadband polarization-control devices for integrated photonic and imaging platforms.