Nearly Full-Stokes Polarization Control Enabled by Geometric Polarization in Broadband Metasurfaces.

The geometric phase has become a foundational element in modern metasurface design for optical phase and wavefront control. Here, we introduce the concept of geometric polarization, in which the orientation of the output polarization is modulated solely by in-plane rotation of anisotropic meta-atoms under circularly polarized illumination. Unlike conventional geometric phase approaches that manipulate converted circular polarization components, geometric polarization enables continuous tuning of the polarization azimuth on the Poincaré sphere while maintaining a fixed output ellipticity. Furthermore, with proper structural optimization, wavelength tuning modifies the amplitude and phase of the co-polarized component and the amplitude of the cross-polarized component, allowing the polarization states to span a broad latitudinal range. Together, geometric polarization and spectral tuning enable nearly full-Stokes polarization control using a single-layer metasurface. This work presents a conceptually distinct strategy for polarization engineering and expands the functional versatility of metasurfaces in optical modulation, imaging, and communication technologies.