Geometric phase wavefront manipulation for arbitrary polarization states based on metasurfaces

Metasurfaces enable the arbitrary manipulation of multiple physical dimensions of photons at the subwavelength scale, offering transformative solutions for lightweight and miniaturized photonic devices. Among the various phase modulation mechanisms of metasurfaces, geometric phase has attracted extensive attention from researchers due to its intuitive concept and relatively facile implementation; additionally, the fixed feature size of geometric phase structures reduces the complexity of micro/nanofabrication. However, the generation of geometric phase is accompanied by the orthogonal conversion of circular polarization states, and this process is confined to the circular polarization of the incident light. To overcome this limitation, a generalized wavefront manipulation method is proposed in this article, which enables geometric phase modulation under arbitrary polarization states. The core concept of this method is to convert arbitrarily polarized light into circularly polarized light using a polarization-modulating waveplate and encode geometric phase into the circularly polarized light, thereby constructing a free-standing bilayer metasurface with a geometric phase array stacked on a polarization-modulating array. As a proof of principle, six arbitrary polarization states (linear, circular, and elliptical polarizations) are selected on the Poincaré sphere, and the corresponding metasurfaces are designed to demonstrate wavefront manipulation functions including focal shift, on-axis zooming, and focused vortex beam. Furthermore, we integrate the geometric phase with the propagation phase to realize holographic imaging based on elliptical polarization multiplexing. The proposed wavefront manipulation method extends geometric phase to arbitrary polarization states, providing a novel paradigm for wavefront manipulation under arbitrary polarization conditions.