Free standing bilayer metasurfaces: a new approach to wavefront control

A new class of bilayer metasurfaces composed of two free-standing TiO₂ nanofins enables broadband, polarization-dependent wavefront shaping. This architecture extends Pancharatnam–Berry phase metasurfaces beyond circular polarization by leveraging a cyclic transformation on the Poincaré sphere. We systematically explored design parameters to identify regimes where the top and bottom nanofins behave as nearly decoupled single-layer metasurfaces, and performed near-field analysis to reveal strong interlayer coupling effects. The fabrication uses a two-step lithography and selective development process for precise alignment and high efficiency. We demonstrate a metasurface grating deflecting x- and y-polarized light into opposite orders and a vortex generator producing polarization-controlled topological charges. Experiments confirm broadband operation with up to 80% diffraction efficiency, promising structured light, holography, and integrated photonics applications.