Twist‐Enabled Transmissive Metasurface with Co‐Polarized Geometric Phase

Abstract Metasurfaces have offered unprecedented control over electromagnetic (EM) waves across a wide range of frequency spectrum by manipulating their phase, amplitude, and polarization at subwavelength scales. Full wavefront control using metasurfaces requires 2π phase modulation, which is essential for advanced optical and photonic engineering. Common approaches, such as the Pancharatnam‐Berry (PB) phases and resonant phases, face stringent limitations: PB phases essentially depend on circular polarization conversion, while resonant phases are inherently narrowband and require a complex design process. To overcome these challenges, a broadband metasurface with a co‐polarized transmissive geometric phase is proposed that achieves 2π phase coverage while conserving the circular polarization of incident EM waves. This co‐polarized phase is enabled by a local twist angle between the upper and lower metallic patterns, forming a branch cut in the parameter space determined by the twist angle and frequency. The branch cut connects phase singularities of opposite chirality, ensuring broadband 2π phase coverage. The presence of the branch cut is experimentally validated and demonstrates broadband generation of arbitrary orbital angular momentum (OAM) for co‐polarized output. The approach provides a versatile method for designing broadband metasurfaces without altering circular polarizations, paving the way for development of compact optical and photonic devices.