High‐Order Rotational Symmetry Induced High‐ <i>Q</i> Generalized Geometric Phases in Nonlocal Metasurfaces

ABSTRACT Nonlocal metasurfaces capable of independent narrowband wavefront shaping with high‐ resonant modes have attracted increasing interest in nanophotonics. However, owing to the complex lattice coupling and multiple resonance modes, conventional nonlocal wavefront shaping theory cannot be extended to higher‐rotational symmetry, leading to a practical limit on multi‐dimensional optical field manipulation. Here, we present a general theoretical framework for high‐ generalized geometric phases in nonlocal metasurfaces composed of high‐order rotational symmetric meta‐atoms (, 3) arranged in a diatomic lattice. Notably, we reveal that such phases exist under arbitrary rotational symmetry and originate primarily from guided mode resonances (GMRs), rather than quasi‐bound states in the continuum (qBICs). Furthermore, reducing the ‐factor of the GMRs enables to generation of geometric phase in the adjacent qBICs mode. As a proof of concept, a novel metagrating is experimentally demonstrated at the near‐infrared band, showing chiral‐dependent narrowband deflection with factors around 963 (LCP) and 733 (RCP). This work may pave the way for versatile applications of high‐order rotational symmetry engineering in narrowband wavefront modulation in linear/nonlinear and quantum optics.