All-dielectric planar metasurface for giant circular dichroism and polarization-selective focusing

Abstract Achieving strong chiroptical effects and polarization-dependent wavefront shaping within a planar platform remains a major challenge in nanophotonics. In this paper, we propose and numerically demonstrate an all-dielectric metasurface composed of individually achiral, crescent-shaped nanopillars arranged in a hexagonal lattice. Through geometric parameter tuning (e.g. offset distance), the metasurface exhibits significant structural anisotropy, giving rise to giant extrinsic circular dichroism ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mtext>CD</mml:mtext> <mml:mo>&gt;</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> </mml:math> ), large optical rotation, and relatively high polarization extinction ratio. Multipole decomposition reveals that the observed chiroptical responses are predominantly governed by toroidal dipole (TD) resonances, with electric and magnetic field distributions confirming the strong confinement and directional asymmetry of TD-induced fields. The strong extrinsic chirality arises from the combination of the achiral metasurface supporting TD and oblique incident waves. Additionally, full 2 π geometric phase modulation with enough high transmittance across the visible range is also achieved, enabling the realization of a polarization-selective metalens that achieves broadband, diffraction-limited focusing performance for circularly polarized light. These results suggest that our design approach may offer a compact platform for on-chip integration, with promising applications in chiral sensing, optical communication, and chiroptical spectroscopy.