High efficiency spin-decoupled modulation using chiral C2-symmetric meta-atoms

Orthogonal circularly polarized light is essential for multiplexing tunable metasurfaces. Mainstream spin-decoupled metasurfaces, consisting of numerous meta-atoms with mirror symmetry, rely on the cooperative modulation of the Pancharatnam-Berry (PB) phase and the propagation phase. This paper demonstrates spin-decoupled functionality through the synergistic utilization of planar chiral meta-atom phase response and PB phase. Based on the Jones calculus, it has been found that meta-atoms with chiral C2-symmetry owns a larger geometric parameter range with high cross-polarization ratio compared to those with mirror symmetry or higher symmetries at the same aspect ratio. This characteristic is advantageous in terms of enabling high-efficiency manipulation and enhancing the signal-to-noise ratio. As an example, 10 kinds of C2-symmetry chiral meta-atoms with a H-like shape are selected by the self-adaptive genetic algorithm to attain a full 2$\pi$ phase span with an interval of $\pi$/5. To mitigate the additional propagation phase change of the guided modes originated from the arrangement alternation upon the rotation of the meta-atoms, the enantiomer of chiral meta-atoms and its PB phase delay are adopted to minimize the difference between the actual and desired target phases. A polarization-insensitive metalens and a chiral virtual-moving metalens array are designed to demonstrate the spin-decoupled function with both high efficiency and signal-to-noise ratio. The work in this paper may trigger more exciting and interesting spin-decoupled multiplexing metasurfaces and broaden the prospect of chiroptical applications.