Spin-decoupled phase modulation for second harmonic generation via single-meta-atom metasurfaces

Nonlinear metasurfaces, integrating wavefront engineering and frequency conversion, have emerged as a promising platform for nonlinear photonic devices. Spin-decoupled phase modulation offers enhanced degrees of freedom for optical field control. However, existing strategies often rely on diatomic or multi-atomic configurations, which compromise spatial resolution. Here, we present a single-meta-atom design for spin-decoupled second-harmonic (SH) phase modulation. Leveraging a C3-symmetric meta-atom, we achieve spin-independent phase control by varying the arm length. Combined with the nonlinear Pancharatnam-Berry phase, this approach enables continuous and independent phase modulation for left and right circularly polarized states. As a proof-of-concept demonstration, we design a spin-decoupled SH spin-Hall device that achieves spin-nonconjugate angular deflection. The proposed metasurface platform may pave the way for advanced applications in nonlinear vector optical field modulation and entangled photon sources.