Fig. 1: Conceptual illustration of non-invasive, fully two-dimensional visualization of transparent flow fields using photonic spin-decoupled metasurface.

a Left panel: schematic of the experimental setup. The distance between the first (M₁) and second (M₂) parabolic mirrors is ~5 m to ensure a sufficiently large test area. Both parabolic mirrors have the same diameter (15 cm) and focal length (1.5 m). The metasurface is positioned at the focal plane of the parabolic mirror M₂. The incident light is non-polarized with a central wavelength of 650 nm and a full-width at half-maximum about 20 nm. Right panel: The entry of the LCP state (\({|L}{\rm{\rangle }}\)) into the visualization system allows for the acquisition of horizontal dimension information of the transparent flow field, while the RCP state (\({|R}{\rm{\rangle }}\)) provides vertical dimension information. Thus, the incidence of unpolarized light (UP) permits real-time observation of both dimensions. b Top-view and (c) perspective view of SEM images of the fabricated metasurface. The bottom inset shows the unit cell of the metasurface. The silicon nanopillar has a height of h and a period of P