Water-Immersed GaP Huygens' Meta-Optics for Visible Structured Light Generation.

Underwater photonics is essential for imaging and analysis in biomedical technologies, as well as for communication and autonomous vehicle networks in the Internet of Underwater Things (IoUT). Structured light enables spatial control of beams, offering new opportunities for underwater sensing, imaging, and signal transmission. However, compact and robust water-immersed photonic devices remain challenging to realize. Among various approaches, dielectric metasurfaces are particularly promising for underwater optics due to their flat form factor and subwavelength phase control. Nevertheless, current platforms suffer from instability in liquids, incomplete phase modulation, or strong sensitivity to fabrication imperfections. Here, ultrathin water-immersed metasurfaces composed of gallium phosphide (GaP) Huygens' integrated resonance units (HIRUs) operating in the visible regime are demonstrated. By leveraging the high refractive index and low optical loss of GaP, and integrating both forward and inverse design strategies, metasurfaces with planar geometry (≈1/5 of the operating wavelength), enhanced fabrication tolerance, and stable in-liquid operation are realized. These metasurfaces generate various structured light beams in water, including abruptly autofocusing (AAF) beams, Bessel beams, optical vortices, and Gaussian focusing beams. This work establishes a scalable and reliable platform for underwater nanophotonics and emerging IoUT-related applications.