Chimney-board metastructures for birefraction and asymmetric transmission of water waves

Birefringence, a cornerstone of optics for over three centuries, describes the splitting of a single incident wave into two refracted beams in anisotropic crystals. However, achieving birefraction in water waves has remained a long-standing fundamental challenge due to the intrinsic single mode. Here, we theoretically propose and experimentally demonstrate a chimney-board metastructure that creates double modes for low-frequency water waves. This enables the first realization of high-order lattice diffraction-free, and broadband birefraction in water waves, with an effective refractive index difference of 1.55 and an experimental separation angle between the two refracted waves greater than 40°. Unlike conventional water wave control strategies relying on brute-force blocking or energy dissipation, our metastructure achieves precise wave steering and refined energy flux management via stratified beam transmission. By tuning geometric parameters and operating frequencies, we further realize broadband asymmetric transmission, which blocks destructive incoming ocean waves while allowing waves generated inside harbors to escape, offering a proof of concept for harbor protection. This work establishes a generic platform for stratified water wave control, with potential applications in wave‑energy redirection and coastal protection.