Remote Water‐to‐Air Eavesdropping with a Phase‐Engineered Impedance Matching Metasurface

Abstract Efficiently receiving underwater sound remotely from air is a long‐standing challenge in acoustics hindered by the large impedance mismatch at the water–air interface. Here, a phase‐engineered water–air impedance matching metasurface is proposed and experimentally demonstrated for remote and efficient water‐to‐air eavesdropping. The judiciously designed metasurface with near‐unity transmission efficiency, long monitoring distance, and high mechanical stiffness is capable of making the water–air interface acoustically transparent and, at the same time, freewheelingly patterning the transmitted wavefront. This enables efficient control over the effective spatial location of a distant airborne sensor such that it can measure underwater signals with large signal‐to‐noise ratios as if placed close to the physical underwater source. Such airborne eavesdropping of underwater sound is experimentally demonstrated with a measured sensitivity enhancement of nearly 10⁴ at 8 kHz, far from achievable with the current state‐of‐the‐art methods. Moreover, the opportunities of using the proposed metasurface for cross‐media orbital‐angular‐momentum‐multiplexed communication and underwater acoustic window are also demonstrated. This metasurface opens new avenues for communication and sensing in inhomogeneities with totally reflective interfaces, which may be translated to nano‐optics and radio frequencies.