Thickness-insensitive coding metasurface for broadband underwater acoustic radar cross section reduction

In underwater acoustics, reducing target detectability typically requires tailoring material thickness to specific frequencies, posing a fundamental constraint for broadband stealth. Here, we break this thickness-frequency dependency by introducing a 1-bit coding metasurface (CM) that achieves effective acoustic scattering suppression from 10 to 35 kHz with minimal sensitivity to cavity geometry. The CM comprises two deep-subwavelength units exhibiting a 180° reflection-phase difference: a metal-backed "0" unit that mimics a rigid boundary, and a "1" unit that encapsulates a stabilized air bubble with a polymer membrane to approximate a pressure-release surface. Through optimized spatial coding, the CM redirects incident plane waves into diffuse scattering patterns, substantially attenuating specular reflection. Both simulations and experiments confirm greater than 10 dB radar-cross section reduction over a broad angular span (up to 45°). Crucially, the reflection phase remains stable against variations in air-cavity thickness across the operational band, demonstrating a thickness-decoupled design paradigm. This work provides a robust, fabrication-friendly strategy for broadband underwater acoustic signature control, with potential applications in sonar camouflage and wavefront manipulation.