A holographic GRIN lens for broadband, highly directive, and aberration-free echo detection

Directing wave energy through strongly aberrating media poses a fundamental challenge in wave physics. This problem is especially critical in underwater acoustics, where the hydrodynamic domes of autonomous underwater vehicles inevitably distort sonar wavefronts. This aberration leads to degraded beam quality and a significantly reduced detection range. Here, we address this long-standing challenge with a passive and broadband holographic gradient-index lens designed based on time-reversal invariance and high-frequency approximation principles. The lens is realized with a soft, water-impedance-matched tungsten-silicone metamaterial with a highly stable refractive index under key marine environmental factors such as temperature and salinity. This allows the lens to powerfully correct severe aberrations while simultaneously collimating sound into a highly directive beam over a wide operational band of 20-45 kHz. This metamaterial-based approach achieves significant performance improvements, achieving approximately four-fold beam sharpening from over 65° to 16°-30° and increasing the on-axis source level by more than 10 dB. In practical underwater detection scenarios, it achieves over 10 dB of reverberation suppression and significantly increases the effective detection range. Our work establishes a new and versatile design platform for advanced wavefront engineering with broad application prospects in underwater acoustic sensing and beyond.