Ultrathin Electromagnetic-Acoustic Amphibious Stealth Coats

Radar and sonar technologies are generally used to detect objects in air space and water. In some cases, both technologies are involved in probing waterborne objects like submarines. However, the simultaneous stealth regarding these two detection methods is not well explored due to the difficulty in satisfying material properties for two different physical systems. In this work, an artificial ultrathin amphibious stealth coat with a minimum thickness less than 0.5 mm achieving free-space microwave and underwater acoustic absorption is proposed and demonstrated. It is realized by combining an electromagnetic metasurface and an acoustic metasurface-a flexible printed circuit with multisized metal resonators and a polydimethylsiloxane film with multisized air cavities. These two alien metasurfaces are engineered with minimized disturbance to each other by controlling the thicknesses of the resonators as thin as possible. A free-space microwave absorptivity over 80% in the 12.38-13.7 GHz region and an underwater acoustic reflectance less than 10% in the 80-160 kHz region are obtained experimentally. The current work may shed light on advanced stealth technologies of the free-space microwaves and underwater acoustic waves.