Ultrathin Acoustic Parity-Time Symmetric Metasurface Cloak

Invisibility or unhearability cloaks have been made possible by using metamaterials enabling light or sound to fow around obstacle without the trace of refections or shadows.Metamaterials are known for being fexible building units that can mimic a host of unusual and extreme material responses,which are essential when engineering artifcial material properties to realize a coordinate transforming cloak.Bending and stretching the coordinate grid in space require stringent material parameters;therefore,small inaccuracies and inevitable material losses become sources for unwanted scattering that are decremental to the desired efect.Tese obstacles further limit the possibility of achieving a robust concealment of sizeable objects from either radar or sonar detection.By using an elaborate arrangement of gain and lossy acoustic media respecting parity-time symmetry,we built a one-way unhearability cloak able to hide objects seven times larger than the acoustic wavelength.Generally speaking,our approach has no limits in terms of working frequency,shape,or size,specifcally though we demonstrate how,in principle,an object of the size of a human can be hidden from audible sound.