Ultrathin Acoustic Parity-Time Symmetric Metasurface Cloak

(a) A rigid cylinder is covered by an ultrathin symmetric metasurface. The left (right) semicircle metasurface contains acoustic loss (gain) to fully absorb incoming (reemit outgoing) sound waves. (b) Three scenarios are exemplified: no cloak, comprising strong backscattering and a shadow; camouflaging through complete absorption with a lossy semicircle metasurface only; cloaking via symmetry.