Unidirectional Cloaking Based on Metasurfaces with Balanced Loss and Gain

We prove that balanced loss-gain distributions provide an elegant path towards realizing loss-free unidirectional cloaks for objects much larger than the wavelength. By coating a scatterer with an ultrathin metasurface with balanced loss and gain, such as a parity-time (PT)-symmetric metasurface in the special case of a symmetric scatterer, we show that it is possible to ideally cloak the scatterer from a particular direction. The passive portion of the cloak ensures that the incident power on the illuminated side of the object is fully absorbed, while the active portion radiates the required pattern on the opposite side, automatically synchronized with the impinging signal, eliminating shadows and making the object invisible. The proposed approach substitutes complex metamaterial cloaks and achieves cloaking with relaxed limitations on size, and in an ultrathin, loss-free, and stable design. Beyond introducing a new venue to cloaking of large objects, our paper shows the unique potential for scattering manipulation introduced by structures with balanced loss and gain, pointing to a new direction in the field of PT-symmetric systems.