Parity-Time Symmetry in Acoustics: Theory, Devices, and Potential Applications

Parity-time (PT) symmetric systems based on balanced distributions of gain and loss have attracted significant attention in wave physics, enabling the possibility of compensating material losses in photonic systems, and of inducing exceptional scattering phenomena, offering new possibilities for wave manipulation. A successful transition of these theoretical concepts to devices and practical applications requires a sufficient level of control over the frequency dispersion of the involved gain and loss elements, in order to engineer a PT-symmetric system that is stable and practical. Here, we show that such a degree of control can be achieved for acoustic waves, and obtain stable PT-symmetric acoustic systems that can manipulate sound in unprecedented ways. We describe the theory, design and potentials of PT-based negative refraction and phase compensation devices in free-space, induced by a pair of PT-symmetric acoustic metasurfaces engineered from active electro-acoustic resonators. Our study proves that acoustic systems represent an ideal platform to apply the unique scattering phenomena associated with PT-symmetric systems operated at exceptional points. We envision applications in acoustic imaging, furtive sensing, and noise control, and extension of these concepts to electromagnetics, optics, and nanophotonic systems.