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

Parity-time (PT) symmetric systems based on bal- anced distributions of gain and loss have attracted significant attention in wave physics, enabling the possibility of compen- sating material losses in photonic systems, and of inducing ex- ceptional 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 un- precedented 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 metasur- faces 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 system