Two-face god in sound: Directionality beyond spin-directed acoustics

Understanding unidirectional and topological wave phenomena requires the unveiling of intrinsic geometry and symmetry for wave dynamics. This is an essential challenging for the flexible control of near-field evanescent waves. However, exploitations of near-field waves are limited due to the lack of fundamental understanding about inherent near-field symmetries and directional couplings at subwavelengths, especially for longitudinal waves. Here, Chinese scientists show the efficient selective couplings enabled by near-field symmetry properties based on the acoustic wave platform.