Acoustic Bianisotropic Impedance and Hyperbolic Propagation in Metasurfaces

The strong analogies between electromagnetic and acoustic wave propagation have led to the discoveries of various forms of electromagnetic metamaterials and of their acoustic counterparts. However, the longitudinal properties of acoustics waves in fluid make the effective bulk modulus and effective impedance in acoustics inherently isotropic, which marks a strong difference, and a limitation of acoustic metamaterials and metasurfaces, compared to electromagnetics. Several important concepts in electromagnetics inherently require anisotropic impedance profiles, such as hyperbolic wave propagation, and therefore cannot find a direct counterpart in acoustics. Motivated by this limitation, in this talk we discuss a way to realize acoustic bianisotropy metasurfaces, relying on strong nonlocalities over the surface. Based on this approach, we introduce a design for hyperbolic acoustic metasurfaces with enhanced local density of states for sound, and inherent imaging and canalization properties. These design approaches based on strong nonlocality may also be extended to electromagnetics, offering a new path towards hyperbolic wave propagation.