Defect-Immune Sound Radiation in a Topologically Nontrivial Hyperbolic Metamaterial

Topological insulators remain steadfast at the forefront of condensed matter physics due to their unique electronic properties. The well-known bulk-boundary correspondence ensures the existence of robust edge or interface states, which are protected by the topological characteristics of the material's bulk band structure. Moving beyond the bound edge states traditionally emphasized in the literature, we explore how topologically protected states extend to nontrivial radiation away from the confining interfaces. Here, we design a metamaterial whose engineered hyperbolic dispersion entails both chiral and mirror symmetries. We discuss how the associated spin polarization degree of freedom enables defect-immune radiation with a variety of tunable directive far-field patterns. Our experiments unequivocally showcase the topological resilience of acoustic radiation in the wake of willfully added absorptive and obstructive defects, which has the potential to stimulate research into advanced room and urban acoustics.