Observation of Bulk Polarization Transitions and Second-Order Acoustic Corner States Protected by Generalized Chiral Symmetry

A new class of topological lattice characterized by bulk polarization has been introduced recently, such system has been shown to host Wannier-type higher order states. Here, we introduce and measure topological bulk polarization in 3D printed two-dimensional acoustic meta-structures, and observe topological transitions as the design parameters are tuned. We also demonstrate that our topological meta-structure hosts both 1D edge and Wannier-type second-order corner states with unique acoustic properties. The edge states have the angular momentum that reverses for opposite propagation direction, thus supporting directional excitation. We observe the second order topological states protected by the generalized chiral symmetry of the meta-structure, which are localized at the corners and are pinned to ‘zero energy’. Interestingly, unlike the corner states protected by the conventional chiral symmetry, the generalized chiral symmetry of our three-atom sublattice enables their spectral overlap with the continuum of bulk states without leakage. The confinement and inherent robustness of the corner states is theoretically analyzed and experimentally confirmed by deliberately introducing disorder. Our findings open new directions for advanced sound propagation and manipulation.