Observations of Tamm modes in acoustic topological insulators

The existence of topologically protected boundary states in the Su–Schrieffer–Heeger (SSH) model has been widely explored in various systems in the past decades. On the other hand, a seemingly irrelevant phenomenon of Tamm mode, a defect mode caused by elaborately introducing perturbation into the trivial structures, has also attracted enormous interest. In this Letter, we bridge the gap between acoustic Tamm mode and its correlation rule with typical topological acoustic mode. Starting from the 1D acoustic cavity-based SSH model, the topologically protected edge modes in a complete bandgap are achieved. Through modulating the height of boundary cavity, we find two types of Tamm edge modes both numerically and experimentally and grasp their distinct variation tendency from the topologically protected edge modes. Furthermore, we extend the study to 2D higher-order topological acoustic systems and the lower-dimensional Tamm corner modes are clearly observed. We foresee that the multi-frequency sound confinement introduced by the Tamm localized modes may open a revealing landscape for acoustic functional devices.