Observation of merged topological corner states in the continuum

Merging bound states in the continuum (BICs) has significant promise for wave manipulation since it can provide an ultra-high Q factor when compared to the isolated BICs. However, the study of merging topological bound states in the continuum (TBICs) remains largely unexplored. In this study, we introduce a straightforward structure for crafting the merged higher order TBICs, i.e., the merged topological corner states in the continuum (MTCICs), via a synthetic way. Two identical topological insulators, supporting topological corner states within the bulk band, are mirror-stacked. The eigenstates inherited from a single layer experience energy spectrum shifts without hybridization as interlayer couplings change, leading to the emergence of MTCICs. With acoustic crystals, we experimentally identify and characterize the MTCICs. The results evidently confirm the superior energy confinement capabilities of MTCICs over TBICs and gapped topological corner states (TCSs). Our work may deepen the understandings of both topological and BIC physics to inspire the development of devices with ultra-high Q factor and enhanced robustness.