Acoustic non-Hermitian higher-order topological bound states in the continuum

Recently, the concept of bound states in the continuum (BICs) has been extended to topological physics, inspiring investigations into higher-order topological BICs (TBICs) and related ultra-strong wave localization, which not only enriches the realm of topological physics but also bestows the BICs with inherent topological protection. However, previous explorations toward higher-order TBICs have been limited to the Hermitian assumption, omitting the nonconservative characteristics present in many artificial materials. In this work, we propose and experimentally demonstrate an acoustic lattice model supporting higher-order TBICs that solely rely on non-Hermiticity, in which the non-Hermiticity is implemented by strategically applying additional loss to specific sites in the lattice. Importantly, these in-band corner states are protected by chiral symmetry and can be spectrally switched by introducing perturbations to the corner sites or couplings. Our findings highlight the distinctive role of non-Hermiticity in constructing higher-order TBICs, which may inspire sophisticated and externally tunable approaches for designing high-Q devices in wave-based technologies.