Shuowei An

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.

Constructing a highly localized wave field by means of bound states in the continuum (BICs) promotes enhanced wave-matter interaction and offers approaches to high-sensitivity devices. Elastic waves can carry complex polarizations and thus differ from electromagnetic waves and other scalar mechanical waves in the formation of BICs, which is yet to be fully explored and exploited. Here, we report the investigation of local resonance modes supported by a Lamb waveguide side-branched with two pairs of resonant pillars and show the emergence of two groups of elastic BICs with different polarizations or symmetries. Particularly, the two groups of BICs exhibit distinct responses to external perturbations, based on which a label-free sensing scheme with enhanced-sensitivity is proposed. Our study reveals the rich properties of BICs arising from the complex wave dynamics in elastic media and demonstrates their unique functionality for sensing and detection.

Non-Hermiticity, usually represented in the context of gain and loss, gives rise to many exotic topological phenomena and offers more opportunities to steering topological functions. In modern acoustics, it has been widely perceived that the topological mode will be altered if topological phase changes. Our work shows otherwise in non-Hermitian acoustic crystals. We experimentally demonstrate an acoustic quadrupole topological insulator, whose topological corner, edge, and bulk modes could be arbitrarily engineered at any desired positions with its topological phase maintained. These non-Hermiticity-controlled topological modes bestow a bulk structure with unique features beyond the classical bulky state, offering a reconfigurable and versatile approach to manipulating topological phenomena. This non-Hermitian scheme can be readily generalized to other topological systems in various dimensions, such as the three-dimensional photonic/phononic lattices, which offer advanced and externally controllable recipes for manipulating topological phenomena.

Topologically protected localized states are commonly existent inside bulk band gaps with topological features, which, however, can be shifted to coexist with a continuous spectrum under particular coupling modulations, forming the so-called topological bound states in the continuum (BICs). These embedded topological states have so far been derived under the Hermitian assumptions that neglect the influence of nonconservative characteristics of classical wave systems on the topological properties. In this study, with a one-dimensional trimerized elastic lattice, we report the experimental demonstration of topological edge states in the continuum produced solely by non-Hermiticity, without additional coupling modulation. The trimerized elastic lattice is a chain of plates connected through thin beams, with the non-Hermiticity introduced by attaching constrained damping layers to particular sites in the chain assembly. We prove that appropriately tailored non-Hermitian modulation can induce topological edge states that appear in the bulk spectrum rather than exist in the band gap. Besides, the existence of such topological edge states is observed to be closely linked to the configurations of the lattice boundaries. This study affirms that non-Hermiticity can play an important role in creating topological BICs, gaining insight into the non-Hermitian topological physics and offering interesting avenues for exploring sophisticated non-Hermitian topological phenomena including higher-order topology in the continuum.