Haiyan Fan

The nonlinear force generated by the magnetic device facilitates the width of the effective operating range of the system and thus provides a promising approach for designing self-powered sensing devices. However, existing energy harvesting devices still fall short in terms of efficiency and adjustability. This paper addresses the challenges by introducing the concepts of compactness and tunability, combined with nonlinear characteristics, multiple degrees of freedom, and time-varying potential energy. A multi-stable dual-beam nonlinear energy harvesting device featuring magnetic coupling and dual piezoelectric transducers is proposed. As a key performance indicator, the induced voltage RMS value quantitatively characterizes energy conversion effectiveness, while system stability and harvesting efficiency are comparatively analyzed across varying frequency ratios and potential energy configurations. The topological configuration of basins of attraction is computationally analyzed under static potential energy conditions, providing phase-space visualization of the system’s dynamic stability characteristics. The numerical results show that the ratio of frequencies of the system and the initial position of the magnet will directly affect the potential energy form. The combination of the two degrees of freedom of the piezoelectric beam and the mode of the trolley drive would be critical to the stability and energy harvesting performance of the device.

Conventional ventilation sound insulation requires closed cavities to sustain strong acoustic resonance for reflecting incident waves. In this Letter, we propose a different meta-structure design that uses honeycomb hollow-out acoustic metasurface (HHAM) to realize ventilation sound insulation. The HHAM unit exhibits dual capabilities of very high ventilation sound insulation at resonance and also effective broadband sound attenuation. The well-designed multi-unit HHAM design achieves fine performance with an ultra-thin profile of only 53 mm (0.17λ) and 30% open area with average sound transmission loss exceeding 15 dB from 1100 to 2500 Hz. The proposed design offers a compelling combination of ultra-broadband, effective ventilation, structural simplicity, and fabrication feasibility. Experimental results agree well with numerical simulations and theoretical predictions. Our findings offer a promising pathway for the design of ventilated acoustic metasurfaces and may facilitate practical applications in noise control and architectural acoustics.

We propose a metasurface design filled with porous medium inside Helmholtz resonators, which can project multiple predesigned images at different frequencies. It is easier to fabricate and more practical than the conventional multiplexed acoustic holographic metasurface of collateral Helmholtz resonators. Moreover, the amplitude and phase of the reflected acoustic wave it modulates have decoupled characteristics, which means there is no need to search for amplitude-phase space by algorithms. We theoretically, numerically, and experimentally demonstrate the meta-unit can achieve arbitrary amplitude-phase combinations at four frequencies. Then, the fourfold acoustic holographic images are numerically generated by one metasurface at 1180, 1490, 2340, and 3480 Hz, respectively. These proposed strategies significantly increase the degree of freedom of the classical metasurfaces, which may open up additional avenues for multifunctional compact acoustic devices.

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.