Tuo Liu

This paper presents a novel square Maxwell fish-eye lens (SMFEL), designed using the Schwarz–Christoffel conformal transformation method, aimed at effectively controlling the propagation of flexural waves and achieving high-resolution imaging. Traditional Maxwell fish-eye lenses, due to their unique refractive index distribution, can focus waves from any point without aberrations to their antipodal point. However, in practical applications, they often encounter impedance mismatch issues at the boundary. To address this issue, the proposed SMFEL resolves the impedance mismatch induced by boundary effects in conventional lens designs by leveraging a thin plate structure with varying thickness—all while preserving outstanding imaging capabilities. To explore the lens’s imaging traits under different excitation positions and frequency conditions, researchers carried out numerical simulations and experimental verifications. The results show that the SMFEL can achieve subwavelength resolution imaging over a wide frequency range (10–30 kHz) and demonstrates good focusing effects at various frequencies. Further experimental verification reveals that the imaging performance of the SMFEL is highly consistent with the numerical simulation results, proving its potential applications in fields such as structural health monitoring, non-destructive testing, and high-resolution acoustic imaging. This study provides a new approach to flexural wave control and demonstrates the broad application prospects of square lens designs in flexural wave imaging.

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.

Overcoming the diffraction limit has been a long-lasting pursuit for researchers owing to the great potential it offers in going beyond the fundamental resolution restriction in imaging processes. In acoustics, meta-lenses have been a promising way to achieve sub-wavelength imaging, the practical application of which, however, has been limited by expensive material manufacturing, complex system setup, and material loss. Here, we propose a set of procedures equivalent to a virtual super-lens that selectively amplifies the evanescent wave components in the measured acoustic field spectrum, thereby enabling super-resolution imaging without any auxiliary setups or purposely designed super-lens. The proposed virtual super-lens is experimentally verified by considering the imaging of an irregularly shaped sample with sub-wavelength features. We further demonstrate the robustness of the high-quality imaging performance remains acceptable with some environment background noises. In the light of the simple experimental setup involved, our proposed method is flexible and can be readily applied to various practical imaging scenarios.

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.

We study theoretically and experimentally the acoustic Purcell effect induced by quasi-bound states in the continuum (quasiBICs). A theoretical framework describing the acoustic Purcell effect of a resonant system is developed based on the system’s radiative and dissipative factors, which reveals the critical emission condition for achieving optimum Purcell factors. We show that the quasiBICs contribute to highly confined acoustic field and bring about greatly enhanced acoustic emission, leading to strong Purcell effect. Our concept is demonstrated via two coupled resonators supporting a Friedrich–Wintgen quasiBIC, and the theoretical results are validated by the experiments observing emission enhancement of the sound source by nearly two orders of magnitude. Our work bridges the gap between the acoustic Purcell effect and acoustic BICs essential for enhanced wave-matter interaction and acoustic emission, which may contribute to the research of high-intensity sound sources, high-quality-factor acoustic devices and nonlinear acoustics.