Yan-Feng Wang

Waves described by Bessel functions of different orders have recently attracted widespread attention. Owing to their potential to provide additional available channels in acoustic communications. Their orthogonality in the azimuthal plane enables spatial multiplexing at a given frequency. The number of available channels is thereby increased, facilitating the development of faster and more reliable communication systems. In this paper, traveling waves with different radial wavenumbers as well as azimuthal indices are employed, leading to a significant increase in potential spatial channels. These waves are termed complex Bessel modes (CBMs) and are rigorously orthogonal when confined within a waveguide. Compared with multiplexing based on orthogonality in the azimuthal plane only, CBMs offer additional spatial degrees of freedom for channel multiplexing. However, a sophisticated requirement on modulation arises naturally, owing to the resulting inhomogeneous amplitude and phase distribution. Generalized impedance metasurfaces are therefore introduced to address this challenge. They convert efficiently between different CBMs, achieving 100% power flow conversion efficiency in numerical simulations. Two demonstration generators for CBMs with zero azimuthal index are fabricated with practical structures obtained through optimization. The input waveguide of the generators can be designed to have one-quarter the cross-sectional area of the output waveguide, owing to the flexibility of the generalized impedance theory. Generation of CBMs can therefore be achieved using a single speaker, eliminating the requirement for a large array of speakers and thereby leading to substantial cost reduction and improved stability. High-purity CBMs are demonstrated in both numerical simulations and experiments. This work reveals the potential of CBMs to serve as additional orthogonal spatial channels for acoustic multiplexing. This extension of generalized impedance theory paves the way for more reliable acoustic communications.

Flexible manipulation of flexural waves is of great importance for structural vibration control and health monitoring. While existing tunable elastic metasurfaces for elastic wave manipulations primarily rely on mechanical reconfiguration, magnetic fields or shunting circuits, the potential of harnessing shape memory alloys (SMAs) to construct elastic metasurfaces remains an unexplored frontier. In this study, we propose a temperature-controlled switchable elastic metasurface composed of SMA pillars to function as an ON/OFF gating for flexural wave manipulation. This switching mechanism arises from the temperature-induced stiffness disparity inherent in martensite-austenite phase transition. Specifically, one thermal state is designed to provide the full 2 π phase shift required for wavefront steering, actively turning the anomalous manipulation ON. Conversely, the alternate state yields limited phase variations that suppress anomalous reflection, effectively turning the functionality OFF. Numerical simulations and experimental validations demonstrate the on-demand activation and deactivation of anomalous reflection of flexural waves. This work presents a novel method for designing tunable elastic metasurfaces, offering prospects for programmable and smart elastic devices.

Transmission of sound waves through the water–air interface is a critical question for acoustic communications. The significant contrast in the parameters of both media indeed results in a severe challenge for wave manipulation. In this paper, the concept of acoustic Willis metasurface is proposed for transmedia beam steering with complete transmission. Such a metasurface supplies arbitrary phase shifts together with perfect transmission in one dimension, by appropriate modulation of material parameters, owing to the presence of Willis coupling terms. A hybrid Willis coupling element is proposed as the unit structure of transmedia impedance metasurfaces. Impedance theory is first recalled for two-dimensional beam steering, and the relationship between impedance components and material parameters is then given explicitly. A two-step optimization process ensures independent cell design while mitigating transverse acoustic-structure interaction. As a result, perfect power flow manipulation through the water–air interface is achieved with extreme steering angle. The present work provides a new perspective for the application of Willis coupling materials and an alternative theoretical support for transmedia metasurface design.

• A reconfigurable coding metasurface enables efficient bidirectional wavefront control across the water-air interface. • Sidewall-induced flexural resonance in discrete units simultaneously ensures high transmission and tunable phase modulation. • The compact single-layer design supports programmable functions like focusing, splitting, and Airy-beam generation. • Experiments confirm enhanced transmission and reconfigurable field control in both propagation directions.

We employ a combined approach of numerical simulation and experimental validation to design a two-dimensional valley topological phononic crystal plate, in which cup-shaped cavities are connected to a perforated plate via thin beams and are arranged periodically. We systematically investigate the propagation characteristics of elastic waves in the phononic crystal plate featuring Valley-Hall edge states. Based on the principle of local resonance, low-frequency Dirac degeneracies are introduced. By modulating the volume of liquid in different resonators, the lattice symmetry is broken such that the energies of the upper and lower dispersion bands at the high-symmetry K point became unequal. This leads to the splitting of the Dirac cone degeneracy and to the formation of a topological bandgap, within which elastic wave propagation with topological protection is observed. Our work demonstrates topologically protected edge states propagating along interfacial boundaries between domains with distinct topological invariants. Furthermore, the acoustoelastic waveguides can be designed by selectively filling the liquid into the resonators, and the locally resonant frequencies can be continuously tuned by adjusting the liquid volume and concentration. Leveraging the concentration-dependent tuning of the resonant frequencies, we develop a phononic crystal sensor for precise detection of liquid concentration. This study offers an innovative solution for reconfigurable acoustic devices, smart sensing systems, and elastic wave information processing technologies.