Space-coiling acoustic metasurfaces have been largely exploited and shown their outstanding wave manipulation capacity. However, they are complex in realization and cannot directly manipulate acoustic near-fields by controlling the effective path length. Here, we propose a comprehensive paradigm for acoustic metasurfaces to extend the wave manipulations to both far- and near-fields and markedly reduce the implementation complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate. A semi-analytical approach for such a design is established using a microscopic coupled-wave model, which reveals that the acoustic diffractive pattern at every slit exit is the sum of the initial transmission and the secondary scatterings of the coupled fields from other slits. For proof-of-concept, we examine two metasurface lenses for sound focusing within and beyond the diffraction limit. This work provides a feasible strategy for creating ultra-compact acoustic components with versatile potentials.
Corrosion is a major safety and economic concern to various industries. In this paper, a novel ultrasonic guided wave tomography (GWT) system based on self-designed piezoelectric sensors is presented for on-line corrosion monitoring of large plate-like structures. Accurate thickness reconstruction of corrosion damages is achieved by using the dispersive regimes of selected guided waves and a reconstruction algorithm based on full waveform inversion (FWI). The system makes use of an array of miniaturised piezoelectric transducers that are capable of exciting and receiving highly dispersive A0 Lamb wave mode at low frequencies. The scattering from transducer array has been found to have a small effect on the thickness reconstruction. The efficiency and the accuracy of the new system have been demonstrated through continuous forced corrosion experiments. The FWI reconstructed thicknesses show good agreement with analytical predictions obtained by Faraday’s law and laser measurements, and more importantly, the thickness images closely resemble the actual corrosion sites.
Focusing of ultrasonic waves in water plays an important role in various scenarios ranging from biomedical imaging to nondestructive testing. Acoustic metasurfaces have been largely explored for acoustic focusing, but they are generally narrowband and mainly implemented for airborne sound because of their structural complexity. Nevertheless, our previous development of metasurfaces provides a great opportunity to solve the challenges. Here, we present numerically and experimentally the broadband focusing of ultrasonic waves in water with a metasurface lens consisting of an array of deep-subwavelength sized and spaced slots. The slot widths of the metasurface are optimized based on microscopic coupled-wave theory. Due to the non-resonant arrangement, the focusing effect is demonstrated over a broad band of frequencies. The metasurface lens with simplicity and an ultra-compact size provides a feasible means for the design of thin and lightweight ultrasonic devices and is suitable for practical applications in biomedical and industrial fields.
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