Based on transformation optics, we introduce another set of generalized laws of reflection and refraction (differs from that of [Science 334, 333 (2011)]), through which a transformation media slab is derived as a meta-surface, producing anomalous reflection and refraction for all polarizations of incident light.

Generalized laws of reflection and refraction from transformation optics

Causal nature of the acoustic response, for any materials or structures, dictates an inequality that relates the absorption spectrum of the sample to its thickness. We present a general recipe for constructing sound-absorbing structures that can attain near-equality for the causal relation with very high absorption performance; such structures are denoted optimal. Our strategy involves using carefully designed acoustic metamaterials as backing to a thin layer of conventional sound absorbing material, e.g., acoustic sponge. By using this design approach, we have realized a 12 cm-thick structure that exhibits broadband, near-perfect flat absorption spectrum starting at around 400 Hz. From the causal relation, the calculated minimum sample thickness is 11.5 cm for the observed absorption spectrum. We present the theory that underlies such absorption performance, involving the evanescent waves and their interaction with a dissipative medium, and show the excellent agreement with the experiment.

Optimal Sound Absorbing Structures

Artificial microstructures, which allow us to control and change the properties of wave fields through changing the geometrical parameters and the arrangements of microstructures, have attracted plenty of attentions in the past few decades. Some artificial microstructure based research areas, such as metamaterials, metasurfaces and phononic topological insulators, have seen numerous novel applications and phenomena. The manipulation of different dimensions (phase, amplitude, frequency or polarization) of wave fields, particularly, can be easily achieved at subwavelength scales by metasurfaces. In this review, we focus on the recent developments of wave field manipulations based on artificial microstructures and classify some important applications from the viewpoint of different dimensional manipulations of wave fields. The development tendency of wave field manipulation from single-dimension to multidimensions provides a useful guide for researchers to realize miniaturized and integrated optical and acoustic devices.

Multidimensional manipulation of wave fields based on artificial microstructures

A review of additive manufacturing of metamaterials and developing trends

Acoustic Metamaterials: A Review of Theories, Structures, Fabrication Approaches, and Applications

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. Here, the authors propose an acoustic metasurface design to extend the wave manipulations to both far- and near-fields while reducing the complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate.

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Deep-subwavelength control of acoustic waves in an ultra-compact metasurface lens.

Detection of damage in welded joints using high order feature guided ultrasonic waves

We present laser-induced full-matrix imaging of complex-shaped objects that combines the advantages of noncontact laser ultrasound inspection and high-contrast array imaging based on total focusing method (TFM). Full-matrix data were acquired by synthesizing a laser ultrasonic array through an alternate scanning of the generation and detection laser beams. Taking advantage of the simultaneous generation of all wave modes, surface Rayleigh waves were extracted for surface profiling, and longitudinal waves were utilized for imaging. To gain a better understanding, we analyzed the full-wave dynamics in generation, propagation, and scattering by using numerical simulations. TFM was adapted to accommodate complex surface and suppress interference from other waves, including surface waves and straightforward traveling bulk waves between emission and reception. Numerical and experimental studies on an aluminum sample with a sinusoidal surface and side-drilled holes were conducted, and results agreed well.

Laser-Induced Full-Matrix Ultrasonic Imaging of Complex-Shaped Objects

Remote characterization of surface slots by enhanced laser-generated ultrasonic Rayleigh waves.

Surface-breaking cracks are typical defects in tubular structures. Compared with other types of defects such as internal voids, surface cracks often impose more serious threats to structural integrity. This study presents an approach to detect and characterize surface-breaking cracks on tubular samples through the use of ultrasonic phased array technology with surface acoustic waves (SAWs). A Rayleigh type surface wave is selected in our work as it is nondispersive and highly sensitive to surface and subsurface defects. Finite element (FE) analysis is used to simulate the interaction between Rayleigh waves and surface-breaking cracks with varying depths, inclined angles and profiles. The reflection coefficient of Rayleigh waves is calculated based on simulation results and fitted to a proper model. Crack depths and inclined angles can be evaluated from the fitted curves. Furthermore, a full matrix capture (FMC) data acquisition strategy is simulated in FE models with phased array to collect pulse-echo signals of Rayleigh waves. An array imaging algorithm is applied to FMC data and adapted to curved surface. The profile and location of surface cracks are reconstructed from imaging results. The configuration of phased array is optimized to increase the resolution of the method. The proposed approach is validated numerically and provides an efficient way to measure the length, depth and inclined angle of surface-breaking cracks on tubular component.

Jing Xiao

Papers

Deep-subwavelength control of acoustic waves in an ultra-compact metasurface lens.

Detection of damage in welded joints using high order feature guided ultrasonic waves

Laser-Induced Full-Matrix Ultrasonic Imaging of Complex-Shaped Objects

Remote characterization of surface slots by enhanced laser-generated ultrasonic Rayleigh waves.

Characterization of Surface-Breaking Cracks on Tubular Structures Using Ultrasonic Phased Array with Rayleigh Waves