Three-dimensional collimated self-accelerating beam through acoustic metascreen

Recent Progress in Resonant Acoustic Metasurfaces

Acoustic metasurfaces, as two-dimensional acoustic metamaterials, are a current research topic for their sub-wavelength thickness and excellent acoustic wave manipulation. They hold significant promise in noise reduction and isolation, cloaking, camouflage, acoustic imaging, and focusing. Resonant structural units are utilized to construct acoustic metasurfaces with the unique advantage of controlling large wavelengths within a small size. In this paper, the recent research progresses of the resonant metasurfaces are reviewed, covering the design mechanisms and advances of structural units, the classification and application of the resonant metasurfaces, and the tunable metasurfaces. Finally, research interest in this field is predicted in future.

A transfer matrix method for calculating the transmission and reflection coefficient of labyrinthine metamaterials

Labyrinthine unit cells have existed for many years and have been central to the design of numerous metamaterial solutions. However, the literature does not present a reproducible analytical model to predict their behaviour both in transmission and reflection, thus limiting design optimization in terms of bandwidth of operation and space occupied. In this work, we present an analytical model based on the transfer matrix method for phase shift and intensity of transmission/reflection-based labyrinthine unit cells. We benchmark our analytical model by finding agreement with finite element method simulations - using commercial software - within 1 dB in amplitude and a 1° in phase. Finally, we compare our predictions with measurements on transmissive/reflective units with 4 and 6 horizontal baffles ("bars"), using different experimental methods. We found that some of the measurement methods lead to an agreement within 2 dB, while others are completely out of range, thus pointing out the challenges in characterizing this type of acoustic metamaterial.

Modulation of out-of-plane reflected waves by using acoustic metasurfaces with tapered corrugated holes

In this paper, modulation of reflected wavefront out of the incident plane by a tunable acoustic metasurface is investigated based on the fully generalized Snell's law in the three-dimensional space. The metasurface is constructed by a square lattice of circular holes with gradient annular bumps. The phase shift is tuned by changing the volume of water filled in the holes. The acoustic wave steering out of the incident plane and the out-of-plane acoustic focusing with the oblique incidence at the subwavelength scale are demonstrated numerically by selecting suitable distributions of water depth. The numerical results show that the wavefront of the reflected wave can be manipulated over a wide frequency range; and the gradient design of the unit cells can suppress the parasitic reflection. The present work is relevant to the practical design of novel acoustic devices.

Wavefront manipulation based on transmissive acoustic metasurface with membrane-type hybrid structure

We designed and demonstrated a gradient acoustic metasurface to manipulate the transmissive wavefront. The gradient metasurface is composed of eight elements based on membrane-type hybrid structures, whose thickness and width are about 1/5 and 1/20 of the incident wavelength, respectively. Here, we employ acoustic theory to analyze the transmission spectrum and phase gradient of the metasurface, the properties of high transmission efficiency and discrete phase shifts over the full \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2\pi $$\end{document}2π range can be achieved simultaneously. By appropriate selection of the phase profile along the transverse coordinate of the metasurface or the angle of incident wave, the transmissive wavefront manipulations based on metasurface can be obtained as expected from the generalized Snell’s law, such as anomalous refraction, acoustic cloak based on flat focusing, acoustic self-bending beam, conversion of propagating wave to surface wave and negative refraction. Our gradient metasurface may have potential application in low-loss acoustic devices.

Observation of Self-Bending and Focused Ultrasound Beams in the Megahertz Range

Self-bending (or self-accelerating) and nondiffracting acoustic beams, such as Airy beams, have the potential to focus around obstacles that are directly in the beam path. Here, we demonstrate the self-bending and focusing properties of Airy beams in the ultrasound domain using finite difference time-domain simulations at 5.2 MHz. The phase profiles of self-bending Airy beams are determined from the Airy function. This beam is then transmitted experimentally using a linear array transducer connected to a 128 channel Vantage Verasonics operating at 5.2 MHz. The performance of self-bending beams is compared to conventional focused ultrasound beams in the presence of a strong scattering obstacle (steel rod). The ability of self-bending Airy beams to bypass obstacles is characterized in terms of their relative energy retention at peak intensity, that was found experimentally to be 50.5% for traditional focused beams whereas 71.5% for Airy beams, proving that self-bending beams performed better than conventional beams in terms of relative energy retention with no significant change in the focal profiles. However, it is observed that, in absolute terms, self-bending beams focus less energy than traditional focused beams.

Manipulation of acoustic wavefront by gradient metasurface based on Helmholtz Resonators

We designed a gradient acoustic metasurface to manipulate acoustic wavefront freely. The broad bandwidth and high efficiency transmission are achieved by the acoustic metasurface which is constructed with a series of unit cells to provide desired discrete acoustic velocity distribution. Each unit cell is composed of a decorated metal plate with four periodically arrayed Helmholtz resonators (HRs) and a single slit. The design employs a gradient velocity to redirect refracted wave and the impedance matching between the metasurface and the background medium can be realized by adjusting the slit width of unit cell. The theoretical and numerical results show that some excellent wavefront manipulations are demonstrated by anomalous refraction, non-diffracting Bessel beam, sub-wavelength flat focusing, and effective tunable acoustic negative refraction. Our designed structure may offer potential applications for the imaging system, beam steering and acoustic lens.

Metamaterial bricks and quantization of meta-surfaces

Controlling acoustic fields is crucial in diverse applications such as loudspeaker design, ultrasound imaging and therapy or acoustic particle manipulation. The current approaches use fixed lenses or expensive phased arrays. Here, using a process of analogue-to-digital conversion and wavelet decomposition, we develop the notion of quantal meta-surfaces. The quanta here are small, pre-manufactured three-dimensional units-which we call metamaterial bricks-each encoding a specific phase delay. These bricks can be assembled into meta-surfaces to generate any diffraction-limited acoustic field. We apply this methodology to show experimental examples of acoustic focusing, steering and, after stacking single meta-surfaces into layers, the more complex field of an acoustic tractor beam. We demonstrate experimentally single-sided air-borne acoustic levitation using meta-layers at various bit-rates: from a 4-bit uniform to 3-bit non-uniform quantization in phase. This powerful methodology dramatically simplifies the design of acoustic devices and provides a key-step towards realizing spatial sound modulators.