Anisotropic Complementary Acoustic Metamaterial for Canceling out Aberrating Layers

Abstract

In this paper, we investigate a type of anisotropic, acoustic complementary metamaterial (CMM) and its application in restoring acoustic fields distorted by aberrating layers. The proposed quasi two-dimensional (2D), nonresonant CMM consists of unit cells formed by membranes and side branches with open ends. Simultaneously, anisotropic and negative density is achieved by assigning membranes facing each direction (x and y directions) different thicknesses, while the compressibility is tuned by the side branches. Numerical examples demonstrate that the CMM, when placed adjacent to a strongly aberrating layer, could acoustically cancel out that aberrating layer. This leads to dramatically reduced acoustic field distortion and enhanced sound transmission, therefore virtually removing the layer in a noninvasive manner. In the example where a focused beam is studied, using the CMM, the acoustic intensity at the focus is increased from 28% to 88% of the intensity in the control case (in the absence of the aberrating layer and the CMM). The proposed acoustic CMM has a wide realm of potential applications, such as cloaking, all-angle antireflection layers, ultrasound imaging, detection, and treatment through aberrating layers. In many medical ultrasound or nondestructive evaluation (NDE) applications, ultrasound needs to be transmitted through an aberrating layer [1][2][3][4][5][6][7], where either the transmission is desired to be maximized or the reflection needs to be minimized. One of the most representative examples is transcranial ultrasound beam focusing, which could find usage in both brain imaging and treatment [6,7]. However, transcranial beam focusing is extremely challenging because of the presence of the skull. A common approach to achieve transcranial beam focusing is based on the timereversal or phase-conjugate technique and ultrasound phased arrays [8,9]. Although the focal position can be corrected, one significant shortcoming of this strategy is that it does not compensate for the large acoustic energy loss due to the impedance mismatch between the skull and the background medium (water). Recent development of acoustic metamaterials [10][11][12] could open up the possibility for noninvasive ultrasound transmission through aberrating layers. For example, an acoustic metamaterial could be used to cancel out or cloak the aberrating layer, allowing the acoustic wave to pass through the layer without energy loss (Fig. 1). Conventional cloaking strategies [11,13,14], however, compress the space and hide the object inside an enclosure in which there is no interaction with the outside world; therefore, it is not suited to the problem of interest in this study. Lai et al. demonstrated that cloaking or illusion based on electromagnetic wave (EM) complementary metamaterials (CMM) [15] can open up a virtual hole in a wall without distortion [16,17]. In addition, this type of approach does not require the cloaked object to be inside an enclosure or cloaking shell, and it is valid in free space [18]. Because of the s...

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