Development and progress in acoustic phase-gradient metamaterials for wavefront modulation

Acoustic metamaterials (AMs) for sound wave manipulation have attracted significant attention due to their fascinating functionalities, such as anomalous reflection/refraction, acoustic cloaking, sound absorption, acoustic imaging, etc. The acoustic phase-gradient metamaterials possess the capability of wavefront manipulation, thus, are fundamental to designing these fascinating functionalities. The underlying mechanism is controlling the acoustic responses (the phase and/or amplitude) of the units by varying the parameters so that one can redirect the wavefront in the desired manner. In this article, we review the state-of-the-art on development of phase-gradient metamaterials for wavefront manipulation. The governing principles of the phase-gradient metamaterials for wave control in static and moving media are first introduced. Then, according to the unit type, the phase-gradient metamaterials are roughly classified into three categories: the locally resonant structures, the space-coiling structures and the material-filling structures. Afterwards, three representative functionalities of the gradient metamaterials are reviewed, including acoustic cloaking, sound absorption/isolation and acoustic lens. Finally, the limitations of present metamaterials and possible future directions for development are concluded.