Unprecedented wave control with active elastic metamaterials
Authors:   , Wu Qian 
Published: February 2023

[EMBARGOED UNTIL 8/1/2023] The research of elastic metamaterials has received tremendous attention from both academic and industrial communities over the past decade or so. Elastic metamaterials are a type of structured artificial composites whose macroscopic properties are determined by their periodic subwavelength building blocks. Due to their extreme obtainable material parameters, elastic metamaterials have been used to illustrate numerous exotic physical phenomena such as negative refraction and superlensing. Their potential applications include vibration and wave mitigations, waveguiding, energy harvesting, cloaking and so forth. In recent years, passive elastic metamaterials cannot meet the needs from modern wave functional devices, because their material parameters cannot be altered once they are fabricated. This drawback has eventually led to the development of active elastic metamaterials, which involves reprogrammable components and provides real time reconfigurability. Among the existing active control strategies, shunted piezoelectric materials have received considerable attention as they provide reprogrammable interactions between mechanical and electrical parts with large degree of freedom. They have been utilized to enhance the performance of conventional wave manipulation strategies such as broadband wave and/or vibration mitigations and reconfigurable wave beam steering. In this dissertation, some prototypes of active elastic metamaterials and metasurfaces are proposed for unconventional wave manipulation, by leveraging the circuit control concept of shunted piezoelectric materials. Theoretical and experimental approaches are utilized for illustrating design principles, characterizing system properties, and validating theoretical predictions. Specifically, various types of reprogrammable reciprocal/nonreciprocal interactions are employed to break certain symmetries in order to observe some nonreciprocal and asymmetric wave propagation behaviors. First, a type of non-Hermitian parity-time symmetric metabeam enable by balanced positive and negative resistance is introduced, which features unidirectional reflectionlessness at phase transition points. Then, a space-time modulated surface of a semi-infinite medium is investigated to support nonreciprocal frequency and mode conversions for Rayleigh waves. In addition, by leveraging the nonreciprocal coupling between piezoelectric sensors and actuators, a nonlocal metabeam and an active solid with odd mass density are proposed to realize nonreciprocal wave amplification/attenuation for flexural waves in dimension one and in-plane waves in dimension two, respectively. Last, in combination of time-dependent transfer functions and piezoelectric sensing-actuating system, a linear active metasurface capable of independently converting a flexural incidence into arbitrary frequency components with phases and amplitudes on demand is presented with theoretical characterizations and experimental validations. It is emphasized that the dissertation focuses primarily on discovering and characterizing the fundamentals of some unconventional wave manipulation strategies in sold structures. The examples involved provide conceptual designs which could be readily transformed to the micro/nano scales. It is also hoped that the content covered by the dissertation could pave the ways for the development of next-generation wave control devices, in the areas of structural health monitoring, one-way mechanical devices as well as ultrasensitive sensing.