Abstract: Elastic wave absorption at subwavelength scale is of significance in many engineering applications. Non-Hermitian metamaterials show the ability in high-efficiency wave absorption. However, the single functionality of metamaterials is an important limitation on their practical applications for lack of tunability and reconfigurability. Here, we propose a tunable and reconfigurable non-Hermitian piezoelectric metamaterial bar, in which piezoelectric bars connect with resonant circuits, to achieve asymmetric unidirectional perfect absorption (UPA) and symmetric bidirectional perfect absorption (PA) at low frequencies. The two functions can be arbitrarily switched by rearranging shunted circuits. Based on the reverberation-ray matrix (RRM) method, an approach is provided to achieve UPA by setting an exceptional point (EP) in the coupled resonant bandgap. By using the transfer matrix method (TMM) and the finite element method (FEM), it is observed that a non-Hermitian pseudo-band forms between two resonant bandgaps, and the EP appears at the bottom of the pseudo-band. In addition, the genetic algorithm is used to accurately and efficiently design the shunted circuits for desired low-frequency UPA and PA. The present work may provide new strategies for vibration suppression and guided waves manipulation in wide potential applications.

Key words: non-Hermitian metamaterial, exceptional point (EP), unidirectional perfect absorption (UPA), piezoelectric metamaterial, reverberation-ray matrix (RRM) method, genetic algorithm