Non-Hermitian metagrating for perfect absorption of elastic waves.

Beyond the fashion of Hermitian physics, non-Hermiticity has inspired, most recently, a surge of nontrivial principles and significant applications in both open quantum and classical systems characterized by gain or loss. However, research on elastic wave manipulation is still predominantly focused on conservative Hermitian systems, overlooking the energy interaction with the environment. The unavoidable energy loss, originating from the inherent material properties, is normally ignored. Additionally, most existing materials for elastic wave absorption suffer from complex configurations, sophisticated designs, and large volumes. Achieving highly efficient absorption properties in advanced artificial materials with ultrathin size and easy fabrication is still a challenge. This work proposes a design strategy based on non-Hermitian modulation to address such a challenge. The proposed non-Hermitian metagrating (NHMG), featured with the subwavelength unit cell, achieves the perfect absorption of elastic waves under specific low-loss conditions. The loss-induced non-Hermitian effects for perfect absorption are theoretically elucidated and a design framework is established in the NHMG with irregular and arbitrary shapes. The robust performance of omnidirectional and perfect absorption capabilities with respect to the boundary shape, rotation angle, and wave source location is numerically and experimentally verified. Consequently, a cloaking device based on the NHMG is further designed to avoid arbitrary-shaped targets being detected. Our study enriches the ways to elastic wave manipulation in non-Hermitian materials and provides an ultra-compact solution for wave absorption in engineering applications.