On the design of non-Hermitian elastic metamaterial for broadband perfect absorbers

Elastic metamaterial, an engineered artificial material, has received much attention in physics and engineering communities due to its functional properties unavailable in natural materials. However, most elastic metamaterials, especially for their two-dimensional structures, belong to the Hermitian category, making them difficult to adapt to real lossy structures and explore their loss modulation. In the present study, non-Hermitian loss-modulation beam and plate models based on complex wavenumber plane, structural dynamics, and mode-coupling scattering theory are established to design lossy elastic metamaterials (LEMs) for any solid material. Based on a unified closed-form solution for absorption, the subwavelength lightweight LEM can cope with the conventional challenge of achieving broadband and near-omnidirectional elastic wave perfect absorption. We reveal here the high-performance absorption of the LEM from greatly enhanced wave-energy dissipation by a combination of dissipation-radiation-balance and multiple reflections in the non-Hermitian elastic wave system. We numerically and experimentally verify the effectiveness of the theoretical model and demonstrate broadband and perfect absorption in a plate-like structure. Our work not only opens a new route to achieve broadband low-frequency vibration suppression in macro devices and microelectromechanical systems, more essentially, but it also provides an effective paradigm to wave engineering in non-Hermitian elastic wave systems.