This thesis aims to present the underlying design principle and implementation of broadband and optimal metamaterial absorbers near the causal limit, realized in the microwave and underwater acoustic systems, respectively. A universal theoretical framework for describing resonance-based metamaterials will be constructed for gaining insights into the absorption phenomenon in a coherent perspective. We propose two typical routes to achieve the causal optimality by either increasing the dissipation or the mode density, which are used in the design of microwave and underwater acoustic absorbers, respectively. This is because the dissipation can be easily tuned in microwave systems by introducing chip resistors onto the metallic structures. In contrast, more modes can be introduced in acoust...[ Read more ]

This thesis aims to present the underlying design principle and implementation of broadband and optimal metamaterial absorbers near the causal limit, realized in the microwave and underwater acoustic systems, respectively. A universal theoretical framework for describing resonance-based metamaterials will be constructed for gaining insights into the absorption phenomenon in a coherent perspective. We propose two typical routes to achieve the causal optimality by either increasing the dissipation or the mode density, which are used in the design of microwave and underwater acoustic absorbers, respectively. This is because the dissipation can be easily tuned in microwave systems by introducing chip resistors onto the metallic structures. In contrast, more modes can be introduced in acoustic systems and tailored to be well decoupled to compensate for the relatively fixed dissipation. Simulations based on the finite element method (FEM) help examine the performance of the proposed absorbers. The extracted absorption spectra are used to calculate the minimal thickness dictated by the causal limit, which we show is close to the practical sample thickness. Therefore, the microwave and underwater acoustic absorbers are causally optimal in the aspect of numerical simulation. Meanwhile, we confirm the causal optimality of the microwave absorber by experiments in a dark room. For underwater acoustic absorbers, the measured absorption is slightly lower than the simulation data due to the complexity of underwater experiments in a water pool.

Compared with other existing works on metamaterial absorption, we first experimentally realize the microwave and underwater acoustic counterparts of the causally optimal broadband absorption. The microwave absorber has a simple metallic structure and holds the potential for low-cost and large-scale applications. Going a step further, we attempt to mitigate the stringent constraint imposed by causal limit in underwater systems by using soft composite materials with large mass density. By enhancing energy density and suppressing the wavelength in materials, such treatments will lead to thinner structure thickness without losing the absorption performance. I conclude in the final part by looking into the future trends in the development of metamaterial absorbers.