THESIS
2022
1 online resource (xxvi, 89 pages) : color illustrations
Abstract
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...[
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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.
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