We demonstrate that the metamaterial perfect absorber behaves as ameta-cavity bounded between a resonant metasurface and a metallic thin-filmreflector. The perfect absorption is achieved by the Fabry-Perot cavityresonance via multiple reflections between the "quasi-open" boundary ofresonator and the "closed" boundary of reflector. The characteristic featuresincluding angle independence, ultra-thin thickness and strong fieldlocalization can be well explained by this model. With this model, metamaterialperfect absorber can be redefined as a meta-cavity exhibiting high Q-factor,strong field enhancement and extremely high photonic density of states, therebypromising novel applications for high performance sensor, infraredphotodetector and cavity quantum electrodynamics devices.
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