Metamaterial absorber (MMA), which is a kind of thin electromagnetic absorber consisting of sub-wavelength metamaterial resonators and can exhibit near-perfect absorption characteristics, has been widely investigated in recently years. The impedance matching theory was proposed to analyze the configuration of MMA in most literatures. Such theory, however, may not suitable to analyze the interactions of metamaterial resonators and the ground plane. The interference theory, on another hand, can play effective approach for this kind of problem presented in recent studies, whereas little attention has been paid on the oblique incidence conditions. In this paper, we firstly extend the interference theory model to make it applicable for oblique incident waves and analyze MMA using the extended interference theory model. Secondly, we further explore the sufficient condition for the maximum absorptivity at both normal and oblique incidence cases. Thirdly, with the sufficient condition, we can obtain the absorbing frequency directly if the thickness of MMA is given. These theoretical results have significant effects on the design and analyze of MMA. And lastly, we point out that absorptivity is not absolutely insensitive to the incidence angle in TM mode as what previous study claims, but insensitive when the dielectric slab is high loss, which can also be explained by interference theory

In this paper, we numerically and theoretically discuss the novel absorption properties of a conventional metasurface-based terahertz (THz) electromagnetic (EM) absorber with different dielectric thicknesses. Two absorption modes are presented in the considered frequency band due to the increased dielectric thickness, and both modes can achieve near-unity absorptions when the dielectric layers reach additional nλ(d)/2 (n=1, 2) thicknesses, where λ(d) is the operating wavelength at the peak absorption in the dielectric slabs. The surface currents between the metasurface resonators and ground plane are not associated any longer, different from the conventional thin absorbers. Moreover, the EM wave energies are completely absorbed by the metasurface resonators and dielectric layer, and the main function of ground plane is to reflect the incident EM waves back to the resonators. The discussed novel absorption properties are analyzed and explained by classical EM theory and interference theory after numerical demonstrations. These findings can broaden the potential applications of the metasurface-based absorbers in the THz frequency range for different requirements.

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