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Role of Wood’s anomaly in the performance of metamaterial absorbers with periodicity comparable to wavelength

We examined simple metamaterial absorbers (MMAs) in a frequency regime where the periodicity of the unit cell of MM was comparable to the wavelength of the incident electromagnetic wave, and found that Wood’s anomaly caused by in-plane propagation played an important role. The investigated structure is composed of a cut-wire array and a continuous metal plate on the front and rear sides of the FR-4 substrate, respectively. We observed two absorption peaks corresponding to the fundamental and its third-harmonic resonances. Besides these two peaks, a minor peak located at a slightly higher frequency than that of the third-harmonic peak was observed. The existence of the minor peak was not manifested in the simulation, in which periodic boundary conditions (PBCs) were routinely used. By using a non-PBC simulation the minor peak was successfully reproduced. The so-called Wood’s anomaly plays a crucial role not only in the appearance of the minor peak but also the angle-of-incidence dependence of both the major and minor peaks, in which the structural conditions of the metallic patterns on the front side is key. Although our investigations are restricted to MMAs, the results obtained can be easily extended to the performance of other MM-based devices, especially, when operating in frequency ranges that are recommended to be avoided due to diffraction effects.