Radiative Quality Factor in Thin Resonant Metamaterial Absorbers

Perfect electromagnetic absorption in an array of thin resonators is analyzed by means of the quality factor involving separately the contribution of losses and a coupling with free space. An equivalent electrical circuit based on an open-resonator model is introduced for an absorber made of dielectric cubes arranged in a square lattice deposited onto a metallic ground plane. From full-wave simulations, two regimes depending on the lattice period are pointed out. A quadratic dependence of the radiative quality factor is shown for largest periods, whereas the radiative Q-factor is governed by a coupling between resonators at small periods. It results that an optimal period for obtaining a unitary absorption can be deduced from a single simulation and that a maximal absorption bandwidth emerges from a tradeoff between these two regimes. Finally, the radiative Q-factor concept is applied to analyze an absorber made of patch resonators with the goal to broaden the absorption bandwidth. Particularly, the performances of a multisized patch absorber are experimentally evaluated in the W-band (75-110 GHz) with a good agreement when compared to simulations. Such an analysis of Q-factor appears as a powerful tool for designing singlesized and multisized resonator absorbers targeting a specific absorption spectrum.