Enabling Ultrathin Metamaterial Absorbers with Narrow Slits

Metamaterial absorbers (MAs) consisting of subwavelength units offer flexible design freedom and prominent advantages over traditional absorbers. Here, it is found that narrow slits provide the MAs a significant passageway to control the absorption behavior and obtain an ultrasmall thickness. As narrowing the slits in an MA gradually, two effects appear in turn, of which the former is that the slits help the localized energy leak away from the resonant units to balance the material absorption and radiation leakage rates and sustain perfect absorption, and the latter is that acting as a capacitor, the slits red-shift the absorption peak. A record ultrathin microwave MA with a thickness of only 1/474 of the working wavelength is demonstrated. Colossal electric field is excited inside ultranarrow slits by the assistance of localized resonance in collecting the incident wave. The finding can be extended to other frequencies, and is fascinating for some areas requiring ultrathin absorption structures or strong interaction between waves and matters.