A three-dimensional gravity tailored ultra-broadband absorber based on the high-impedance surface

In this paper, we present a novel, to the best of our knowledge, 3D ultra-broadband tunable metamaterial absorber (TMA), which is innovatively tuned by the gravity field with changing the position of the liquid metal (eutectic gallium-indium) in theory. The given TMA consists of a 3D glass cavity filled with the liquid metal, two-layer high-impedance surfaces, two dielectric layers, and a metallic plate on the bottom. The ultra-broadband absorption is achieved by the high-impedance surface and the liquid metal. Furthermore, it is noted that the liquid metal is poured into the different parts of the glass cavities via rotation under the action of the gravity field to obtain the tunable absorption. When such a TMA is not rotated, for the TE wave, the absorption rate exceeds 90% from 1.8 to 57.5 GHz with a wide relative absorption bandwidth (AB) of 187.8%. However, for the TM wave, the absorptivity goes beyond 90% at 5.6–56.4 GHz. The proposed TMA exhibits ultra-broadband absorption in the frequency regime of 1.6–45.3 GHz (the absorption rate is higher than 90%) with such a TMA rotated 180° in the x−y plane, while the absorption rate is near zero from 1.6 to 60 GHz when the proposed TMA is rotated 180° in the y−z plane. Moreover, the surface current distributions, the near-field amplitude |Ez|, the phase φz, and the magnetic field distributions are investigated to describe the operating mechanism of the designed TMA. Besides, the numerical results indicate that such a TMA can realize high absorption under a wider incident angle. Compared with traditional tunable devices, the gravity field regulation, which can change the operating frequencies of an absorber in a noncontact way, has the advantages of easy implementation and resource savings. The proposed absorber tuned by the gravity will have potential applications in frequency selection. With the change of liquid metal and structure design, this design can also be potentially applied in other microwave and terahertz devices. Taken together, this study suggests a role for such an absorber in promoting a novel application for tunable devices.