Broadband optically transparent microwave absorber made of interdigital metasurfaces in rotational symmetry with a single air spacer

A broadband optically transparent metasurface microwave absorber (MMA) is designed and experimentally studied. The MMA is made of two indium tin oxide (ITO) resistive films deposited on two transparent polyethylene terephthalate substrates respectively, between which is sandwiched a single air spacer. The top ITO resistive film is etched with periodic interdigital metasurface patterns in rotational symmetry, while the bottom ITO resistive film is an integrated sheet with a low resistance working as the backplane. By carefully optimizing the functional interdigital metasurface structures in a numerical solver, a desirable 4-octave broadband MMA is achieved. The absorbing bandwidth is 4.53–18.71 GHz (122.03%) in the numerical predictions for the perpendicular incidence, in which the absorptivity is greater than 90%. Its total thickness is only 5.8 mm or 0.088λL, where λL is the wavelength (66.23 mm) at the lowest 4.53 GHz. The absorber is validated in experiments. Results are observed in good agreement with the simulated ones. The interdigital MMA is polarization-insensitive and able to operate for wide-angle incidences up to 45°. These properties are demonstrated in both simulations and experiments.