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The special electromagnetic properties of metamaterials have contributed to the development of terahertz technology, and terahertz broadband absorbers for various applications have been investigated. The design of metamaterial absorbers with tunability is in a particularly attractive position. In this work, a tunable broadband terahertz metamaterial absorber is proposed based on the phase transition material vanadium dioxide (VO2). The simulation results show that an excellent absorption bandwidth reaches 3.78 THz with the absorptivity over 90% under normal incidence. The absorptivity of the proposed structure can be dynamically tuned from 2.7% to 98.9% by changing the conductivity of VO2, which changes the structure from a perfect reflector to an absorber. An excellent amplitude modulation with the absorptivity is realized. The mechanism of broadband absorption is explored by analyzing the electric field distribution of the absorber based on impedance matching theory. In addition, it also has the advantage of polarization and incident angle insensitivity. The proposed absorber may have a wide range of promising applications in areas such as terahertz imaging, sensing, and detection.

In order to widen the absorption band of the metamaterial absorber to expand its application field, using the electromagnetic characteristics of water in the microwave band, a water-based metamaterial absorber with a structure different from the traditional “metal layer–dielectric layer–metal layer” one is proposed. The numerical simulation results show that its absorptivity of incident electromagnetic waves exceeds 90% in the ultra-wide operating band of 7.84–74.16 GHz and the absorption bandwidth can cover five bands including the X-band, Ku-band, K-band, Ka-band, and U-band. Based on the theories of equivalent medium and impedance matching, the mechanism of broadband absorption was explored by analyzing the electromagnetic field distribution and power loss density of the absorber. The designed absorber has the absorption performance of polarization independence, and it can work under a wide incident angle. In addition, the absorber has excellent thermal stability which can still maintain stable operation when the temperature changes from 0 °C to 100 °C. This water-based metamaterial absorber is of low cost, and its manufacturing process is relatively convenient, so it has broad application prospects in the fields of radar stealth, microwave darkroom, and prevention of electromagnetic radiation.