Metamaterial absorbers have been widely studied and continuously concerned owing to their excellent resonance features of ultra-thin thickness, light-weight, and high absorbance. Their applications, however, are typically restricted by the intrinsic dispersion of materials and strong resonant features of patterned arrays (mainly referring to narrow absorption bandwidth). It is, therefore essential to reassert the principles of building broadband metamaterial absorbers (BMAs). Herein, the research progress of BMAs from principles, design strategies, tunable properties to functional applications are comprehensively and deeply summarized. Physical principles behind broadband absorption are briefly discussed, typical design strategies in realizing broadband absorption are further emphasized, such as top-down lithography, bottom-up self-assembly, and emerging 3D printing technology. Diversified active components choices, including optical response, temperature response, electrical response, magnetic response, mechanical response, and multi-parameter responses, are reviewed in achieving dynamically tuned broadband absorption. Following this, the achievements of various interdisciplinary applications for BMAs in energy-harvesting, photodetectors, radar-IR dual stealth, bolometers, noise absorbing, imaging, and fabric wearable are summarized. Finally, the challenges and perspectives for future development of BMAs are discussed.

This paper presents a scheme to achieve a broadband metamaterial absorber in the terahertz region, which is consisted of an anti-symmetric T-shaped resonator (formed by two mutually perpendicular metallic strips) on top of a metallic back layer separated by an insulator dielectric layer. The anti-symmetric T-shaped resonator can have a strong electromagnetic interaction with the incident beam, resulting in four different absorption peaks having adjacent frequencies, and the superposition effect of these adjacent peaks produces a broadband absorption. The resonance bandwidth of greater than 80% absorption can have 0.58 THz ranging from 1.54 to 2.12 THz, and its relative resonance bandwidth is 31.69%. The resonance bandwidth and relative resonance bandwidth can be scaled by controlling the parameters of two mutually perpendicular metallic strips in the anti-symmetric T-shaped resonator. Moreover, the parameter optimization of the anti-symmetric T-shaped resonator can realize five discrete absorption peaks with adjacent frequencies, and their combined effect can obtain not only broadband absorption but also a greater absorption bandwidth compared with the case of broadband absorption having four absorption peaks.

Triple-band terahertz metamaterial absorber with design of miniaturization and compactness is presented in this work. The unit cell of the terahertz absorber is formed by an analogy I-typed resonator (a rectangular patch with two small notches) deposited on top of dielectric sheet and metallic mirror. The miniaturized structure design exhibits three discrete frequency points with near-perfect absorption at terahertz regime. The three absorption peaks could be ascribed to localized resonances of analogy I-typed resonator, while the response positions of these absorption peaks at the analogy I-typed resonator are different by analyzing the near-field patterns of these resonance peaks. Changes in structure parameters of the analogy I-typed resonator are also investigated. Simulation results revealed that the notch sizes of the rectangular patch are the key factor to form the triple-band near-perfect absorption. Further structure optimization is given to demonstrate triple-band polarization insensitive performance. Moreover, actively tunable absorption properties are realized by inserting or introducing vanadium dioxide with adjustable conductivity into the metamaterial structure. It is revealed that the insulator–metal phase transition of vanadium dioxide is the main reason for the modulation of absorption performance. Compared with previous multiple-band absorbers, the device given here has excellent features of high degrees of simplification, miniaturization, and active modulation, these are important in practical applications.

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