▎ 摘　　要

Terahertz electromagnetic ( EM) wave absorbers are vital in photonics, however, they suffer from limited bandwidth. A new approach for ultra- wideband ( UWB) terahertz absorber design is proposed with metal and graphene metasurfaces. The UWB characteristics are owing to three factors: ( 1) the metal metasurface boosts the surface plasmon- polaritons ( SPPs) of the graphene metasurface which leads to confined field enhancement, ( 2) the merging and interaction of the resonances of the metal and graphene metasurfaces, and ( 3) the multiple reflections and superpositions between the metasurfaces and the gold layer. A prototype designed using a dual- ring metal metasurface, fishnet graphene metasurface, polyimide substrate and gold reflecting layer is proposed. One cell of the prototype includes one metal dual- ring unit and four graphene fishnet units. The proposed absorber has an UWB bandwidth of 6.46 THz ( 145%) for absorptivity larger than 0.9, with a high octave of 6.21. The proposed absorber is also insensitive to the polarization state and incident angle of the illuminating EM waves. Besides, the amplitude modulation depth in the 5- 6 THz band is up to 95.4%. The physical mechanisms of the wideband operation are also discussed. The research in this work could offer a new thought for UWB absorber design, and has potential applications in terahertz imaging, sensors, photodetectors and modulators ( e. g. [ L. Peng, X. M. Li, X. Liu, X. Jiang and S. M. Li, Nanoscale Adv., 2000, 35, 3523]).