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Switchable metamaterial absorbers/reflectors (MAs/MRs) are important bifunctional electromagnetic devices and have been the subject of numerous scientific studies. However, there is a lack of bifunctional devices that operate in the terahertz band. Here, we theoretically propose a broadband switchable MA with many excellent properties, such as good thermal stability, high insensitivity to inferior film quality of the graphene, excitation polarization and wide incident angles, and outstanding structural parameter tolerance. The bandwidth of the proposed broadband MA is 3.4 THz with an absorptivity over 90% in the frequency band of 1.6-5 THz. The proposed absorber can switch to a reflector with a reflectivity over 93% by tuning the chemical potential of the graphene and reducing the temperature. Therefore, the switching intensity of the proposed MA exceeds 83%. The physical mechanisms of the broadband absorption of the proposed structure are investigated using the impedance matching theory and the multiple reflection interference theory. The reflection mechanism of the proposed broadband reflector is discussed by analyzing the effective parameters. The absorption and switching mechanism are theoretically investigated by performing detailed numerical calculations to analyze the surface loss intensity, electric field, and magnetic field. These findings can accelerate the development of terahertz broadband switchable devices.

Magnetic resonators based on metamaterials are valuable for numerous applications including perfect absorber, sensing and medical imaging. However, due to the existence of the difficulty in magnetic excitation in planar metasurface, the study of magnetic metamaterial resonators is less reported, particularly in contrast to the electrical resonators. In this paper, the three-dimensional split ring resonator (SRR) metamaterials featuring dual-band magnetic plasmonic resonance modes are proposed and numerically analyzed. We calculate the electromagnetic field distributions of the three-dimensional metamaterials at the resonant frequencies to elucidate the resonant characteristics of the dual modes (fundamental LC mode and high-order magnetic plasmonic resonance mode). The influences of geometric parameters of the constituent meta-atoms on the resonance frequencies of the two resonance modes are analyzed by numerical calculations. The results show that the resonance frequency of the high-order magnetic resonance mode is nearly independent of the standing columns of the metamaterials owing to the special resonant characteristic of the high-order mode. Benefiting from the independence, the frequency interval between the dual modes can be customized by adjusting the dimensions of the column. In addition, by tuning the dimension of the columns, the quality factors of the resonances can reach the highest value of 176 for LC mode and 270 for the high-order mode, respectively. In light of practical application, we evaluate refractive index sensing performance of the proposed metamaterials, the maximum sensitivity can reach 474 GHz/RIU for LC mode and 446 GHz/RIU for high-order resonance, respectively. We also explore the universality of the special resonant characteristic of the high-order mode in other similar three-dimension SRR metamaterials, which shows all of these three-dimension magnetic plasmonic metamaterials provide potential platforms for multiband magnetic metamaterial applications.