Versatile Protrusive VO<sub>2</sub> Metamaterials Enabling Tunable Ultra-broadband Terahertz Absorption

Until now, it has been conventional to design tunable metamaterial absorbers with single layer scheme of metal-dielectric-VO<sub>2</sub>. For further expansion of the operating bandwidth, the construction method stacking several layers of dielctric-VO<sub>2</sub> has been utilized. However, the single layer configuration of metal-dielectric-VO<sub>2</sub> cannot extend the absorption bandwidth to the entire THz band. Meanwhile, In the stacking method, other VO<sub>2</sub> resonators excepting the topmost VO<sub>2</sub> resonator are embedded in the dielectric substrate. This unavoidably leads to the delay in response time to external stimuli. So, apart from these conventional methods, we have conceived the protrusive VO<sub>2</sub> metamaterials, which are effective for overcoming the above limitations. The protrusive VO<sub>2</sub> metamaterials consist of a VO<sub>2</sub> square ring, a protrusive VO<sub>2</sub> square patch and a polyimide substrate backed with a gold bottom plane. Based on the proposed design idea, we demonstrate a tunable ultra-broadband THz metamaterial absorber (MA) with bandwidth of 9.39 THz. That is, the protrusive VO<sub>2</sub> metamaterials greatly enlarge the operating bandwidth with perfect absorption. The proposed MA can obtain the absorptivity above 90% in range of 1.67-11.06 THz, with relative absorption bandwidth of 147.52%. Moreover, the protrusive VO<sub>2</sub> metamaterials can realize the absorption tunability by modifying the conductivity of VO<sub>2</sub>. Additionally, we also demonstrate that the idea of protrusive VO<sub>2</sub> metamaterials is very useful for designing several tunable ultra-broadband THz MAs. Owing to the excellent structural versatility, the protrusive VO<sub>2</sub> metamaterials are of great value for THz applications.