A bifunctional tunable terahertz absorber based on a Fabry–Perot cavity: enabling broadband perfect absorption and refractive index sensing

Abstract

In this paper, a terahertz (THz) metamaterial absorber (MA) based on the Fabry–Perot cavity is proposed. The combination of graphene and vanadium dioxide (VO₂) enables switching between broadband perfect absorption and tri-narrowband absorption. When VO₂ acts as a metal, the MA can realize an absorption rate exceeding 99% with a bandwidth of 3.14 THz. Additionally, by tuning the Fermi energy level of graphene, the bandwidth of perfect absorption can be extended to 4.23 THz. When VO₂ is transformed into the insulating state, the proposed MA exhibits tri-narrowband absorption, making it suitable for refractive index (RI) sensing. The sensitivities (S) of the three peaks are 0.847 THz per RIU, 0.828 THz per RIU, and 1.326 THz per RIU, respectively. Compared to previous MAs, the proposed structure significantly broadens the bandwidth of perfect absorption, thereby enhancing the absorption efficiency. Moreover, the integration of the sensing capabilities expands the potential applications of the THz MA.