Wide-band and narrow-band switchable terahertz absorber based on graphene

A three-layer terahertz dual-function absorber with broadband absorption and narrowband absorption switching is proposed based on the special electrically tunable non-equilibrium optical response of graphene. Instead of using phase change materials, the structure uses patterned graphene in the top layer to achieve the switching effect. Patterned graphene consists of a cross and an external square frame. The relaxation time is adjusted by applying an electric field to the graphene to achieve broadband and narrowband switching. The structure can achieve broadband absorption when the relaxation time of graphene is 0.2 ps. From 1.27 to 2.16 THz, the absorbance of the structure can exceed 90%, and the average absorption rate can reach 96.88%. At 1.68-1.96 THz, the absorbance of the structure can even exceed 99%. When the relaxation time of graphene is 3 ps, the structure achieves narrowband absorption at 1.72 THz with a maximum absorption rate of 99%, achieving "perfect" absorption. Simultaneously, the proposed structure is highly symmetric, both modes are stable in the incidence angle range of 0-60 degrees, and polarization insensitive. Once the absorber achieves broadband absorption, we control the chemical potential of graphene by adjusting the applied voltage to switch the absorption rate from 0% to 99%. Once the absorber achieves narrow-band absorption, we verify that the structure can be used as a sensor by changing the refractive index of the surrounding environment. Accordingly, we demonstrate that the absorber proposed in this study has potential applications in the fields of photoelectric switches, terahertz energy harvesting, and refractive index sensing.