Tunable mid-infrared absorption in a compound graphene grating waveguide structure supporting quasi-bound states in the continuum

In this work, we introduce a stacked pair of graphene ribbons array into a slab waveguide to achieve tunable optical absorption of a structure in the mid-infrared region. The quasi-bound states in the continuum (quasi-BICs) based on the selective guided resonances can be realized by alternatively changing the Fermi energy of graphene ribbons. Moreover, the radiation coupling rate of quasi-BIC can be modulated by changing the Fermi energy of graphene, which contributes to tunable optical absorption of the structure. Different from the effect of plasmon resonance of graphene ribbons, the resonance frequency of the quasi-BIC is insensitive to the Fermi energy of graphene. The absorption modulation depth can be larger than 80%. The studied results may have potential applications in switchable infrared photodetectors and thermal radiation systems.