Excitation and tuning of a dual graphene plasmonic wave based on a trapezoidal grating structure

The independent excitation and tuning of a dual-band graphene plasmonic wave are realized in a hybrid structure that consists of two graphene monolayers placed above and below the trapezoidal grating. Because of the transparency of graphene in the mid-infrared range, the incident light can travel through the first graphene layer to be diffracted by the grating structure and couple its energy to both graphene layers. Numerical simulations are performed using the finite difference time domain method. Results show that the plasmon resonances corresponding to the two graphene monolayers are excited at 9.8 and 10.9 μm, which agrees well with the theoretical analysis. Because of the fast and efficient electrical tunability of graphene, the resonance wavelengths can be tuned individually by changing the chemical potential of the corresponding graphene. Furthermore, the effects of geometric parameters and the refractive index of the surrounding media are studied. The results show that the structure can achieve an optimal sensing coefficient at 0.4 and 0.5 eV for the top and bottom graphene plasmon resonances, respectively. The proposed structure provides an alternative option to engineer the proposed structure for sensing with high detection accuracy at mid-infrared wavelengths.