Nano-infrared imaging of surface plasmons in twisted bilayer graphene

One of the major goals of the field of graphene plasmonics is to achieve functional nanophotonic circuits operating in the terahertz to infrared spectral regions. To achieve that, it is necessary to develop a large selection of graphene-based building blocks with plasmonic parameters that are stable even without continuous electrical gating. Here, we report the observation of tailored plasmonic responses in twisted bilayer graphene (tBLG) – two graphene layers stacked with a twist angle. Through near-field imaging of tBLG single crystals with a wide distribution of twist angles, we found that tBLG supports confined infrared plasmons that are sensitively dependent on the twist angle. More specifically speaking, as the twist angle varies from 0 to 30 degrees, the plasmon wavelength of tBLG increases and the plasmon damping rate decreases systematically. Further analysis and modeling indicate that the observed twist-angle-dependence of tBLG plasmons is mainly due to the Fermi-velocity renormalization, a unique characteristic of tBLG originated from the interlayer electron coupling.