▎ 摘　　要

We report an infrared (IR) imaging study of trilayer graphene (TLG) with both ABA (Bernal) and ABC (rhombohedral) stacking orders using the scattering-type scanning near-field optical microscope (SSNOM). With the SSNOM operating in the mid-IR region, we map in real space the surface plasmon polaritons of ABA TLG and ABC TLG, which are tunable with electrical gating. Through quantitative modeling of plasmonic imaging data, we find that the plasmon wavelength of ABA TLG is signifi-cantly larger than that of ABC TLG, resulting in a sizable impedance mismatch and hence a strong plasmon reflection at the ABA/ABC lateral junction. Further analysis indicates that the different plas-monic responses of the two types of TLG are directly linked to their distinct electronic structures and carrier properties. Our work uncovers the physics behind the stacking-dependent plasmonic responses of TLG and sheds light on future applications of TLG and the ABA/ABC junctions in IR plasmonics and planar nano-optics.