▎ 摘　　要

Plasmon is the quantum of the collective oscillation of electrons. How plasmon loses its energy (or damping) plays a pivotal role in plasmonic science and technology. Graphene plasmon is of particular interest, partly because of its potentially low damping rate. However, to date, damping pathways have not been clearly unravelled experimentally. Here, we demonstrate mid-infrared (4-15 mu m) plasmons in graphene nanostructures with dimensions as small as 50 nm (with a mode area of similar to 1x10(-3) mu m(2)). We also reveal damping channels via graphene intrinsic optical phonons and scattering from the edges. Plasmon lifetimes of 20 fs or less are observed when damping via the emission of graphene optical phonons is allowed. Furthermore, surface polar phonons in the SiO2 substrate under graphene nanostructures lead to a significantly modified plasmon dispersion and damping, in contrast to the case of a nonpolar diamond-like-carbon substrate. Our study paves the way for applications of graphene in plasmonic waveguides, modulators and detectors from sub-terahertz to mid-infrared regimes.