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 μm) plasmons in graphene nanostructures with dimensions as small as 50 nm (with a mode area of ∼1 × 10 -3 μm 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 SiO 2 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.
Bibliographical noteFunding Information:
The authors thank B. Ek, J. Bucchignano and S. (Jay) Chey for technical assistance, and V. Perebeinos and Z. Li of the National High Magnetic Field Laboratory and T.F. Heinz of Columbia University for discussions. F.X. thanks C. Gmachl of Princeton University and Y. Yao of Harvard University for help in the planning stage of the project. T.L. and F.G. acknowledge the hospitality of KITP, supported in part by the National Science Foundation (grant no. NSF PHY11-25915). T.L. also acknowledges partial support from NRI-INDEX, and F.G. is also supported by the Spanish MICINN (FIS2008-00124, CONSOLIDER CSD2007-00010) and ERC grant 290846.